element in HTML private HREF="#RefHTML"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [5].
Transfer Encoding The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient. This differs from the Content-Encoding in that the transfer coding is a property of the message, not of the original resource entity. Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding
Transfer codings are defined in section ref Transfer_Codings \n 7.6. An example is: Transfer-Encoding: chunked
Many older HTTP/1.0 applications do not understand the Transfer-Encoding header.
Upgrade The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like to use if the server finds it appropriate to switch protocols. The server MUST use the Upgrade header field within a 101 (Switching Protocols) response to indicate which protocol(s) are being switched. Upgrade = "Upgrade" ":" 1#product
For example, Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11
The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a �better� protocol if available (where �better� is determined by the server, possibly according to the nature of the method and/or resource being requested).
The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request containing the Upgrade header field.
The Upgrade header field only applies to the immediate connection. Therefore, the �upgrade� keyword MUST be supplied within a Connection header field (section ref Connection \n 18.11) whenever Upgrade is present in an HTTP/1.1 message.
The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response.
This specification only defines the protocol name �HTTP� for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of section ref HTTP_Version \n 7.1 and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both the client and server associate the name with the same protocol.
User-Agent The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. Although it is not required, user agents SHOULD include this field with requests. The field can contain multiple product tokens (section ref Product \n 7.8) and comments identifying the agent and any subproducts which form a significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the application. User-Agent = "User-Agent" ":" 1*( product Example: User-Agent: CERN-LineMode/2.15 libwww/2.17b3
Vary
The Vary response-header field is used by an origin server to signal that the resource identified by the current request is a generic) resource. A generic resource has multiple entities associated with it, all of which are representations of the content of the resource. If a GET or HEAD request on a generic resource is received, the origin server will select one of the associated entities as the entity best matching the request. Selection of this entity is based on the contents of particular header fields in the request message, or on other information pertaining to the request, like the network address of the sending client.
A resource being generic has an important effect on cache management, particularly for caching proxies which service a diverse set of user agents. All 200 (OK) responses from generic resources MUST contain at least one Vary header (section ref Vary \n 18.46) or Alternates header (section ref Alternates \n 18.8) to signal variance. If no Vary headers and no Alternates headers are present in a 200 (OK) response, then caches may assume, as long as the response is fresh, that the resource in question is plain, and has only one associated entity. Note however that this entity can still change through time, as possibly indicated by a Cache-Control response header (section ref Cache_Control \n 18.10).
After selection of the entity best matching the current request, the origin server will usually generate a 200 (OK) response, but it can also generate other responses like 206 (Partial Content) or 304 (Not Modified) if headers which modify the semantics of the request, like Range (section ref Range \n 18.38) or If-Match (section ref If_Match \n 18.26), are present. An origin server need not be capable of selecting an entity for every possible incoming request on a generic resource; it can choose to generate a 3xx (redirection) or 4xx (client error) type response for some requests.
In a request message on a generic resource, the selecting request headers are those request headers whose contents were used by the origin server to select the entity best matching the request. The Vary header field specifies the selecting request headers and any other selection parameters that were used by the origin server. Vary = "Vary" ":" 1#selection-parameter selection-parameter = request-header-name | "{accept-headers}" | "{other}" request-header-name = field-name extension-parameter = token
The presence of a request-header-name signals that the request-header field with this name is selecting. Note that the name need not belong to a request-header field defined in this specification, and that header names are case-insensitive. The presence of the �{accept-headers}� parameter signals that all request headers whose names start with �accept� are selecting.
The inclusion of the �{other}� parameter in a Vary field signals that parameters other than the contents of request headers, for example the network address of the sending party, play a role in the selection of the response.
Note: This specification allows the origin server to express that other parameters were used, but does not allow the origin server to specify the exact nature of these parameters. This is left to future extensions.
If an extension-parameter unknown to the cache is present in a Vary header, the cache MUST treat it as the �{other}� parameter. If multiple Vary and Alternates header fields are present in a response, these MUST be combined to give all selecting parameters.
The field name �Host� MUST never be included in a Vary header; clients MUST ignore it if it is present. The names of fields which change the semantics of a GET request, like �Range� and �If-Match� MUST also never be included, and MUST be ignored when present. Servers which use access authentication are not obliged to send �Vary: Authorization� headers in responses. It MUST be assumed that requests on authenticated resources can always produce different responses for different users. Note that servers can signal the absence of authentication by including �Cache-Control: public� header in the response.
A cache MAY store and refresh 200 (OK) responses from a generic resource according to the rules in section ref Constructing_Respons\n 16.4. The partial entities in 206 (Partial Content) responses from generic resources MAY also be used by the cache.
When getting a request on a generic resource, a cache can only return a cached 200 (OK) response to one of its clients in two particular cases.
First, if a cache gets a request on a generic resource for which it has cached one or more responses with Vary or Alternates headers, it can relay that request towards the origin server, adding an If-NoneMatch header listing the etag-info values in the ETag headers (section ref CVal \n Error! Reference source not found.) of the cached responses which have variant-IDs. If it then gets back a 304 (Not Modified) response with the etag-info of a cached 200 (OK) response in its ETag header, it can return this cached 200 (OK) response to its client, after merging in any of the 304 response headers as specified in section ref Non_Modifiable_Headr\n 16.4.2.
Second, if a cache gets a request on a generic resource, it can return to its client a cached, fresh 200 (OK) response which has Vary or Alternates headers, provided that
� the Vary and Alternates headers of this fresh response specify that only request header fields are selecting parameters,
� the specified selecting request header fields of the current request match the specified selecting request header fields of a previous request on the resource relayed towards the origin server,
� this previous request got a 200 (OK) or 304 (Not Modified) response which had the same etag-info value in its ETag header as the cached, fresh 200 (OK) response.
Two sequences of selecting request header fields match if and only if the first sequence can be transformed into the second sequence by only adding or removing whitespace at places in fields where this is allowed according to the syntax rules in this specification.
If a cached 200 (OK) response MAY be returned to a request on a generic resource which includes a Range request header, then a cache MAY also use this 200 (OK) response to construct and return a 206 (Partial Content) response with the requested range.
Note: Implementation of support for the second case above is mainly interesting in user agent caches, as a user agent cache will generally have an easy way of determining whether the sequence of request header fields of the current request equals the sequence sent in an earlier request on the same resource. Proxy caches supporting the second case would have to record diverse sequences of request header fields previously relayed; the implementation effort associated with this may not be balanced by a sufficient payoff in traffic savings. A planned specification of a content negotiation mechanism will define additional cases in which proxy caches can return a cached 200 (OK) response without contacting the origin server. The implementation effort associated with support for these additional cases is expected to have a much better cost/benefit ratio.
Via The Via general-header field MUST be used by gateways and proxies to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses. It is analogous to the �Received� field of RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9] and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain. Via = "Via" ":" 1#( received-protocol received-by [ comment ] ) received-protocol = [ protocol-name "/" ] protocol-version protocol-name = token protocol-version = token received-by = ( host [ ":" port ] ) The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients.
The protocol-name is optional if and only if it would be �HTTP�. The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol.
Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications.
Comments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message.
For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named �fred�, which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the following Via header field: Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports of hosts within the firewall region. This information SHOULD only be propagated if explicitly enabled. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host.
For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example, Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy could be collapsed to Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. Applications MUST NOT combine entries which have different received-protocol values.
Note: The Via header field replaces the Forwarded header field which was present in earlier drafts of this specification.
Warning
Warning headers are sent with responses using: Warning = "Warning" ":" warn-code SP warn-agent SP warn-text warn-code = 2DIGIT warn-agent = ( host [ ":" port ] ) A response may carry more than one Warning header.
The warn-text should be in a natural language and character set that is most likely to be intelligible to the human user receiving the response. This decision may be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the Content-Language field in a response, etc. The default language is English and the default character set is ISO-8599-1.
If a character set other than ISO-8599-1 is used, it must be encoded in the warn-text using the method described in RFC 1522 [14].
Any server or cache may add Warning headers to a response. New Warning headers should be added after any existing Warning headers. A cache MUST NOT delete any Warning header that it received with a response. However, if a cache successfully validates a cache entry, it SHOULD remove any Warning headers previously attached to that entry. It MUST then add any Warning headers received in the validating response. In other words, Warning headers are those that would be attached to the most recent relevant response.
When multiple Warning headers are attached to a response, the user agent SHOULD display as many of them as possible, in the order that they appear in the response. If it is not possible to display all of the warnings, the user agent should follow these heuristics:
� Warnings that appear early in the response take priority over those appearing later in the response.
� Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes and warn-agents.
Systems that generate multiple Warning headers should order them with this user-agent behavior in mind.
This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning.
10 Response is stale MUST be included whenever the returned response is stale. A cache may add this warning to any response, but may never remove it until the response is known to be fresh.
11 Revalidation failed MUST be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability to reach the server. A cache may add this warning to any response, but may never remove it until the response is successfully revalidated.
12 Disconnected operation SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time.
99 Miscellaneous warning The warning text may include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action. WWW-Authenticate The WWW-Authenticate response-header field MUST be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI. WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge
The HTTP access authentication process is described in section ref AA \n 14. User agents MUST take special care in parsing the WWW-Authenticate field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters.
Security Considerations This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.1 as described by this document. The discussion does not include definitive solutions to the problems revealed, though it does make some suggestions for reducing security risks.
Authentication of Clients As mentioned in section ref AA \n 14, the Basic authentication scheme is not a secure method of user authentication, nor does it in any way protect the Entity-Body, which is transmitted in clear text across the physical network used as the carrier. HTTP does not prevent additional authentication schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes to use one-time passwords) to Basic authentication.
The most serious flaw in Basic authentication is that it results in the essentially clear text transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address.
Because Basic authentication involves the clear text transmission of passwords it SHOULD never be used (without enhancements) to protect sensitive or valuable information.
A common use of Basic authentication is for identification purposes -
If a server permits users to select their own passwords, then the threat is not only illicit access to documents on the server but also illicit access to the accounts of all users who have chosen to use their account password. If users are allowed to choose their own password that also means the server must maintain files containing the (presumably encrypted) passwords. Many of these may be the account passwords of users perhaps at distant sites. The owner or administrator of such a system could conceivably incur liability if this information is not maintained in a secure fashion.
Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting to a host containing information protected by basic authentication when in fact he is connecting to a hostile server or gateway then the attacker can request a password, store it for later use, and feign an error. This type of attack is not possible with Digest Authentication[26]. Server implementers SHOULD guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous for a server to simply turn over a connection to a gateway since that gateway can then use the persistent connection mechanism to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable by the client.
Safe Methods The writers of client software should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they may take which may have an unexpected significance to themselves or others.
In particular, the convention has been established that the GET and HEAD methods should never have the significance of taking an action other than retrieval. These methods should be considered �safe. � This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.
Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.
Abuse of Server Log Information A server is in the position to save personal data about a user's requests which may identify their reading patterns or subjects of interest. This information is clearly confidential in nature and its handling may be constrained by law in certain countries. People using the HTTP protocol to provide data are responsible for ensuring that such material is not distributed without the permission of any individuals that are identifiable by the published results.
Transfer of Sensitive Information Like any generic data transfer protocol, HTTP cannot regulate the content of the data that is transferred, nor is there any a priori method of determining the sensitivity of any particular piece of information within the context of any given request. Therefore, applications SHOULD supply as much control over this information as possible to the provider of that information. Four header fields are worth special mention in this context: Server, Via, Referer and From.
Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementers SHOULD make the Server header field a configurable option.
Proxies which serve as a portal through a network firewall SHOULD take special precautions regarding the transfer of header information that identifies the hosts behind the firewall. In particular, they SHOULD remove, or replace with sanitized versions, any Via fields generated behind the firewall.
The Referer field allows reading patterns to be studied and reverse links drawn. Although it can be very useful, its power can be abused if user details are not separated from the information contained in the Referer. Even when the personal information has been removed, the Referer field may indicate a private document's URI whose publication would be inappropriate.
The information sent in the From field might conflict with the user's privacy interests or their site's security policy, and hence it SHOULD NOT be transmitted without the user being able to disable, enable, and modify the contents of the field. The user MUST be able to set the contents of this field within a user preference or application defaults configuration.
We suggest, though do not require, that a convenient toggle interface be provided for the user to enable or disable the sending of From and Referer information.
Attacks Based On File and Path Names Implementations of HTTP origin servers SHOULD be careful to restrict the documents returned by HTTP requests to be only those that were intended by the server administrators. If an HTTP server translates HTTP URIs directly into file system calls, the server MUST take special care not to serve files that were not intended to be delivered to HTTP clients. For example, UNIX, Microsoft Windows, and other operating systems use �..� as a path component to indicate a directory level above the current one. On such a system, an HTTP server MUST disallow any such construct in the Request-URI if it would otherwise allow access to a resource outside those intended to be accessible via the HTTP server. Similarly, files intended for reference only internally to the server (such as access control files, configuration files, and script code) MUST be protected from inappropriate retrieval, since they might contain sensitive information. Experience has shown that minor bugs in such HTTP server implementations have turned into security risks.
Personal Information
HTTP clients are often privy to large amounts of personal information (e.g. the user's name, location, mail address, passwords, encryption keys, etc.), and SHOULD be very careful to prevent unintentional leakage of this information via the HTTP protocol to other sources. We very strongly recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers and implementers be particularly careful in this area. History shows that errors in this area are often both serious security and/or privacy problems, and often generate very adverse publicity for the implementer's company.
Privacy Issues Connected to Accept headers
Accept request headers can reveal information about the user to all servers which are accessed. The Accept-Language header in particular can reveal information the user would consider to be of a private nature, because the understanding of particular languages is often strongly correlated to the membership of a particular ethnic group. User agents which offer the option to configure the contents of an Accept-Language header to be sent in every request are strongly encouraged to let the configuration process include a message which makes the user aware of the loss of privacy involved. An approach that limits the loss of privacy would be for a user agent to omit the sending of Accept-Language headers by default, and to ask the user whether it should start sending Accept-Language headers to a server if it detects, by looking for any Vary or Alternates response headers generated by the server, that such sending could improve the quality of service.
Elaborate user-customized accept header fields sent in every request, in particular if these include quality values, can be used by servers as relatively reliable and long-lived user identifiers. Such user identifiers would allow content providers to do click-trail tracking, and would allow collaborating content providers to match cross-server click-trails or form submissions of individual users. Note that for many users not behind a proxy, the network address of the host running the user agent will also serve as a long-lived user identifier. In environments where proxies are used to enhance privacy, user agents should be conservative in offering accept header configuration options to end users. As an extreme privacy measure, proxies could filter the accept headers in relayed requests. General purpose user agents which provide a high degree of header configurability should warn users about the loss of privacy which can be involved.
DNS Spoofing
Clients using HTTP rely heavily on the Domain Name Service, and are thus generally prone to security attacks based on the deliberate miss-association of IP addresses and DNS names. The deployment of DNSSECprivate HREF="#RefDNSSEC" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [27] should help this situation. In advance of this deployment, however, clients need to be cautious in assuming the continuing validity of an IP number/DNS name association.
In particular, HTTP clients SHOULD rely on their name resolver for confirmation of an IP number/DNS name association, rather than caching the result of previous host name lookups. Many platforms already can cache host name lookups locally when appropriate, and they SHOULD be configured to do so. These lookups should be cached, however, only when the TTL (Time To Live) information reported by the name server makes it likely that the cached information will remain useful.
If HTTP clients cache the results of host name lookups in order to achieve a performance improvement, they MUST observe the TTL information reported by DNS.
If HTTP clients do not observe this rule, they could be spoofed when a previously-accessed server's IP address changes. As renumbering is expected to become increasingly commonprivate HREF="#Ref1900" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [24], the possibility of this form of attack will grow. Observing this requirement thus reduces this potential security vulnerability.
This requirement also improves the load-balancing behavior of clients for replicated servers using the same DNS name and reduces the likelihood of a user's experiencing failure in accessing sites which use that strategy.
Location Headers and Spoofing
If a single server supports multiple organizations that do not trust one another, then it must check the values of Location and Content-Location headers in responses that are generated under control of said organizations to make sure that they do not attempt to invalidate resources over which they have no authority.
Acknowledgments This specification makes heavy use of the augmented BNF and generic constructs defined by David H. Crocker for RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]. Similarly, it reuses many of the definitions provided by Nathaniel Borenstein and Ned Freed for MIME private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]. We hope that their inclusion in this specification will help reduce past confusion over the relationship between HTTP and Internet mail message formats.
The HTTP protocol has evolved considerably over the past four years. It has benefited from a large and active developer community--the many people who have participated on the www-talk mailing list--and it is that community which has been most responsible for the success of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker, H�kon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining early aspects of the protocol.
This document has benefited greatly from the comments of all those participating in the HTTP-WG. In addition to those already mentioned, the following individuals have contributed to this specification: Gary Adams Harald Tveit Alvestrand Keith Ball Brian Behlendorf Paul Burchard Maurizio Codogno Mike Cowlishaw Roman Czyborra Michael A. Dolan Alan Freier Marc Hedlund Koen Holtman Alex Hopmann Bob Jernigan Shel Kaphan Rohit Khare Martijn Koster Alexei Kosut David M. Kristol Daniel LaLiberte Paul J. Leach Albert Lunde John C. Mallery Jean-Philippe Martin-Flatin Larry Masinter Mitra Gavin Nicol Scott Powers Bill Perry Jeffrey Perry Owen Rees Luigi Rizzo David Robinson Marc Salomon Rich Salz Jim Seidman Chuck Shotton Eric W. Sink Simon E. Spero Richard N. Taylor Robert S. Thau Fran�ois Yergeau Mary Ellen Zurko David Morris Greg Herlihy Bill (BearHeart) Weinman Allan M. Schiffman Much of the content and presentation of the caching design is due to suggestions and comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, Larry Masinter, and Roy Fielding.
Most of the specification of ranges is based on work originally done by Ari Luotonen and John Franks, with additional input from Steve Zilles and Roy Fielding.
XXX need acks for subgroup work. References [1] H. Alvestrand. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1766.txt" MACROBUTTON HtmlResAnchor Tags for the identification of languages.� RFC 1766, UNINETT, March 1995. [2] F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, B. Alberti. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1436.txt" MACROBUTTON HtmlResAnchor The Internet Gopher Protocol: (a distributed document search and retrieval protocol)�, RFC 1436, University of Minnesota, March 1993. [3] T. Berners-Lee. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1630.txt" MACROBUTTON HtmlResAnchor Universal Resource Identifiers in WWW� A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web.� RFC 1630, CERN, June 1994. [4] T. Berners-Lee, L. Masinter, M. McCahill. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1738.txt" MACROBUTTON HtmlResAnchor Uniform Resource Locators (URL).� RFC 1738, CERN, Xerox PARC, University of Minnesota, December 1994. [5] T. Berners-Lee, D. Connolly. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1866.txt" MACROBUTTON HtmlResAnchor HyperText Markup Language Specification [6] T. Berners-Lee, R. Fielding, H. Frystyk. PRIVATE HREF="http://www.ics.uci.edu/pub/ietf/http/"MACROBUTTON HtmlResAnchor "Hypertext Transfer Protocol [7] N. Borenstein, N. Freed. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1521.ps" MACROBUTTON HtmlResAnchor MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies." RFC 1521, Bellcore, Innosoft, September 1993. [8] R. Braden. � PRIVATE HREF="http://ds.internic.net/std/std3.txt" MACROBUTTON HtmlResAnchor Requirements for Internet hosts [9] D. H. Crocker. � PRIVATE HREF="http://ds.internic.net/std/std11.txt" MACROBUTTON HtmlResAnchor Standard for the Format of ARPA Internet Text Messages.� STD 11, RFC 822, UDEL, August 1982. [10] F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J. Sui, M. Grinbaum. �WAIS Interface Protocol Prototype Functional Specification.� (v1.5), Thinking Machines Corporation, April 1990. [11] R. Fielding. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1808.txt" MACROBUTTON HtmlResAnchor Relative Uniform Resource Locators.� RFC 1808, UC Irvine, June 1995. [12] M. Horton, R. Adams. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1036.txt" MACROBUTTON HtmlResAnchor Standard for interchange of USENET messages.� RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories, Center for Seismic Studies, December 1987. [13] B. Kantor, P. Lapsley. � PRIVATE HREF="http://ds.internic.net/rfc/rfc977.txt" MACROBUTTON HtmlResAnchor Network News Transfer Protocol A Proposed Standard for the Stream-Based Transmission of News.� RFC 977, UC San Diego, UC Berkeley, February 1986. [14] K. Moore. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1522.txt" MACROBUTTON HtmlResAnchor MIME (Multipurpose Internet Mail Extensions) Part Two : Message Header Extensions for Non-ASCII Text.� RFC 1522, University of Tennessee, September 1993. [15] E. Nebel, L. Masinter. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1867.txt" MACROBUTTON HtmlResAnchor Form-based File Upload in HTML.� RFC 1867, Xerox Corporation, November 1995. [16] J. Postel. � PRIVATE HREF="http://ds.internic.net/std/std10.txt" MACROBUTTON HtmlResAnchor Simple Mail Transfer Protocol.� STD 10, RFC 821, USC/ISI, August 1982. [17] J. Postel. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1590.txt" MACROBUTTON HtmlResAnchor Media Type Registration Procedure.� RFC 1590, USC/ISI, March 1994. [18] J. Postel, J. K. Reynolds. � PRIVATE HREF="http://ds.internic.net/std/std9.txt" MACROBUTTON HtmlResAnchor File Transfer Protocol (FTP)� STD 9, RFC 959, USC/ISI, October 1985. [19] J. Reynolds, J. Postel. � PRIVATE HREF="http://ds.internic.net/std/std2.txt" MACROBUTTON HtmlResAnchor Assigned Numbers.� STD 2, RFC 1700, USC/ISI, October 1994. [20] K. Sollins, L. Masinter. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1737.txt" MACROBUTTON HtmlResAnchor Functional Requirements for Uniform Resource Names.� RFC 1737, MIT/LCS, Xerox Corporation, December 1994. [21] US-ASCII. Coded Character Set [22] ISO-8859. International Standard -
[23] Meyers, M. Rose � PRIVATE HREF="http://ds.internic.net/rfc/rfc1864.txt" MACROBUTTON HtmlResAnchor The Content-MD5 Header Field.� RFC 1864, Carnegie Mellon, Dover Beach Consulting, October, 1995.
[24] B. Carpenter, Y. Rekhter, � PRIVATE HREF="http://ds.internic.net/rfc/rfc1900.txt" MACROBUTTON HtmlResAnchor Renumbering Needs Work�. RFC 1900, IAB, February 1996.
[25] Gzip is available from the GNU project at <URL: PRIVATE HREF="ftp://prep.ai.mit.edu/pub/gnu/" MACROBUTTON HtmlResAnchor ftp://prep.ai.mit.edu/pub/gnu/>. A more formal specification is currently a work in progress. [26] Work In Progress for Digest authentication of the IETF HTTP working group.
[27] TBS, Work in progress (XXX should put RFC in here� )
[28] Mills, D, � PRIVATE HREF="http://ds.internic.net/rfc/rfc1305.txt" MACROBUTTON HtmlResAnchor Network Time Protocol, Version 3�, Specification, Implementation and Analysis RFC 1305, University of Delaware, March, 1992.
[29] Work in progress of the HTTP working group (XXX is this correct reference for incomplete work?).
[30] S. Spero. �Analysis of HTTP Performance Problems� URL:http://sunsite.unc.edu/mdma-release/http-prob.html
[31] E. Rescorla, A. Schiffman �The Secure HyperText Transfer Protocol� Internet-Draft (work in progress).
[32] A. Freier, P Karlton, P. Kocher. �SSL Version 3.0" Internet-Draft� (work in progress).
[33] Jeffrey C. Mogul. �The Case for Persistent-Connection HTTP�. In Proc.SIGCOMM '95 Symposium on Communications Architectures and Protocols, pages 299-313. Cambridge, MA, August, 1995.
[34] Jeffrey C. Mogul. �The Case for Persistent-Connection HTTP�. Research, Report 95/4, Digital Equipment Corporation Western Research Laboratory, May, 1995., <URL: PRIVATE HREF="http://www.research.digital.com/wrl/techreports/abstracts/95.4.html" MACROBUTTON HtmlResAnchor http://www.research.digital.com/wrl/techreports/abstracts/95.4.html
[35] Work in progress of the HTTP working group on state management.
Authors' Addresses private HREF="http://www.ics.uci.edu/~fielding/"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor Roy T. Fielding PRIVATE HREF="http://www.ics.uci.edu/~fielding/"MACROBUTTON HtmlResAnchor Roy T. FieldingDepartment of Information and Computer ScienceUniversity of CaliforniaIrvine, CA 92717-3425, USAFax: +1 (714) 824-4056Email: fielding@ics.uci.edu
PRIVATE HREF="http://www.w3.org/pub/WWW/People/Frystyk/"MACROBUTTON HtmlResAnchor Henrik Frystyk NielsenW3 ConsortiumMIT Laboratory for Computer Science545 Technology SquareCambridge, MA 02139, USAFax: +1 (617) 258 8682Email: frystyk@w3.org
PRIVATE HREF="http://www.w3.org/pub/WWW/People/Berners-Lee/"MACROBUTTON HtmlResAnchor Tim Berners-LeeDirector, W3 ConsortiumMIT Laboratory for Computer Science545 Technology SquareCambridge, MA 02139, USAFax: +1 (617) 258 8682Email: timbl@w3.org
PRIVATE HREF="http://www.w3.org/pub/WWW/People/Gettys/" MACROBUTTON HtmlResAnchor Jim GettysMIT Laboratory for Computer Science545 Technology SquareCambridge, MA 02139, USAFax: +1 (617) 258 8682Email: jg@w3.org
PRIVATE HREF="http://www.research.digital.com/wrl/people/mogul/bio.html" MACROBUTTON HtmlResAnchor Jeffrey C. MogulWestern Research LaboratoryDigital Equipment Corporation250 University AvenuePalo Alto, California, 94305, U.S.A.Email: mogul@wrl.dec.com Appendices These appendices are provided for informational reasons only -
Internet Media Type message/http In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type �message/http�. The following is to be registered with IANA private HREF="#RefMediaType"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [17]. Media Type name: message Media subtype name: http Required parameters: none Optional parameters: version, msgtype version: The HTTP-Version number of the enclosed message (e.g., "1.1"). If not present, the version can be determined from the first line of the body. msgtype: The message type - Encoding considerations: only "7bit", "8bit", or "binary" are permitted Security considerations: none
Tolerant Applications Although this document specifies the requirements for the generation of HTTP/1.1 messages, not all applications will be correct in their implementation. We therefore recommend that operational applications be tolerant of deviations whenever those deviations can be interpreted unambiguously.
Clients SHOULD be tolerant in parsing the Status-Line and servers tolerant when parsing the Request-Line. In particular, they SHOULD accept any amount of SP or HT characters between fields, even though only a single SP is required.
The line terminator for HTTP-header fields is the sequence CRLF. However, we recommend that applications, when parsing such headers, recognize a single LF as a line terminator and ignore the leading CR.
Differences Between HTTP Bodies and RFC 1521 Internet Message Bodies
HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]) and the Multipurpose Internet Mail Extensions (MIME private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]) to allow entities to be transmitted in an open variety of representations and with extensible mechanisms. However, RFC 1521 discusses mail, and HTTP has a few features that are different than those described in RFC 1521. These differences were carefully chosen to optimize performance over binary connections, to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients.
At the time of this writing, it is expected that RFC 1521 will be revised. The revisions may include some of the practices found in HTTP/1.1 but not in RFC 1521.
This appendix describes specific areas where HTTP differs from RFC 1521. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions may be required.
Conversion to Canonical Form RFC 1521 requires that an Internet mail entity be converted to canonical form prior to being transferred, as described in Appendix G of RFC 1521 private HREF="#RefMIME1" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]. Section ref Canonical_Text_Defau\n 7.7.1 of this document describes the forms allowed for subtypes of the �text� media type when transmitted over HTTP. RFC 1521 requires that content with a typeof �text� represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content when a message is transmitted over HTTP.
Where it is possible, a proxy or gateway from HTTP to a strict RFC 1521 environment SHOULD translate all line breaks within the text media types described in section ref Canonical_Text_Defau\n 7.7.1 of this document to the RFC 1521 canonical form of CRLF. Note, however, that this may be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some character sets which do not use octets 13 and 10 to represent CR and LF, as is the case for some multi-byte character sets.
Conversion of Date Formats HTTP/1.1 uses a restricted set of date formats (section ref HTTP_Date \n 7.3.1) to simplify the process of date comparison. Proxies and gateways from other protocols SHOULD ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary.
Introduction of Content-Encoding RFC 1521 does not include any concept equivalent to HTTP/1.1's Content-Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols MUST either change the value of the Content-Type header field or decode the Entity-Body before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of �;conversions=� to perform an equivalent function as Content-Encoding. However, this parameter is not part of RFC 1521.)
No Content-Transfer-Encoding HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 1521. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any non-identity CTE (�quoted-printable� or �base64�) encoding prior to delivering the response message to an HTTP client.
Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where �safe transport� is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.
HTTP Header Fields in Multipart Body-Parts In RFC 1521, most header fields in multipart body-parts are generally ignored unless the field name begins with �Content-�. In HTTP/1.1, multipart body-parts may contain any HTTP header fields which are significant to the meaning of that part.
Introduction of Transfer-Encoding HTTP/1.1 introduces the Transfer-Encoding header field (section ref Transfer_Encoding \n 18.43). Proxies/gateways MUST remove any transfer coding prior to forwarding a message via a MIME-compliant protocol. The process for decoding the �chunked� transfer coding (section ref Transfer_Codings \n 7.6) can be represented in pseudo-code as: length := 0 read chunk-size and CRLF while (chunk-size MIME-Version HTTP is not a MIME-compliant protocol (see Appendix ref MIME \n 23.3). However, HTTP/1.1 messages may include a single MIME-Version general-header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field indicates that the message is in full compliance with the MIME protocol (as defined in private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]). Proxies/gateways are responsible for ensuring full compliance (where possible) when exporting HTTP messages to strict MIME environments. MIME-Version = "MIME-Version" ":" 1DIGIT "." 1DIGIT
MIME version �1.0� is the default for use in HTTP/1.1. However, HTTP/1.1 message parsing and semantics are defined by this document and not the MIME specification.
Changes from HTTP/1.0 This section will summarize major differences between versions HTTP/1.0 and HTTP/1.1.
Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses
The requirements that clients and servers support the Host request-header, report an error if the Host request-header (section ref Host \n 18.24) is missing from an HTTP/1.1 request, and accept absolute URIs (Section ref Request_URI \n 9.1.2) are among the most important changes from HTTP/1.0.
In HTTP/1.0 there is a one-to-one relationship of IP addresses and servers. There is no other way to distinguish the intended server of a request than the IP address to which that request is directed. The HTTP/1.1 change will allow the Internet, once HTTP/1.0 clients and servers are no longer common, to support multiple Web sites from a single IP address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created serious problems. The Internet will also be able to recover the IP addresses that have been used for the sole purpose of allowing root-level domain names to be used in HTTP URLs. Given the rate of growth of the Web, and the number of servers already deployed, it is extremely important that implementations of HTTP/1.1 correctly implement these new requirements:
� both clients and servers MUST support the Host request-header
� Host request-headers are required in HTTP/1.1 requests.
� servers MUST report an error if an HTTP/1.1 request does not include a Host request-header
� servers MUST accept absolute URIs
Additional Features This appendix documents protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementers should be aware of these features, but cannot rely upon their presence in, or interoperability with, other HTTP/1.1 applications. Some of these describe proposed experimental features, and some describe features that experimental deployment found lacking that are now addressed in the base HTTP/1.1 specification.
Additional Request Methods PATCH The PATCH method is similar to PUT except that the entity contains a list of differences between the original version of the resource identified by the Request-URI and the desired content of the resource entity after the PATCH action has been applied. The list of differences is in a format defined by the media type of the entity (e.g., �application/diff�) and MUST include sufficient information to allow the server to recreate the changes necessary to convert the original version of the resource entity to the desired version.
If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.
For compatibility with HTTP/1.0 applications, all PATCH requests MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. When sending a PATCH request to an HTTP/1.1 server, a client MUST use a valid Content-Length or the �chunked� Transfer-Encoding. The server SHOULD respond with a 400 (Bad Request) message if it cannot determine the length of the request message's content, or with 411 (Length Required) if it wishes to insist on receiving a valid Content-Length.
The actual method for determining how the patched resource is placed, and what happens to its predecessor, is defined entirely by the origin server. If the original version of the resource being patched included a Content-Version header field, the request entity MUST include a Derived-From header field corresponding to the value of the original Content-Version header field. Applications are encouraged to use these fields for constructing versioning relationships and resolving version conflicts.
PATCH requests must obey the entity transmission requirements set out in section ref Entity_Transmission_\n 13.4.1.
Caches that implement PATCH should invalidate cached responses as defined in section ref Invalidation_After_U\n 16.10 for PUT.
LINK The LINK method establishes one or more Link relationships between the existing resource identified by the Request-URI and other existing resources. The difference between LINK and other methods allowing links to be established between resources is that the LINK method does not allow any Entity-Body to be sent in the request and does not directly result in the creation of new resources.
If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.
Caches that implement LINK should invalidate cached responses as defined in section ref Invalidation_After_U\n 16.10 for PUT.
UNLINK The UNLINK method removes one or more Link relationships from the existing resource identified by the Request-URI. These relationships may have been established using the LINK method or by any other method supporting the Link header. The removal of a link to a resource does not imply that the resource ceases to exist or becomes inaccessible for future references.
If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable.
Caches that implement UNLINK should invalidate cached responses as defined in section ref Invalidation_After_U\n 16.10 for PUT.
PUT
To support the PATCH method, if the entity being PUT was derived from an existing resource which included a Content-Version header field, the new entity MUST include a Derived-From header field corresponding to the value of the original Content-Version header field. Multiple Derived-From values may be included if the entity was derived from multiple resources with Content-Version information. Applications are encouraged to use these fields for constructing versioning relationships and resolving version conflicts.
Additional Header Field Definitions
Content-Version The Content-Version entity-header field defines the version tag associated with a rendition of an evolving entity. Together with the Derived-From field described in section ref Derived_From \n 23.5.2.2, it allows a group of people to work simultaneously on the creation of a work as an iterative process. The field SHOULD be used to allow evolution of a particular work along a single path. It SHOULD NOT be used to indicate derived works or renditions in different representations. It MAY also me used as an opaque value for comparing a cached entity's version with that of the current resource entity. Content-Version = "Content-Version" ":" quoted-string
Examples of the Content-Version field include: Content-Version: "2.1.2" Content-Version: "Fred 19950116-12:26:48" Content-Version: "2.5a4-omega7"
The value of the Content-Version field SHOULD be considered opaque to all parties but the origin server. A user agent MAY suggest a value for the version of an entity transferred via a PUT request; however, only the origin server can reliably assign that value.
Derived-From The Derived-From entity-header field can be used to indicate the version tag of the resource from which the enclosed entity was derived before modifications were made by the sender. This field is used to help manage the process of merging successive changes to a resource, particularly when such changes are being made in parallel and from multiple sources. Derived-From = "Derived-From" ":" quoted-string
An example use of the field is: Derived-From: "2.1.1"
The Derived-From field is required for PUT and PATCH requests if the entity being sent was previously retrieved from the same URI and a Content-Version header was included with the entity when it was last retrieved.
Link The Link entity-header field provides a means for describing a relationship between two resources, generally between the requested resource and some other resource. An entity MAY include multiple Link values. Links at the metainformation level typically indicate relationships like hierarchical structure and navigation paths. The Link field is semantically equivalent to the <LINK Link = "Link" ":" #("<" URI ">" ( ";" link-param ) link-param = ( ( "rel" "=" relationship ) | ( "rev" "=" relationship ) | ( "title" "=" quoted-string ) | ( "anchor" "=" <"> URI <"> ) link-extension = token [ "=" ( token relationship = sgml-name | ( <"> sgml-name ( SP sgml-name) <" sgml-name = ALPHA *( ALPHA | DIGIT | "." Relationship values are case-insensitive and MAY be extended within the constraints of the sgml-name syntax. The title parameter MAY be used to label the destination of a link such that it can be used as identification within a human-readable menu. The anchor parameter MAY be used to indicate a source anchor other than the entire current resource, such as a fragment of this resource or a third resource.
Examples of usage include: Link: http://www.cern.ch/TheBook/chapter2; rel="Previous" Link: mailto:timbl@w3.org; rev="Made"; title="Tim Berners-Lee"
The first example indicates that chapter2 is previous to this resource in a logical navigation path. The second indicates that the person responsible for making the resource available is identified by the given e-mail address.
URI
The URI header field has, in past versions of this specification, been used as a combination of the existing Location, Content-Location, and Alternates header fields. Its primary purpose has been to include a list of additional URIs for the resource, including names and mirror locations. However, it has become clear that the combination of many different functions within this single field has been a barrier to consistently and correctly implementing any of those functions. Furthermore, we believe that the identification of names and mirror locations would be better performed via the Link header field. The URI header field is therefore deprecated in favor of those other fields. URI-header = "URI" ":" 1#( "<" URI ">" )
Compatibility with HTTP/1.0 Persistent Connections
Some clients and servers may wish to be compatible with some previous implementations of persistent connections in HTTP/1.0 clients and servers. These implementations are faulty, and the new facilities in HTTP/1.1 are designed to rectify these problems. The fear was that some existing 1.0 clients may be sending Keep-Alive to a proxy server that doesn't understand Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection and result in a dead 1.0 proxy waiting for the close on the response. The result is that 1.0 clients must be prevented from using Keep-Alive when talking to proxies.
However, talking to proxies is the most important use of persistent connections, so that is clearly unacceptable. Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when talking to an old proxy that ignores Connection. As it turns out, there are two ways to accomplish that:
Introduce a new keyword (persist) which is declared to be valid only when received from an HTTP/1.1 message.
Declare persistence to be the default for HTTP/1.1 messages and introduce a new keyword (close) for declaring non-persistence.
The following describes the original, buggy form of persistent connections.
When connecting to an origin server an HTTP client MAY send the Keep-Alive connection-token in addition to the Persist connection-token: Connection: Keep-Alive,Persist
An HTTP/1.0 server would then respond with the Keep-Alive connection token and the client may proceed with an HTTP/1.0 (or Keep-Alive) persistent connection.
An HTTP/1.1 server may also establish persistent connections with HTTP/1.0 clients upon receipt of a Keep-Alive connection token. However, a persistent connection with an HTTP/1.0 client cannot make use of the chunked transfer-coding, and therefore MUST use a Content-Length for marking the ending boundary of each Entity-Body.
A client MUST NOT send the Keep-Alive connection token to a proxy server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing the Connection header field.
The Keep-Alive Header
When the Keep-Alive connection-token has been transmitted with a request or a response a Keep-Alive header field MAY also be included. The Keep-Alive header field takes the following form: Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param keepalive-param = param-name "=" value
The Keep-Alive header itself is optional, and is used only if a parameter is being sent. HTTP/1.1 does not define any parameters.
If the Keep-Alive header is sent, the corresponding connection token MUST be transmitted. The Keep-Alive header MUST be ignored if received without the connection token.
Compatibility with Previous Versions
It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP/1.1 was deliberately designed, however, to make supporting previous versions easy. While we are contemplating a separate document containing advice to implementers, we feel it worth noting that at the time of composing this specification, we would expect commercial HTTP/1.1 servers to:
� recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 requests;
� understand any valid request in the format of HTTP/0.9, 1.0, or 1.1;
� respond appropriately with a message in the same major version used by the client. And we would expect HTTP/1.1 clients to:
� recognize the format of the Status-Line for HTTP/1.0 and 1.1 responses;
� understand any valid response in the format of HTTP/0.9, 1.0, or 1.1. For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the server after sending the response. A few implementations implement the Keep-Alive version of persistent connections described in section ref Keep_Alive_Header \n 23.5.2.5.1.
INTERNET-DRAFT Hypertext Transfer Protocol Fielding, Frystyk, Berners-Lee, Gettys, and Mogul [Page page 64] Fielding, Frystyk, Berners-Lee, Gettys and Mogul [Page page 9] page # "'Page: '#''" Page: 5CONTENT
page # "'Page: '#''" Page: 5CHUNKED
page # "'Page: '#''" Page: 5MEDIATYPES
page # "'Page: '#''" Page: 5FULLURL
page # "'Page: '#''" Page: 5FULLURL, HOST
page # "'Page: '#''" Page: 5REWRITE issue
page # "'Page: '#''" Page: 5 Placeholder for Range proposal.
page # "'Page: '#''" Page: 5CODES
page # "'Page: '#''" Page: 5Version not supported
page # "'Page: '#''" Page: 5TRACE
page # "'Page: '#''" Page: 5CONNEG
page # "'Page: '#''" Page: 5INTEGOK, CONTENT-MD5. Updated to draft of 4/2/96
page # "'Page: '#''" Page: 5HOST, FULLURL
page # "'Page: '#''" Page: 5TRACE
page # "'Page: '#''" Page: 5UPGRADE
page # "'Page: '#''" Page: 5FORWARDED
page # "'Page: '#''" Page: 5Basic authentication Warning, John Franks.
page # "'Page: '#''" Page: 5SECFILE
page # "'Page: '#''" Page: 5PRIVACY
page # "'Page: '#''" Page: 5ACCEPT
page # "'Page: '#''" Page: 5DNS
PAGE # "'Page: '#''" Page: 5CONTENT XXX check if an RFC has been issued by publication.
page # "'Page: '#''" Page: 5MEDIATYPES
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_Toc355709902
_Toc355711049
_Toc355712020Message
_Toc355696061
_Toc355709903
_Toc355710124
_Toc355711050
_Toc355712021
Message_Types
_Toc355696062
_Toc355709904
_Toc355710125
_Toc355711051
_Toc355712022Message_Headers
_Toc355696063
_Toc355709905
_Toc355710126
_Toc355711052
_Toc355712023General_Header
_Toc355696064
_Toc355709906
_Toc355710127
_Toc355711053
_Toc355712024Request
_Toc355696065
_Toc355709907
_Toc355710128
_Toc355711054
_Toc355712025Request_Line
_Toc355696066
_Toc355709908
_Toc355710129
_Toc355711055
_Toc355712026Method
_Toc355696067
_Toc355709909
_Toc355710130
_Toc355711056
_Toc355712027Request_URI
_Toc355696068
_Toc355709910
_Toc355710131
_Toc355711057
_Toc355712028Request_Header
_Toc355696069
_Toc355709911
_Toc355710132
_Toc355711058
_Toc355712029
_Toc355696070
_Toc355709912
_Toc355710133
_Toc355711059
_Toc355712030Response
_Toc355696071
_Toc355709913
_Toc355710134
_Toc355711060
_Toc355712031Status_Line
_Toc355696072
_Toc355709914
_Toc355710135
_Toc355711061
_Toc355712032Status_Code
_Toc355696073
_Toc355709915
_Toc355710136
_Toc355711062
_Toc355712033Response_Header
_Toc355696074
_Toc355709916
_Toc355710137
_Toc355711063
_Toc355712034Entity
_Toc355696075
_Toc355709917
_Toc355710138
_Toc355711064
_Toc355712035
Entity_Header
_Toc355696076
_Toc355709918
_Toc355710139
_Toc355711065
_Toc355712036Entity_Body
_Toc355696077
_Toc355709919
_Toc355710140
_Toc355711066
_Toc355712037BodyType
_Toc355696078
_Toc355709920
_Toc355710141
_Toc355711067
_Toc355712038
BodyLength
_Toc355696079
_Toc355709921
_Toc355710142
_Toc355711068
_Toc355712039Status_Codes
_Toc355696080
_Toc355709922
_Toc355710143
_Toc355711069
_Toc355712040Code1xx
_Toc355696081
_Toc355709923
_Toc355710144
_Toc355711070
_Toc355712041Code100Code101Code2xx
_Toc355696082
_Toc355709924
_Toc355710145
_Toc355711071
_Toc355712042Code200Code201Code202Code203Code204Code205Code206Code3xx
_Toc355696083
_Toc355709925
_Toc355710146
_Toc355711072
_Toc355712043Code300Code301Code302Code303Code304Code305Code4xx
_Toc355696084
_Toc355709926
_Toc355710147
_Toc355711073
_Toc355712044Code400Code401Code402Code403Code404Code405Code406Code407Code408Code409Code410Code411Code412Code413Code414Code415Code5xx
_Toc355696085
_Toc355709927
_Toc355710148
_Toc355711074
_Toc355712045Code500Code501Code502Code503Code504Code505Methods
_Toc355696086
_Toc355709928
_Toc355710149
_Toc355711075
_Toc355712046OPTIONS
_Toc355696087
_Toc355709929
_Toc355710150
_Toc355711076
_Toc355712047GET
_Toc355696088
_Toc355709930
_Toc355710151
_Toc355711077
_Toc355712048HEAD
_Toc355696089
_Toc355709931
_Toc355710152
_Toc355711078
_Toc355712049POST
_Toc355696090
_Toc355709932
_Toc355710153
_Toc355711079
_Toc355712050
_Toc355696091
_Toc355710154
_Toc355709933
_Toc355711080
_Toc355712051Entity_Transmission_PUT
_Toc355696092
_Toc355709934
_Toc355710155
_Toc355711081
_Toc355712052DELETE
_Toc355696093
_Toc355709935
_Toc355710156
_Toc355711082
_Toc355712053TRACE
_Toc355696094
_Toc355709936
_Toc355710157
_Toc355711083
_Toc355712054AA
_Toc355696095
_Toc355709937
_Toc355710158
_Toc355711084
_Toc355712055BasicAA
_Toc355696096
_Toc355709938
_Toc355710159
_Toc355711085
_Toc355712056DigestAA
_Toc355696097
_Toc355709939
_Toc355710160
_Toc355711086
_Toc355712057Content_Negotiation
_Toc355696098
_Toc355709940
_Toc355710161
_Toc355711087
_Toc355712058
_Toc355696099Negotiation_Faciliti
_Toc355709941
_Toc355710162
_Toc355711088
_Toc355712059Caching_In_HTTP
_Toc355696100
_Toc355709942
_Toc355710163
_Toc355711089
_Toc355712060Semantic_Transparenc
_Toc355696101
_Toc355709943
_Toc355710164
_Toc355711090
_Toc355712061
_Toc355696102Cache_Correctness
_Toc355709944
_Toc355710165
_Toc355711091
_Toc355712062Expiration_ModelCache_Control_Mechan
_Toc355696103
_Toc355709945
_Toc355710166
_Toc355711092
_Toc355712063Warnings
_Toc355696104
_Toc355709946
_Toc355710167
_Toc355711093
_Toc355712064Explicit_User_Agent_
_Toc355696105
_Toc355709947
_Toc355710168
_Toc355711094
_Toc355712065Exceptions_to_the_Ru
_Toc355696106
_Toc355709948
_Toc355710169
_Toc355711095
_Toc355712066Client_Controlled_Be
_Toc355696107
_Toc355709949
_Toc355710170
_Toc355711096
_Toc355712067
_Toc355696108
_Toc355709950
_Toc355710171
_Toc355711097
_Toc355712068Server_Specified_Exp
_Toc355696109
_Toc355709951
_Toc355710172
_Toc355711098
_Toc355712069Limitations_On_Expir
_Toc355696110
_Toc355709952
_Toc355710173
_Toc355711099
_Toc355712070Heuristic_Expiration
_Toc355696111
_Toc355709953
_Toc355710174
_Toc355711100
_Toc355712071Age_Calculations
_Toc355696112
_Toc355709954
_Toc355710175
_Toc355711101
_Toc355712072Expiration_Calculati
_Toc355696113
_Toc355709955
_Toc355710176
_Toc355711102
_Toc355712073Disambiguating_Expir
_Toc355696114
_Toc355710177Scope_Of_Expiration
_Toc355709956
_Toc355711103
_Toc355712074
_Toc355696115
_Toc355709957
_Toc355710178
_Toc355711104
_Toc355712075Disambiguating_Multi
_Toc355696116
_Toc355709958
_Toc355710179
_Toc355711105
_Toc355712076Validation_Model
_Toc355696117
_Toc355709959
_Toc355710180
_Toc355711106
_Toc355712077Last_Modified_Dates
_Toc355696118
_Toc355709960
_Toc355710181
_Toc355711107
_Toc355712078Tags
_Toc355696119
_Toc355709961
_Toc355710182
_Toc355711108
_Toc355712079Weak_and_Strong_Tags
_Toc355696120
_Toc355709962
_Toc355710183
_Toc355711109
_Toc355712080Rules_For_Tags_and_L
_Toc355696121
_Toc355709963
_Toc355710184
_Toc355711110
_Toc355712081
_Toc355696122
_Toc355710185Nonvalidating_Condit
_Toc355709964
_Toc355711111
_Toc355712082Constructing_Respons
_Toc355696123
_Toc355709965
_Toc355710186
_Toc355711112
_Toc355712083EtoE_and_HbyH_Header
_Toc355696124
_Toc355709966
_Toc355710187
_Toc355711113
_Toc355712084Non_Modifiable_Headr
_Toc355696125
_Toc355709967
_Toc355710188
_Toc355711114
_Toc355712085Combining_Headers
_Toc355696126
_Toc355709968
_Toc355710189
_Toc355711115
_Toc355712086Combining_Byte_Range
_Toc355696127
_Toc355709969
_Toc355710190
_Toc355711116
_Toc355712087Caching_and_Varying_
_Toc355696128
_Toc355709970
_Toc355710191
_Toc355711117
_Toc355712088Vary_Header_Use
_Toc355696129
_Toc355709971
_Toc355710192
_Toc355711118
_Toc355712089
_Toc355696130
_Toc355710193Alternates_Header_Us
_Toc355709972
_Toc355711119
_Toc355712090Variant_ID_Use
_Toc355696131
_Toc355709973
_Toc355710194
_Toc355711120
_Toc355712091Shared_and_Non_Share
_Toc355696132
_Toc355709974
_Toc355710195
_Toc355711121
_Toc355712092
_Toc355711122
_Toc355709975
_Toc355696134
_Toc355709976
_Toc355710197
_Toc355711123
_Toc355712093
_Toc355709977
_Toc355710198
_Toc355711124
_Toc355712094
_Toc355709978
_Toc355710199
_Toc355711125
_Toc355712095Errors_Or_Incomplete
_Toc355696138
_Toc355709979
_Toc355710200
_Toc355711126
_Toc355712096Caching_and_Status_C
_Toc355696139
_Toc355709980
_Toc355710201
_Toc355711127
_Toc355712097
_Toc355696140
_Toc355709981
_Toc355710202
_Toc355711128
_Toc355712098
_Toc355696141
_Toc355710203Side_Effects_of_Get_
_Toc355709982
_Toc355711129
_Toc355712099
_Toc355696142
_Toc355710204Invalidation_After_U
_Toc355709983
_Toc355711130
_Toc355712100Write_Through_Mandan
_Toc355696143
_Toc355709984
_Toc355710205
_Toc355711131
_Toc355712101
_Toc355696144Varying_Resources_An
_Toc355709985
_Toc355710206
_Toc355711132
_Toc355712102Varying_Resources_an
_Toc355696145
_Toc355710207
_Toc355709986
_Toc355711133
_Toc355712103
_Toc355696146
_Toc355710208Caching_of_Negative_
_Toc355709987
_Toc355711134
_Toc355712104
History_Lists
_Toc355696147
_Toc355709988
_Toc355710209
_Toc355711135
_Toc355712105Persistent_Connectio
_Toc355696148
_Toc355709989
_Toc355710210
_Toc355711136
_Toc355712106Persist_Purpose
_Toc355696149
_Toc355709990
_Toc355710211
_Toc355711137
_Toc355712107Persist_Overall_Oper
_Toc355696150
_Toc355709991
_Toc355710212
_Toc355711138
_Toc355712108Persist_Negotiation
_Toc355696151
_Toc355709992
_Toc355710213
_Toc355711139
_Toc355712109Persist_Pipe_Lining
_Toc355696152
_Toc355709993
_Toc355710214
_Toc355711140
_Toc355712110Persist_Entity_Bodie
_Toc355696153
_Toc355709994
_Toc355710215
_Toc355711141
_Toc355712111Persist_Proxy_Server
_Toc355696154
_Toc355709995
_Toc355710216
_Toc355711142
_Toc355712112Persist_Interactions
_Toc355696155
_Toc355709996
_Toc355710217
_Toc355711143
_Toc355712113Persist_Practical_Co
_Toc355696156
_Toc355709997
_Toc355710218
_Toc355711144
_Toc355712114HeaderFields
_Toc355696157
_Toc355709998
_Toc355710219
_Toc355711145
_Toc355712115Accept
_Toc355696158
_Toc355709999
_Toc355710220
_Toc355711146
_Toc355712116 OLE_LINK5Accept_Charset
_Toc355696159
_Toc355710000
_Toc355710221
_Toc355711147
_Toc355712117Accept_Encoding
_Toc355696160
_Toc355710001
_Toc355710222
_Toc355711148
_Toc355712118Accept_Language
_Toc355696161
_Toc355710002
_Toc355710223
_Toc355711149
_Toc355712119 OLE_LINK8
_Toc355696162
_Toc355710224
Accept_Ranges
_Toc355710003
_Toc355711150
_Toc355712120Age
_Toc355696163
_Toc355710004
_Toc355710225
_Toc355711151
_Toc355712121Allow
_Toc355696164
_Toc355710005
_Toc355710226
_Toc355711152
_Toc355712122
Authorization
Alternates
_Toc355696165
_Toc355710006
_Toc355710227
_Toc355711153
_Toc355712123
_Toc355696166
_Toc355710007
_Toc355710228
_Toc355711154
_Toc355712124
Cache_Control
_Toc355696167
_Toc355710008
_Toc355710229
_Toc355711155
_Toc355712125
_Toc355696168
_Toc355710230What_is_Cachable
_Toc355710009
_Toc355711156
_Toc355712126What_May_be_Stored_b
_Toc355696169
_Toc355710010
_Toc355710231
_Toc355711157
_Toc355712127Modifications_of_Exp
_Toc355696170
_Toc355710011
_Toc355710232
_Toc355711158
_Toc355712128
_Toc355696171
_Toc355710233Cache_Revalidation_a
_Toc355710012
_Toc355711159
_Toc355712129Cache_Miscellaneous_
_Toc355696172
_Toc355710013
_Toc355710234
_Toc355711160
_Toc355712130
Connection
_Toc355696173
_Toc355710014
_Toc355710235
_Toc355711161
_Toc355712131Content_Base
_Toc355696174
_Toc355710015
_Toc355710236
_Toc355711162
_Toc355712132Content_Encoding
_Toc355696175
_Toc355710016
_Toc355710237
_Toc355711163
_Toc355712133Content_Language
_Toc355696176
_Toc355710017
_Toc355710238
_Toc355711164
_Toc355712134Content_Length
_Toc355696177
_Toc355710018
_Toc355710239
_Toc355711165
_Toc355712135Content_Location
_Toc355696178
_Toc355710019
_Toc355710240
_Toc355711166
_Toc355712136
_Toc355711167
_Toc355696179Content_MD5
_Toc355710020
_Toc355710241
_Toc355712137
_Toc355696180
_Toc355710242
Content_Range
_Toc355710021
_Toc355711168
_Toc355712138MIME_byteranges
_Toc355696181
_Toc355710022
_Toc355710243
_Toc355711169
_Toc355712139Additional_Rules_For
_Toc355696182
_Toc355710023
_Toc355710244
_Toc355711170
_Toc355712140Content_Type
_Toc355696183
_Toc355710024
_Toc355710245
_Toc355711171
_Toc355712141Date
_Toc355696184
_Toc355710025
_Toc355710246
_Toc355711172
_Toc355712142
_Toc355696185ETag
_Toc355710026
_Toc355710247
_Toc355711173
_Toc355712143
_Toc355696186
_Toc355710248Expires
_Toc355710027
_Toc355711174
_Toc355712144From
_Toc355696187
_Toc355710028
_Toc355710249
_Toc355711175
_Toc355712145Host
_Toc355696188
_Toc355710029
_Toc355710250
_Toc355711176
_Toc355712146If_Modified_Since
_Toc355696189
_Toc355710030
_Toc355710251
_Toc355711177
_Toc355712147If_ValidIf_Match
_Toc355696190
_Toc355710031
_Toc355710252
_Toc355711178
_Toc355712148
_Toc355696191
_Toc355710032
_Toc355710253
_Toc355711179
_Toc355712149
_Toc355696192
_Toc355710254Range_If
_Toc355710033
_Toc355711180
_Toc355712150Unless_Modified_Sinc
_Toc355696193
_Toc355710034
_Toc355710255
_Toc355711181
_Toc355712151
Last_Modified
_Toc355696194
_Toc355710035
_Toc355710256
_Toc355711182
_Toc355712152Location
_Toc355696195
_Toc355710036
_Toc355710257
_Toc355711183
_Toc355712153Max_Forwards
_Toc355696196
_Toc355710037
_Toc355710258
_Toc355711184
_Toc355712154Persist
_Toc355696197
_Toc355710038
_Toc355710259
_Toc355711185
_Toc355712155Pragma
_Toc355696198
_Toc355710039
_Toc355710260
_Toc355711186
_Toc355712156Proxy_Authenticate
_Toc355696199
_Toc355710040
_Toc355710261
_Toc355711187
_Toc355712157Proxy_Authorization
_Toc355696200
_Toc355710041
_Toc355710262
_Toc355711188
_Toc355712158Public
_Toc355696201
_Toc355710042
_Toc355710263
_Toc355711189
_Toc355712159Range
_Toc355696202
_Toc355710043
_Toc355710264
_Toc355711190
_Toc355712160Referer
_Toc355696203
_Toc355710044
_Toc355710265
_Toc355711191
_Toc355712161Retry_After
_Toc355696204
_Toc355710045
_Toc355710266
_Toc355711192
_Toc355712162Server
_Toc355696205
_Toc355710046
_Toc355710267
_Toc355711193
_Toc355712163Title
_Toc355696206
_Toc355710047
_Toc355710268
_Toc355711194
_Toc355712164Transfer_Encoding
_Toc355696207
_Toc355710048
_Toc355710269
_Toc355711195
_Toc355712165Upgrade
_Toc355696208
_Toc355710049
_Toc355710270
_Toc355711196
_Toc355712166
User_Agent
_Toc355696209
_Toc355710050
_Toc355710271
_Toc355711197
_Toc355712167WWW_AuthenticateVary
_Toc355696210
_Toc355710051
_Toc355710272
_Toc355711198
_Toc355712168 ForwardedVia
_Toc355696211
_Toc355710052
_Toc355710273
_Toc355711199
_Toc355712169
_Toc355696212Warning
_Toc355710053
_Toc355710274
_Toc355711200
_Toc355712170
_Toc355696213
_Toc355710054
_Toc355710275
_Toc355711201
_Toc355712171Security
_Toc355696214
_Toc355710055
_Toc355710276
_Toc355711202
_Toc355712172AuthSecurity
_Toc355696215
_Toc355710056
_Toc355710277
_Toc355711203
_Toc355712173SafeMethods
_Toc355696216
_Toc355710057
_Toc355710278
_Toc355711204
_Toc355712174LogAbuse
_Toc355696217
_Toc355710058
_Toc355710279
_Toc355711205
_Toc355712175 Sensitive
_Toc355696218
_Toc355710059
_Toc355710280
_Toc355711206
_Toc355712176PathNameSecurity
_Toc355696219
_Toc355710060
_Toc355710281
_Toc355711207
_Toc355712177SecPersonal
_Toc355696220
_Toc355710061
_Toc355710282
_Toc355711208
_Toc355712178SecPrivacyAccept
_Toc355696221
_Toc355710062
_Toc355710283
_Toc355711209
_Toc355712179SecDNS
_Toc355696222
_Toc355710063
_Toc355710284
_Toc355711210
_Toc355712180
_Toc355696223
_Toc355710285Location_Headers_and
_Toc355710064
_Toc355711211
_Toc355712181Acknowledgments
_Toc355696224
_Toc355710065
_Toc355710286
_Toc355711212
_Toc355712182
References
_Toc355696225
_Toc355710066
_Toc355710287
_Toc355711213
_Toc355712183Authors
_Toc355696226
_Toc355710067
_Toc355710288
_Toc355711214RefLangTags RefGopherRefURIRefURLRefHTML RefHTTP10RefMIME1RefSTD3RefSTD11RefWAIS RefRelURL RefUSENETRefNNTPRefMIME2
RefFileUploadRefSMTPRefMediaTypeRefFTPRefIANARefURNRefASCII
RefISO8859Ref1864Ref1900GZIP DigestRef RefDNSSECNTPRefSHTTPRefSSL
_Toc355712184
Appendices
_Toc355696227
_Toc355710068
_Toc355710289
_Toc355711215
_Toc355712185message_http
_Toc355696228
_Toc355710069
_Toc355710290
_Toc355711216
_Toc355712186Tolerant
_Toc355696229
_Toc355710070
_Toc355710291
_Toc355711217
_Toc355712187MIME
_Toc355696230
_Toc355710071
_Toc355710292
_Toc355711218
_Toc355712188Conversion_To_Canoni
_Toc355696231
_Toc355710072
_Toc355710293
_Toc355711219
_Toc355712189Conversion_of_Date_F
_Toc355696232
_Toc355710073
_Toc355710294
_Toc355711220
_Toc355712190MIME_CE
_Toc355696233
_Toc355710074
_Toc355710295
_Toc355711221
_Toc355712191No_Content_Transfer_
_Toc355696234
_Toc355710075
_Toc355710296
_Toc355711222
_Toc355712192
MIME_parts
_Toc355696235
_Toc355710076
_Toc355710297
_Toc355711223
_Toc355712193MIME_TE
_Toc355696236
_Toc355710077
_Toc355710298
_Toc355711224
_Toc355712194MIME_Version
_Toc355696237
_Toc355710078
_Toc355710299
_Toc355711225
_Toc355712195Changes
_Toc355696238
_Toc355710079
_Toc355710300
_Toc355711226
_Toc355712196 OLE_LINK4
_Toc355696239
_Toc355710080
_Toc355710301
_Toc355711227
_Toc355712197AppHost
Additional
_Toc355696240
_Toc355710081
_Toc355710302
_Toc355711228
_Toc355712198Additional_Methods
_Toc355696241
_Toc355710082
_Toc355710303
_Toc355711229
_Toc355712199PATCHLINKUNLINKAdditional_Headers
_Toc355696242
_Toc355710083
_Toc355710304
_Toc355711230
_Toc355712200Content_VersionDerived_FromLink_Header
URI_HeaderCompatibility_With_1Keep_Alive_HeaderCompatibility_With_P
_Toc355696243
_Toc355710084
_Toc355710305
_Toc355711231
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Hypertext Transfer Protocol -- HTTP/1.1 Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or made obsolete by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as �work in progress�. To learn the current status of any Internet-Draft, please check the �1id-abstracts.txt� listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Distribution of this document is unlimited. Please send comments to the HTTP working group at http-wg@cuckoo.hpl.hp.com. Discussions of the working group are archived at . General discussions about HTTP and the applications which use HTTP should take place on the www-talk@w3.org mailing list. NOTE: This specification is for discussion purposes only. It is not claimed to represent the consensus of the HTTP working group, and contains a number of proposals that either have not been discussed or are controversial. The working group is discussing significant changes in many areas, including - support for caching, persistent connections, range retrieval, content negotiation, MIME compatibility, authentication, timing of the PUT operation. Abstract The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. It is a generic, stateless, object-oriented protocol which can be used for many tasks, such as name servers and distributed object management systems, through extension of its request methods (commands). A feature of HTTP is the typing and negotiation of data representation, allowing systems to be built independently of the data being transferred. HTTP has been in use by the World-Wide Web global information initiative since 1990. This specification defines the protocol referred to as �HTTP/1.1�. Note to Readers of This Document We believe this draft to be very close to consensus of the working group in terms of functionality for HTTP/1.1, and the text substantially correct. One final technical change NOT reflected in this draft is to make persistent connections the default behavior for HTTP/1.1; editorial changes to reflect this in the next, and we hope final draft, are being circulated in the working group mailing list. This draft has undergone extensive reorganization to improve presentation. Let us know if there are remaining problems. The terminology used in this draft has changed to reduce confusion. While we are converging on a shared set of terminology and definitions, it is possible there will be a final set of terminology adopted in the next draft. Despite any terminology changes that may occur to improve the presentation of the specification, we do not expect to change the name of any header field or parameter name. There are a very few remaining issues indicated by Editor�s Note: in bold font. Table of Contents TOC \o "1-4" Hypertext Transfer Protocol -- HTTP/1.1 GOTOBUTTON _Toc355711982 PAGEREF _Toc355711982 1 1 Status of this Memo GOTOBUTTON _Toc355711983 PAGEREF _Toc355711983 1 2 Abstract GOTOBUTTON _Toc355711984 PAGEREF _Toc355711984 1 3 Note to Readers of This Document GOTOBUTTON _Toc355711985 PAGEREF _Toc355711985 1 4 Table of Contents GOTOBUTTON _Toc355711986 PAGEREF _Toc355711986 3 5 Introduction GOTOBUTTON _Toc355711987 PAGEREF _Toc355711987 9 5.1 Purpose GOTOBUTTON _Toc355711988 PAGEREF _Toc355711988 9 5.2 Requirements GOTOBUTTON _Toc355711989 PAGEREF _Toc355711989 9 5.3 Terminology GOTOBUTTON _Toc355711990 PAGEREF _Toc355711990 10 5.4 Overall Operation GOTOBUTTON _Toc355711991 PAGEREF _Toc355711991 12 5.5 HTTP and MIME GOTOBUTTON _Toc355711992 PAGEREF _Toc355711992 13 6 Notational Conventions and Generic Grammar GOTOBUTTON _Toc355711993 PAGEREF _Toc355711993 13 6.1 Augmented BNF GOTOBUTTON _Toc355711994 PAGEREF _Toc355711994 13 6.2 Basic Rules GOTOBUTTON _Toc355711995 PAGEREF _Toc355711995 15 7 Protocol Parameters GOTOBUTTON _Toc355711996 PAGEREF _Toc355711996 16 7.1 HTTP Version GOTOBUTTON _Toc355711997 PAGEREF _Toc355711997 16 7.2 Uniform Resource Identifiers GOTOBUTTON _Toc355711998 PAGEREF _Toc355711998 16 7.2.1 General Syntax GOTOBUTTON _Toc355711999 PAGEREF _Toc355711999 17 7.2.2 http URL GOTOBUTTON _Toc355712000 PAGEREF _Toc355712000 17 7.2.3 URI Canonicalization GOTOBUTTON _Toc355712001 PAGEREF _Toc355712001 18 7.3 Date/Time Formats GOTOBUTTON _Toc355712002 PAGEREF _Toc355712002 18 7.3.1 Full Date GOTOBUTTON _Toc355712003 PAGEREF _Toc355712003 18 7.3.2 Delta Seconds GOTOBUTTON _Toc355712004 PAGEREF _Toc355712004 19 7.4 Character Sets GOTOBUTTON _Toc355712005 PAGEREF _Toc355712005 20 7.5 Content Codings GOTOBUTTON _Toc355712006 PAGEREF _Toc355712006 20 7.6 Transfer Codings GOTOBUTTON _Toc355712007 PAGEREF _Toc355712007 21 7.7 Media Types GOTOBUTTON _Toc355712008 PAGEREF _Toc355712008 22 7.7.1 Canonicalization and Text Defaults GOTOBUTTON _Toc355712009 PAGEREF _Toc355712009 22 7.7.2 Multipart Types GOTOBUTTON _Toc355712010 PAGEREF _Toc355712010 23 7.8 Product Tokens GOTOBUTTON _Toc355712011 PAGEREF _Toc355712011 23 7.9 Quality Values GOTOBUTTON _Toc355712012 PAGEREF _Toc355712012 24 7.10 Language Tags GOTOBUTTON _Toc355712013 PAGEREF _Toc355712013 24 7.11 Entity Tags GOTOBUTTON _Toc355712014 PAGEREF _Toc355712014 24 7.12 Variant IDs GOTOBUTTON _Toc355712015 PAGEREF _Toc355712015 25 7.13 Variant Sets GOTOBUTTON _Toc355712016 PAGEREF _Toc355712016 25 7.14 Range Protocol Parameters GOTOBUTTON _Toc355712017 PAGEREF _Toc355712017 25 7.14.1 Range Units GOTOBUTTON _Toc355712018 PAGEREF _Toc355712018 25 7.14.2 Byte Ranges GOTOBUTTON _Toc355712019 PAGEREF _Toc355712019 25 7.14.3 Content Ranges GOTOBUTTON _Toc355712020 PAGEREF _Toc355712020 27 8 HTTP Message GOTOBUTTON _Toc355712021 PAGEREF _Toc355712021 27 8.1 Message Types GOTOBUTTON _Toc355712022 PAGEREF _Toc355712022 27 8.2 Message Headers GOTOBUTTON _Toc355712023 PAGEREF _Toc355712023 27 8.3 General Header Fields GOTOBUTTON _Toc355712024 PAGEREF _Toc355712024 28 9 Request GOTOBUTTON _Toc355712025 PAGEREF _Toc355712025 28 9.1 Request-Line GOTOBUTTON _Toc355712026 PAGEREF _Toc355712026 29 9.1.1 Method GOTOBUTTON _Toc355712027 PAGEREF _Toc355712027 29 9.1.2 Request-URI GOTOBUTTON _Toc355712028 PAGEREF _Toc355712028 29 9.2 The Resource Identified by a Request GOTOBUTTON _Toc355712029 PAGEREF _Toc355712029 30 9.3 Request Header Fields GOTOBUTTON _Toc355712030 PAGEREF _Toc355712030 31 10 Response GOTOBUTTON _Toc355712031 PAGEREF _Toc355712031 31 10.1 Status-Line GOTOBUTTON _Toc355712032 PAGEREF _Toc355712032 31 10.1.1 Status Code and Reason Phrase GOTOBUTTON _Toc355712033 PAGEREF _Toc355712033 31 10.2 Response Header Fields GOTOBUTTON _Toc355712034 PAGEREF _Toc355712034 33 11 Entity GOTOBUTTON _Toc355712035 PAGEREF _Toc355712035 34 11.1 Entity Header Fields GOTOBUTTON _Toc355712036 PAGEREF _Toc355712036 34 11.2 Entity Body GOTOBUTTON _Toc355712037 PAGEREF _Toc355712037 34 11.2.1 Type GOTOBUTTON _Toc355712038 PAGEREF _Toc355712038 35 11.2.2 Length GOTOBUTTON _Toc355712039 PAGEREF _Toc355712039 35 12 Status Code Definitions GOTOBUTTON _Toc355712040 PAGEREF _Toc355712040 35 12.1 Informational 1xx GOTOBUTTON _Toc355712041 PAGEREF _Toc355712041 36 12.2 Successful 2xx GOTOBUTTON _Toc355712042 PAGEREF _Toc355712042 36 12.3 Redirection 3xx GOTOBUTTON _Toc355712043 PAGEREF _Toc355712043 37 12.4 Client Error 4xx GOTOBUTTON _Toc355712044 PAGEREF _Toc355712044 39 12.5 Server Error 5xx GOTOBUTTON _Toc355712045 PAGEREF _Toc355712045 41 13 Method Definitions GOTOBUTTON _Toc355712046 PAGEREF _Toc355712046 42 13.1 OPTIONS GOTOBUTTON _Toc355712047 PAGEREF _Toc355712047 42 13.2 GET GOTOBUTTON _Toc355712048 PAGEREF _Toc355712048 43 13.3 HEAD GOTOBUTTON _Toc355712049 PAGEREF _Toc355712049 43 13.4 POST GOTOBUTTON _Toc355712050 PAGEREF _Toc355712050 43 13.4.1 SLUSHY: Entity Transmission Requirements GOTOBUTTON _Toc355712051 PAGEREF _Toc355712051 44 13.5 PUT GOTOBUTTON _Toc355712052 PAGEREF _Toc355712052 45 13.6 DELETE GOTOBUTTON _Toc355712053 PAGEREF _Toc355712053 46 13.7 TRACE GOTOBUTTON _Toc355712054 PAGEREF _Toc355712054 46 14 Access Authentication GOTOBUTTON _Toc355712055 PAGEREF _Toc355712055 47 14.1 Basic Authentication Scheme GOTOBUTTON _Toc355712056 PAGEREF _Toc355712056 47 14.2 Digest Authentication Scheme GOTOBUTTON _Toc355712057 PAGEREF _Toc355712057 48 15 Content Negotiation GOTOBUTTON _Toc355712058 PAGEREF _Toc355712058 48 15.1 Negotiation Facilities Defined in this Specification GOTOBUTTON _Toc355712059 PAGEREF _Toc355712059 49 16 Caching in HTTP GOTOBUTTON _Toc355712060 PAGEREF _Toc355712060 49 16.1 Semantic Transparency GOTOBUTTON _Toc355712061 PAGEREF _Toc355712061 49 16.1.1 Cache Correctness GOTOBUTTON _Toc355712062 PAGEREF _Toc355712062 50 16.1.2 Cache-control Mechanisms GOTOBUTTON _Toc355712063 PAGEREF _Toc355712063 50 16.1.3 Warnings GOTOBUTTON _Toc355712064 PAGEREF _Toc355712064 50 16.1.4 Explicit User Agent Warnings GOTOBUTTON _Toc355712065 PAGEREF _Toc355712065 51 16.1.5 Exceptions to the Rules and Warnings GOTOBUTTON _Toc355712066 PAGEREF _Toc355712066 51 16.1.6 Client-controlled Behavior GOTOBUTTON _Toc355712067 PAGEREF _Toc355712067 51 16.2 Expiration Model GOTOBUTTON _Toc355712068 PAGEREF _Toc355712068 52 16.2.1 Server-Specified Expiration GOTOBUTTON _Toc355712069 PAGEREF _Toc355712069 52 16.2.2 Limitations on the Effect of Expiration Times GOTOBUTTON _Toc355712070 PAGEREF _Toc355712070 52 16.2.3 Heuristic Expiration GOTOBUTTON _Toc355712071 PAGEREF _Toc355712071 52 16.2.4 Age Calculations GOTOBUTTON _Toc355712072 PAGEREF _Toc355712072 53 16.2.5 Expiration Calculations GOTOBUTTON _Toc355712073 PAGEREF _Toc355712073 54 16.2.6 Scope of Expiration GOTOBUTTON _Toc355712074 PAGEREF _Toc355712074 55 16.2.7 Disambiguating Expiration Values GOTOBUTTON _Toc355712075 PAGEREF _Toc355712075 55 16.2.8 Disambiguating Multiple Responses GOTOBUTTON _Toc355712076 PAGEREF _Toc355712076 55 16.3 Validation Model GOTOBUTTON _Toc355712077 PAGEREF _Toc355712077 55 16.3.1 Last-modified Dates GOTOBUTTON _Toc355712078 PAGEREF _Toc355712078 56 16.3.2 Entity Tags GOTOBUTTON _Toc355712079 PAGEREF _Toc355712079 56 16.3.3 Weak and Strong Validators GOTOBUTTON _Toc355712080 PAGEREF _Toc355712080 57 16.3.4 Rules for When to Use Entity Tags and Last-modified Dates GOTOBUTTON _Toc355712081 PAGEREF _Toc355712081 58 16.3.5 Non-validating Conditionals GOTOBUTTON _Toc355712082 PAGEREF _Toc355712082 59 16.4 Constructing Responses From Caches GOTOBUTTON _Toc355712083 PAGEREF _Toc355712083 59 16.4.1 End-to-end and Hop-by-hop Headers GOTOBUTTON _Toc355712084 PAGEREF _Toc355712084 59 16.4.2 Non-modifiable Headers GOTOBUTTON _Toc355712085 PAGEREF _Toc355712085 60 16.4.3 Combining Headers GOTOBUTTON _Toc355712086 PAGEREF _Toc355712086 60 16.4.4 Combining Byte Ranges GOTOBUTTON _Toc355712087 PAGEREF _Toc355712087 61 16.5 Caching and Generic Resources GOTOBUTTON _Toc355712088 PAGEREF _Toc355712088 61 16.5.1 Vary Header Use GOTOBUTTON _Toc355712089 PAGEREF _Toc355712089 61 16.5.2 Alternates Header Use GOTOBUTTON _Toc355712090 PAGEREF _Toc355712090 61 16.5.3 Variant-ID Use GOTOBUTTON _Toc355712091 PAGEREF _Toc355712091 61 16.6 Shared and Non-Shared Caches GOTOBUTTON _Toc355712092 PAGEREF _Toc355712092 62 16.7 Selecting a Cached Response GOTOBUTTON _Toc355712093 PAGEREF _Toc355712093 62 16.7.1 Plain Resources GOTOBUTTON _Toc355712094 PAGEREF _Toc355712094 62 16.7.2 Generic Resources GOTOBUTTON _Toc355712095 PAGEREF _Toc355712095 63 16.8 Errors or Incomplete Response Cache Behavior GOTOBUTTON _Toc355712096 PAGEREF _Toc355712096 63 16.8.1 Caching and Status Codes GOTOBUTTON _Toc355712097 PAGEREF _Toc355712097 63 16.8.2 Handling of Retry-After GOTOBUTTON _Toc355712098 PAGEREF _Toc355712098 63 16.9 Side Effects of GET and HEAD GOTOBUTTON _Toc355712099 PAGEREF _Toc355712099 64 16.10 Invalidation After Updates or Deletions GOTOBUTTON _Toc355712100 PAGEREF _Toc355712100 64 16.11 Write-Through Mandatory GOTOBUTTON _Toc355712101 PAGEREF _Toc355712101 64 16.12 Generic Resources and HTTP/1.0 Proxy Caches GOTOBUTTON _Toc355712102 PAGEREF _Toc355712102 65 16.13 Cache Replacement GOTOBUTTON _Toc355712103 PAGEREF _Toc355712103 65 16.14 Caching of Negative Responses GOTOBUTTON _Toc355712104 PAGEREF _Toc355712104 65 16.15 History Lists GOTOBUTTON _Toc355712105 PAGEREF _Toc355712105 65 17 Persistent Connections GOTOBUTTON _Toc355712106 PAGEREF _Toc355712106 65 17.1 Purpose GOTOBUTTON _Toc355712107 PAGEREF _Toc355712107 65 17.2 Overall Operation GOTOBUTTON _Toc355712108 PAGEREF _Toc355712108 66 17.2.1 Negotiation GOTOBUTTON _Toc355712109 PAGEREF _Toc355712109 66 17.2.2 Pipe-lining GOTOBUTTON _Toc355712110 PAGEREF _Toc355712110 66 17.2.3 Delimiting Entity-Bodies GOTOBUTTON _Toc355712111 PAGEREF _Toc355712111 67 17.3 Proxy Servers GOTOBUTTON _Toc355712112 PAGEREF _Toc355712112 67 17.4 Interaction with Security Protocols GOTOBUTTON _Toc355712113 PAGEREF _Toc355712113 67 17.5 Practical Considerations GOTOBUTTON _Toc355712114 PAGEREF _Toc355712114 67 18 Header Field Definitions GOTOBUTTON _Toc355712115 PAGEREF _Toc355712115 68 18.1 Accept GOTOBUTTON _Toc355712116 PAGEREF _Toc355712116 68 18.2 Accept-Charset GOTOBUTTON _Toc355712117 PAGEREF _Toc355712117 69 18.3 Accept-Encoding GOTOBUTTON _Toc355712118 PAGEREF _Toc355712118 70 18.4 Accept-Language GOTOBUTTON _Toc355712119 PAGEREF _Toc355712119 70 18.5 Accept-Ranges GOTOBUTTON _Toc355712120 PAGEREF _Toc355712120 71 18.6 Age GOTOBUTTON _Toc355712121 PAGEREF _Toc355712121 71 18.7 Allow GOTOBUTTON _Toc355712122 PAGEREF _Toc355712122 71 18.8 Alternates GOTOBUTTON _Toc355712123 PAGEREF _Toc355712123 72 18.9 Authorization GOTOBUTTON _Toc355712124 PAGEREF _Toc355712124 72 18.10 Cache-Control GOTOBUTTON _Toc355712125 PAGEREF _Toc355712125 73 18.10.1 Cache-Control Restrictions on What is Cachable GOTOBUTTON _Toc355712126 PAGEREF _Toc355712126 73 18.10.2 What May be Stored by Caches GOTOBUTTON _Toc355712127 PAGEREF _Toc355712127 74 18.10.3 Modifications of the Basic Expiration Mechanism GOTOBUTTON _Toc355712128 PAGEREF _Toc355712128 75 18.10.4 Cache Revalidation and Reload Controls GOTOBUTTON _Toc355712129 PAGEREF _Toc355712129 75 18.10.5 Miscellaneous Restrictions GOTOBUTTON _Toc355712130 PAGEREF _Toc355712130 77 18.11 Connection GOTOBUTTON _Toc355712131 PAGEREF _Toc355712131 77 18.12 Content-Base GOTOBUTTON _Toc355712132 PAGEREF _Toc355712132 77 18.13 Content-Encoding GOTOBUTTON _Toc355712133 PAGEREF _Toc355712133 78 18.14 Content-Language GOTOBUTTON _Toc355712134 PAGEREF _Toc355712134 78 18.15 Content-Length GOTOBUTTON _Toc355712135 PAGEREF _Toc355712135 78 18.16 Content-Location GOTOBUTTON _Toc355712136 PAGEREF _Toc355712136 79 18.17 Content-MD5 GOTOBUTTON _Toc355712137 PAGEREF _Toc355712137 79 18.18 Content-Range GOTOBUTTON _Toc355712138 PAGEREF _Toc355712138 80 18.18.1 MIME multipart/byteranges Content-type GOTOBUTTON _Toc355712139 PAGEREF _Toc355712139 80 18.18.2 Additional Rules for Content-Range GOTOBUTTON _Toc355712140 PAGEREF _Toc355712140 81 18.19 Content-Type GOTOBUTTON _Toc355712141 PAGEREF _Toc355712141 81 18.20 Date GOTOBUTTON _Toc355712142 PAGEREF _Toc355712142 81 18.21 ETag GOTOBUTTON _Toc355712143 PAGEREF _Toc355712143 82 18.22 Expires GOTOBUTTON _Toc355712144 PAGEREF _Toc355712144 82 18.23 From GOTOBUTTON _Toc355712145 PAGEREF _Toc355712145 83 18.24 Host GOTOBUTTON _Toc355712146 PAGEREF _Toc355712146 83 18.25 If-Modified-Since GOTOBUTTON _Toc355712147 PAGEREF _Toc355712147 84 18.26 If-Match GOTOBUTTON _Toc355712148 PAGEREF _Toc355712148 84 18.27 If-NoneMatch GOTOBUTTON _Toc355712149 PAGEREF _Toc355712149 85 18.28 If-Range GOTOBUTTON _Toc355712150 PAGEREF _Toc355712150 86 18.29 If-Unmodified-Since GOTOBUTTON _Toc355712151 PAGEREF _Toc355712151 86 18.30 Last-Modified GOTOBUTTON _Toc355712152 PAGEREF _Toc355712152 87 18.31 Location GOTOBUTTON _Toc355712153 PAGEREF _Toc355712153 87 18.32 Max-Forwards GOTOBUTTON _Toc355712154 PAGEREF _Toc355712154 87 18.33 Persist GOTOBUTTON _Toc355712155 PAGEREF _Toc355712155 88 18.34 Pragma GOTOBUTTON _Toc355712156 PAGEREF _Toc355712156 88 18.35 Proxy-Authenticate GOTOBUTTON _Toc355712157 PAGEREF _Toc355712157 88 18.36 Proxy-Authorization GOTOBUTTON _Toc355712158 PAGEREF _Toc355712158 89 18.37 Public GOTOBUTTON _Toc355712159 PAGEREF _Toc355712159 89 18.38 Range GOTOBUTTON _Toc355712160 PAGEREF _Toc355712160 89 18.39 Referer GOTOBUTTON _Toc355712161 PAGEREF _Toc355712161 90 18.40 Retry-After GOTOBUTTON _Toc355712162 PAGEREF _Toc355712162 90 18.41 Server GOTOBUTTON _Toc355712163 PAGEREF _Toc355712163 90 18.42 Title GOTOBUTTON _Toc355712164 PAGEREF _Toc355712164 90 18.43 Transfer Encoding GOTOBUTTON _Toc355712165 PAGEREF _Toc355712165 91 18.44 Upgrade GOTOBUTTON _Toc355712166 PAGEREF _Toc355712166 91 18.45 User-Agent GOTOBUTTON _Toc355712167 PAGEREF _Toc355712167 92 18.46 Vary GOTOBUTTON _Toc355712168 PAGEREF _Toc355712168 92 18.47 Via GOTOBUTTON _Toc355712169 PAGEREF _Toc355712169 94 18.48 Warning GOTOBUTTON _Toc355712170 PAGEREF _Toc355712170 95 18.49 WWW-Authenticate GOTOBUTTON _Toc355712171 PAGEREF _Toc355712171 96 19 Security Considerations GOTOBUTTON _Toc355712172 PAGEREF _Toc355712172 96 19.1 Authentication of Clients GOTOBUTTON _Toc355712173 PAGEREF _Toc355712173 96 19.2 Safe Methods GOTOBUTTON _Toc355712174 PAGEREF _Toc355712174 97 19.3 Abuse of Server Log Information GOTOBUTTON _Toc355712175 PAGEREF _Toc355712175 97 19.4 Transfer of Sensitive Information GOTOBUTTON _Toc355712176 PAGEREF _Toc355712176 97 19.5 Attacks Based On File and Path Names GOTOBUTTON _Toc355712177 PAGEREF _Toc355712177 98 19.6 Personal Information GOTOBUTTON _Toc355712178 PAGEREF _Toc355712178 98 19.7 Privacy Issues Connected to Accept headers GOTOBUTTON _Toc355712179 PAGEREF _Toc355712179 98 19.8 DNS Spoofing GOTOBUTTON _Toc355712180 PAGEREF _Toc355712180 99 19.9 Location Headers and Spoofing GOTOBUTTON _Toc355712181 PAGEREF _Toc355712181 99 20 Acknowledgments GOTOBUTTON _Toc355712182 PAGEREF _Toc355712182 99 21 References GOTOBUTTON _Toc355712183 PAGEREF _Toc355712183 100 22 Authors' Addresses GOTOBUTTON _Toc355712184 PAGEREF _Toc355712184 102 23 Appendices GOTOBUTTON _Toc355712185 PAGEREF _Toc355712185 103 23.1 Internet Media Type message/http GOTOBUTTON _Toc355712186 PAGEREF _Toc355712186 103 23.2 Tolerant Applications GOTOBUTTON _Toc355712187 PAGEREF _Toc355712187 103 23.3 Differences Between HTTP Bodies and RFC 1521 Internet Message Bodies GOTOBUTTON _Toc355712188 PAGEREF _Toc355712188 104 23.3.1 Conversion to Canonical Form GOTOBUTTON _Toc355712189 PAGEREF _Toc355712189 104 23.3.2 Conversion of Date Formats GOTOBUTTON _Toc355712190 PAGEREF _Toc355712190 104 23.3.3 Introduction of Content-Encoding GOTOBUTTON _Toc355712191 PAGEREF _Toc355712191 104 23.3.4 No Content-Transfer-Encoding GOTOBUTTON _Toc355712192 PAGEREF _Toc355712192 104 23.3.5 HTTP Header Fields in Multipart Body-Parts GOTOBUTTON _Toc355712193 PAGEREF _Toc355712193 105 23.3.6 Introduction of Transfer-Encoding GOTOBUTTON _Toc355712194 PAGEREF _Toc355712194 105 23.3.7 MIME-Version GOTOBUTTON _Toc355712195 PAGEREF _Toc355712195 105 23.4 Changes from HTTP/1.0 GOTOBUTTON _Toc355712196 PAGEREF _Toc355712196 105 23.4.1 Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses GOTOBUTTON _Toc355712197 PAGEREF _Toc355712197 105 23.5 Additional Features GOTOBUTTON _Toc355712198 PAGEREF _Toc355712198 106 23.5.1 Additional Request Methods GOTOBUTTON _Toc355712199 PAGEREF _Toc355712199 106 23.5.2 Additional Header Field Definitions GOTOBUTTON _Toc355712200 PAGEREF _Toc355712200 107 23.5.3 Compatibility with Previous Versions GOTOBUTTON _Toc355712201 PAGEREF _Toc355712201 109 Introduction Purpose The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypermedia information systems. HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, referred to as HTTP/0.9, was a simple protocol for raw data transfer across the Internet. HTTP/1.0, as defined by RFC xxxx private HREF="#RefHTTP10"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [6], improved the protocol by allowing messages to be in the format of MIME-like entities, containing metainformation about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 does not sufficiently take into consideration the effect of hierarchical proxies , caching, the need for persistent connections and virtual hosts.. In addition, the proliferation of incompletely-implemented applications calling themselves �HTTP/1.0� has necessitated a protocol version change in order for two communicating applications to determine each other's true capabilities. This specification defines the protocol referred to as �HTTP/1.1�. This protocol is backwards-compatible with HTTP/1.0, but includes more stringent requirements in order to ensure reliable implementation of its features. Practical information systems require more functionality than simple retrieval, including search, front-end update, and annotation. HTTP allows an open-ended set of methods that indicate the purpose of a request. It builds on the discipline of reference provided by the Uniform Resource Identifier (URI) private HREF="#RefURI"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [3], as a location (URL) private HREF="#RefURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [4] or name (URN) private HREF="#RefURN"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [20], for indicating the resource to which a method is to be applied. Messages are passed in a format similar to that used by Internet Mail private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9] and the Multipurpose Internet Mail Extensions (MIME) private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]. HTTP is also used as a generic protocol for communication between user agents and proxies/gateways to other Internet protocols, such as SMTP private HREF="#RefSMTP"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [16], NNTP private HREF="#RefNNTP"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [13], FTP private HREF="#RefFTP"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [18], Gopher private HREF="#RefGopher"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [2], and WAIS private HREF="#RefWAIS"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [10], allowing basic hypermedia access to resources available from diverse applications and simplifying the implementation of user agents. Requirements This specification uses the same words as RFC 1123 private HREF="#RefSTD3"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [8] for defining the significance of each particular requirement. These words are: MUST This word or the adjective �required� means that the item is an absolute requirement of the specification. SHOULD This word or the adjective �recommended� means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course. MAY This word or the adjective �optional� means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item. An implementation is not compliant if it fails to satisfy one or more of the MUST requirements for the protocols it implements. An implementation that satisfies all the MUST and all the SHOULD requirements for its protocols is said to be �unconditionally compliant�; one that satisfies all the MUST requirements but not all the SHOULD requirements for its protocols is said to be �conditionally compliant�. Terminology This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication. connection A transport layer virtual circuit established between two programs for the purpose of communication. message The basic unit of HTTP communication, consisting of a structured sequence of octets matching the syntax defined in section ref Message \n 8 and transmitted via the connection. request An HTTP request message as defined in section ref Request \n 9. response An HTTP response message as defined in section ref Response \n 10. resource A network data object or service that can be identified by a URI (section ref URI \n 7.2). At any point in time, a resource may be either a plain resource, which corresponds to only one possible representation, or a generic resource. generic resource A resource that is a set of closely related representations of the same document, form, applet, etc. A generic resource is always identified by a URI. The individual representations may each be identified by a unique URI, or by the combination of the generic resource's URI and a variant-ID, or by the combination of the generic resource�s URI and some �content-negotiation� mechanism. In this case, other URIs may exist which identify a resource more specifically. plain resource A resource that is not a generic resource. A plain resource is always identified by a URI. entity The set of information transferred as the payload of a request or response An entity consists of metainformation in the form of Entity-Header fields and content in the form of an Entity-Body, as described in section ref Entity \n 11. resource entity A specific representation, rendition, encoding, or presentation of a network data object or service, either a plain resource or a specific member of a generic resource. A resource entity might be identified by a URI, or by the combination of a URI and a variant-ID, or by the combination of a URI and some other mechanism. An plain resource MUST be bound to a single resource entity at any instant in time. variant A resource entity that is a member of at least one generic resource. Sometimes called a resource variant. Note that the set of variants of a generic resource may change over time as well. content negotiation The mechanism for selecting the appropriate variant of a generic resource when servicing a request, as described in section ref Content_Negotiation \n 15. entity tag An opaque string associated with an entity and used to distinguish it from other entities of the requested resource . A �strong entity tag� is one that may be shared by two entities of a resource only if they are equivalent by octet equality. A �weak entity tag� is one that may be shared by two entities of a resource if they are equivalent and could be substituted for each other with no significant change in semantics. A given entity tag value may be used for entities obtained by requests on different URIs without implying anything about the equivalence of these entities. client An application program that establishes connections for the purpose of sending requests. user agent The client which initiates a request. These are often browsers, editors, spiders (web-traversing robots), or other end user tools. server An application program that accepts connections in order to service requests by sending back responses. Any given program MAY be capable of being both a client and a server; our use of these terms refers only to the role being performed by the program for a particular connection, rather than to the program's capabilities in general. Likewise, any server MAY act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request. origin server The server on which a given resource resides or is to be created. proxy An intermediary program which acts as both a server and a client for the purpose of making requests on behalf of other clients. Requests are serviced internally or by passing them on, with possible translation, to other servers. A proxy MUST interpret and, if necessary, rewrite a request message before forwarding it. Proxies are often used as client-side portals through network firewalls and as helper applications for handling requests via protocols not implemented by the user agent. gateway A server which acts as an intermediary for some other server. Unlike a proxy, a gateway receives requests as if it were the origin server for the requested resource; the requesting client may not be aware that it is communicating with a gateway. Gateways are often used as server-side portals through network firewalls and as protocol translators for access to resources stored on non-HTTP systems. tunnel An intermediary program which is acting as a blind relay between two connections. Once active, a tunnel is not considered a party to the HTTP communication, though the tunnel may have been initiated by an HTTP request. The tunnel ceases to exist when both ends of the relayed connections are closed. Tunnels are used when a portal is necessary and the intermediary cannot, or should not, interpret the relayed communication. cache A program's local store of response messages and the subsystem that controls its message storage, retrieval, and deletion. A cache stores cachable responses in order to reduce the response time and network bandwidth consumption on future, equivalent requests. Any client or server MAY include a cache, though a cache cannot be used by a server that acts acting as a tunnel. cachable A response is cachable if a cache is allowed to store a copy of the response message for use in answering subsequent requests. The rules for determining the cachability of HTTP responses are defined in Section ref Caching_In_HTTP \n 16. Even if a resource is cachable, there may be additional constraints on when and if a cache can use the cached copy for a particular request. firsthand A response is firsthand if it comes directly and without unnecessary delay from the origin server, perhaps via one or more proxies. A response is also firsthand if its validity has just been checked directly with the origin server. explicit expiration time The time at which the origin server intends that an entity should no longer be returned by a cache without further validation. heuristic expiration time An expiration time assigned by a cache when no explicit expiration time is available. age The age of a response is the time since it was generated by, or successfully validated with, the origin server. freshness lifetime The length of time between the generation of a response and its expiration time. fresh A response is fresh if its age has not yet exceeded its freshness lifetime. stale A response is stale if its age has passed its freshness lifetime. A cache may use a fresh response without validating it, but �normally� may not use a stale response without first validating it. (�Normally� means �unless configured to provide better performance at the expense of transparency.�) Therefore, what expires is the cache's authority to use a cached response, without validation, in its reply to a subsequent request. semantically transparent Ideally, an HTTP/1.1 cache would be �semantically transparent.� That is, use of the cache would not affect either the clients or the servers in any way except to improve performance. When a client makes a request via a semantically transparent cache, it receives exactly the same entity headers and entity body it would have received if it had made the same request to the origin server, at the same time. validator An entity tag, or a Last-Modified time, which is used to find out whether a cache entry is a semantically transparent copy of a resource entity. A cache entry is semantically transparent if its validator exactly matches the validator that the server would provide for current instance of that resource entity. Overall Operation The HTTP protocol is a request/response protocol. A client sends a request to the server in the form of a request method, URI, and protocol version, followed by a MIME-like message containing request modifiers, client information, and possible body content over a connection with a server. The server responds with a status line, including the message's protocol version and a success or error code, followed by a MIME-like message containing server information, entity metainformation, and possible entity body content. Most HTTP communication is initiated by a user agent and consists of a request to be applied to a resource on some origin server. In the simplest case, this may be accomplished via a single connection (v) between the user agent (UA) and the origin server (O). request chain ------------------------> UA -------------------v------------------- O <----------------------- response chain A more complicated situation occurs when one or more intermediaries are present in the request/response chain. There are three common forms of intermediary: proxy, gateway, and tunnel. A proxy is a forwarding agent, receiving requests for a URI in its absolute form, rewriting all or part of the message, and forwarding the reformatted request toward the server identified by the URI. A gateway is a receiving agent, acting as a layer above some other server(s) and, if necessary, translating the requests to the underlying server's protocol. A tunnel acts as a relay point between two connections without changing the messages; tunnels are used when the communication needs to pass through an intermediary (such as a firewall) even when the intermediary cannot understand the contents of the messages. request chain --------------------------------------> UA -----v----- A -----v----- B -----v----- C -----v----- O <------------------------------------- response chain The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain MUST pass through four separate connections. This distinction is important because some HTTP communication options may apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant may be engaged in multiple, simultaneous communications. For example, B may be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request. Any party to the communication which is not acting as a tunnel may employ an internal cache for handling requests. The effect of a cache is that the request/response chain is shortened if one of the participants along the chain has a cached response applicable to that request. The following illustrates the resulting chain if B has a cached copy of an earlier response from O (via C) for a request which has not been cached by UA or A. request chain ----------> UA -----v----- A -----v----- B - - - - - - C - - - - - - O <--------- response chain Not all responses are cachable, and some requests may contain modifiers which place special requirements on cache behavior. HTTP requirements for cache behavior and cachable responses are defined in section ref Caching_In_HTTP \n 16. HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80 private HREF="#RefIANA"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [19], but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification. However, HTTP/1.1 implementations SHOULD implement persistent connections (See section ref Persistent_Connectio\n 17). Both clients and servers MUST be capable of handling cases where either party closes the connection prematurely, due to user action, automated time-out, or program failure. In any case, the closing of the connection by either or both parties always terminates the current request, regardless of its status. HTTP and MIME HTTP/1.1 uses many of the constructs defined for MIME, as defined in RFC 1521 private HREF="#RefMIME1" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]. Appendix ref MIME \n 23.3 describes the ways in which the context of HTTP allows for different use of Internet Media Types than is typically found in Internet mail, and gives the rationale for those differences. Notational Conventions and Generic Grammar Augmented BNF All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar to that used by RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]. Implementers will need to be familiar with the notation in order to understand this specification. The augmented BNF includes the following constructs: name = definition The name of a rule is simply the name itself (without any enclosing "<" and ">") and is separated from its definition by the equal character "=". Whitespace is only significant in that indentation of continuation lines is used to indicate a rule definition that spans more than one line. Certain basic rules are in uppercase, such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are used within definitions whenever their presence will facilitate discerning the use of rule names. "literal" Quotation marks surround literal text. Unless stated otherwise, the text is case-insensitive. rule1 | rule2 Elements separated by a bar ("I") are alternatives, e.g., "yes | no" will accept yes or no. (rule1 rule2) Elements enclosed in parentheses are treated as a single element. Thus, �(elem (foo | bar) elem)� allows the token sequences �elem foo elem� and �elem bar elem�. *rule The character �*� preceding an element indicates repetition. The full form is �*element� indicating at least and at most occurrences of element. Default values are 0 and infinity so that �*(element)� allows any number, including zero; �1*element� requires at least one; and �1*2element� allows one or two. [rule] Square brackets enclose optional elements; �[foo bar]� is equivalent to �*1(foo bar)�. N rule Specific repetition: �(element)� is equivalent to �*(element)�; that is, exactly occurrences of (element). Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three alphabetic characters. #rule A construct "#" is defined, similar to "*", for defining lists of elements. The full form is �#element � indicating at least and at most elements, each separated by one or more commas (",") and optional linear whitespace (LWS). This makes the usual form of lists very easy; a rule such as �( *LWS element *( *LWS "," *LWS element )) � can be shown as �1#element�. Wherever this construct is used, null elements are allowed, but do not contribute to the count of elements present. That is, �(element), , (element) � is permitted, but counts as only two elements. Therefore, where at least one element is required, at least one non-null element MUST be present. Default values are 0 and infinity so that "#(element) � allows any number, including zero; �1#element� requires at least one; and �1#2element� allows one or two. ; comment A semi-colon, set off some distance to the right of rule text, starts a comment that continues to the end of line. This is a simple way of including useful notes in parallel with the specifications. implied *LWS The grammar described by this specification is word-based. Except where noted otherwise, linear whitespace (LWS) can be included between any two adjacent words (token or quoted-string), and between adjacent tokens and delimiters (tspecials), without changing the interpretation of a field. At least one delimiter (tspecials) MUST exist between any two tokens, since they would otherwise be interpreted as a single token. However, applications SHOULD attempt to follow �common form� when generating HTTP constructs, since there exist some implementations that fail to accept anything beyond the common forms. Basic Rules The following rules are used throughout this specification to describe basic parsing constructs. The US-ASCII coded character set is defined by private HREF="#RefASCII"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [21]. OCTET = CHAR = UPALPHA = LOALPHA = ALPHA = UPALPHA | LOALPHA DIGIT = CTL = CR = LF = SP = HT = <"> = HTTP/1.1 defines the octet sequence CR LF as the end-of-line marker for all protocol elements except the Entity-Body (see appendix ref Tolerant \n 23.2 for tolerant applications). The end-of-line marker within an Entity-Body is defined by its associated media type, as described in section ref Media_Types \n 7.7. CRLF = CR LF HTTP/1.1 headers can be folded onto multiple lines if the continuation line begins with a space or horizontal tab. All linear whitespace, including folding, has the same semantics as SP. LWS = [CRLF] 1*( SP | HT ) The TEXT rule is only used for descriptive field contents and values that are not intended to be interpreted by the message parser. Words of *TEXT MAY contain octets from character sets other than US-ASCII only when encoded according to the rules of RFC 1522 private HREF="#RefMIME2"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [14]. TEXT = Recipients of header field TEXT containing octets outside the US-ASCII character set range MAY assume that they represent ISO-8859-1 characters if there is no other encoding indicated by an RFC 1522 mechanism. Hexadecimal numeric characters are used in several protocol elements. HEX = "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT Many HTTP/1.1 header field values consist of words separated by LWS or special characters. These special characters MUST be in a quoted string to be used within a parameter value. word = token | quoted-string token = 1* tspecials = "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" | "\" | <"> | "/" | "[" | "]" | "?" | "=" | "{" | "}" | SP | HT Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing �comment� as part of their field value definition. In all other fields, parentheses are considered part of the field value. comment = "(" *( ctext | comment ) ")" ctext = A string of text is parsed as a single word if it is quoted using double-quote marks. quoted-string = ( <"> *(qdtext) <"> ) qdtext = and CTLs, but including LWS> The backslash character (�\�) may be used as a single-character quoting mechanism only within quoted-string and comment constructs. quoted-pair = "\" CHAR Protocol Parameters HTTP Version HTTP uses a �.� numbering scheme to indicate versions of the protocol. The protocol versioning policy is intended to allow the sender to indicate the format of a message and its capacity for understanding further HTTP communication, rather than the features obtained via that communication. No change is made to the version number for the addition of message components which do not affect communication behavior or which only add to extensible field values. The number is incremented when the changes made to the protocol add features which do not change the general message parsing algorithm, but which may add to the message semantics and imply additional capabilities of the sender. The number is incremented when the format of a message within the protocol is changed. The version of an HTTP message is indicated by an HTTP-Version field in the first line of the message. If the protocol version is not specified, the recipient MUST assume that the message is in the simple HTTP/0.9 format private HREF="#RefHTTP10"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [6]. HTTP-Version = "HTTP" "/" 1*DIGIT "." 1*DIGIT Note that the major and minor numbers SHOULD be treated as separate integers and that each MAY be incremented higher than a single digit. Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower than HTTP/12.3. Leading zeros SHOULD be ignored by recipients and never generated by senders. Applications sending Full-Request or Full-Response messages, as defined by this specification, MUST include an HTTP-Version of �HTTP/1.1�. Use of this version number indicates that the sending application is at least conditionally compliant with this specification. Proxy and gateway applications MUST be careful in forwarding requests that are received in a format different from that of the application's native HTTP version. Since the protocol version indicates the protocol capability of the sender, a proxy/gateway MUST never send a message with a version indicator which is greater than its native version; if a higher version request is received, the proxy/gateway MUST either downgrade the request version, respond with an error, or switch to tunnel behavior. Requests with a version lower than that of the application's native format MAY be upgraded before being forwarded; the proxy/gateway's response to that request MUST follow the server requirements listed above. Note: Converting between versions of HTTP may involve addition or deletion of headers required or forbidden by the version involved. It is likely more involved than just changing the version indicator. Uniform Resource Identifiers URIs have been known by many names: WWW addresses, Universal Document Identifiers, Universal Resource Identifiers private HREF="#RefURI"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [3], and finally the combination of Uniform Resource Locators (URL) private HREF="#RefURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [4] and Names (URN) private HREF="#RefURN"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [20]. As far as HTTP is concerned, Uniform Resource Identifiers are simply formatted strings which identify--via name, location, or any other characteristic--a network resource. General Syntax URIs in HTTP can be represented in absolute form or relative to some known base URI private HREF="#RefRelURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [11], depending upon the context of their use. The two forms are differentiated by the fact that absolute URIs always begin with a scheme name followed by a colon. URI = ( absoluteURI | relativeURI ) [ "#" fragment ] absoluteURI = scheme ":" *( uchar | reserved ) relativeURI = net_path | abs_path | rel_path net_path = "//" net_loc [ abs_path ] abs_path = "/" rel_path rel_path = [ path ] [ ";" params ] [ "?" query ] path = fsegment *( "/" segment ) fsegment = 1*pchar segment = *pchar params = param *( ";" param ) param = *( pchar | "/" ) scheme = 1*( ALPHA | DIGIT | "+" | "-" | "." ) net_loc = *( pchar | ";" | "?" ) query = *( uchar | reserved ) fragment = *( uchar | reserved ) pchar = uchar | ":" | "@" | "&" | "=" | "+" uchar = unreserved | escape unreserved = ALPHA | DIGIT | safe | extra | national escape = "%" HEX HEX reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" extra = "!" | "*" | "'" | "(" | ")" | "," safe = "$" | "-" | "_" | "." unsafe = CTL | SP | <"> | "#" | "%" | "<" | ">" national = For definitive information on URL syntax and semantics, see RFC 1738 private HREF="#RefURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [4] and RFC 1808 private HREF="#RefRelURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [11]. The BNF above includes national characters not allowed in valid URLs as specified by RFC 1738, since HTTP servers are not restricted in the set of unreserved characters allowed to represent the rel_path part of addresses, and HTTP proxies may receive requests for URIs not defined by RFC 1738. The HTTP protocol does not place any a priori limit on the length of a URI. Servers MUST be able to handle the URI of any resource they serve, and SHOULD be able to handle URIs of unbounded length if they provide GET-based forms that could generate such URIs. A server SHOULD return a status code of 414 Request-URI Too Large if a URI is longer than the server can handle. See section ref Code414 \n 12.4.1.15. Note: Servers should be cautious about depending on URI lengths above 255 bytes, because some older client or proxy implementations may not properly support these. All client and proxy implementations MUST be able to handle a URI of any finite length. http URL The �http� scheme is used to locate network resources via the HTTP protocol. This section defines the scheme-specific syntax and semantics for http URLs. http_URL = "http:" "//" host [ ":" port ] [ abs_path ] host = port = *DIGIT If the port is empty or not given, port 80 is assumed. The semantics are that the identified resource is located at the server listening for TCP connections on that port of that host, and the Request-URI for the resource is abs_path. The use of IP addresses in URL's SHOULD be avoided whenever possible. See RFC 1900private HREF="#Ref1900" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [24]. If the abs_path is not present in the URL, it MUST be given as �/� when used as a Request-URI for a resource (section ref Request_URI \n 9.1.2). Note: Although the HTTP protocol is independent of the transport layer protocol, the http URL only identifies resources by their TCP location, and thus non-TCP resources MUST be identified by some other URI scheme. The canonical form for �http� URLs is obtained by converting any UPALPHA characters in host to their LOALPHA equivalent (hostnames are case-insensitive), eliding the [ ":" port ] if the port is 80, and replacing an empty abs_path with �/�. URI Canonicalization A cache, when comparing two URIs to decide if they match or not, a cache MUST use a case-sensitive octet-by-octet comparison of the entire URIs, with these exceptions: Following the rules from section ref http_URL \n 7.2.2: � A port that is empty or not given is equivalent to port 80. � Comparisons of host names MUST be case-insensitive. � Comparisons of scheme names MUST be case-insensitive. � An empty abs_path is equivalent to an abs_path of �/� Characters except those in the reserved set and the unsafe set (see section ref URI \n 7.2) are equivalent to their �"%" HEX HEX� encodings. For example, the following three URIs are equivalent: http://abc.com:80/\~smith/home.htmlhttp://ABC.com/%7Esmith/home.htmlhttp://ABC.com:/%7esmith/home.html Date/Time Formats Full Date HTTP applications have historically allowed three different formats for the representation of date/time stamps: Sun, 06 Nov 1994 08:49:37 GMT ; RFC 822, updated by RFC 1123 Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036 Sun Nov 6 08:49:37 1994 ; ANSI C's asctime() format The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 private HREF="#RefSTD3"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [8] (an update to RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]). The second format is in common use, but is based on the obsolete RFC 850 private HREF="#RefUSENET"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [12] date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value MUST accept all three formats, though they MUST generate only the RFC 1123 format for representing date/time stamps in HTTP message fields. Note: Recipients of date values are encouraged to be robust in accepting date values that may have been generated by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP. All HTTP date/time stamps MUST be represented in Universal Time (UT), also known as Greenwich Mean Time (GMT), without exception. This is indicated in the first two formats by the inclusion of �GMT� as the three-letter abbreviation for time zone, and SHOULD be assumed when reading the asctime format. HTTP-date = rfc1123-date | rfc850-date | asctime-date rfc1123-date = wkday "," SP date1 SP time SP "GMT" rfc850-date = weekday "," SP date2 SP time SP "GMT" asctime-date = wkday SP date3 SP time SP 4DIGIT date1 = 2DIGIT SP month SP 4DIGIT ; day month year (e.g., 02 Jun 1982) date2 = 2DIGIT "-" month "-" 2DIGIT ; day-month-year (e.g., 02-Jun-82) date3 = month SP ( 2DIGIT | ( SP 1DIGIT )) ; month day (e.g., Jun 2) time = 2DIGIT ":" 2DIGIT ":" 2DIGIT ; 00:00:00 - 23:59:59 wkday = "Mon" | "Tue" | "Wed" | "Thu" | "Fri" | "Sat" | "Sun" weekday = "Monday" | "Tuesday" | "Wednesday" | "Thursday" | "Friday" | "Saturday" | "Sunday" month = "Jan" | "Feb" | "Mar" | "Apr" | "May" | "Jun" | "Jul" | "Aug" | "Sep" | "Oct" | "Nov" | "Dec" Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc. Additional rules for requirements on parsing and representation of dates and other potential problems with date representations include: � HTTP/1.1 clients and caches should assume that an RFC-850 date which appears to be more than 50 years in the future is in fact in the past (this helps solve the �year 2000� problem). � An HTTP/1.1 implementation may internally represent a parsed Expires date as earlier than the proper value, but MUST NOT internally represent a parsed Expires date as later than the proper value. � All expiration-related calculations must be done in Universal Time (GMT). The local time zone MUST NOT influence the calculation or comparison of an age or expiration time. � If an HTTP header incorrectly carries a date value with a time zone other than GMT, it must be converted into GMT using the most conservative possible conversion. Delta Seconds Some HTTP header fields allow a time value to be specified as an integer number of seconds, represented in decimal, after the time that the message was received. This format SHOULD only be used to represent short time periods or periods that cannot start until receipt of the message. delta-seconds = 1*DIGIT Character Sets HTTP uses the same definition of the term �character set� as that described for MIME: The term �character set� is used in this document to refer to a method used with one or more tables to convert a sequence of octets into a sequence of characters. Note that unconditional conversion in the other direction is not required, in that not all characters may be available in a given character set and a character set may provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encodings, from simple single-table mappings such as US-ASCII to complex table switching methods such as those that use ISO 2022's techniques. However, the definition associated with a MIME character set name MUST fully specify the mapping to be performed from octets to characters. In particular, use of external profiling information to determine the exact mapping is not permitted. Note: This use of the term �character set� is more commonly referred to as a �character encoding.� However, since HTTP and MIME share the same registry, it is important that the terminology also be shared. HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry private HREF="#RefIANA"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [19]. However, because that registry does not define a single, consistent token for each character set, we define here the preferred names for those character sets most likely to be used with HTTP entities. These character sets include those registered by RFC 1521 private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7] -- the US-ASCII private HREF="#RefASCII"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [21] and ISO-8859 private HREF="#RefISO8859"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [22] character sets -- and other names specifically recommended for use within MIME charset parameters. charset = "US-ASCII" | "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3" | "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6" | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9" | "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR" | "UNICODE-1-1" | "UNICODE-1-1-UTF-7" | "UNICODE-1-1-UTF-8" | token Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry private HREF="#RefIANA"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [19] MUST represent the character set defined by that registry. Applications SHOULD limit their use of character sets to those defined by the IANA registry. The character set of an entity body SHOULD be labeled as the lowest common denominator of the character codes used within that body, with the exception that no label is preferred over the labels US-ASCII or ISO-8859-1. Content Codings Content coding values indicate an encoding transformation that has been or can be applied to a resource entity. Content codings are primarily used to allow a document to be compressed or encrypted without losing the identity of its underlying media type. Typically, the resource entity is stored in this encoding and only decoded before rendering or analogous usage. content-coding = "gzip" | "x-gzip" | "compress" | "x-compress" | token Note: For historical reasons, HTTP applications SHOULD consider �x-gzip� and �x-compress� to be equivalent to �gzip� and �compress�, respectively. All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (section ref Accept_Encoding \n 18.3) and Content-Encoding (section ref Content_Encoding \n 18.13) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding. Note that a single program MAY be capable of decoding multiple content-coding formats. Two values are defined by this specification: gzip An encoding format produced by the file compression program �gzip� (GNU zip) developed by Jean-loup Gaillyprivate HREF="#GZIP" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [25]. This format is typically a Lempel-Ziv coding (LZ77) with a 32 bit CRC. compress The encoding format produced by the file compression program �compress�. This format is an adaptive Lempel-Ziv-Welch coding (LZW). Note: Use of program names for the identification of encoding formats is not desirable and should be discouraged for future encodings. Their use here is representative of historical practice, not good design. HTTP defines a registration process which uses the Internet Assigned Numbers Authority (IANA) as a central registry for content-coding value tokens. Additional content-coding value tokens beyond the four defined in this document (gzip x-gzip compress x-compress) SHOULD be registered with the IANA. To allow interoperability between clients and servers, specifications of the content coding algorithms used to implement a new value SHOULD be publicly available and adequate for independent implementation, and MUST conform to the purpose of content coding defined in this section. Transfer Codings Transfer coding values are used to indicate an encoding transformation that has been, can be, or may need to be applied to an Entity-Body in order to ensure safe transport through the network. This differs from a content coding in that the transfer coding is a property of the message, not of the original resource entity. transfer-coding = "chunked" | transfer-extension transfer-extension = token All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer coding values in the Transfer-Encoding header field (section ref Transfer_Encoding \n 18.43). Transfer codings are analogous to the Content-Transfer-Encoding values of MIME private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7], which were designed to enable safe transport of binary data over a 7-bit transport service. However, �safe transport� has a different focus for an 8bit-clean transfer protocol. In HTTP, the only unsafe characteristic of message bodies is the difficulty in determining the exact body length (section ref BodyLength \n 11.2.2), or the desire to encrypt data over a shared transport. All HTTP/1.1 applications MUST be able to receive and decode the �chunked� transfer coding , and MUST ignore transfer coding extensions they do not understand. A server which receives a an entity-body with a transfer-coding it does not understand SHOULD return 501(Unimplemented), and close the connection. A server MUST NOT send transfer-codings to a client that were not defined in the version of HTTP used in the client's request. Clients sending entity-bodies with transfer-codings SHOULD must be prepared for the connection to be closed if the server doesn't understand the transfer-coding. The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an optional footer containing entity-header fields. This allows dynamically-produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message. Chunked-Body = *chunk "0" CRLF footer CRLF chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF chunk-size = hex-no-zero *HEX chunk-ext = *( ";" chunk-ext-name [ "=" chunk-ext-value ] ) chunk-ext-name = token chunk-ext-val = token | quoted-string chunk-data = chunk-size(OCTET) footer = *<> hex-no-zero = Note that the chunks are ended by a zero-sized chunk, followed by the footer and terminated by an empty line. An example process for decoding a Chunked-Body is presented in appendix ref MIME_TE \n 23.3.6. Media Types HTTP uses Internet Media Types private HREF="#RefMediaType"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [17] in the Content-Type (section ref Content_Type \n 18.19) and Accept (section ref Accept \n 18.1) header fields in order to provide open and extensible data typing and type negotiation. media-type = type "/" subtype *( ";" parameter ) type = token subtype = token Parameters may follow the type/subtype in the form of attribute/value pairs. parameter = attribute "=" value attribute = token value = token | quoted-string The type, subtype, and parameter attribute names are case-insensitive. Parameter values may or may not be case-sensitive, depending on the semantics of the parameter name. LWS MUST NOT be generated between the type and subtype, nor between an attribute and its value. Upon receipt of a media type with an unrecognized parameter, a user agent SHOULD treat the media type as if the unrecognized parameter and its value were not present. Some older HTTP applications do not recognize media type parameters. HTTP/1.1 applications SHOULD only use media type parameters when they are necessary to define the content of a message. Media-type values are registered with the Internet Assigned Number Authority (IANA private HREF="#RefIANA" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [19]). The media type registration process is outlined in RFC 1590 private HREF="#RefMediaType" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [17]. Use of non-registered media types is discouraged. Canonicalization and Text Defaults Internet media types are registered with a canonical form. In general, an Entity-Body transferred via HTTP MUST be represented in the appropriate canonical form prior to its transmission; the exception is �text� types, as defined in the next paragraph.. when in canonical form , media subtypes of the �text� type use CRLF as the text line break. However, HTTP allows the transport of text media with plain CR or LF alone representing a line break when if it is done consistently for an entire Entity-Body.. HTTP applications MUST accept CRLF, bare CR, and bare LF as being representative of a line break in text media received via HTTP.In addition, if the text media is represented in a character set that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character sets, HTTP allows the use of whatever octet sequences are defined by that character set to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the Entity-Body; a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries). If an Entity-Body is encoded with a Content-Encoding, the underlying data MUST be in a form defined above prior to being encoded. The �charset� parameter is used with some media types to define the character set (section ref Charset \n 7.4) of the data. When no explicit charset parameter is provided by the sender, media subtypes of the �text� type are defined to have a default charset value of �ISO-8859-1� when received via HTTP. Data in character sets other than �ISO-8859-1� or its subsets MUST be labeled with an appropriate charset value in order to be consistently interpreted by the recipient. Note: Many current HTTP servers provide data using charsets other than �ISO-8859-1� without proper labeling. This situation reduces interoperability and is not recommended. To compensate for this, some HTTP user agents provide a configuration option to allow the user to change the default interpretation of the media type character set when no charset parameter is given. Multipart Types MIME provides for a number of �multipart� types -- encapsulations of one or more entities within a single message's Entity-Body. All multipart types share a common syntax, as defined in section 7.2.1 of RFC 1521 private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7], and MUST include a boundary parameter as part of the media type value. The message body is itself a protocol element and MUST therefore use only CRLF to represent line breaks between body-parts. Unlike in RFC 1521, the epilogue of any multipart message MUST be empty; HTTP applications MUST NOT transmit the epilogue even if the original resource entity contains an epilogue. In HTTP, multipart body-parts MAY contain header fields which are significant to the meaning of that part. In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to �multipart/mixed�. Note: The �multipart/form-data� type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in RFC 1867 private HREF="#RefFileUpload"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [15]. Product Tokens Product tokens are used to allow communicating applications to identify themselves via a simple product token, with an optional slash and version designator. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by whitespace. By convention, the products are listed in order of their significance for identifying the application. product = token ["/" product-version] product-version = token Examples: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 Server: Apache/0.8.4 Product tokens should be short and to the point -- use of them for advertising or other non-essential information is explicitly forbidden. Although any token character may appear in a product-version, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value). Quality Values HTTP content negotiation (section ref Content_Negotiation \n 15) uses short �floating point� numbers to indicate the relative importance (�weight�) of various negotiable parameters. The weights are normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. In order to discourage misuse of this feature, HTTP/1.1 applications MUST NOT generate more than three digits after the decimal point. User configuration of these values SHOULD also be limited in this fashion. qvalue = ( "0" [ "." 0*3DIGIT ] ) | ( "1" [ "." 0*3("0") ] ) �Quality values� is a slight misnomer, since these values actually measure relative degradation in perceived quality. Thus, a value of �0.8� represents a 20% degradation from the optimum rather than a statement of 80% quality. Language Tags A language tag identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to other human beings. Computer languages are explicitly excluded. HTTP uses language tags within the Accept-Language, and Content-Language fields. The syntax and registry of HTTP language tags is the same as that defined by RFC 1766 private HREF="#RefLangTags"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [1]. In summary, a language tag is composed of 1 or more parts: A primary language tag and a possibly empty series of subtags: language-tag = primary-tag *( "-" subtag ) primary-tag = 1*8ALPHA subtag = 1*8ALPHA Whitespace is not allowed within the tag and all tags are case-insensitive. The name space of language tags is administered by the IANA. Example tags include: en, en-US, en-cockney, i-cherokee, x-pig-latin where any two-letter primary-tag is an ISO 639 language abbreviation and any two-letter initial subtag is an ISO 3166 country code. (The last three tags above are not registered tags; all but the last are examples of tags which could be registered in future.) Entity Tags Entity tags are quoted strings whose internal structure is not visible to clients or caches. Entity tags are used as cache validators in HTTP/1.1. entity-tag = strong-entity-tag | weak-entity-tag | null-entity-tag strong-entity-tag = quoted-string weak-entity-tag = quoted-string "/W" null-entity-tag = <"> <"> Note that the �/W� tag is considered part of a weak entity tag; it MUST NOT be removed by any cache or client. There are two comparison functions on validators: � The strong comparison function: in order to be considered equal, both validators must be identical in every way, and neither may be weak. � The weak comparison function: in order to be considered equal, both validators must be identical in every way, except for the presence or absence of a �weak� tag. The weak comparison function MAY be used for simple (non-subrange) GET requests. The strong comparison function MUST be used in all other cases. The null validator is a special value, defined as never matching the current validator of an existing resource entity, and always matching the �current� validator of a resource entity that does not exist. Variant IDs A cache stores instances of resource entities, not instances of generic resources per se. Therefore, the URI of a generic resource is not sufficient for use as an identifier for a specific resource entity. In certain interactions between a cache and an origin server, it is convenient to encode that identifier using a more compact representation than the full set of selecting request headers (which may not even be possible if the selection criteria are not known to the cache). For these reasons, the HTTP protocol provides an optional mechanism for identifying a specific entity source of a generic resource, called a variant-ID. Variant-IDs are used to identify specific variants of a generic resource; see section ref Variant_ID_Use \n 16.5.3 for how they are used. variant-id = quoted-string Variant-IDs are compared using string octet-equality; case is significant. All responses from generic resources SHOULD include variant-IDs. If these are not present, the resource author can expect caches to correctly handle requests on the generic resource, but cannot expect the caching to be efficient. Variant Sets Validator sets are used for doing conditional retrievals on generic resources; see section ref Variant_ID_Use \n 16.5.3. variant-set = 1#variant-set-item variant-set-item = opaque-validator ";" variant-id Range Protocol Parameters This section defines certain HTTP protocol parameters used in range requests and related responses. Range Units A resource entity may be broken down into subranges according to various structural units. range-unit = bytes-unit | other-range-unit bytes-unit = "bytes" other-range-unit = token The only range unit defined by HTTP/1.1 is �bytes�. HTTP/1.1 implementations may ignore ranges specified using other units. Byte Ranges Since all HTTP entities are represented in HTTP messages as sequences of bytes, the concept of a byte range is meaningful for any HTTP entity. (However, not all clients and servers need to support byte-range operations.) Byte range specifications in HTTP apply to the sequence of bytes that would be transferred by the protocol if no transfer-coding were being applied. This means that if Content-coding is applied to the data, the byte range specification applies to the resulting content-encoded byte stream, not to the unencoded byte stream. It also means that if the entity-body's media-type is a composite type (e.g., multipart/* and message/rfc822), then the composite�s body-parts may have their own content-encoding and content-transfer-encoding, and the byte range applies to the result of the those encodings. A byte range operation may specify a single range of bytes, or a set of ranges within a single entity. ranges-specifier = byte-ranges-specifier byte-ranges-specifier = bytes-unit "=" byte-range-set byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec ) byte-range-spec = first-byte-pos "-" [last-byte-pos] first-byte-pos = 1*DIGIT last-byte-pos = 1*DIGIT The first-byte-pos value in a byte-range-spec gives the byte-offset of the first byte in a range. The last-byte-pos value gives the byte-offset of the last byte in the range; that is, the byte positions specified are inclusive. Byte offsets start at zero. If the last-byte-pos value is present, it must be greater than or equal to the first-byte-pos in that byte-range-spec, or the byte-range-spec is invalid. The recipient of an invalid byte-range-spec must ignore it. If the last-byte-pos value is absent, it is assumed to be equal to the current length of the entity in bytes. If the last-byte-pos value is larger than the current length of the entity, it is assumed to be equal to the current length of the entity. suffix-byte-range-spec = "-" suffix-length suffix-length = 1*DIGIT A suffix-byte-range-spec is used to specify the suffix of the entity, of a length given by the suffix-length value. (That is, this form specifies the last N bytes of an entity.) If the entity is shorter than the specified suffix-length, the entire entity is used. Examples of byte-ranges-specifier values (assuming an entity of length 10000): � The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-499 � The second 500 bytes (byte offsets 500-999, inclusive): bytes=500-999 � The final 500 bytes (byte offsets 9500-9999, inclusive): bytes=-500 � Or bytes=9500- � The first and last bytes only (bytes 0 and 9999): bytes=0-0,-1 � Several legal but not canonical specifications of the second 500 bytes (byte offsets 500-999, inclusive): bytes=500-600,601-999 bytes=500-700,601-999 Content Ranges When a server returns a partial response to a client, it must describe both the extent of the range covered by the response, and the length of the entire entity. content-range-spec = byte-content-range-spec byte-content-range-spec = bytes-unit SP first-byte-pos "-" last-byte-pos "/" entity-length entity-length = 1*DIGIT Unlike byte-ranges-specifier values, a byte-content-range-spec may only specify one range, and must contain absolute byte positions for both the first and last byte of the range. A byte-content-range-spec whose last-byte-pos value is less than its first-byte-pos value, or whose entity-length value is less than or equal to its last-byte-pos value, is invalid. The recipient of an invalid byte-content-range-spec MUST ignore it and any content transferred along with it. Examples of byte-content-range-spec values, assuming that the entity contains a total of 1234 bytes: � The first 500 bytes: bytes 0-499/1234 � The second 500 bytes: bytes 500-999/1234 � All except for the first 500 bytes: bytes 500-1233/1234 � The last 500 bytes: bytes 734-1233/1234 HTTP Message Message Types HTTP messages consist of requests from client to server and responses from server to client. HTTP-message = Full-Request ; HTTP/1.1 messages | Full-Response Full-Request and Full-Response use the generic message format of RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9] for transferring entities. Both messages may include optional header fields (also known as �headers�) and an entity body. The entity body is separated from the headers by a null line (i.e., a line with nothing preceding the CRLF). Message Headers HTTP header fields, which include General-Header (Section ref General_Header \n 8.3), Request-Header (Section ref Request_Header \n 9.2), Response-Header (Section ref Response_Header \n 10.2), and Entity-Header (Section ref Entity_Header \n 11.1) fields, follow the same generic format as that given in Section 3.1 of RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]. Each header field consists of a name followed by a colon (�:�) and the field value. Field names are case-insensitive. The field value may be preceded by any amount of LWS, though a single SP is preferred. Header fields can be extended over multiple lines by preceding each extra line with at least one SP or HT. HTTP-header = field-name ":" [ field-value ] CRLF field-name = token field-value = *( field-content | LWS ) field-content = The order in which header fields with differing field names are received is not significant. However, it is �good practice� to send General-Header fields first, followed by Request-Header or Response-Header fields, and ending with the Entity-Header fields. Multiple HTTP-header fields with the same field-name may be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one �field-name: field-value� pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. Thus, the order in which multiple header fields with the same field-name are received may be significant to the interpretation of the combined field-value. General Header Fields There are a few header fields which have general applicability for both request and response messages, but which do not apply to the entity being transferred. These headers apply only to the message being transmitted. General-Header = Cache-Control ; Section ref Cache_Control \n 18.10 | Connection ; Section ref Connection \n 18.11 | Date ; Section ref Date \n 18.20 | Via ; Section ref Via \n 18.47 | Keep-Alive ; Section ref Keep_Alive_Header \n 23.5.2.5.1 | Pragma ; Section ref Pragma \n 18.34 | Upgrade ; Section ref Upgrade \n 18.44 General header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields may be given the semantics of general header fields if all parties in the communication recognize them to be general header fields. Unrecognized header fields are treated as Entity-Header fields. Request A request message from a client to a server includes, within the first line of that message, the method to be applied to the resource, the identifier of the resource, and the protocol version in use. For backwards compatibility with the more limited HTTP/0.9 protocol, there are two valid formats for an HTTP request: Request = Full-Request Full-Request = Request-Line ; Section ref Request_Line \n 9.1 *( General-Header ; Section ref General_Header \n 8.3 | Request-Header ; Section ref Request_Header \n 9.2 | Entity-Header ) ; Section ref Entity_Header \n 11.1 CRLF [ Entity-Body ] ; Section ref Entity_Body \n 11.2 Request-Line The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The elements are separated by SP characters. No CR or LF are allowed except in the final CRLF sequence. Request-Line = CRLF | Method SP Request-URI SP HTTP-Version CRLF In the interest of robustness, HTTP/1.1 servers SHOULD ignore null request lines (ones that comprise just CRLF). An HTTP/1.1 client MUST NOT preface a request with CRLF. Method The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case-sensitive. Method = "OPTIONS" ; Section ref OPTIONS \n 13.1 | "GET" ; Section ref GET \n 13.2 | "HEAD" ; Section ref HEAD \n 13.3 | "POST" ; Section ref POST \n 13.4 | "PUT" ; Section ref PUT \n 13.5 | "DELETE" ; Section ref DELETE \n 13.6 | "TRACE" ; Section ref TRACE \n 13.7 | extension-method extension-method = token The list of methods acceptable by a plain resource can be specified in an Allow header field (section ref Allow \n 18.7). However, the client is always notified through the return code of the response whether a method is currently allowed on a plain resource, as this can change dynamically. Servers SHOULD return the status code 405 (method not allowed) if the method is known by the server but not allowed for the requested resource, and 501 (not implemented) if the method is unrecognized or not implemented by the server. The list of methods known by a server can be listed in a Public response header field (section ref Public \n 18.37). The methods GET and HEAD MUST be supported by all general-purpose servers. Servers which provide Last-Modified dates for resources MUST also support the conditional GET method. All other methods are optional; however, if the above methods are implemented, they MUST be implemented with the same semantics as those specified in section ref Methods \n 13. Request-URI The Request-URI is a Uniform Resource Identifier (section ref URI \n 7.2) and identifies the resource upon which to apply the request. Request-URI = "*" | absoluteURI | abs_path The three options for Request-URI are dependent on the nature of the request. The asterisk �*� means that the request does not apply to a particular resource, but to the server itself, and is only allowed when the Method used does not necessarily apply to a resource. One example would be OPTIONS * HTTP/1.1 The absoluteURI form is required when the request is being made to a proxy. The proxy is requested to forward the request or service it from a valid cache, and return the response.. Note that the proxy MAY forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request loops, a proxy MUST be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example Request-Line would be: GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1 To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies. The Host request-header field MUST be ignored in requests using an absoluteURL as the Request-URI. The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute path of the URI MUST be transmitted (see ref URI_syntax \n 7.2.1, abs_path) as the Request-URI, and the network location of the URI (net_loc) MUST be transmitted in a Host header field.. For example, a client wishing to retrieve the resource above directly from the origin server would create a TCP connection to port 80 of the host �www.w3.org� and send the lines: GET /pub/WWW/TheProject.html HTTP/1.1 Host:www.w3.org followed by the remainder of the Full-Request. Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as �/� (the server root). If a proxy receives a request without any path in the Request-URI and the method specified is capable of supporting the asterisk form of request, then the last proxy on the request chain MUST forward the request with �*� as the final Request-URI. For example, the request OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1 would be forwarded by the proxy as OPTIONS * HTTP/1.1 Host: www.ics.uci.edu:8001 after connecting to port 8001 of host �www.ics.uci.edu�. The Request-URI is transmitted as an encoded string, where some characters may be escaped using the �% HEX HEX� encoding defined by RFC 1738 private HREF="#RefURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [4]. The origin server MUST decode the Request-URI in order to properly interpret the request. In requests that they forward, proxies MUST NOT rewrite the �abs_path� part of a Request-URI in any way except as noted above to replace a null abs_path with �*�. Illegal Request-URIs SHOULD be responded to with an appropriate status code. Proxies MAY transform the Request-URI for internal processing purposes, but SHOULD NOT send such a transformed Request-URI in forwarded requests. The main reason for this rule is to make sure that the form of Request-URI is well specified, to enable future extensions without fear that they will break in the face of some rewritings. Another is that one consequence of rewriting the Request-URI is that integrity or authentication checks by the server may fail; since rewriting MUST be avoided in this case, it may as well be proscribed in general. Implementers should be aware that some pre-HTTP/1.1 proxies do some rewriting. The Resource Identified by a Request HTTP/1.1 origin servers SHOULD be aware that the exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field. An origin server that does not allow resources to differ by the requested host MAY ignore the Host header field. An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or vanity hostnames) MUST use the following rules for determining the requested resource on an HTTP/1.1 request:. 1. If Request-URI is an absoluteURI, the host is included in the Request-URI. Any Host header field in the request MUST be ignored. 2. If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host header field. 3. If the request-URI is not an absoluteURI and no Host header field is present (or does not represent a valid host on that server), the response MUST be a 400 (Bad Request) error message. Recipients of an HTTP/1.0 request lacking a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine what exact resource is being requested. Request Header Fields The request header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method (procedure) invocation. Request-Header = Accept ; Section ref Accept \n 18.1 | Accept-Charset ; Section ref Accept_Charset \n 18.2 | Accept-Encoding ; Section ref Accept_Encoding \n 18.3 | Accept-Language ; Section ref Accept_Language \n 18.4 | Authorization ; Section ref Authorization \n 18.8 | From ; Section ref From \n 18.23 | Host ; Section ref Host \n 18.24 | If-Modified-Since ; Section ref If_Modified_Since \n 18.25 | If-Range ; Section ref Range_If \n 18.28 | Proxy-Authorization ; Section ref Proxy_Authorization \n 18.36 | Range ; Section ref Range \n 18.38 | Referer ; Section ref Referer \n 18.39 | User-Agent ; Section ref User_Agent \n 18.45 | Max-Forwards ; Section ref Max_Forwards \n 18.32 Request-Header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of request header fields if all parties in the communication recognize them to be request header fields. Unrecognized header fields are treated as Entity-Header fields. Response After receiving and interpreting a request message, a server responds in the form of an HTTP response message. Response = Full-Response Full-Response = Status-Line ; Section ref Status_Line \n 10.1 *( General-Header ; Section ref General_Header \n 8.3 | Response-Header ; Section ref Response_Header \n 10.2 | Entity-Header ) ; Section ref Entity_Header \n 11.1 CRLF [ Entity-Body ] ; Section ref Entity_Body \n 11.2 Status-Line The first line of a Full-Response message is the Status-Line, consisting of the protocol version followed by a numeric status code and its associated textual phrase, with each element separated by SP characters. No CR or LF is allowed except in the final CRLF sequence. Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF Status Code and Reason Phrase The Status-Code element is a 3-digit integer result code of the attempt to understand and satisfy the request. The Reason-Phrase is intended to give a short textual description of the Status-Code. The Status-Code is intended for use by automata and the Reason-Phrase is intended for the human user. The client is not required to examine or display the Reason-Phrase. The first digit of the Status-Code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit: � 1xx: Informational - Request received, continuing process � 2xx: Success - The action was successfully received, understood, and accepted � 3xx: Redirection - Further action must be taken in order to complete the request � 4xx: Client Error - The request contains bad syntax or cannot be fulfilled � 5xx: Server Error - The server failed to fulfill an apparently valid request The individual values of the numeric status codes defined for HTTP/1.1, and an example set of corresponding Reason-Phrase's, are presented below. The reason phrases listed here are only recommended -- they may be replaced by local equivalents without affecting the protocol. These codes are fully defined in section ref Status_Codes \n 12. Status-Code = "100" ; Continue | "101" ; Switching Protocols | "200" ; OK | "201" ; Created | "202" ; Accepted | "203" ; Non-Authoritative Information | "204" ; No Content | "205" ; Reset Content | "206" ; Partial Content | "300" ; Multiple Choices | "301" ; Moved Permanently | "302" ; Moved Temporarily | "303" ; See Other | "304" ; Not Modified | "305" ; Use Proxy | "400" ; Bad Request | "401" ; Unauthorized | "402" ; Payment Required | "403" ; Forbidden | "404" ; Not Found | "405" ; Method Not Allowed | "406" ; Not Acceptable | "407" ; Proxy Authentication Required | "408" ; Request Time-out | "409" ; Conflict | "410" ; Gone | "411" ; Length Required | "412" ; Precondition Failed | "413" ; Request Entity Too Large | "414" ; Request URI Too Large | "415" ; Unsupported Media Type | "500" ; Internal Server Error | "501" ; Not Implemented | "502" ; Bad Gateway | "503" ; Service Unavailable | "504" ; Gateway Time-out | "505" ; HTTP Version not supported | extension-code extension-code = 3DIGIT Reason-Phrase = *<text, excluding="" cr,="" lf=""> HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response MUST NOT be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents SHOULD present to the user the entity returned with the response, since that entity is likely to include human-readable information which will explain the unusual status. Response Header Fields The response header fields allow the server to pass additional information about the response which cannot be placed in the Status-Line. These header fields give information about the server and about further access to the resource identified by the Request-URI. Response-Header = Location ; Section ref Location \n 18.31 | Proxy-Authenticate ; Section ref Proxy_Authenticate \n 18.35 | Public ; Section ref Public \n 18.37 | Retry-After ; Section ref Retry_After \n 18.40 | Server ; Section ref Server \n 18.41 | WWW-Authenticate ; Section ref WWW_Authenticate \n 18.46 Response-Header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of response header fields if all parties in the communication recognize them to be response header fields. Unrecognized header fields are treated as Entity-Header fields. Entity Full-Request and Full-Response messages MAY transfer an entity within some requests and responses. An entity consists of Entity-Header fields and (usually) an Entity-Body. In this section, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity. Entity Header Fields Entity-Header fields define optional metainformation about the Entity-Body or, if no body is present, about the resource identified by the request. Entity-Header = Allow ; Section ref Allow \n 18.7 | Content-Base ; Section ref Content_Base \n 18.12 | Content-Encoding ; Section ref Accept_Encoding \n 18.3 | Content-Language ; Section ref Content_Language \n 18.14 | Content-Length ; Section ref Content_Length \n 18.15 | Content-Location ; Section ref Content_Location \n 18.16 | Content-MD5 ; Section ref Content_MD5 \n 0 | Content-Range ; Section ref Content_Range \n 18.18 | Content-Type ; Section ref Content_Type \n 18.19 | Expires ; Section ref Expires \n 18.22 | Last-Modified ; Section ref Last_Modified \n 18.30 | Title ; Section ref Title \n 18.42 | Transfer-Encoding ; Section ref Transfer_Encoding \n 18.43 | extension-header extension-header = HTTP-header The extension-header mechanism allows additional Entity-Header fields to be defined without changing the protocol, but these fields cannot be assumed to be recognizable by the recipient. Unrecognized header fields SHOULD be ignored by the recipient and forwarded by proxies. Entity Body The entity body (if any) sent with an HTTP request or response is in a format and encoding defined by the Entity-Header fields. Entity-Body = *OCTET An entity body MUST ONLY be included with a request message when the request method calls for one. The presence of an entity body in a request is signaled by the inclusion of a Content-Length and/or Content-Type header field in the request message headers. For response messages, whether or not an entity body is included with a message is dependent on both the request method and the response code. All responses to the HEAD request method MUST NOT include a body, even though the presence of entity header fields may lead one to believe they do. All 1xx (informational), 204 (no content), and 304 (not modified) responses MUST NOT include a body. All other responses MUST include an entity body or a Content-Length header field defined with a value of zero (0). Type When an entity body is included with a message, the data type of that body is determined via the header fields Content-Type, Content-Encoding, and Transfer-Encoding. These define a three-layer, ordered encoding model: entity-body := Transfer-Encoding( Content-Encoding( Content-Type( data ) ) ) The default for both encodings is none (i.e., the identity function). Content-Type specifies the media type of the underlying data. Content-Encoding may be used to indicate any additional content codings applied to the type, usually for the purpose of data compression, that are a property of the resource entity requested. Transfer-Encoding may be used to indicate any additional transfer codings applied by an application to ensure safe and proper transfer of the message. Note that Transfer-Encoding is a property of the message, not of the resource entity. Any HTTP/1.1 message containing an entity body SHOULD include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a Content-Type header, the recipient may attempt to guess the media type via inspection of its content and/or the name extension(s) of the URL used to identify the resource. If the media type remains unknown, the recipient SHOULD treat it as type �application/octet-stream�. Length When an entity body is included with a message, the length of that body may be determined in one of several ways. If a Content-Length header field is present, its value in bytes represents the length of the entity body. Otherwise, the body length is determined by the Transfer-Encoding (if the �chunked� transfer coding has been applied) or by the server closing the connection. Note: Any response message which MUST NOT include an entity body (such as the 1xx, 204, and 304 responses and any response to a HEAD request) is always terminated by the first empty line after the header fields, regardless of the entity header fields present in the message. Closing the connection cannot be used to indicate the end of a request body, since it leaves no possibility for the server to send back a response. For compatibility with HTTP/1.0 applications, HTTP/1.1 requests containing an entity body MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. HTTP/1.1 servers MUST accept the �chunked� transfer coding (section ref Transfer_Codings \n 7.6), thus allowing this mechanism to be used for a request when Content-Length is unknown. If a request contains an entity body and Content-Length is not specified, the server SHOULD respond with 400 (bad request) if it cannot determine the length of the request message's content, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length. Messages MUST NOT include both a Content-Length header field and the �chunked� transfer coding. If both are received, the Content-Length MUST be ignored. When a Content-Length is given in a message where an entity body is allowed, its field value MUST exactly match the number of OCTETs in the entity body. HTTP/1.1 user agents MUST notify the user when an invalid length is received and detected. Status Code Definitions Each Status-Code is described below, including a description of which method(s) it can follow and any metainformation required in the response. Informational 1xx This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions. 100 Continue The client may continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed. 101 Switching Protocols The server understands and is willing to comply with the client's request, via the Upgrade message header field (section ref Upgrade \n 18.44), for a change in the application protocol being used on this connection. The server will switch protocols to those defined by the response's Upgrade header field immediately after the empty line which terminates the 101 response. The protocol should only be switched when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol may be advantageous when delivering resources that use such features. Successful 2xx This class of status code indicates that the client's request was successfully received, understood, and accepted. 200 OK The request has succeeded. The information returned with the response is dependent on the method used in the request, as follows: GET an entity corresponding to the requested resource is sent in the response; HEAD the response MUST only contain the header information and no Entity-Body; POST an entity describing or containing the result of the action; TRACE an entity containing the request message as received by the end server; otherwise, an entity describing the result of the action; If the entity corresponds to a resource, the response MAY include a Content-Location header field giving the actual location of that plain resource for later reference. 201 Created The request has been fulfilled and resulted in a new resource being created. The newly created resource can be referenced by the URI(s) returned in the entity of the response, with the most specific URL for the resource given by a Location header field. The origin server SHOULD create the resource before returning this status code. If the action cannot be carried out immediately, the server MUST include in the response body a description of when the resource will be available; otherwise, the server SHOULD respond with 202 (Accepted). 202 Accepted The request has been accepted for processing, but the processing has not been completed. The request MAY or MAY NOT eventually be acted upon, as it MAY be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this. The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The entity returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled. 203 Non-Authoritative Information The returned metainformation in the Entity-Header is not the definitive set as available from the origin server, but is gathered from a local or a third-party copy. The set presented MAY be a subset or superset of the original version. For example, including local annotation information about the resource MAY result in a superset of the metainformation known by the origin server. Use of this response code is not required and is only appropriate when the response would otherwise be 200 (OK). 204 No Content The server has fulfilled the request but there is no new information to send back. If the client is a user agent, it SHOULD NOT change its document view from that which caused the request to be generated. This response is primarily intended to allow input for actions to take place without causing a change to the user agent's active document view. The response MAY include new metainformation in the form of entity headers, which SHOULD apply to the document currently in the user agent's active view. The 204 response MUST NOT include an entity body, and thus is always terminated by the first empty line after the header fields. 205 Reset Content The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be generated. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action. The response MUST include a Content-Length with a value of zero (0) and no entity body. 206 Partial Content The server has fulfilled the partial GET request for the resource. The request MUST have included a Range header field (section ref Range \n 18.38) indicating the desired range. The response MUST include a Content-Range header field (section ref Content_Range \n 18.18) indicating the range included with this response. All entity header fields in the response MUST describe the partial entity transmitted rather than what would have been transmitted in a full response. In particular, the Content-Length header field in the response MUST match the actual number of OCTETs transmitted in the entity body. It is assumed that the client already has the complete entity's header field data. Redirection 3xx This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. The action required MAY be carried out by the user agent without interaction with the user if and only if the method used in the second request is GET or HEAD. A user agent SHOULD NOT automatically redirect a request more than 5 times, since such redirections usually indicate an infinite loop. 300 Multiple Choices This status code is reserved for future use by a planned content negotiation mechanism. HTTP/1.1 user agents receiving a 300 response which includes a Location header field can treat this response as they would treat a 303 (See Other) response. If no Location header field is included, the appropriate action is to display the entity enclosed in the response to the user. 301 Moved Permanently The requested resource has been assigned a new permanent URI and any future references to this resource SHOULD be done using one of the returned URIs. Clients with link editing capabilities SHOULD automatically re-link references to the Request-URI to one or more of the new references returned by the server, where possible. This response is cachable unless indicated otherwise. If the new URI is a location, its URL MUST be given by the Location field in the response. Unless it was a HEAD request, the Entity-Body of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s). If the 301 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued. Note: When automatically redirecting a POST request after receiving a 301 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request. 302 Moved Temporarily The requested resource resides temporarily under a different URI. Since the redirection may be altered on occasion, the client SHOULD continue to use the Request-URI for future requests. This response is only cachable if indicated by a Cache-Control or Expires header field. If the new URI is a location, its URL MUST be given by the Location field in the response. Unless it was a HEAD request, the Entity-Body of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s). If the 302 status code is received in response to a request other than GET or HEAD, the user agent MUST NOT automatically redirect the request unless it can be confirmed by the user, since this might change the conditions under which the request was issued. Note: When automatically redirecting a POST request after receiving a 302 status code, some existing HTTP/1.0 user agents will erroneously change it into a GET request. 303 See Other The response to the request can be found under a different URI and SHOULD be retrieved using a GET method on that resource. This method exists primarily to allow the output of a POST-activated script to redirect the user agent to a selected resource. The new resource is not a update reference for the original Request-URI. The 303 response is not cachable, but the response to the second request MAY be cachable. If the new URI is a location, its URL MUST be given by the Location field in the response. Unless it was a HEAD request, the Entity-Body of the response SHOULD contain a short hypertext note with a hyperlink to the new URI(s). 304 Not Modified If the client has performed a conditional GET request and access is allowed, but the document has not been modified since the date and time specified in the If-Modified-Since field, the server MUST respond with this status code and not send an Entity-Body to the client. Header fields contained in the response SHOULD only include information which is relevant to cache managers or which MAY have changed independently of the entity's Last-Modified date. Examples of relevant header fields include: Date, Server, Content-Length, Content-MD5, Content-Version, Cache-Control and Expires. A cache SHOULD update its cached entity to reflect any new field values given in the 304 response. If the new field values indicate that the cached entity differs from the current resource entity (as would be indicated by a change in Content-Length, Content-MD5, or Content-Version), then the cache MUST disregard the 304 response and repeat the request without an If-Modified-Since field. The 304 response MUST NOT include an entity body, and thus is always terminated by the first empty line after the header fields. 305 Use Proxy The requested resource MUST be accessed through the proxy given by the Location field in the response. In other words, this is a proxy redirect. Client Error 4xx The 4xx class of status code is intended for cases in which the client seems to have erred. If the client has not completed the request when a 4xx code is received, it SHOULD immediately cease sending data to the server. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. Note: If the client is sending data, server implementations using TCP SHOULD be careful to ensure that the client acknowledges receipt of the packet(s) containing the response prior to closing the input connection. If the client continues sending data to the server after the close, the server's controller will send a reset packet to the client, which may erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application. 400 Bad Request The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications. 401 Unauthorized The request requires user authentication. The response MUST include a WWW-Authenticate header field (section ref WWW_Authenticate \n 18.46) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section ref Authorization \n 18.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity MAY include relevant diagnostic information. HTTP access authentication is explained in section ref AA \n 14. 402 Payment Required This code is reserved for future use. 403 Forbidden The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD not be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity body. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable. 404 Not Found The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. 405 Method Not Allowed The method specified in the Request-Line is not allowed for the resource identified by the Request-URI. The response MUST include an Allow header containing a list of valid methods for the requested resource. 406 Not Acceptable The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request. HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept headers sent in the request. In some cases, this may even be preferable to sending a 406 response. User agents are encouraged to inspect the headers of an incoming response to determine if it is acceptable. If the response is not acceptable, user agents SHOULD interrupt the receipt of the response if doing so would save network resources. If it is unknown whether an incoming response would be acceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on furtheractions. 407 Proxy Authentication Required This code is similar to 401 (Unauthorized), but indicates that the client MUST first authenticate itself with the proxy. The proxy MUST return a Proxy-Authenticate header field (section ref Proxy_Authenticate \n 18.35) containing a challenge applicable to the proxy for the requested resource. The client MAY repeat the request with a suitable Proxy-Authorization header field (section ref Proxy_Authorization \n 18.36). HTTP access authentication is explained in section ref AA \n 14. 408 Request Timeout The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time. 409 Conflict The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user MAY be able to resolve the conflict and resubmit the request. The response body SHOULD include enough information for the user to recognize the source of the conflict. Ideally, the response entity would include enough information for the user or user-agent to fix the problem; however, that MAY not be possible and is not required. Conflicts are most likely to occur in response to a PUT request. If versioning is being used and the entity being PUT includes changes to a resource which conflict with those made by an earlier (third-party) request, the server MAY use the 409 response to indicate that it can't complete the request. In this case, the response entity SHOULD contain a list of the differences between the two versions in a format defined by the response Content-Type. 410 Gone The requested resource is no longer available at the server and no forwarding address is known. This condition SHOULD be considered permanent. Clients with link editing capabilities SHOULD delete references to the Request-URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) SHOULD be used instead. This response is cachable unless indicated otherwise. The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as �gone� or to keep the mark for any length of time -- that is left to the discretion of the server owner. 411 Length Required The server refuses to accept the request without a defined Content-Length. The client MAY repeat the request if it adds a valid Content-Length header field containing the length of the entity body in the request message. 412 Precondition Failed The precondition given in one or more of the request header fields evaluated to false when it was tested on the server. This response code allows the client to place preconditions on the current resource metainformation (header field data) and thus prevent the requested method from being applied to a resource other than the one intended. 413 Request Entity Too Large The server is refusing to process a request because it considers the request entity to be larger than it is willing or able to process. The server SHOULD close the connection if that is necessary to prevent the client from continuing the request. If the client manages to read the 413 response, it MUST honor it and SHOULD reflect it to the user. If this restriction is considered temporary, the server MAY include a Retry-After header field to indicate that it is temporary and after what time the client MAY try again. 414 Request-URI Too Long The server is refusing to service the request because the Request-URI is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URL �black hole� of redirection (e.g., a redirected URL prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the Request-URI. 415 Unsupported Media Type The server is refusing to service the request because the entity body of the request is in a format not supported by the requested resource for the requested method. Server Error 5xx Response status codes beginning with the digit �5� indicate cases in which the server is aware that it has erred or is incapable of performing the request. If the client has not completed the request when a 5xx code is received, it SHOULD immediately cease sending data to the server. Except when responding to a HEAD request, the server SHOULD include an entity containing an explanation of the error situation, and whether it is a temporary or permanent condition. These response codes are applicable to any request method and there are no required header fields. 500 Internal Server Error The server encountered an unexpected condition which prevented it from fulfilling the request. 501 Not Implemented The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource. 502 Bad Gateway The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request. 503 Service Unavailable The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response. Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers MAY wish to simply refuse the connection. 504 Gateway Timeout The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server it accessed in attempting to complete the request. 505 HTTP Version Not Supported The server does not support, or refuses to support, the HTTP protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in section ref HTTP_Version \n 7.1, other than with this error message. The response SHOULD contain an entity describing why that version is not supported and what other protocols are supported by that server. Method Definitions The set of common methods for HTTP/1.1 is defined below. Although this set can be expanded, additional methods cannot be assumed to share the same semantics for separately extended clients and servers. The Host request-header field (section ref Host \n 18.24) MUST accompany all HTTP/1.1 requests. OPTIONS The OPTIONS method represents a request for information about the communication options available on the request/response chain identified by the Request-URI. This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval. Unless the server's response is an error, the response MUST NOT include entity information other than what can be considered as communication options (e.g., Allow is appropriate, but Content-Type is not) and MUST include a Content-Length with a value of zero (0). Responses to this method are not cachable. If the Request-URI is an asterisk (�*�), the OPTIONS request is intended to apply to the server as a whole. A 200 response SHOULD include any header fields which indicate optional features implemented by the server (e.g., Public), including any extensions not defined by this specification, in addition to any applicable general or response header fields. As described in section ref Request_URI \n 9.1.2, an �OPTIONS *� request can be applied through a proxy by specifying the destination server in the Request-URI without any path information. If the Request-URI is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource. A 200 response SHOULD include any header fields which indicate optional features implemented by the server and applicable to that resource (e.g., Allow), including any extensions not defined by this specification, in addition to any applicable general or response header fields. If the OPTIONS request passes through a proxy, the proxy MUST edit the response to exclude those options known to be unavailable through that proxy. GET The GET method means retrieve whatever information (in the form of an entity) is identified by the Request-URI. If the Request-URI refers to a data-producing process, it is the produced data which shall be returned as the entity in the response and not the source text of the process, unless that text happens to be the output of the process. The semantics of the GET method change to a �conditional GET� if the request message includes an If-Modified-Since header field. A conditional GET method requests that the identified resource entity be transferred only if it has been modified since the date given by the If-Modified-Since header, as described in section ref If_Modified_Since \n 18.25. The conditional GET method is intended to reduce unnecessary network usage by allowing cached entities to be refreshed without requiring multiple requests or transferring data already held by the client. The semantics of the GET method change to a �partial GET� if the request message includes a Range header field. A partial GET requests that only part of the identified resource entity be transferred, as described in section ref Range \n 18.38. The partial GET method is intended to reduce unnecessary network usage by allowing partially-retrieved entities to be completed without transferring data already held by the client. The response to a GET request may be cachable if and only if it meets the requirements for HTTP caching described in section ref Caching_In_HTTP \n 16. HEAD The HEAD method is identical to GET except that the server MUST NOT return any Entity-Body in the response. The metainformation contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metainformation about the resource entity identified by the Request-URI without transferring the Entity-Body itself. This method is often used for testing hypertext links for validity, accessibility, and recent modification. The response to a HEAD request may be cachable in the sense that the information contained in the response may be used to update a previously cached entity from that resource. If the new field values indicate that the cached entity differs from the current resource entity (as would be indicated by a change in Content-Length, Content-MD5, or Content-Version), then the cache MUST mark the cache entry stale. There is no �conditional HEAD� or �partial HEAD� request analogous to those associated with the GET method. If an If-Modified-Since and/or Range header field is included with a HEAD request, they SHOULD be ignored. POST The POST method is used to request that the destination server accept the entity enclosed in the request as a new subordinate of the resource identified by the Request-URI in the Request-Line. POST is designed to allow a uniform method to cover the following functions: � Annotation of existing resources; � Posting a message to a bulletin board, newsgroup, mailing list, or similar group of articles; � Providing a block of data, such as the result of submitting a form private HREF="#RefHTML"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [5], to a data-handling process; � Extending a database through an append operation. The actual function performed by the POST method is determined by the server and is usually dependent on the Request-URI. The posted entity is subordinate to that URI in the same way that a file is subordinate to a directory containing it, a news article is subordinate to a newsgroup to which it is posted, or a record is subordinate to a database. For compatibility with HTTP/1.0 applications, all POST requests MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. When sending a POST request to an HTTP/1.1 server, a client MUST use a valid Content-Length or the �chunked� Transfer-Encoding. The server SHOULD respond with a 400 (bad request) message if it cannot determine the length of the request message's content, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length. A successful POST does not require that the entity be created as a resource on the origin server or made accessible for future reference. That is, the action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (no content) is the appropriate response status, depending on whether or not the response includes an entity that describes the result. If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain an entity (preferably of type �text/html�) which describes the status of the request and refers to the new resource. Responses to this method are not cachable. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cachable resource. POST requests must obey the entity transmission requirements set out in section ref Entity_Transmission_\n 13.4.1. SLUSHY: Entity Transmission Requirements Editor�s Note: The issues here around reliable transmission of large entities to servers, particularly HTTP/1.0 servers, are complicated and subtle, particularly since we�d like optimistic transmission to be the normal situation. We would like it if we can redraft this section to be simpler in the next draft General requirements: � HTTP/1.1 servers should maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion. � An HTTP/1.1 (or later) client doing a PUT-like method SHOULD monitor the network connection for an error status while it is transmitting the request. If the client sees an error status, it should immediately cease transmitting the body. If the body is being sent using a �Chunked� encoding, a zero length chunk is used to mark the end of the message. If the body was preceded by a Content-length header, the client MUST close the connection. � An HTTP/1.1 (or later) client MUST be prepared to accept a �100 Continue� status followed by a regular response. � An HTTP/1.1 (or later) server that receives a request from a HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue) response; it SHOULD either wait for the request to be completed normally (thus avoiding an interrupted request) or close the connection prematurely. Upon receiving a method subject to these requirements from an HTTP/1.1 (or later) client, an HTTP/1.1 (or later) server MUST either immediately respondwith 100 (continue) and continue to read from the input stream, or respond with an error status. If it responds with an error status, it MAY close the transport (TCP) connection or it MAY continue to read and discard the rest of the request. It MUST NOT perform the requested method if it returns an error status. If an HTTP/1.1 client has seen an HTTP/1.1 or later response from the server (clients SHOULD remember the version number of at least the most recently used server), and it sees the connection close before receiving any status from the server, the client SHOULD retry the request. If the client does retry the request, it MUST first send the request headers, and then MUST wait for the server to respond with either a 100 (continue) response, in which case the client should continue, or with an error status. If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from the server, it should assume that the server implements HTTP/1.0 or older and will not use the 100 (Continue) response. If in this case the client sees the connection close before receiving any status from the server, the client SHOULD retry the request. If the client does retry the request, it should use the following �binary exponential backoff� algorithm to be assured of obtaining a reliable response: Initiate a new connection to the server Transmit the request headers Initialize a variable R to the estimated round-trip time to the server (e.g., based on the time it took to establish the connection), or to a constant value of 5 seconds if the round-trip time is not available. Compute T = R * (2**N), where N is the number of previous retries of this request. Wait either for an error response from the server, or for T seconds (whichever comes first) If no error response is received, after T seconds transmit the body of the request. If client sees that the connection is closed prematurely, repeat from step 1 until the request is accepted, an error response is received, or the user becomes impatient. No matter what the server version, if an error status is received, the client MUST NOT continue and MUST close the connection if it has not already completed sending the full request body including any encoding mechanism used to transmit the body. An HTTP/1.1 (or later) client that sees the connection close after receiving a 100 (continue) but before receiving any other status SHOULD retry the request, and need not wait for 100 (continue) response (but MAY do so if this simplifies the implementation). PUT The PUT method requests that the enclosed entity be stored under the supplied Request-URI. If the Request-URI refers to an already existing resource, the enclosed entity SHOULD be considered as a modified version of the one residing on the origin server. If the Request-URI does not point to an existing resource, and that URI is capable of being defined as a new resource by the requesting user agent, the origin server can create the resource with that URI. If a new resource is created, the origin server MUST inform the user agent via the 201 (created) response. If an existing resource is modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent to indicate successful completion of the request. If the resource could not be created or modified with the Request-URI, an appropriate error response SHOULD be given that reflects the nature of the problem. If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable. The fundamental difference between the POST and PUT requests is reflected in the different meaning of the Request-URI. The URI in a POST request identifies the resource that will handle the enclosed entity as an appendage. That resource may be a data-accepting process, a gateway to some other protocol, or a separate entity that accepts annotations. In contrast, the URI in a PUT request identifies the entity enclosed with the request -- the user agent knows what URI is intended and the server MUST NOT attempt to apply the request to some other resource. If the server desires that the request be applied to a different URI, it MUST send a 301 (Moved Permanently) response; the user agent MAY then make its own decision regarding whether or not to redirect the request. A single resource MAY be identified by many different URIs. For example, an article may have a URI for identifying �the current version� which is separate from the URI identifying each particular version. In this case, a PUT request on a general URI may result in several other URIs being defined by the origin server. For compatibility with HTTP/1.0 applications, all PUT requests MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. When sending a PUT request to an HTTP/1.1 server, a client MUST use a valid Content-Length or the �chunked� Transfer-Encoding. The server SHOULD respond with a 400 (bad request) message if it cannot determine the length of the request message's content, or with 411 (length required) if it wishes to insist on receiving a valid Content-Length. The actual method for determining how the resource entity is placed, and what happens to its predecessor, is defined entirely by the origin server. PUT requests must obey the entity transmission requirements set out in section ref Entity_Transmission_\n 13.4.1. DELETE The DELETE method requests that the origin server delete the resource identified by the Request-URI. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD not indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location. A successful response SHOULD be 200 (OK) if the response includes an entity describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the response is OK but does not include an entity. If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable. TRACE The TRACE method is used to invoke a remote, application-layer loop-back of the request message. The final recipient of the request SHOULD reflect the message received back to the client as the entity body of a 200 (OK) response. The final recipient is either the origin server or the first proxy or gateway to receive a Max-Forwards value of zero (0) in the request (see section ref Max_Forwards \n 18.32). A TRACE request MUST NOT include an entity. TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (section ref Via \n 18.47) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop. If successful, the response SHOULD contain the entire request message in the entity body, with a Content-Type of �message/http�, �application/http�, or �text/plain�. Responses to this method MUST NOT be cached. Access Authentication HTTP provides a simple challenge-response authentication mechanism which MAY be used by a server to challenge a client request and by a client to provide authentication information. It uses an extensible, case-insensitive token to identify the authentication scheme, followed by a comma-separated list of attribute-value pairs which carry the parameters necessary for achieving authentication via that scheme. auth-scheme = token auth-param = token "=" quoted-string The 401 (Unauthorized) response message is used by an origin server to challenge the authorization of a user agent. This response MUST include a WWW-Authenticate header field containing at least one challenge applicable to the requested resource. challenge = auth-scheme 1*SP realm *( "," auth-param ) realm = "realm" "=" realm-value realm-value = quoted-string The realm attribute (case-insensitive) is required for all authentication schemes which issue a challenge. The realm value (case-sensitive), in combination with the canonical root URL of the server being accessed, defines the protection space. These realms allow the protected resources on a server to be partitioned into a set of protection spaces, each with its own authentication scheme and/or authorization database. The realm value is a string, generally assigned by the origin server, which may have additional semantics specific to the authentication scheme. A user agent that wishes to authenticate itself with a server--usually, but not necessarily, after receiving a 401 or 411 response--MAY do so by including an Authorization header field with the request. The Authorization field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested. credentials = basic-credentials | auth-scheme 0#auth-param The domain over which credentials can be automatically applied by a user agent is determined by the protection space. If a prior request has been authorized, the same credentials MAY be reused for all other requests within that protection space for a period of time determined by the authentication scheme, parameters, and/or user preference. Unless otherwise defined by the authentication scheme, a single protection space cannot extend outside the scope of its server. If the server does not wish to accept the credentials sent with a request, it SHOULD return a 401 (Unauthorized) response. The response MUST include a WWW-Authenticate header field containing the (possibly new) challenge applicable to the requested resource and an entity explaining the refusal. The HTTP protocol does not restrict applications to this simple challenge-response mechanism for access authentication. Additional mechanisms MAY be used, such as encryption at the transport level or via message encapsulation, and with additional header fields specifying authentication information. However, these additional mechanisms are not defined by this specification. Proxies MUST be completely transparent regarding user agent authentication. That is, they MUST forward the WWW-Authenticate and Authorization headers untouched, and MUST NOT cache the response to a request containing Authorization. HTTP/1.1 allows a client to pass authentication information to and from a proxy via the Proxy-Authenticate and Proxy-Authorization headers. Basic Authentication Scheme The �basic� authentication scheme is based on the model that the user agent must authenticate itself with a user-ID and a password for each realm. The realm value should be considered an opaque string which can only be compared for equality with other realms on that server. The server will service the request only if it can validate the user-ID and password for the protection space of the Request-URI. There are no optional authentication parameters. Upon receipt of an unauthorized request for a URI within the protection space, the server SHOULD respond with a challenge like the following: WWW-Authenticate: Basic realm="WallyWorld" where �WallyWorld� is the string assigned by the server to identify the protection space of the Request-URI. To receive authorization, the client sends the user-ID and password, separated by a single colon (�:�) character, within a base64 private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7] encoded string in the credentials. basic-credentials = "Basic" SP basic-cookie basic-cookie = user-pass = userid ":" password userid = [ token ] password = *TEXT If the user agent wishes to send the user-ID �Aladdin� and password �open sesame�, it would use the following header field: Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ== The basic authentication scheme is a non-secure method of filtering unauthorized access to resources on an HTTP server. It is based on the assumption that the connection between the client and the server can be regarded as a trusted carrier. As this is not generally true on an open network, the basic authentication scheme should be used accordingly. In spite of this, clients SHOULD implement the scheme in order to communicate with servers that use it. Digest Authentication Scheme The �digest� authentication scheme is [currently described in an expired Internet-Draft, and this description will have to be improved to reference a new draft or include the old one]. Content Negotiation A generic resource has multiple entities associated with it, all of which are representations of the content of the resource. Content negotiation is the process of selecting the best representation when a GET or HEAD request is made on the generic resource. HTTP/1.1 has provisions for two kinds of content negotiation: opaque negotiation and transparent negotiation. With opaque negotiation, the selection of the best representation is done by an algorithm located at the origin server, and unknown to the proxies and user agents involved. Selection is based on the contents of particular header fields in the request message, or on other information pertaining to the request, like the network address of the sending client. A typical example of opaque negotiation would be the selection of a text/html response in a particular language based on the contents of the Accept-Language request header field. A disadvantage of opaque negotiation is that the request headers may not always contain enough information to allow for selection. If the Accept header Accept: text/*: q=0.3, text/html, */*: q=0.5 is sent in a request on a generic resource which has a video/mpeg and a video/quicktime representation, the selection algorithm in the origin server will either have to make a default choice, or return an error response which allows the user to decide on further actions. With transparent negotiation, the selection of the best representation is done by a distributed algorithm which can perform computation steps in the origin server, in proxies, or in the user agent. Transparent negotiation guarantees that, if the user agent supports the transparent negotiation algorithm and is correctly configured, the request will always correctly yield either the video/mpeg representation, the video/quicktime representation, or an error message indicating that the resource cannot be displayed by the user agent. Negotiation Facilities Defined in this Specification This specification defines all protocol facilities for opaque negotiation, but does not define the distributed algorithm for transparent negotiation. This specification only defines the basic facilities (Vary, Alternates, Accept) in the core protocol allowing requests on transparently negotiated resources to be correctly handled by HTTP/1.1 caches. All other information about transparent content negotiation is found in a separate document[29]. If a generic resource is opaquely negotiated, successful responses to requests on the resource will always include a Vary header. If a generic resource is transparently negotiated, successful responses to requests on the resource will always include an Alternates header. If a successful response contains an Alternates header, it will also always contain a Content-Location header. A future specification may allow a combination of opaque and transparent negotiation that would lead to the inclusion of both a Vary header and an Alternates header in a response. Caching in HTTP The World Wide Web is a distributed system, and so its performance can be improved by the use of caches. These caches are typically placed at proxies and in the clients themselves. The HTTP/1.1 protocol includes a number of elements intended to make caching work as well as possible. Because these elements are inextricable from other aspects of the protocol, and because they interact with each other, it is useful to describe the basic caching design of HTTP separately from the detailed descriptions of methods, headers, response codes, etc. Semantic Transparency Requirements for performance, availability, and disconnected operation require us to be able to relax the goal of semantic transparency. The HTTP/1.1 protocol allows origin servers, caches, and clients to explicitly reduce transparency when necessary. However, because non-transparent operation may confuse non-expert users, and may be incompatible with certain server applications (such as those for ordering merchandise), the protocol requires that transparency may not be relaxed � without an explicit protocol-level request (when relaxed by client or origin server) � without a means for warning the end user (when relaxed by cache or client) Therefore, the HTTP/1.1 protocol provides these important elements: 1. Protocol features that provide full semantic transparency when this is desired by all parties. 2. Protocol features that allow an origin server or end-user client to explicitly request and control non-transparent operation. 3. Protocol features that allow a cache to attach warnings to responses that do not preserve semantic transparency. A basic principle is that it must be possible for the clients to detect any potential breakdown of semantic transparency. Caching would be useless if it did not significantly improve performance. The goal of caching in HTTP/1.1 is to eliminate the need to send requests in many cases, and to eliminate the need to send full responses in many other cases. The former reduces the number of network round-trips required for many operations; we use an �expiration� mechanism for this purpose (see section ref Expiration_Model \n 16.1.2). The latter reduces network bandwidth requirements; we use a �validation� mechanism for this purpose (see section 13.3). The server, cache, or client implementer may be faced with design decisions not explicitly discussed in this specification. If a decision may affect semantic transparency, the implementer ought to err on the side of maintaining transparency unless a careful and complete analysis shows significant benefits in breaking transparency. Cache Correctness If the cache can communicate with the origin-server, then a correct cache MUST respond to a request with a response that meets all the following conditions: 1. its end-to-end headers (see section ref EtoE_and_HbyH_Header\n 16.4.1) and entity-body value are equivalent to what the server would have returned for that request if the resource had not been modified since the response was cached. This may be accomplished by revalidating the response with the origin server, if is not fresh. 2. it is �fresh enough� (see section ref Expiration_Model \n 16.1.2). In the default case, this means it meets the least restrictive freshness requirement of the client, server, and cache (see section ref Cache_Control \n 18.10); if the origin-server so specifies, it is the freshness requirement of the origin-server alone. 3. it includes a warning if the freshness demand of the client or the origin-server is violated (see section ref Exceptions_to_the_Ru\n 16.1.5 and ref Warning \n 18.48). 4. it is the most up-to-date response appropriate to the request the cache has seen (see section ref Disambiguating_Expir\n 16.2.6, ref Disambiguating_Multi\n 16.2.8, and ref Varying_Resources_an\n 16.13). If the cache can not communicate with the origin server, then a correct cache SHOULD respond as above if the response can be correctly served from the cache; if not it MUST return an error or warning indicating that there was a communication. Cache-control Mechanisms The basic cache mechanisms in HTTP/1.1 (server-specified expiration times and validators) are implicit directives to caches. In some cases, a server or client may need to provide explicit directives to the HTTP caches. We use the Cache-Control header for this purpose. The Cache-Control header allows a client or server to transmit a variety of directives in either requests or responses. These directives typically override the default caching algorithms. As a general rule, if there is any apparent conflict between header values, the most restrictive interpretation should be applied (that is, the one that is most likely to preserve semantic transparency). However, in some cases, Cache-Control directives are explicitly specified as weakening semantic transparency (for example, �max-stale� or �public�). The Cache-Control directives are described in detail in section ref Cache_Control \n 18.10. Warnings Whenever a cache returns a response that is not semantically transparent, it must attach a warning to that effect, using a Warning response header. This warning allows clients and user agents to take appropriate action. Warnings may be used for other purposes, both cache-related and otherwise. The use of a warning, rather than an error status code, distinguish these responses from true failures. Warnings are always cachable, because they never weaken the transparency of a response. This means that warnings can be passed to HTTP/1.0 caches without danger; such caches will simply pass the warning along as a entity header in the response. Warnings are assigned numbers between 0 and 99. This specification defines the code numbers and meanings of each warning, allowing a client or cache to take automated action in some (but not all) cases. Warnings also carry a warning message text in any appropriate natural language (perhaps based on the client's Accept headers), and an optional indication of what language and character set are used. Multiple warning messages may be attached to a response (either by the origin server or by a cache), including multiple warnings with the same code number. For example, a server may provide the same warning with texts in both English and Basque. When multiple warnings are attached to a response, it may not be practical or reasonable to display all of them to the user. This version of HTTP does not specify strict priority rules for deciding which warnings to display and in what order, but does suggest some heuristics. The Warning header and the currently defined warnings are described in section ref Warning \n 18.48. Explicit User Agent Warnings Many user agents make it possible for users to override the basic caching mechanisms. For example, the user agent may allow the user to specify that cached entities (even explicitly stale ones) are never validated. Or the user agent might habitually add �Cache-Control: max-stale=3600� or �Cache-Control: reload� to every request. We recognize that there may be situations which require such overrides, although user agents SHOULD NOT default to any behavior contrary to the HTTP/1.1 specification. That is, the user should have to explicitly request either non-transparent behavior, or behavior that results in abnormally ineffective caching. If the user has overridden the basic caching mechanisms, the user agent should explicitly indicate to the user whenever this results in the display of information that might not meet the server's transparency requirements (in particular, if the displayed entity is known to be stale). Since the protocol normally allows the user agent to determine if responses are stale or not, this indication need only be displayed when this actually happens. The indication need not be a dialog box; it could be an icon (for example, a picture of a rotting fish) or some other visual indicator. If the user has overridden the caching mechanisms in a way that would abnormally reduce the effectiveness of caches, the user agent should continually display an indication (for example, a picture of currency in flames) so that the user does not inadvertently consume excess resources or suffer from excessive latency. Exceptions to the Rules and Warnings In some cases, the operator of a cache may choose to configure it to return stale responses even when not requested by clients. This decision not be made lightly, but may be necessary for reasons of availability or performance, especially when the cache is poorly connected to the origin server. Whenever a cache returns a stale response, it MUST mark it as such (using a Warning header). This allows the client software to alert the user that there may be a potential problem. It also allows the user to take steps to obtain a firsthand or fresh response, if the user so desires. For this reason, a cache MUST NOT return a stale response if the client explicitly requests a first-hand or fresh one, unless it is impossible to comply. Client-controlled Behavior While the origin server (and to a lesser extent, intermediate caches, by their contribution to the age of a response) are the primary source of expiration information, in some cases the client may need to control a cache's decision about whether to return a cached response without validating it. Clients do this using several directives of the Cache-Control header. A client's request may specify the maximum age it is willing to accept for an unvalidated response; specifying a value of zero forces the cache(s) to revalidate all responses. A client may also specify the minimum time remaining before a response expires. Both of these options increase constraints on the behavior of caches, and so cannot decrease semantic transparency. A client may also specify that it will accept stale responses, up to some maximum amount of staleness. This loosens the constraints on the caches, and so may violate semantic transparency, but may be necessary to support disconnected operation, or high availability in the face of poor connectivity. Expiration Model Server-Specified Expiration HTTP caching works best when caches can entirely avoid making requests to the origin server. The primary mechanism for avoiding requests is for an origin server to provide an explicit expiration time in the future, indicating that a response may be used to satisfy subsequent requests. In other words, a cache can return a fresh response without first contacting the server. Our expectation is that servers will assign future explicit expiration times to responses in the belief that the entity is not likely to change, in a semantically significant way, before the expiration time is reached. This normally preserves semantic transparency, as long as the server's expiration times are carefully chosen. If an origin server wishes to force a semantically transparent cache to validate every request, it may assign an explicit expiration time in the past. This means that the response is always stale, and so the cache SHOULD validate it before using it for subsequent requests. (See section ref Cache_Revalidation_a\n 18.10.4 for a more restrictive way to force revalidation). Note that a firsthand response MUST always be returned to the requesting client, independent of its expiration time, unless the connection to the client is lost. If an origin server wishes to force any HTTP/1.1 cache, no matter how it is configured, to validate every request, it should use the �must-revalidate� Cache-Control directive. See section ref Cache_Control \n 18.10. Servers specify explicit expiration times using either the Expires header, or the max-age directive of the Cache-Control header. Limitations on the Effect of Expiration Times An expiration time cannot be used to force a user agent to refresh its display or reload a resource entity; its semantics apply only to caching mechanisms, and such mechanisms need only check a resource's expiration status when a new request for that resource is initiated. User agents often have history mechanisms, such as �Back� buttons and history lists, which can be used to redisplay an entity retrieved earlier in a session. By default, an expiration time does not apply to history mechanisms. If the entity is still in storage, a history mechanism should display it even if the entity has expired, unless the user has specifically configured the agent to refresh expired history documents. Heuristic Expiration Since origin servers do not always provide explicit expiration times, HTTP caches typically assign heuristic expiration times, employing algorithms that use other header values (such as the Last-Modified time) to estimate a plausible expiration time. The HTTP/1.1 specification does not provide specific algorithms, but does impose worst-case constraints on their results. Since heuristic expiration times may compromise semantic transparency, they should be used cautiously, and we encourage origin servers to provide explicit expiration times as much as possible. Age Calculations In order to know if a cached entry is fresh, a cache needs to know if its age exceeds its freshness lifetime. We discuss how to calculate the latter in section ref Expiration_Calculati\n 0; this section describes how to calculate the age of a response or cache entry. In this discussion, we use the term �now� to mean �the current value of the clock at the host performing the calculation.� All HTTP implementations, but especially origin servers and caches, should use NTP [RFC1305] or some similar protocol to synchronize their clocks to a globally accurate time standard. Also note that HTTP/1.1 requires origin servers to send a Date header with every response, giving the time at which the response was generated. We use the term �date_value� to denote a representation of the value of the Date header, in a form appropriate for arithmetic operations. HTTP/1.1 uses the �Age� response header to help convey age information between caches. The Age header value is the sender's estimate of the amount of time since the response was generated at the origin server. In the case of a cached response that has been revalidated with the origin server, the Age value is based on the time of revalidation, not of the original response. In essence, the Age value is the sum of the time that the response has been resident in each of the caches along the path from the origin server, plus the amount of time it has been in transit along network paths. We use the term �age_value� to denote a representation of the value of the Age header, in a form appropriate for arithmetic operations. An response's age can be calculated in two entirely independent ways: 1. now - date_value, if the local clock is reasonably well synchronized to the origin server's clock. If the result is negative, this is replaced by zero. 2. age_value, if all of the caches along the response path implement HTTP/1.1. Given that we have two independent ways to compute the age of a response when it is received, we can combine these as corrected_received_age = max(now - date_value, age_value) and as long as we have either nearly synchronized clocks or all-HTTP/1.1 paths, one gets a reliable (conservative) result. Note that this correction is applied at each HTTP/1.1 cache along the path, so that if there is an HTTP/1.0 cache in the path, the correct received age is computed as long as the receiving cache's clock is nearly in sync. We don't need end-to-end clock synchronization (although it is good to have), and there is no explicit clock synchronization step. Because of network-imposed delays, some significant interval may pass from the time that a server generates a response, and the time it is received at the next outbound cache or client. If uncorrected, this delay could result in improperly low ages. Because the request that resulted in the returned Age value must have been initiated prior to that Age value's generation, we can correct for delays imposed by the network by recording the time at which the request was initiated. Then, when an Age value is received, it MUST be interpreted relative to the time the request was initiated, not the time that the response was received. This algorithm results in conservative behavior no matter how much delay is experienced. So, we compute: corrected_initial_age = corrected_received_age + (now - request_time) where �request_time� is the time (according to the local clock) when the request that elicited this response was sent. Summary of age calculation algorithm, when a cache receives a response: /* * age_value * is the value of Age: header received by the cache with * this response. * date_value * is the value of the origin server's Date: header * request_time * is the (local) time when the cache made the request * that resulted in this cached response * response_time * is the (local) time when the cache received the * response * now * is the current (local) time */ apparent_age = max(0, now - date_value); corrected_received_age = max(apparent_age, age_value); response_delay = now - request_time; corrected_initial_age = corrected_received_age + response_delay; resident_time = now - response_time; current_age = corrected_initial_age + resident_time; When a cache sends a response, it must add to the corrected_initial_age the amount of time that the response was resident locally. It must then transmit this total age, using the Age header, to the next recipient cache. Note that a client can usually tell if a response is firsthand by comparing the Date to its local request-time, and hoping that the clocks are not badly skewed. Expiration Calculations In order to decide whether a response is fresh or stale, we need to compare its freshness lifetime to its age. The age is calculated as described in section ref Age_Calculations \n 16.2.4; this section describes how to calculate the freshness lifetime, and to determine if a response has expired. We use the term �expires_value� to denote a representation of the value of the Expires header, in a form appropriate for arithmetic operations. We use the term �max_age_value� to denote an appropriate representation of the number of seconds carried by the max-age directive of the Cache-Control header in a response (see section ref Connection \n 18.11). The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply: freshness_lifetime = max_age_value Otherwise, if Expires is present in the response, the calculation is: freshness_lifetime = expires_value - date_value Note that neither of these calculations is vulnerable to clock skew, since all of the information comes from the origin server. If neither Expires nor Cache-Control max-age appears in the response, and the response does not include other restrictions on caching, the cache MAY compute a freshness lifetime using a heuristic. This heuristic is subject to certain limitations; the minimum value may be zero, and the maximum value MUST be no more than 24 hours. Also, if the response does have a Last-Modified time, the heuristic expiration value SHOULD be no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%. The calculation to determine if a response has expired is quite simple: response_is_fresh = (freshness_lifetime > current_age) Scope of Expiration HTTP/1.1�s expiration model is that as soon as any variant of a URI becomes stale, all variants becomes stale as well. Thus, �freshness� applies to all the variants of URI, rather than any particular variant. Dates and expires etc. apply to any cached variant that a proxy might have with a URI and not just the one particular entity. Editor�s note: This restriction may be dropped in the next draft; there are still discussions about whether this restriction is needed. Disambiguating Expiration Values Because expiration values are assigned optimistically, it is possible that two caches may contain fresh values for the same resource that are different. If a client performing a retrieval receives a non-firsthand response for a resource entity that was already fresh in its own cache, and the Date header in its existing cache entry is newer than the Date on the new response, then the client MAY ignore the response. If so, it MAY retry the request with a �Cache-Control: max-age=0� directive (see section ref Cache_Control \n 18.10), to force a check with the origin server. If a cache that is pooling cached responses from other caches sees two fresh responses for the same resource entity with different validators, it SHOULD use the one with the newer Date header. Disambiguating Multiple Responses Because a client may be receiving responses via multiple paths, so that some responses flow through one set of caches and other responses flow through a different set of caches, a client may receive responses in an order different from that in which the origin server generated them. We would like the client to use the most recently generated response, even if older responses are still apparently fresh. Neither the entity tag nor the expiration value can impose an ordering on responses, since it is possible that a later response intentionally carries an earlier expiration time. However, the HTTP/1.1 specification requires the transmission of Date headers on every response, and the Date values are ordered to a granularity of one second. If a client performs a request for a resource entity that it already has in its cache, and the response it receives contains a Date header that appears to be older than the one it already has in its cache, then the client SHOULD repeat the request unconditionally, and include Cache-Control: max-age=0 to force any intermediate caches to validate their copies directly with the origin server, or Cache-Control: no-cache to force any intermediate caches to obtain a new copy from the origin server. This prevents certain paradoxes arising from the use of multiple caches. If the Date values are equal, then the client may use either response (or may, if it is being extremely prudent, request a new response). Servers MUST NOT depend on clients being able to choose deterministically between responses generated during the same second, if their expiration times overlap. Validation Model When a cache has a stale entry that it would like to use as a response to a client's request, it first has to check with the origin server (or possibly an intermediate cache with a fresh response) to see if its cached entry is still usable. We call this �validating� the cache entry. Since we do not want to have to pay the overhead of retransmitting the full response if the cached entry is good, and we do not want to pay the overhead of an extra round trip if the cached entry is invalid, the HTTP/1.1 protocol supports the use of conditional methods. The key protocol features for supporting conditional methods are those concerned with �cache validators.� When an origin server generates a full response, it attaches some sort of validator to it, which is kept with the cache entry. When a client (end-user or cache) makes a conditional request for a resource for which it has a cache entry, it includes the associated validator in the request. The server then checks that validator against the current validator for the resource entity, and, if they match, it responds with a special status code (usually, �304 Not Modified�) and no entity body. Otherwise, it returns a full response (including entity body). Thus, we avoid transmitting the full response if the validator matches, and we avoid an extra round trip if it does not match. Note: the comparison functions used to decide if validators match are defined in section ref Weak_and_Strong_Tags\n 16.3.3. In HTTP/1.1, a conditional request looks exactly the same as a normal request for the same resource, except that it carries a special header (which includes the validator) that implicitly turns the method (usually, GET) into a conditional. The protocol includes both positive and negative senses of cache-validating conditions. That is, it is possible to request either that a method be performed if and only if the validators match, or if and only if the validators do not match. Note: a response that lacks a cache validator may still be cached, and served from cache until it expires, unless this is explicitly prohibited by a Cache-Control directive. However, a cache cannot do a conditional retrieval if it does not have a cache validator for the entity, which means it will not be refreshable after it expires. Last-modified Dates In HTTP/1.0, the only cache validator is the Last-Modified time carried by a response. Clients validate entities using the If-Modified-Since header. In simple terms, a cache entry is considered to be valid if the actual resource entity has not been modified since the original response was generated. Entity Tags HTTP/1.1 introduces the possibility of using an �opaque� validator, called an �entity tag,� for situations where the Last-Modified date is not appropriate. This may include server implementations where it is not convenient to store modification dates, or where the one-second resolution of HTTP date values is insufficient, or where the origin server wishes to avoid certain paradoxes that may arise from the use of modification dates. An entity tag is simply a string of octets whose internal structure is not known to clients or caches. Caches store entity tags and return them when making conditional requests. Also, when a cache receives a conditional request for a resource for which it has a fresh cache entry, it may compare entity tags using strict octet-equality. Otherwise, entity tags have no semantic value to clients or caches. To preserve compatibility with HTTP/1.0 clients and caches, and because the Last-Modified date may be useful for purposes other than cache validation, HTTP/1.1 servers SHOULD send Last-Modified whenever feasible. The headers used to convey entity tags are described in sections ref CVal \n Error! Reference source not found., ref If_Nomatch \n Error! Reference source not found., ref If_Match \n 18.26, and ref Vary \n 18.46. Weak and Strong Validators Since both origin servers and caches will compare two validator values to decide if they represent the same or different resource entities, one normally would expect that if the resource entity (the entity body or any entity headers) changes in any way, then the associated validator would change as well. If this is true, then we call this validator a �strong validator.� However, there may be cases when a server prefers to change the validator only on semantically significant changes, and not when insignificant aspects of the resource entity change. A validator that does not always change when the resource changes is a �weak validator.� One can think of a strong validator as one that changes whenever the bits of an entity changes, while a weak value changes whenever the meaning of an entity changes. Alternatively, one can think of a strong validator as part of an identifier for a specific entity, while a weak validator is part of an identifier for a set of semantically equivalent entities. Note: One example of a strong validator is an integer that is incremented in stable storage every time an entity is changed. An entity's modification time, if represented with one-second resolution, could be a weak validator, since it is possible that the resource entity may be modified twice during a single second. Entity tags are normally �strong validators,� but the protocol provides a mechanism to tag an entity tag as �weak.� A �use� of a validator is either when a client generates a request and includes the validator in a validating header field, or when a server compares two validators. Strong validators are usable in any context. Weak validators are only usable in contexts that do not depend on exact equality of an entity. For example, either kind is usable for a conditional GET of a full entity. However, only a strong validator is usable for a sub-range retrieval, since otherwise the client may end up with an internally inconsistent entity body. The only function that the HTTP/1.1 protocol defines on validators is comparison. There are two validator comparison functions, depending on whether the comparison context allows the use of weak validators or not: � The strong comparison function: in order to be considered equal, both validators must be identical in every way, and neither may be weak. � The weak comparison function: in order to be considered equal, both validators must be identical in every way, but either or both of them may be tagged as �weak� without affecting the result. The weak comparison function SHOULD be used for simple (non-subrange) GET requests. The strong comparison function MUST be used in all other cases. An entity tag is strong unless it is explicitly tagged as weak. Section ref Validation_Model \n 16.3 gives the syntax for entity tags. A Last-Modified time, when used as a validator in a request, is implicitly weak unless it is possible to deduce that it is strong, using the following rules: � The validator is being compared by an origin server to the actual current validator for the entity and, � That origin server reliably knows that the associated entity did not change twice during the second covered by the presented validator. or � The validator is about to be used by a client in an If-Modified-Since or If-Unmodified-Since header, because the client has a cache entry for the associated entity, and � That cache entry includes a Date value, which gives the time when the origin server generated the original response, and � The presented Last-Modified time is at least 60 seconds before the Date value. or � The validator is being compared by an intermediate cache to the validator stored in its cache entry for the entity, and � That cache entry includes a Date value, which gives the time when the origin server generated the original response, and � The presented Last-Modified time is at least 60 seconds before the Date value. This method relies on the fact that if two different responses were generated by the origin server during the same second, but both had the same Last-Modified time, then at least one of those responses would have a Date value equal to its Last-Modified time. The arbitrary 60-second limit guards against the possibility that the Date and Last-Modified values are generated from different clocks, or at somewhat different times during the preparation of the response. An implementation may use a value larger than 60 seconds, if it is believed that 60 seconds is too short. If a client wishes to perform a sub-range retrieval on a value for which it has only a Last-Modified time and no opaque validator, it may do this only if the Last-Modified time is strong in the sense described here. A cache or origin server receiving a cache-conditional request, other than a full-body GET request, must use the strong comparison function to evaluate the condition. These rules allow HTTP/1.1 caches and clients to safely perform sub-range retrievals on values that have been obtained from HTTP/1.0 servers. Rules for When to Use Entity Tags and Last-modified Dates We adopt a set of rules and recommendations for origin servers, clients, and caches regarding when various validator types should be used, and for what purposes. HTTP/1.1 origin servers: � SHOULD send an entity tag validator unless performance considerations support the use of weak entity tags, or unless it is unfeasible to send a strong entity tag. � MAY send a weak entity tag instead of a strong one. � MAY send no entity tag if it is not feasible to generate one. � SHOULD send a Last-Modified value if it is feasible to send one, unless the risk of a breakdown in semantic transparency that could result from using this date in an If-Modified-Since header would lead to serious problems. In other words, the preferred behavior for an HTTP/1.1 origin server is to send both a strong entity tag and a Last-Modified value. In order to be legal, a strong entity tag MUST change whenever the associated entity value changes in any way. A weak entity tag SHOULD change whenever the associated entity changes in a semantically significant way. Note: in order to provide semantically transparent caching, an origin server should avoid reusing a specific strong entity tag value for two different resource entities, or reusing a specific weak entity tag value for two semantically different instances of a resource entity. Cache entries may persist for arbitrarily long periods, regardless of expiration times, so it may be inappropriate to expect that a cache will never again attempt to validate an entry using a validator that it obtained at some point in the past. HTTP/1.1 clients: � If an entity tag has been provided by the origin server, MUST use that entity tag in any cache-conditional request (using If-Match or If-NoneMatch). � If only a Last-Modified value has been provided by the origin server, SHOULD use that value in non-subrange cache-conditional requests (using If-Modified-Since). � If only a Last-Modified value has been provided by an HTTP/1.0 origin server, MAY use that value in subrange cache-conditional requests (using If-Unmodified-Since:). The user agent should provide a way to disable this, in case of difficulty. � If both an entity tag and a Last-Modified value have been provided by the origin server, SHOULD use both validators in cache-conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches to respond appropriately. An HTTP/1.1 cache, upon receiving a request, MUST use the most restrictive validator when deciding whether the client's cache entry matches the cache's own cache entry. This is only an issue when the request contains both an entity tag and a last-modified-date validator (If-Modified-Since or If-Unmodified-Since). A note on rationale: The general principle behind these rules is that HTTP/1.1 servers and clients should transmit as much non-redundant information as is available in their responses and requests. HTTP/1.1 systems receiving this information will make the most conservative assumptions about the validators they receive. HTTP/1.0 clients and caches will ignore entity tags. Generally, last-modified values received or used by these systems will support transparent and efficient caching, and so HTTP/1.1 origin servers should provide Last-Modified values. In those rare cases where the use of a Last-Modified value as a validator by an HTTP/1.0 system could result in a serious problem, then HTTP/1.1 origin servers should not provide one. Non-validating Conditionals The principle behind entity tags is that only the service author knows the semantics of a resource well enough to select an appropriate cache validation mechanism, and the specification of any validator comparison function more complex than byte-equality would open up a can of worms. Thus, comparisons of any other headers (except Last-Modified, for compatibility with HTTP/1.0) are never used for purposes of validating a cache entry. Constructing Responses From Caches The purpose of an HTTP cache is to store information received in response to requests, for use in responding to future requests. In many cases, a cache simply returns the appropriate parts of a response to the requester. However, if the cache holds a cache entry based on a previous response, it may have to combine parts of a new response with what is held in the cache entry. End-to-end and Hop-by-hop Headers For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP headers into two categories: � End-to-end headers, which must be transmitted to the ultimate recipient of a request or response. End-to-end headers in responses must be stored as part of a cache entry and transmitted in any response formed from a cache entry. � Hop-by-hop headers, which are meaningful only for a single transport-level connection, and are not stored by caches or forwarded by proxies. The following HTTP/1.1 headers are hop-by-hop headers: � Connection � Keep-Alive � Upgrade � Public � Proxy-Authenticate � Transfer-Encoding All other headers defined by HTTP/1.1 are end-to-end headers. Hop-by-hop headers introduced in future versions of HTTP MUST be listed in a Connection header, as described in section ref Connection \n 18.11. Non-modifiable Headers Some features of the HTTP/1.1 protocol, such as Digest Authentication, depend on the value of certain end-to-end headers. A cache or non-caching proxy SHOULD NOT modify an end-to-end header unless the definition of that header requires or specifically allows that. A cache or non-caching proxy MUST NOT modify any of the following fields in a request or response, nor may it add any of these fields if not already present: � Content-Type � Content-Encoding � Content-Length � Expires � Last-Modified � Content-Range � Content-Location Warning: unnecessary modification of end-to-end headers may cause authentication failures if stronger authentication mechanisms are introduced in later versions of HTTP. Such authentication mechanisms may rely on the values of header fields not listed here. Combining Headers When a cache makes a validating request to a server, and the server provides a 304 Not Modified response, the cache must construct a response to send to the requesting client. The cache uses the entity-body stored in the cache entry as the entity-body of this outgoing response. It uses the end-to-end headers from the incoming response, not from the cache entry. Unless it decides to remove the cache entry, it must also replace the end-to-end headers stored with the cache entry with those received in the incoming response. In other words, the complete set of end-to-end headers received in the incoming response overrides all end-to-end headers stored with the cache entry. The cache may add Warning headers (see section ref Warning \n 18.48) to this set. A cache MUST preserve the order of all headers as received in an incoming response. These rule allows an origin server to completely control the response seen by the client of a cache when the cache revalidates an entry, and may be necessary for preserving semantic transparency or for certain kinds of security mechanisms or future extensions. Combining Byte Ranges A response may transfer only a subrange of the bytes of an entity, either because the request included one or more Range specifications, or because a connection was broken prematurely. After several such transfers, a cache may have received several ranges of the same entity. If a cache has a stored non-empty set of subranges for an entity, and an incoming response transfers another subrange, the cache MAY combine the new subrange with the existing set if both the following conditions are met: � Both the incoming response and the cache entry must have a cache validator. � The two cache validators must match using the strong comparison function (see section ref Weak_and_Strong_Tags\n 16.3.3). If either requirement is not meant, the cache must use only the most recent partial response (based on the Date values transmitted with every response, and using the incoming response if these values are equal or missing), and must discard the other partial information. Caching and Generic Resources Generic resources interacts with caching in several ways: � A generic resource (one subject to content negotiation) may be bound to more than one entity. Each of these entities is called a �variant� of the resource. � The request-URI may be only one part of the cache key. Vary Header Use Origin servers may respond to requests for generic resources use the Vary header (see section ref Vary \n 18.46 for a full description) to inform the cache which header fields of the request were used to select the variant returned in the response. A cache can use that response to reply to a subsequent request only if the two requests not only specify the same URI, but also have the same value for all headers specified in the Vary response-header. The Vary header may also inform the cache that the variant was selected using criteria not limited to the request headers; in this case, the response MUST NOT be used in a reply to a subsequent request except if the cache relays the new request to the origin server in a conditional request, and the origin server responds with 304 (Not Modified) and includes the same variant-ID (see 13.8.3). Alternates Header Use The Alternates header is present in the HTTP/1.1 to enable caching of entities from the planned content negotiation facilities. If a cache receives an Alternates header in a response from the origin server (and implement these planned facilities), it should act as if the response carried a �Vary:{accept-headers}� header. This means that the response may be returned in reply to a subsequent request with Accept-* headers identical to those in the current request. Variant-ID Use If an origin server chooses to use the variant-ID mechanism, it assigns a variant-ID (see section ref Variant_IDs \n 7.12) to each distinct resource entity (variant). This assignment can only be made by the origin server. It then returns the appropriate variant-ID with each response that applies to a specific resource entity (variant), using the ETag header (see ref CVal \n Error! Reference source not found.). When sending an entity derived from a particular variant in a response, an origin server SHOULD include a variant-ID identifying the variant in the ETag header (see section ref CVal \n Error! Reference source not found.). This variant-ID can be used for cache replacement and in conditional requests on the generic resource. When a cache receives a successful response with a variant-ID, it SHOULD use this information to replace any existing cache entries for the same variant of the corresponding URI. That is, it forms a cache key using the URI of the request and the variant-ID of the response. If this key matches the key of an existing cache entry, it SHOULD replace the existing entry with the new response (subject to all of the other rules on caching). See section ref Cache_Keys \n Error! Reference source not found. for more details on update. When a cache performs a conditional request on a generic resource, and it has one or more cache entries for the resource that include variant-IDs, the cache MUST transmit the (cache-validator, variant-ID) tuples in the conditional request, using the variant-set mechanism (see section ref Variant_Sets \n 7.13). This tells the server which variants are currently in the requester�s cache. The client MAY choose to transmit only a subset of the (cache-validator, variant-ID) tuples corresponding to its cache entries for this resource. When a server receives a conditional request that includes a variant-set, and the server is able to reply with an appropriate variant (either because it is the origin server, or because it is an intermediate cache that can properly implement the variant selection algorithm), once it has selected the variant it should examine the elements of the supplied variant-set. If one of these matches the variant-ID of the selected variant, and if the cache validators match, the server SHOULD reply with a 304 (Not Modified) response, including the variant-ID of the selected variant. Otherwise, the server should reply as if the request were unconditional. The server may optionally use the variant-set information in its selection algorithm. For example, if the selection algorithm yields several variants with equal preference, and one of these is already in the requester�s cache, the server could select that variant and avoid an extra data transfer. This is a performance optimization; otherwise, the variant-selection mechanism is orthogonal to the variant-ID mechanism. Shared and Non-Shared Caches For reasons of security and privacy, it is necessary to make a distinction between �shared� and �non-shared� caches. A non-shared cache is one that is accessible only to a single user. Accessibility in this case SHOULD be enforced by appropriate security mechanisms. All other caches are considered to be �shared.� Other sections of this specification place certain constraints on the operation of shared caches in order to prevent loss of privacy or failure of access controls Selecting a Cached Response When a cache receives a request it tries to see if it has a cached response appropriate for that request, using the matching rules in this section. If such a response exists, then the cache can decide if it is fresh enough (using the expiration model in section REF Expiration_Model \n 16.1.2 and the freshness requirements of client and origin-server expressed in the Cache-Control headers of the request and cached response) to return in reply to the request. If on a cache lookup there are two or more fresh entries that appear to match the request, then the one with the most recent Date value MUST be used. Plain Resources If the cached response was for a plain resource (that is, the response includes no Vary or Alternates headers), it matches if the Request-URI of the request matches the Request-URI of the of the request that caused the cached response to be stored. Request-URIs match if their canonical forms (see section REF URI_Canonicalization \n 7.2.3) are equal. Generic Resources If the cached response was for a generic resource (that is, the response includes Vary, or Alternates headers), it matches if the Request-URI of the request matches the Request-URI of the request that caused the cached response to be stored, and the selecting request header field values of the request match those of the request that caused the cached response to be stored. (See section REF Vary \n 18.46 on Vary, which defines the canonical form for selecting request headers and the matching rules for them.) If the response contains �Vary: {other}�, then the selecting request header field values for its request are defined as never matching a set of request headers. Errors or Incomplete Response Cache Behavior A cache that receives an incomplete response (for example, with fewer bytes of data than specified in a Content-length: header) may store the response. However, the cache MUST treat this as a partial response. Partial responses may be combined as described in section ref Combining_Byte_Range\n 16.4.4; the result might be a full response or might still be partial. A cache MUST NOT return a partial response to a client without explicitly marking it as such, using the 206 (Partial Content) status code. A cache MUST NOT return a partial response using a status code of 200 (OK). A cache that receives a response with a zero-length Entity-body and no explicit indication that the correct length is zero (such as �Content-Length: 0�) MUST NOT store the response. The same rule applies to a response of any length received without an explicit length indication if the transport connection was terminated in any unusual way. If a cache receives a response carrying Retry-After header (see section ref Retry_After \n 18.40), it may either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it MAY return a previously received response, although it MUST follow all of the rules applying to stale responses. In particular, it MUST NOT override the �must-revalidate� Cache-Control directive (see section ref Cache_Control \n 18.10). Caching and Status Codes A response received with a status code of 200 or 206 may be stored by a cache and used in reply to a subsequent request, subject to the expiration mechanism, unless a Cache-control directive prohibits caching. A response received with any other status code MUST NOT be returned in a reply to a subsequent request unless it carries at least one of the following: � an Expires header � a max-age Cache-control directive � a must-revalidate Cache-control directive � a public Cache-control directive Handling of Retry-After If a cache receives a response carrying a Retry-After header (see section ref Retry_After \n 18.40), it may either forward this response to the requesting client, or act as if the server failed to respond. In the latter case, it MAY return a previously received response, although it MUST follow all of the rules applying to stale responses. In particular, it MUST NOT override the �must-revalidate� Cache-Control directive (see section ref Cache_Control \n 18.10). Side Effects of GET and HEAD Unless the origin server explicitly prohibits the caching of their responses, the application of GET and HEAD methods to any resources SHOULD NOT have side effects that would lead to erroneous behavior if these responses are taken from a cache. They may still have side effects, but a cache is not required to consider such side effects in its caching decisions. Caches are always expected to observe an origin server's explicit restrictions on caching. We note one exception to this rule: since some applications have traditionally used GETs and HEADs with query URLs (those containing a �?� in the rel_path part) to perform operations with significant side effects, caches MUST NOT treat responses to such URLs as fresh unless the server provides an explicit expiration time. This specifically means that responses from HTTP/1.0 servers for such URIs should not be taken from a cache. See section ref SafeMethods \n 19.2 for related information. Invalidation After Updates or Deletions The effect of certain methods at the origin server may cause one or more existing cache entries to become non-transparently invalid. That is, although they may continue to be �fresh,� they do not accurately reflect what the origin server would return for a new request. There is no way for the HTTP protocol to guarantee that all such cache entries are marked invalid. For example, the request that caused the change at the origin server may not have gone through the proxy where a cache entry is stored. However, several rules help reduce the likelihood of erroneous behavior. In this section, the phrase �invalidate an entity� means that the cache should either remove all instances of that entity from its storage, or should mark these as �invalid� and in need of a mandatory revalidation before they can be returned in response to a subsequent request. Some HTTP methods invalidate a single entity. This is either the entity referred to by the Request-URI, or by the Location or Content-Location response headers (if present). These methods are: � PUT � DELETE � POST In order to prevent denial of service attacks, an invalidation based on the URI in a Location or Content-Location header MUST only be performed if the host part is the same as in the Request-URI. Write-Through Mandatory All methods that may be expected to cause modifications to the origin server's resources MUST be written through to the origin server. This currently includes all methods except for GET and HEAD. A cache MUST NOT reply to such a request from a client before having transmitted the request to the inbound server, and having received a corresponding response from the inbound server. The alternative (known as �write-back� or �copy-back� caching) is not allowed in HTTP/1.1, due to the difficulty of providing consistent updates and the problems arising from server, cache, or network failure prior to write-back. Generic Resources and HTTP/1.0 Proxy Caches If the correct handling of responses from a generic resource (Section ref Content_Negotiation \n 15) by HTTP/1.0 proxy caches in the response chain is important, HTTP/1.1 origin servers can include the following Expires (Section ref Expires \n 18.22) response header in all responses from the generic resource: Expires: Thu, 01 Jan 1980 00:00:00 GMT If this Expires header is included, the server should usually also include a Cache-Control header for the benefit of HTTP/1.1 caches, for example Cache-Control: max-age=604800 which overrides the freshness lifetime of zero seconds specified by the included Expires header. Cache Replacement If a new cacheable response (see sections REF What_May_be_Stored_b \n 18.10.2, REF Disambiguating_Expir \n 16.2.6, REF Disambiguating_Multi \n 16.2.8 and REF Errors_Or_Incomplete \n 16.8) is received from a plain resource while any existing responses for the same resource are cached, the cache MUST NOT return any of those older responses to any future requests for the resource. Note: a new response that has an older Date header value than existing cached responses is not cacheable. If a new cacheable response is received from a generic resource with a certain variant-ID while any old responses with the same variant-ID for the same resource are cached, the cache MUST NOT return any of those old responses to any future requests for the resource. Note: In some cases, this may mean that the cache chooses to delete the old response(s) from cache storage to recover space. However, note that there will never be a new response to signal that a variant-ID is no longer in use. It is expected that the cache�s update heuristics will eventually cause such old responses to be deleted. The cache SHOULD use the new response to reply to the current request. It may insert it into cache storage and may, if it meets all other requirements, use it to respond to any future requests that would previously have caused the old response to be returned. If it inserts the new response into cache storage it should follow the rules in section REF Combining_Headers \n 16.4.3. Caching of Negative Responses Caching of negative responses has often been a significant performance advantage in distributed systems. In some future draft or specification we may have more to say about negative caching. History Lists History lists as implemented in many user agents and caches are different. In particular history lists SHOULD NOT try to show a semantically transparent view of the current state of a resource entity. Rather, a history list is meant to show exactly what the user saw at the time when the resource was retrieved . This should not be construed to prohibit the history mechanism from telling the user that a view may be stale. Persistent Connections Purpose HTTP�s greatest strength and its greatest weakness has been its simplicity. Prior to persistent connections, a separate TCP connection was established to fetch each URL, increasing the load on HTTP servers, and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make multiple requests of the same server in a short amount of time. An excellent analysis of these performance problems is available [30]; analysis and results from a prototype implementation are in [33], [34]. Persistent HTTP connections have a number of advantages: � By opening and closing fewer TCP connections, CPU time is saved, and memory used for TCP protocol control blocks is also saved � HTTP requests and responses can be pipe-lined on a connection. Pipe-lining allows a client to make multiple requests without waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time. � Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to determine the congestion state of the network. � HTTP can evolve more gracefully; since errors can be reported without the penalty of closing the TCP connection. Clients using future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old semantics after an error is reported. HTTP implementations SHOULD implement persistent connections. Overall Operation Persistent connections provides a mechanism by which a client and a server can negotiate the use of a TCP connection for an extended conversation. This negotiation takes place using the Connection and Persist header fields. Once this option has been negotiated, the client can make multiple HTTP requests over a single transport connection. Negotiation To request the use of persistent connections, a client sends a Connection header with a connection-token �Persist�. If the server wishes to accept persistent connections, it will respond with the same connection-token. Both the client and server MUST send this connection-token with every request and response for the duration of the persistent connection. If either the client or the server omits the Persist token from the Connection header, that request becomes the last one for the connection. A server MUST NOT establish a persistent connection with an HTTP/1.0 client that uses the above form of the Persist header due to problems with the interactions between HTTP/1.1 clients and HTTP/1.0 proxy servers. (See section ref Compatibility_With_1\n 23.5.2.5 for more information on backwards compatibility with HTTP/1.0 clients.) Pipe-lining Clients and servers which support persistent connections MAY �pipe-line� their requests and responses. When pipe-lining, a client will send multiple requests without waiting for the responses. The server MUST then send all of the responses in the same order that the requests were made. A client MAY assume that a server supports persistent connections if the same server has accepted persistent connections within the past 24 hours. Clients which assume persistent connections and pipeline immediately SHOULD be prepared to retry their connection if the first pipe-lined attempt fails. If a client does such a retry, it MUST NOT pipeline without first receiving an explicit Persist token from the server. Clients MUST also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses. Delimiting Entity-Bodies When using persistent connections, both the client and the server MUST mark the exact endings of transmitted entity-bodies using one of the following three techniques: 1. Send a Content-length field in the header with the exact number of bytes in the entity-body. 2. Send the message using chunked Transfer Coding as described in section ref Transfer_Codings \n 7.6. Chunked Transfer Coding allows the server to transmit the data to the client a piece at a time while still communicating an exact ending of the entity-body. 3. Close the transport connection after the entity body. Sending the Content-length is the preferred technique. Chunked encoding SHOULD be used when the size of the entity-body is not known before beginning to transmit the entity-body. Finally, the connection MAY be closed and fall back to non-persistent connections, if neither 1 or 2 are possible. Clients and servers that support persistent connections MUST correctly support receiving via all three techniques. Proxy Servers It is especially important that proxies correctly implement the properties of the Connection header field as specified in 14.2.1. The proxy server MUST negotiate persistent connections separately with its clients and the origin servers (or other proxy servers) that it connects to. Each persistent connection applies to only one transport link. A proxy server MUST NOT establish a persistent connection with an HTTP/1.0 client. Interaction with Security Protocols It is expected that persistent connections will operate with both SHTTP [31] and SSL [32]. When used in conjunction with SHTTP, the SHTTP request is prepared normally and the persist connection-token is placed in the outermost request block (the one containing the �Secure� method). When used in conjunction with SSL, a SSL session is started as normal and the first HTTP request made using SSL contains the persistent connection header. Practical Considerations Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers might make this a higher value since it is likely that the client will be making more connections through the same server. The use of persistent connections places no requirements on the length of this time-out for either the client or the server. When a client or server wishes to time-out it SHOULD issue a graceful close on the transport connection. Clients and servers SHOULD both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other side�s close promptly it could cause unnecessary resource drain on the network. A client, server, or proxy MAY close the transport connection at any time. For example, a client MAY have started to send a new request at the same time that the server has decided to close the �idle� connection. From the server's point of view, the connection is being closed while it was idle, but from the client's point of view, a request is in progress. This means that clients, servers, and proxies MUST be able to recover from asynchronous close events. Client software SHOULD reopen the transport connection and retransmit the aborted request without user interaction. However, this automatic retry SHOULD NOT be repeated if the second request fails. Servers SHOULD always respond to at least one request per connection, if at all possible. Servers SHOULD NOT close a connection in the middle of transmitting a response, unless a network or client failure is suspected. It is suggested that clients which use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD maintain AT MOST 2 connections with any server of proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion of the Internet or other networks. Header Field Definitions This section defines the syntax and semantics of all standard HTTP/1.1 header fields. For Entity-Header fields, both sender and recipient refer to either the client or the server, depending on who sends and who receives the entity. Accept The Accept request-header field can be used to specify certain media types which are acceptable for the response. Accept headers can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image. The field MAY be folded onto several lines and more than one occurrence of the field is allowed, with the semantics being the same as if all the entries had been in one field value. Accept = "Accept" ":" #( media-range [ ( ":" | ";" ) range-parameter *( ";" range-parameter ) ] | extension-token ) media-range = ( "*/*" | ( type "/" "*" ) | ( type "/" subtype ) ) *( ";" parameter ) range-parameter = ( "q" "=" qvalue ) | extension-range-parameter extension-range-parameter = ( token "=" token ) extension-token = token The asterisk �*� character is used to group media types into ranges, with �*/*� indicating all media types and �type/*� indicating all subtypes of that type. The range-parameter q is used to indicate the media type quality factor for the range, which represents the user�s preference for that range of media types. The default value is q=1. In Accept headers generated by HTTP/1.1 clients, the character separating media-ranges from range-parameters SHOULD be a �:�. HTTP/1.1 servers SHOULD be tolerant of use of the �;� separator by HTTP/1.0 clients. The example Accept: audio/*: q=0.2, audio/basic SHOULD be interpreted as �I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality.� If no Accept header is present, then it is assumed that the client accepts all media types. If Accept headers are present, and if the server cannot send a response which is acceptable according to the Accept headers, then the server SHOULD send an error response with the 406 (not acceptable) status code, though the sending of an unacceptable response is also allowed. A more elaborate example is Accept: text/plain: q=0.5, text/html, text/x-dvi: q=0.8, text/x-c Verbally, this would be interpreted as �text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi entity, and if that does not exist, send the text/plain entity.� Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example, Accept: text/*, text/html, text/html;level=1, */* have the following precedence: 1) text/html;level=1 2) text/html 3) text/* 4) */* The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type. For example, Accept: text/*:q=0.3, text/html:q=0.7, text/html;level=1, */*:q=0.5 would cause the following values to be associated: text/html;level=1 = 1 text/html = 0.7 text/plain = 0.3 image/jpeg = 0.5 text/html;level=3 = 0.7 Note: A user agent MAY be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system which cannot interact with other rendering agents, this default set SHOULD be configurable by the user. Accept-Charset The Accept-Charset request-header field can be used to indicate what character sets are acceptable for the response. This field allows clients capable of understanding more comprehensive or special-purpose character sets to signal that capability to a server which is capable of representing documents in those character sets. The ISO-8859-1 character set can be assumed to be acceptable to all user agents. Accept-Charset = "Accept-Charset" ":" 1#( charset [ ";" "q" "=" qvalue ] ) Character set values are described in section ref Charset \n 7.4. Each charset may be given an associated quality value which represents the user's preference for that charset. The default value is q=1. An example is Accept-Charset: iso-8859-5, unicode-1-1;q=0.8 If no Accept-Charset header is present, the default is that any character set is acceptable. If an Accept-Charset header is present, and if the server cannot send a response which is acceptable according to the Accept-Charset header, then the server SHOULD send an error response with the 406 (not acceptable) status code, though the sending of an unacceptable response is also allowed. Accept-Encoding The Accept-Encoding request-header field is similar to Accept, but restricts the content-coding values (ref Content_Encoding \n 18.13) which are acceptable in the response. Accept-Encoding = "Accept-Encoding" ":" #( content-coding ) An example of its use is Accept-Encoding: compress, gzip If no Accept-Encoding header is present in a request, the server MAY assume that the client will accept any content coding. If an Accept-Encoding header is present, and if the server cannot send a response which is acceptable according to the Accept-Encoding header, then the server SHOULD send an error response with the 406 (not acceptable) status code. Accept-Language The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are preferred as a response to the request. Accept-Language = "Accept-Language" ":" 1#( language-range [ ";" "q" "=" qvalue ] ) language-range = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" ) Each language-range MAY be given an associated quality value which represents an estimate of the user's comprehension of the languages specified by that range. The quality value defaults to �q=1� (100% comprehension).For example, Accept-Language: da, en-gb;q=0.8, en;q=0.7 would mean: �I prefer Danish, but will accept British English (with 80% comprehension) and other types of English(with 70% comprehension).� A language-range matches a language-tag if it exactly equals the tag, or if it exactly equals a prefix (a sub-sequence starting at the first character) of the tag such that the first tag character following the prefix is �-�. The special range �*�, if present in the Accept-Language field, matches every tag not matched by any other ranges present in the Accept-Language field. Note: This use of a prefix matching rule does not imply that language tags are assigned to languages in such a way that it is always true that if a user understands a language with a certain tag, then this user will also understand all languages with tags for which this tag is a prefix. The prefix rule simply allows the use of prefix tags if this is the case. The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the longest language-range in the field that matches the language-range. If no language-range in the field matches the tag, the language quality factor assigned is 0. If no Accept-Language header is present in the request, the server SHOULD assume that all languages are equally acceptable. If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are acceptable. If the server cannot generate a response for an audience capable of understanding at least one acceptable language, it can send a response that uses one or more un-accepted languages. It may be contrary to the privacy expectations of the user to send an Accept-Language header with the complete linguistic preferences of the user in every request. For a discussion of this issue, see section ref SecPrivacyAccept \n 19.7. Note: As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then the Accept-Language header field MUST NOT be given in the request. Accept-Ranges In some cases, a client may want to know if the server accepts range requests using a certain range unit. The server may indicate its acceptance of range requests for a resource entity by providing this header in a response for that resource: Accept-Ranges = "Accept-Ranges" ":" acceptable-ranges acceptable-ranges = 1#range-unit | "none" Origin servers that accept byte-range requests MAY send Accept-Ranges: bytes but are not required to do so. Clients MAY generate byte-range requests without having received this header for the plain resource involved, but the server MAY ignore such requests. Origin servers that do not accept any kind of range request for a plain resource MAY send Accept-Ranges: none to advise the client not to attempt a range request. Age Caches transmit age values using: Age = "Age" ":" age-value age-value = delta-seconds Age values are non-negative decimal integers, representing time in seconds. If a cache receives a value larger than the largest positive integer it can represent, or if any of its age calculations overflows, it MUST transmit an Age header with a value of 2147483648 (2^31). Otherwise, HTTP/1.1 caches MUST send an Age header in every response. Caches SHOULD use a representation with at least 31 bits of range.. Allow The Allow entity-header field lists the set of methods supported by the resource identified by the Request-URI. The purpose of this field is strictly to inform the recipient of valid methods associated with the resource. An Allow header field MUST be present in a 405 (method not allowed) response. The Allow header field is not permitted in a request using the POST method, and thus SHOULD be ignored if it is received as part of a POST entity. Allow = "Allow" ":" 1#method Example of use: Allow: GET, HEAD, PUT This field cannot prevent a client from trying other methods. However, the indications given by the Allow header field value SHOULD be followed. The actual set of allowed methods is defined by the origin server at the time of each request. The Allow header field MAY be provided with a PUT request to recommend the methods to be supported by the new or modified resource. The server is not required to support these methods and SHOULD include an Allow header in the response giving the actual supported methods. A proxy MUST NOT modify the Allow header field even if it does not understand all the methods specified, since the user agent MAY have other means of communicating with the origin server. The Allow header field does not indicate what methods are implemented at the server level. Servers MAY use the Public response header field (section ref Public \n 18.37) to describe what methods are implemented on the server as a whole. Alternates The Alternates response-header field is used by origin servers to signal that the resource identified by the current request has the capability to send different responses depending on the accept headers in the request message. This has an important effect on cache management, particularly for caching proxies which service a diverse set of user agents. This effect is covered in section ref Vary \n 18.46. Alternates = "Alternates" ":" opaque-field opaque-field = field-value The Alternates header is included into HTTP/1.1 to make HTTP/1.1 caches compatible with a planned content negotiation mechanism. HTTP/1.1 allows a future content negotiation standard to define the format of the Alternates header field-value, as long as the defined format satisfies the general rules in section ref Alternates \n 18.8. To ensure compatibility with future experimental or standardized software, caching HTTP/1.1 clients MUST treat all Alternates headers in a response as synonymous to the following Vary header: Vary: {accept-headers} and follow the caching rules associated with the presence of this Vary header, as covered in Section ref Vary \n 18.46. HTTP/1.1 allows origin servers to send Alternates headers under experimental conditions. Authorization A user agent that wishes to authenticate itself with a server--usually, but not necessarily, after receiving a 401 response--MAY do so by including an Authorization request-header field with the request. The Authorization field value consists of credentials containing the authentication information of the user agent for the realm of the resource being requested. Authorization = "Authorization" ":" credentials HTTP access authentication is described in section ref AA \n 14. If a request is authenticated and a realm specified, the same credentials SHOULD be valid for all other requests within this realm. When a shared cache (see section ref Shared_and_Non_Share\n 16.6) receives a request containing an Authorization field, it MUST NOT return the corresponding response as a reply to any other request, unless one of the following specific exceptions holds: 1. If the response includes the �proxy-revalidate� Cache-Control directive, the cache MAY use that response in replying to a subsequent request, but a proxy cache MUST first revalidate it with the origin server, using the request headers from the new request to allow the origin server to authenticate the new request. 2. If the response includes the �must-revalidate� Cache-Control directive, the cache MAY use that response in replying to a subsequent request, but all caches MUST first revalidate it with the origin server, using the request headers from the new request to allow the origin server to authenticate the new request. 3. If the response includes the �public� Cache-Control directive, it may be returned in reply to any subsequent request. Cache-Control The Cache-Control general-header field is used to specify directives that MUST be obeyed by all caching mechanisms along the request/response chain. The directives specify behavior intended to prevent caches from adversely interfering with the request or response. . These directives typically override the default caching algorithms. Cache directives are unidirectional in that the presence of a directive in a request does not imply that the same directive should be given in the response. Cache directives must be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a cache-directive for a specific cache. Cache-Control = "Cache-Control" ":" 1#cache-directive cache-directive = "public" | "private" [ "=" <"> 1#field-name <"> ] | "no-cache" [ "=" <"> 1#field-name <"> ] | "no-store" | "no-transform" | "must-revalidate" | "proxy-revalidate" | "only-if-cached" | "max-age" "=" delta-seconds | "max-stale" "=" delta-seconds | "min-fresh" "=" delta-seconds | "min-vers" "=" HTTP-Version When a directive appears without any 1#field-name parameter, the directive applies to the entire request or response. When such a directive appears with a 1#field-name parameter, it applies only to the named field or fields, and not to the rest of the request or response. This mechanism supports extensibility; implementations of future versions of the HTTP protocol may apply these directives to header fields not defined in HTTP/1.1. The cache-control directives can be broken down into these general categories: � Restrictions on what is cachable; these may only be imposed by the origin server. � Restrictions on what may be stored by a cache; these may be imposed by either the origin server or the end-user client. � Modifications of the basic expiration mechanism; these may be imposed by either the origin server or the end-user client. � Controls over cache revalidation and reload; these may only be imposed by an end-user client. � Restrictions on the number of times a cache entry may be used, and related demographic reporting mechanisms. � Miscellaneous restrictions Caches never add or remove Cache-Control directives to requests or responses. Check: is this true? Cache-Control Restrictions on What is Cachable Unless specifically constrained by a Cache-Control directive, a caching system may always store a successful response (see section ref Errors_Or_Incomplete\n 16.8) as a cache entry, may return it without validation if it is fresh, and may return it after successful validation. If there is neither a cache validator nor an explicit expiration time associated with a response, we do not expect it to be cached, but certain caches may violate this expectation (for example, when little or no network connectivity is available). A client can usually detect that such a response was taken from a cache by comparing the Date header to the current time. Note that some HTTP/1.0 caches are known to violate this expectation without providing any Warning. However, in some cases it may be inappropriate for a cache to retain a resource entity, or to return it in response to a subsequent request. This may be because absolute semantic transparency is deemed necessary by the service author, or because of security or privacy considerations. Certain Cache-Control directives are therefore provided so that the server can indicate that certain resource entities, or portions thereof, may not be cached regardless of other considerations. Note that section ref Authorization \n 18.8 normally prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization header. The following Cache-Control response directives add or remove restrictions on what is cachable: public Overrides the restriction in section ref Authorization \n 18.8 that prevents a shared cache from saving and returning a response to a previous request if that request included an Authorization header. However, any other constraints on caching still apply. private Indicates that all or part of the response message is intended for a single user and MUST NOT be cached by a shared cache. This allows an origin server to state that the specified parts of the response are intended for only one user and are not a valid response for requests by other users. A private (non-shared) cache may ignore this directive. Note: This usage of the word �private� only controls where the response may be cached, and cannot ensure the privacy of the message content. Note in particular that HTTP/1.0 caches will not recognize or obey this directive. no-cache indicates that all or partof the response message MUST NOT be cached anywhere. This allows an origin server to prevent caching even by caches that have been configured to return stale responses to client requests. Note: HTTP/1.0 caches will not recognize or obey this directive. TBS: precedence relations between public, private, and no-cache. What May be Stored by Caches The �no-store� directive applies to the entire message, and may be sent either in a response or in a request. If sent in a request, a cache MUST NOT store any part of either this request or any response to it. If sent in a response, a cache MUST NOT store any part of either this response or the request that elicited it. This directive applies to both non-shared and shared caches. Even when this directive is associated with a response, users may explicitly store such a response outside of the caching system (e.g., with a �Save As� dialog). History buffers may store such responses as part of their normal operation. The purpose of this directive is to meet the stated requirements of certain users and service authors who are concerned about accidental releases of information via unanticipated accesses to cache data structures. While the use of this directive may improve privacy in some cases, we caution that it is NOT in any way a reliable or sufficient mechanism for ensuring privacy. In particular, HTTP/1.0 caches will not recognize or obey this directive, malicious or compromised caches may not recognize or obey this directive, and communications networks may be vulnerable to eavesdropping. The �min-vers� directive applies to the entire message, and may be sent either in a response or in a request. If sent in a request, a cache whose HTTP version number is less than the specified version MUST NOT store any part of either this request or any response to it. If sent in a response, a cache whose HTTP version number is less than the specified version MUST NOT store any part of either this response or the request that elicited it, nor may any cache transmit a stored (non-firsthand) copy of the response to any client with a lower HTTP version number. This directive applies to both non-shared and shared caches, and is made mandatory to allow for future protocol extensions that may affect caching. Note that the lowest version that can be sensibly included in a �min-vers� directive is HTTP/1.1, since HTTP/1.0 caches do not obey it. Modifications of the Basic Expiration Mechanism The expiration time of a resource entity may be specified by the origin server using the Expires header (see section ref Expires \n 18.22). Alternatively, it may be specified using the �max-age� directive in a response. If a response includes both an Expires header and a max-age directive, the max-age directive overrides the Expires header, even if the Expires header is more restrictive. This rule allows an origin server to provide, for a given response, a longer expiration time to an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This may be useful if certain HTTP/1.0 caches improperly calculate ages or expiration times, perhaps due to synchronized clocks. Other directives allow an end-user client to modify the basic expiration mechanism, making it either stricter or looser. These directives may be specified on a request: max-age Indicates that the client is willing to accept a response whose age is no greater than the specified time in seconds. Unless �max-stale� is also included, the client is not willing to accept a stale response. This directive overrides any policy of the cache. min-fresh Indicates that the client is willing to accept a response whose freshness lifetime is no less than its current age plus the specified time in seconds. That is, the client wants a that response will still be fresh for at least the specified number of seconds. max-stale Indicates that the client is willing to accept a response that has exceeded its expiration time by no more than the specified number of seconds. If a cache returns a stale response in response to such a request, it MUST mark it as stale using the Warning header. Note that HTTP/1.0 caches will ignore these directives. If a cache returns a stale response, either because of a max-stale directive on a request, or because the cache is configured to override the expiration time of a response, the cache MUST attach a Warning header to the stale response, using Warning 10 (Response is stale). Cache Revalidation and Reload Controls Sometimes an end-user client may want or need to insist that a cache revalidate its cache entry with the origin server (and not just with the next cache along the path to the origin server), or to reload its cache entry from the origin server. End-to-end revalidation may be necessary if either the cache or the origin server has overestimated the expiration time of the cached response. End-to-end reload may be necessary if the cache entryhas become corrupted for some reason, and the fact that its validator is up-to-date is irrelevant. End-to-end revalidation may be requested either when the client does not have its own local cached copy, in which case we call it �unspecified end-to-end revalidation�, or when the client does have a local cached copy, in which case we call it �specific end-to-end revalidation.� The client can specify these three kinds of action using Cache-Control request directives: End-to-end reload The request includes �Cache-Control: no-cache� or, for compatibility with HTTP/1.0 clients, �Pragma: no-cache�. No field names may be included with the �no-cache� directive in a request. The server MUST NOT use a cached copy when responding to such a request. Specific end-to-end revalidation The request includes �Cache-Control: max-age=0�, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request includes a cache-validating conditional with the client's current validator. Unspecified end-to-end revalidation The request includes �Cache-Control: max-age=0�, which forces each cache along the path to the origin server to revalidate its own entry, if any, with the next cache or server. The initial request does not include a cache-validating conditional; the first cache along the path (if any) that holds a cache entry for this resource includes a cache-validating conditional with its current validator. Note that HTTP/1.0 caches will ignore these directives, except perhaps for �Pragma: no-cache�. When an intermediate cache is forced, by means of a �max-age=0� directive, to revalidate its own cache entry, and the client has supplied its own validator in the request, the supplied validator may differ from the validator currently stored with the cache entry. In this case, the cache may use either validator in making its own request without affecting semantic transparency. However, the choice of validator may affect performance. The best approach is for the intermediate cache to use its own validator when making its request. If the server replies with 304 (Not Modified), then the cache should return its now validated copy to the client with a 200 (OK) response. If the server replies with a new Entity-body and cache validator, however, the intermediate cache should compare the returned validator with the one provided in the client's request, using the strong comparison function. If the client's validator is equal to the origin server's, then the intermediate cache simply returns 304 (Not Modified). Otherwise, it returns the new Entity-body with a 200 (OK) response. If a request includes the �no-cache� directive, it should not include �min-fresh�, �max-stale�, or �max-age�. In some cases, such as times of extremely poor network connectivity, a client may want a cache to return only those responses that it currently has stored, and not to reload or revalidate with the origin server. To do this, the client may include the �only-if-cached� directive in a request. If it receives this directive, a cache SHOULD either respond using a cached entry that is consistent with the other constraints of the request, or respond with a 504 (Gateway Timeout) status. However, if a group of caches is being operated as a unified system with good internal connectivity, such a request MAY be forwarded within that group of caches. Because a cache may be configured to ignore a server's specified expiration time, and because a client request may include a max-stale directive, which has a similar effect, the protocol also includes a mechanism for the origin server to require revalidation of a cache entry on any subsequent use. When the �must-revalidate� directive is present in a response received by a cache, that cache MUST NOT use the entry after it becomes stale to respond to a subsequent request without first revalidating it with the origin server. (I.e., the cache must do an end-to-end revalidation every time, if, based solely on the origin server's Expires or max-age value, the cached response is stale.) The �must-revalidate� directive is necessary to support reliable operation for certain protocol features. In all circumstances an HTTP/1.1 cache MUST obey the �must-revalidate� directive; in particular, if the cache cannot reach the origin server for any reason, it MUST generate a 504 (Gateway Timeout) response. Note that HTTP/1.0 caches will ignore this directive. Servers should send the �must-revalidate� directive if and only if failure to revalidate a request on the entity could result in incorrect operation, such as a silently unexecuted financial transaction. Recipients MUST NOT take any automated action that violates this directive, and MUST NOT automatically provide an unvalidated copy of the entity if revalidation fails. Although this is not recommended, user agents operating under severe connectivity constraints may violate this directive but, if so, MUST explicitly warn the user that an unvalidated response has been provided. The warning MUST be provided on each unvalidated access, and SHOULD require explicit user confirmation. The �proxy-revalidate� directive has the same meaning as the �must-revalidate� directive, except that it does not apply to user-agent caches. Miscellaneous Restrictions In certain circumstances, an intermediate cache (proxy) may find it useful to convert the encoding of an entity body. For example, a proxy might use a compressed content-coding to transfer the body to a client on a slow link. Because end-to-end authentication of entity bodies and/or entity headers relies on the specific encoding of these values, such transformations may cause authentication failures. Therefore, an intermediate cache MUST NOT change the encoding of an entity body if the response includes the �no-transform� directive. Note: the use of hop-by-hop compression in conjunction with Range retrievals may require additional specification in a subsequent draft. Connection HTTP version 1.1 provides a new request and response header field called �Connection�. This header field allows the client and server to specify options which should only exist over that particular connection and MUST NOT be communicated by proxies over further connections. The connection header field MAY have multiple tokens separated by commas (referred to as connection-tokens). HTTP version 1.1 proxies MUST parse the Connection header field and for every connection-token in this field, remove a corresponding header field from the request before the request is forwarded. The use of a connection option is specified by the presence of a connection token in the Connection header field, not by the corresponding additional header field (which may not be present). When a client wishes to establish a persistent connection it MUST send a �Persist� connection-token: Connection: persist The Connection header has the following grammar: Connection-header = "Connection" ":" 1#(connection-token) connection-token = token Content-Base The Content-Base entity-header field may be used to specify the base URI for resolving relative URLs within the entity. This header field is described as �Base� in RFC 1808 private HREF="#RefRelURL"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [11], which is expected to be revised soon. Content-Base = "Content-Base" ":" absoluteURI If no Content-Base field is present, the base URI of an entity is defined either by its Content-Location or the URI used to initiate the request, in that order of precedence. Note, however, that the base URI of the contents within the entity body may be redefined within that entity body. Content-Encoding The Content-Encoding entity-header field is used as a modifier to the media-type. When present, its value indicates what additional content codings have been applied to the resource entity, and thus what decoding mechanisms MUST be applied in order to obtain the media-type referenced by the Content-Type header field. Content-Encoding is primarily used to allow a document to be compressed without losing the identity of its underlying media type. Content-Encoding = "Content-Encoding" ":" 1#content-coding Content codings are defined in section ref Content_Codings \n 7.5. An example of its use is Content-Encoding: gzip The Content-Encoding is a characteristic of the resource entity identified by the Request-URI. Typically, the resource entity is stored with this encoding and is only decoded before rendering or analogous usage. If multiple encodings have been applied to a resource entity, the content codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other Entity-Header fields not defined by this specification. Content-Language The Content-Language entity-header field describes the natural language(s) of the intended audience for the enclosed entity. Note that this may not be equivalent to all the languages used within the entity. Content-Language = "Content-Language" ":" 1#language-tag Language tags are defined in section ref Language_Tags \n 7.10. The primary purpose of Content-Language is to allow a selective consumer to identify and differentiate resource variants according to the consumer's own preferred language. Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is Content-Language: dk If no Content-Language is specified, the default is that the content is intended for all language audiences. This may mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended. Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the �Treaty of Waitangi,� presented simultaneously in the original Maori and English versions, would call for Content-Language: mi, en However, just because multiple languages are present within an entity does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as �A First Lesson in Latin,� which is clearly intended to be used by an English-literate audience. In this case, the Content-Language should only include �en�. Content-Language MAY be applied to any media type -- it SHOULD not be limited to textual documents. Content-Length The Content-Length entity-header field indicates the size of the Entity-Body, in decimal number of octets, sent to the recipient or, in the case of the HEAD method, the size of the Entity-Body that would have been sent had the request been a GET. Content-Length = "Content-Length" ":" 1*DIGIT An example is Content-Length: 3495 Applications SHOULD use this field to indicate the size of the Entity-Body to be transferred, regardless of the media type of the entity. It must be possible for the recipient to reliably determine the end of a HTTP/1.1 request method containing an entity body, e.g., because the request has a valid Content-Length field, uses Transfer-Encoding: chunked or a multipart body. Any Content-Length greater than or equal to zero is a valid value. Section ref BodyLength \n 11.2.2 describes how to determine the length of an Entity-Body if a Content-Length is not given. Note: The meaning of this field is significantly different from the corresponding definition in MIME, where it is an optional field used within the �message/external-body� content-type. In HTTP, it SHOULD be used whenever the entity's length can be determined prior to being transferred. Content-Location The Content-Location entity-header field is used to define the location of the plain resource associated with the entity enclosed in the message. A server SHOULD provide a Content-Location if, when including an entity in response to a GET request on a generic resource, the entity corresponds to a specific, non-negotiated location which can be accessed via the Content-Location URI. A server SHOULD provide a Content-Location with any 200 (OK) response which was internally (not visible to the client) redirected to a resource other than the one identified by the request and for which correct interpretation of that resource MAY require knowledge of its actual location. Content-Location = "Content-Location" ":" absoluteURI If no Content-Base header field is present, the value of Content-Location also defines the base URL for the entity (see Section ref Content_Base \n 18.12). Note that the Content-Location information is advisory, and that there is no guarantee that the URI of the Content-Location actually corresponds in any way to the original request URI. For example, a cache cannot reliably assume that the data returned as a result of the request can be returned from a new request on any URI other than the original request. See section ref Location_Headers_and\n 19.9. Content-MD5 The Content-MD5 entity-header field is an MD5 digest of the entity-body, as defined in RFC 1864 [private HREF="#Ref1864" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor 23], for the purpose of providing an end-to-end message integrity check (MIC) of the entity-body. (Note: an MIC is good for detecting accidental modification of the entity-body in transit, but is not proof against malicious attacks.) ContentMD5 = "Content-MD5" ":" md5-digest md5-digest = The Content-MD5 header may be generated by an origin server to function as an integrity check of the entity-body. Only origin-servers may generate the Content-MD5 header field; proxies and gateways MUST NOT generate it, as this would defeat its value as an end-to-end integrity check. Any recipient of the entity-body, including gateways and proxies, MAY check that the digest value in this header field matches that of the entity-body as received. The MD5 digest is computed based on the content of the entity body, including any Content-Encoding that has been applied, but not including any Transfer-Encoding. If the entity is received with a Transfer-Encoding, that encoding must be removed prior to checking the Content-MD5 value against the received entity. This has the result that the digest is computed on the octets of the entity body exactly as, and in the order that, they would be sent if no Transfer-Encoding were being applied. HTTP extends RFC 1864 to permit the digest to be computed for MIME composite media-types (e.g., multipart/* and message/rfc822), but this does not change how the digest is computed as defined in the preceding paragraph. Note: There are several consequences of this. The entity-body for composite types may contain many body-parts, each with its own MIME and HTTP headers (including Content-MD5, Content-Transfer-Encoding, and Content-Encoding headers). If a body-part has a Content-Transfer-Encoding or Content-Encoding header, it is assumed that the content of the body-part has had the encoding applied, and the body-part is included in the Content-MD5 digest as is -- i.e., after the application. Also, the HTTP Transfer-Encoding header makes no sense within body-parts; if it is present, it is ignored -- i.e. treated as ordinary text. Note: while the definition of Content-MD5 is exactly the same for HTTP as in RFC 1864 for MIME entity-bodies, there are several ways in which the application of Content-MD5 to HTTP entity-bodies differs from its application to MIME entity-bodies. One is that HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does use Transfer-Encoding and Content-Encoding. Another is that HTTP more frequently uses binary content types than MIME, so it is worth noting that in such cases, the byte order used to compute the digest is the transmission byte order defined for the type. Lastly, HTTP allows transmission of text types with any of several line break conventions and not just the canonical form using CRLF. Conversion of all line breaks to CRLF should not be done before computing or checking the digest: the line break convention used in the text actually transmitted should be left unaltered when computing the digest. Content-Range The Content-Range header is sent with a partial entity body to specify where in the full entity body the partial body should be inserted. It also indicates the total size of the entity. Content-Range = "Content-Range" ":" content-range-spec When an HTTP message includes the content of a single range (for example, a response to a request for a single range, or to request for a set of ranges that overlap without any holes), this content is transmitted with a Content-Range header, and a Content-Length header showing the number of bytes actually transferred. For example, HTTP/1.0 206 Partial content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-modified: Wed, 15 Nov 1995 04:58:08 GMT Content-Range: 21010-47021/47022 Content-Length: 26012 Content-Type: image/gif MIME multipart/byteranges Content-type When an HTTP message includes the content of multiple ranges (for example, a response to a request for multiple non-overlapping ranges), these are transmitted as a multipart MIME message. The multipart MIME content-type used for this purpose is defined in this specification to be �multipart/byteranges�. The MIME multipart/byteranges content-type includes two or more parts, each with its own Content-Type and Content-Range fields. The parts are separated using a MIME boundary parameter. For example: HTTP/1.0 206 Partial content Date: Wed, 15 Nov 1995 06:25:24 GMT Last-modified: Wed, 15 Nov 1995 04:58:08 GMT Content-type: multipart/byteranges; boundary=THIS_STRING_SEPARATES --THIS_STRING_SEPARATES Content-type: application/pdf Content-range: bytes 500-999/8000 ...the first range... --THIS_STRING_SEPARATES Content-type: application/pdf Content-range: bytes 7000-7999/8000 ...the second range --THIS_STRING_SEPARATES� Additional Rules for Content-Range A client that cannot decode a MIME multipart/byteranges message should not ask for multiple byte-ranges in a single request. When a client requests multiple byte-ranges in one request, the server SHOULD return them in the order that they appeared in the request. If the server ignores a byte-range-spec because it is invalid, the server should treat the request as if the invalid Range header field did not exist (normally, this means return a 200 response containing the full resource entity). The reason is that the only time a client will make such an invalid request is when the resource entity has changed (shrunk) since the prior request. Content-Type The Content-Type entity-header field indicates the media type of the Entity-Body sent to the recipient or, in the case of the HEAD method, the media type that would have been sent had the request been a GET. Content-Type = "Content-Type" ":" media-type Media types are defined in section ref Media_Types \n 7.7. An example of the field is Content-Type: text/html; charset=ISO-8859-4 Further discussion of methods for identifying the media type of an entity is provided in section ref BodyType \n 11.2.1. Date The Date general-header field represents the date and time at which the message was originated, having the same semantics as orig-date in RFC 822. The field value is an HTTP-date, as described in section ref HTTP_Date \n 7.3.1. Date = "Date" ":" HTTP-date An example is Date: Tue, 15 Nov 1994 08:12:31 GMT If a message is received via direct connection with the user agent (in the case of requests) or the origin server (in the case of responses), then the date can be assumed to be the current date at the receiving end. However, since the date--as it is believed by the origin--is important for evaluating cached responses, origin servers SHOULD always include a Date header. Clients SHOULD only send a Date header field in messages that include an entity body, as in the case of the PUT and POST requests, and even then it is optional. A received message which does not have a Date header field SHOULD be assigned one by the recipient if the message will be cached by that recipient or gatewayed via a protocol which requires a Date. In theory, the date SHOULD represent the moment just before the entity is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value. Note: An earlier version of this document incorrectly specified that this field SHOULD contain the creation date of the enclosed Entity-Body. This has been changed to reflect actual (and proper) usage. Origin servers MUST send a Date field in every response. However, if a cache receives a response without a Date field, it SHOULD attach one with the cache's best estimate of the time at which the response was originally generated. The format of the Date is an absolute date and time as defined by HTTP-date in Section ref DateFormats \n 7.3; it MUST be in RFC1123-date format. ETag The ETag header is used to transmit entity tags with variant id�s in HTTP/1.1 responses. ETag = "ETag" ":" etag-info etag-info = entity-tag [ ";" variant-id ] Examples: ETag: "xyzzy" ETag: "xyzzy"/W ETag: "xyzzy";"3" ETag: "xyzzy"/W;"3" ETag: "" Note that the variant-id is not part of the entity tag. The ETag field is used to transmit a variant-id simply as a matter of compact representation of responses. Expires The Expires entity-header field gives the date/time after which the entity should be considered stale. A stale cache entry may not normally be returned by a cache (either a proxy cache or an end-user cache) unless it is first validated with the origin server (or with an intermediate cache that has a fresh copy of the resource entity). See section ref Expiration_Model \n 16.1.2 for further discussion of the expiration model. The presence of an Expires field does not imply that the original resource will change or cease to exist at, before, or after that time. The format is an absolute date and time as defined by HTTP-date in section ref DateFormats \n 7.3; it MUST be in rfc1123-date format: Expires = "Expires" ":" HTTP-date An example of its use is Expires: Thu, 01 Dec 1994 16:00:00 GMT Note: if a response includes a Cache-Control field with the max-age directive, that directive overrides the Expires field. HTTP/1.1 clients and caches MUST treat other invalid date formats, especially including the value �0�, as in the past (i.e., �already expired�). To mark a response as �already expired,� an origin server should use an Expires date that is equal to the Date header value. (See the rules for expiration calculations in section ref Expiration_Calculati\n 0.) To mark a response as �never expires,� an origin server should use an Expires date approximately one year from the time the response is generated. HTTP/1.1 servers should not send Expires dates more than one year in the future. From The From request-header field, if given, SHOULD contain an Internet e-mail address for the human user who controls the requesting user agent. The address SHOULD be machine-usable, as defined by mailbox in RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9] (as updated by RFC 1123 private HREF="#RefSTD3"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [8]): From = "From" ":" mailbox An example is: From: webmaster@w3.org This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It SHOULD NOT be used as an insecure form of access protection. The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents SHOULD include this header so that the person responsible for running the robot can be contacted if problems occur on the receiving end. The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. For example, when a request is passed through a proxy the original issuer's address SHOULD be used. Note: The client SHOULD not send the From header field without the user's approval, as it may conflict with the user's privacy interests or their site's security policy. It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request. Host The Host request-header field specifies the Internet host and port number of the resource being requested, as obtained from the original URL given by the user or referring resource (generally an HTTP URL, as described in section ref http_URL \n 7.2.2). The Host field value MUST represent the network location of the origin server or gateway given by the original URL. This allows the origin server or gateway to differentiate between internally-ambiguous URLs, such as the root �/� URL of a server for multiple host names on a single IP address. Host = "Host" ":" host [ ":" port ] ; Section ref http_URL \n 7.2.2 A �host� without any trailing port information implies the default port for the service requested (e.g., �80� for an HTTP URL). For example, a request on the origin server for MUST include: GET /pub/WWW/ HTTP/1.1 Host: www.w3.org The Host header field MUST be included in all HTTP/1.1 request messages on the Internet (i.e., on any message corresponding to a request for a URL which includes an Internet host address for the service being requested). If the Host field is not already present, an HTTP/1.1 proxy MUST add a Host field to the request message prior to forwarding it on the Internet. All Internet-based HTTP/1.1 servers MUST respond with a 400 status code to any HTTP/1.1 request message which lacks a Host header field. If-Modified-Since The If-Modified-Since request-header field is used with the GET method to make it conditional: if the requested resource entity has not been modified since the time specified in this field, a copy of the resource entity will not be returned from the server; instead, a 304 (not modified) response will be returned without any Entity-Body. If-Modified-Since = "If-Modified-Since" ":" HTTP-date An example of the field is: If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT A GET method with an If-Modified-Since header and no Range header requests that the identified resource entity be transferred only if it has been modified since the date given by the If-Modified-Since header. The algorithm for determining this includes the following cases: a) If the request would normally result in anything other than a 200 (OK) status, or if the passed If-Modified-Since date is invalid, the response is exactly the same as for a normal GET. A date which is later than the server's current time is invalid. b) If the resource entity has been modified since the If-Modified-Since date, the response is exactly the same as for a normal GET. c) If the resource entity has not been modified since a valid If-Modified-Since date, the server MUST return a 304 (not modified) response. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. Note that the Range request-header field modifies the meaning of If-Modified-Since; see section ref Range \n 18.38 for full details. Note that If-Modified-Since is ignored for generic resources. Note that If-Modified-Since times are interpreted by the server, whose clock may not be synchronized with the client. Note that if a client uses an arbitrary date in the If-Modified-Since header instead of a date taken from the Last-Modified header for the same request, the client should be aware of the fact that this date is interpreted in the server's understanding of time. The client should consider unsynchronized clocks and rounding problems due to the different representations of time between the client and server. This includes the possibility of race conditions if the document has changed between the time it was first requested and the If-Modified-Since date of a subsequent request, and the possibility of clock-skew-related problems if the If-Modified-Date date is derived from the client's clock without correction to the server's clock. Corrections for different time bases between client and server are at best approximate due to network latency. If-Match The If-Match request-header field is used with a method to make it conditional. A client that has a cache entry for the relevant entity supplies the associated entity tag using the If-Match header; if this entity tag matches the server's current entity tag for the entity, the server SHOULD perform the requested operation as if the If-Match header were not present. If the entity tags do not match, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition failed) response with no Entity-Body. This behavior is most useful when the client wants to prevent an updating method, such as PUT or POST, from modifying a resource entity that has changed since the client last checked it. When the If-Match header is used, the server should use the strong comparison function (see section ref If_Match \n 18.26) to compare entity tags. If the If-Match header is used to make a conditional request on generic resource, it may be used to pass a set of validators. This is done using the variant-set mechanism if the client has variant IDs for the corresponding cache entries (see sections REF Variant_ID_Use \n 16.5.3 and REF Variant_Sets \n 7.13 ). The server selects the appropriate variant based on other request headers; if the variant-ID for that resource entity is listed in the If-Match header, and if the entity-tag associated with that variant-ID in the header matches the current entity-tag of the resource entity, then the requested operation SHOULD be performed. Otherwise, it MUST NOT be performed. If-Match = "If-Match" ":" if-match-rhs if-match-rhs = opaque-validator | variant-set An updating request (e.g., a PUT or POST) on a generic resource should include only one variant-set-item, the one associated with the particular variant whose value is being conditionally updated. Examples of plain resource form: If-Match: "xyzzy" If-Match: "xyzzy"/W Examples of generic resource form: If-Match: "xyzzy";"4" If-Match: "xyzzy";"3", "r2d2xxxx";"5", "c3piozzzz";"7" If-Match: "xyzzy"/W; "3", "r2d2xxxx"/W; "5", "c3piozzzz"/W; "7" If the request would, without the If-Match header, result in anything other than a 2xx status, then the If-Match header is ignored. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. It is also used, on updating requests, to prevent inadvertent modification of the wrong variant of a resource. If-NoneMatch The If-NoneMatch request-header field is used with a method to make it conditional. A client that has a cache entry for the relevant entity supplies the associated entity tag using the If-NoneMatch header; if this entity tag matches the server's current entity tag for the entity, the server SHOULD return a 304 (Not Modified) response without any Entity-Body. If the entity tags do not match, the server should treat the request as if the If-NoneMatch header was not present. See section ref If_Match \n 18.26 for rules on how to determine if two entity tags match. If the If-NoneMatch header is used to make a conditional request on generic resource, it may be used to pass a set of validators. This is done using the variant-set mechanism if the client has variant IDs for the corresponding cache entries (see sections ref Variant_ID_Use \n 16.5.3 and ref Variant_Sets \n 7.13). The server selects the appropriate variant based on other request headers; if the variant-ID for that resource entity is listed in the If-NoneMatch header, and if the entity-tag associated with that variant-ID in the header matches the current entity-tag of the resource entity, then the requested operation SHOULD NOT be performed. Otherwise, it SHOULD be performed. If-NoneMatch = "If-NoneMatch" ":" if-nonematch-rhs if-nonematch-rhs = opaque-validator | variant-set Examples of plain resource form: If-NoneMatch: "xyzzy" If-NoneMatch: "xyzzy"/W Examples of generic resource form: If-NoneMatch: "xyzzy";"4" If-NoneMatch: "xyzzy";"3", "r2d2xxxx";"5", "c3piozzzz";"7" If-NoneMatch: "xyzzy"/W; "3", "r2d2xxxx"/W; "5", "c3piozzzz"/W;7 If the request would, without the If-NoneMatch header, result in anything other than a 2xx status, then the If-NoneMatch header is ignored. The purpose of this feature is to allow efficient updates of cached information with a minimum amount of transaction overhead. If-Range If a client has a partial copy of an entity in its cache, and wishes to have an up-to-date copy of the entire entity in its cache, it could use the Range request header with a conditional GET (using either or both of If-Unmodified-Since and If-Match.) However, if the condition fails because the entity has been modified, the client would then have to make a second request to obtain the entire current entity body. The If-Range header allows a client to �short-circuit� the second request. Informally, its meaning is �if the entity is unchanged, send me the part(s) that I am missing; otherwise, send me the entire new entity.'� Range-If = "Range-If" ":" (if-valid-rhs | HTTP-date) If the client has no entity tag for a plain resource, but does have a Last-Modified date, it may use that date in a If-Range header. (The server can detect this because an HTTP-date, unlike any form of if-valid-rhs, does not start with a �"� quotation mark.) Dates may only be used in If-Range for plain resources, not for generic resources. The If-Range header should only be used together with a Range header, and must be ignored if the request does not include a Range header, or if the server does not support the sub-range operation. If the entity tag given in the If-Range header matches the current entity tag for the entity, then the server should provide the specified sub-range of the entity using a 206 (Partial content) response. If the entity tag does not match, then the server should return the entire entity using a 200 (OK) response. If-Unmodified-Since The If-Unmodified-Since request-header field is used with a method to make it conditional. If the requested resource entity has not been modified since the time specified in this field, the server should perform the requested operation as if the If-Unmodified-Since header were not present. If the requested resource entity has been modified since the specified time, the server MUST NOT perform the requested operation, and MUST return a 412 (Precondition Failed) response with no Entity-Body. If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date An example of the field is: If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT If the request normally (i.e., without the If-Unmodified-Since header) would result in anything other than a 2xx status, the If-Unmodified-Since header should be ignored. If the specified date is invalid, the header is ignored. Last-Modified The Last-Modified entity-header field indicates the date and time at which the sender believes the resource entity was last modified. The exact semantics of this field are defined in terms of how the recipient SHOULD interpret it: if the recipient has a copy of this resource entity which is older than the date given by the Last-Modified field, that copy SHOULD be considered stale. Last-Modified = "Last-Modified" ":" HTTP-date An example of its use is Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT The exact meaning of this header field depends on the implementation of the sender and the nature of the original resource. For files, it may be just the file system last-modified time. For entities with dynamically included parts, it may be the most recent of the set of last-modify times for its component parts. For database gateways, it may be the last-update time stamp of the record. For virtual objects, it may be the last time the internal state changed. An origin server MUST NOT send a Last-Modified date which is later than the server's time of message origination. In such cases, where the resource's last modification would indicate some time in the future, the server MUST replace that date with the message origination date. An origin server should obtain the Last-Modified value of the entity as close as possible to the time that it generates the Date value of its response. This allows a recipient to make an accurate assessment of the entity's modification time, especially if the entity changes near the time that the response is generated. Location The Location response-header field is used to redirect the recipient to a location other than the Request-URI for completion of the request or identification of a new resource. For 201 (created) responses, the Location is that of the new resource which was created by the request. For 3xx responses, the location SHOULD indicate the server's preferred URL for automatic redirection to the resource. The field value consists of a single absolute URL. Location = "Location" ":" absoluteURI An example is Location: http://www.w3.org/pub/WWW/People.html Note: The Content-Location header field (section ref Content_Location \n 18.16) differs from Location in that the Content-Location identifies the original location of the entity enclosed in the request. It is therefore possible for a response to contain header fields for both Location and Content-Location. Max-Forwards The Max-Forwards general-header field may be used with the TRACE method (section ref Max_Forwards \n 18.32) to limit the number of times that a proxy or gateway can forward the request to the next inbound server. This can be useful when the client is attempting to trace a request chain which appears to be failing or looping in mid-chain. Max-Forwards = "Max-Forwards" ":" 1*DIGIT The Max-Forwards value is a decimal integer indicating the remaining number of times this request message may be forwarded. Each proxy or gateway recipient of a TRACE request containing a Max-Forwards header field SHOULD check and update its value prior to forwarding the request. If the received value is zero (0), the recipient SHOULD NOT forward the request; instead, it SHOULD respond as the final recipient with a 200 (OK) response containing the received request message as the response entity body (as described in Section ref TRACE \n 13.7). If the received Max-Forwards value is greater than zero, then the forwarded message SHOULD contain an updated Max-Forwards field with a value decremented by one (1). The Max-Forwards header field SHOULD be ignored for all other methods defined by this specification and for any extension methods for which it is not explicitly referred to as part of that method definition. Persist When the Persist connection-token has been transmitted with a request or a response a Persist header field MAY also be included. The Persist header field takes the following form: Persist-header = "Persist" ":" 0#pers-param pers-param = param-name "=" word param-name = token The Persist header itself is optional, and is used only if a parameter is being sent. HTTP/1.1 does not define any parameters. If the Persist header is sent, the corresponding connection token MUST be transmitted. The Persist header MUST be ignored if received without the connection token. Pragma The Pragma general-header field is used to include implementation-specific directives that may apply to any recipient along the request/response chain. All pragma directives specify optional behavior from the viewpoint of the protocol; however, some systems MAY require that behavior be consistent with the directives. Pragma = "Pragma" ":" 1#pragma-directive pragma-directive = "no-cache" | extension-pragma extension-pragma = token [ "=" word ] When the �no-cache� directive is present in a request message, an application SHOULD forward the request toward the origin server even if it has a cached copy of what is being requested. This pragma directive has the same semantics as the �no-cache� cache-directive (see section ref Cache_Control \n 18.10) and is defined here for backwards compatibility with HTTP/1.0. Clients SHOULD include both header fields when a �no-cache� request is sent to a server not known to be HTTP/1.1 compliant. Pragma directives MUST be passed through by a proxy or gateway application, regardless of their significance to that application, since the directives may be applicable to all recipients along the request/response chain. It is not possible to specify a pragma for a specific recipient; however, any pragma directive not relevant to a recipient SHOULD be ignored by that recipient. HTTP/1.1 clients SHOULD NOT send the Pragma request header. HTTP/1.1 caches SHOULD treat �Pragma: no-cache� as if the client had sent �Cache-control: no-cache�. No new Pragma directives will be defined in HTTP. Proxy-Authenticate The Proxy-Authenticate response-header field MUST be included as part of a 407 (Proxy Authentication Required) response. The field value consists of a challenge that indicates the authentication scheme and parameters applicable to the proxy for this Request-URI. Proxy-Authentication = "Proxy-Authentication" ":" challenge The HTTP access authentication process is described in section ref AA \n 14. Unlike WWW-Authenticate, the Proxy-Authenticate header field applies only to the current connection and MUST NOT be passed on to downstream clients. Proxy-Authorization The Proxy-Authorization request-header field allows the client to identify itself (or its user) to a proxy which requires authentication. The Proxy-Authorization field value consists of credentials containing the authentication information of the user agent for the proxy and/or realm of the resource being requested. Proxy-Authorization = "Proxy-Authorization" ":" credentials The HTTP access authentication process is described in section ref AA \n 14. Unlike Authorization, the Proxy-Authorization applies only to the current connection and MUST NOT be passed on to upstream servers. If a request is authenticated and a realm specified, the same credentials SHOULD be valid for all other requests within this realm. Public The Public response-header field lists the set of non-standard methods supported by the server. The purpose of this field is strictly to inform the recipient of the capabilities of the server regarding unusual methods. The methods listed may or may not be applicable to the Request-URI; the Allow header field (section ref Allow \n 18.7) SHOULD be used to indicate methods allowed for a particular URI. This does not prevent a client from trying other methods. The field value SHOULD not include the methods predefined for HTTP/1.1 in section ref Method \n 9.1.1. Public = "Public" ":" 1#method Example of use: Public: OPTIONS, MGET, MHEAD This header field applies only to the server directly connected to the client (i.e., the nearest neighbor in a chain of connections). If the response passes through a proxy, the proxy MUST either remove the Public header field or replace it with one applicable to its own capabilities. Range HTTP retrieval requests using conditional or unconditional GET methods may request one or more sub-ranges of the entity, instead of the entire entity. This is done using the Range request header: Range = "Range" ":" ranges-specifier A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges whenever possible, since this supports efficient recovery from partially failed transfers, and it supports efficient partial retrieval of large entities. If the server supports the Range header and the specified range or ranges are appropriate for the entity: � The presence of a Range header in an unconditional GET modifies what is returned if the GET is otherwise successful. In other words, the response carries a status code of 206 (Partial Content) instead of 200 (OK). � The presence of a Range header in a conditional GET (a request using one or both of If-Modified-Since and If-NoneMatch, or one or both of If-Unmodified-Since and If-Match) modifies what is returned if the GET is otherwise successful and the condition is true. It does not affect the 304 (Not Modified) response returned if the conditional is false. In some cases, it may be more appropriate to use the If-Range header (see section ref Range_If \n 18.28) in addition to the Range header. Referer The Referer[sic] request-header field allows the client to specify, for the server's benefit, the address (URI) of the resource from which the Request-URI was obtained. This allows a server to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. The Referer field MUST NOT be sent if the Request-URI was obtained from a source that does not have its own URI, such as input from the user keyboard. Referer = "Referer" ":" ( absoluteURI | relativeURI ) Example: Referer: http://www.w3.org/hypertext/DataSources/Overview.html If a partial URI is given, it SHOULD be interpreted relative to the Request-URI. The URI MUST NOT include a fragment. Note: Because the source of a link may be private information or may reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information. Retry-After The Retry-After response-header field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response. Retry-After = "Retry-After" ":" ( HTTP-date | delta-seconds ) Two examples of its use are Retry-After: Wed, 14 Dec 1994 18:22:54 GMT Retry-After: 120 In the latter example, the delay is 2 minutes. Server The Server response-header field contains information about the software used by the origin server to handle the request. The field can contain multiple product tokens (section ref Product \n 7.8) and comments identifying the server and any significant subproducts. By convention, the product tokens are listed in order of their significance for identifying the application. Server = "Server" ":" 1*( product | comment ) Example: Server: CERN/3.0 libwww/2.17 If the response is being forwarded through a proxy, the proxy application MUST NOT add its data to the product list. Instead, it SHOULD include a Via field (as described in section ref Via \n 18.47). Note: Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Server implementers are encouraged to make this field a configurable option. Title The Title entity-header field indicates the title of the entity Title = "Title" ":" *TEXT An example of the field is Title: Hypertext Transfer Protocol -- HTTP/1.1 This field is isomorphic with the element in HTML private HREF="#RefHTML"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [5]. Transfer Encoding The Transfer-Encoding general-header field indicates what (if any) type of transformation has been applied to the message body in order to safely transfer it between the sender and the recipient. This differs from the Content-Encoding in that the transfer coding is a property of the message, not of the original resource entity. Transfer-Encoding = "Transfer-Encoding" ":" 1#transfer-coding Transfer codings are defined in section ref Transfer_Codings \n 7.6. An example is: Transfer-Encoding: chunked Many older HTTP/1.0 applications do not understand the Transfer-Encoding header. Upgrade The Upgrade general-header allows the client to specify what additional communication protocols it supports and would like to use if the server finds it appropriate to switch protocols. The server MUST use the Upgrade header field within a 101 (Switching Protocols) response to indicate which protocol(s) are being switched. Upgrade = "Upgrade" ":" 1#product For example, Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11 The Upgrade header field is intended to provide a simple mechanism for transition from HTTP/1.1 to some other, incompatible protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a �better� protocol if available (where �better� is determined by the server, possibly according to the nature of the method and/or resource being requested). The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request containing the Upgrade header field. The Upgrade header field only applies to the immediate connection. Therefore, the �upgrade� keyword MUST be supplied within a Connection header field (section ref Connection \n 18.11) whenever Upgrade is present in an HTTP/1.1 message. The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 301, 302, 303, or 305 redirection response. This specification only defines the protocol name �HTTP� for use by the family of Hypertext Transfer Protocols, as defined by the HTTP version rules of section ref HTTP_Version \n 7.1 and future updates to this specification. Any token can be used as a protocol name; however, it will only be useful if both the client and server associate the name with the same protocol. User-Agent The User-Agent request-header field contains information about the user agent originating the request. This is for statistical purposes, the tracing of protocol violations, and automated recognition of user agents for the sake of tailoring responses to avoid particular user agent limitations. Although it is not required, user agents SHOULD include this field with requests. The field can contain multiple product tokens (section ref Product \n 7.8) and comments identifying the agent and any subproducts which form a significant part of the user agent. By convention, the product tokens are listed in order of their significance for identifying the application. User-Agent = "User-Agent" ":" 1*( product | comment ) Example: User-Agent: CERN-LineMode/2.15 libwww/2.17b3 Vary The Vary response-header field is used by an origin server to signal that the resource identified by the current request is a generic) resource. A generic resource has multiple entities associated with it, all of which are representations of the content of the resource. If a GET or HEAD request on a generic resource is received, the origin server will select one of the associated entities as the entity best matching the request. Selection of this entity is based on the contents of particular header fields in the request message, or on other information pertaining to the request, like the network address of the sending client. A resource being generic has an important effect on cache management, particularly for caching proxies which service a diverse set of user agents. All 200 (OK) responses from generic resources MUST contain at least one Vary header (section ref Vary \n 18.46) or Alternates header (section ref Alternates \n 18.8) to signal variance. If no Vary headers and no Alternates headers are present in a 200 (OK) response, then caches may assume, as long as the response is fresh, that the resource in question is plain, and has only one associated entity. Note however that this entity can still change through time, as possibly indicated by a Cache-Control response header (section ref Cache_Control \n 18.10). After selection of the entity best matching the current request, the origin server will usually generate a 200 (OK) response, but it can also generate other responses like 206 (Partial Content) or 304 (Not Modified) if headers which modify the semantics of the request, like Range (section ref Range \n 18.38) or If-Match (section ref If_Match \n 18.26), are present. An origin server need not be capable of selecting an entity for every possible incoming request on a generic resource; it can choose to generate a 3xx (redirection) or 4xx (client error) type response for some requests. In a request message on a generic resource, the selecting request headers are those request headers whose contents were used by the origin server to select the entity best matching the request. The Vary header field specifies the selecting request headers and any other selection parameters that were used by the origin server. Vary = "Vary" ":" 1#selection-parameter selection-parameter = request-header-name | "{accept-headers}" | "{other}" | "{" extension-parameter "}" request-header-name = field-name extension-parameter = token The presence of a request-header-name signals that the request-header field with this name is selecting. Note that the name need not belong to a request-header field defined in this specification, and that header names are case-insensitive. The presence of the �{accept-headers}� parameter signals that all request headers whose names start with �accept� are selecting. The inclusion of the �{other}� parameter in a Vary field signals that parameters other than the contents of request headers, for example the network address of the sending party, play a role in the selection of the response. Note: This specification allows the origin server to express that other parameters were used, but does not allow the origin server to specify the exact nature of these parameters. This is left to future extensions. If an extension-parameter unknown to the cache is present in a Vary header, the cache MUST treat it as the �{other}� parameter. If multiple Vary and Alternates header fields are present in a response, these MUST be combined to give all selecting parameters. The field name �Host� MUST never be included in a Vary header; clients MUST ignore it if it is present. The names of fields which change the semantics of a GET request, like �Range� and �If-Match� MUST also never be included, and MUST be ignored when present. Servers which use access authentication are not obliged to send �Vary: Authorization� headers in responses. It MUST be assumed that requests on authenticated resources can always produce different responses for different users. Note that servers can signal the absence of authentication by including �Cache-Control: public� header in the response. A cache MAY store and refresh 200 (OK) responses from a generic resource according to the rules in section ref Constructing_Respons\n 16.4. The partial entities in 206 (Partial Content) responses from generic resources MAY also be used by the cache. When getting a request on a generic resource, a cache can only return a cached 200 (OK) response to one of its clients in two particular cases. First, if a cache gets a request on a generic resource for which it has cached one or more responses with Vary or Alternates headers, it can relay that request towards the origin server, adding an If-NoneMatch header listing the etag-info values in the ETag headers (section ref CVal \n Error! Reference source not found.) of the cached responses which have variant-IDs. If it then gets back a 304 (Not Modified) response with the etag-info of a cached 200 (OK) response in its ETag header, it can return this cached 200 (OK) response to its client, after merging in any of the 304 response headers as specified in section ref Non_Modifiable_Headr\n 16.4.2. Second, if a cache gets a request on a generic resource, it can return to its client a cached, fresh 200 (OK) response which has Vary or Alternates headers, provided that � the Vary and Alternates headers of this fresh response specify that only request header fields are selecting parameters, � the specified selecting request header fields of the current request match the specified selecting request header fields of a previous request on the resource relayed towards the origin server, � this previous request got a 200 (OK) or 304 (Not Modified) response which had the same etag-info value in its ETag header as the cached, fresh 200 (OK) response. Two sequences of selecting request header fields match if and only if the first sequence can be transformed into the second sequence by only adding or removing whitespace at places in fields where this is allowed according to the syntax rules in this specification. If a cached 200 (OK) response MAY be returned to a request on a generic resource which includes a Range request header, then a cache MAY also use this 200 (OK) response to construct and return a 206 (Partial Content) response with the requested range. Note: Implementation of support for the second case above is mainly interesting in user agent caches, as a user agent cache will generally have an easy way of determining whether the sequence of request header fields of the current request equals the sequence sent in an earlier request on the same resource. Proxy caches supporting the second case would have to record diverse sequences of request header fields previously relayed; the implementation effort associated with this may not be balanced by a sufficient payoff in traffic savings. A planned specification of a content negotiation mechanism will define additional cases in which proxy caches can return a cached 200 (OK) response without contacting the origin server. The implementation effort associated with support for these additional cases is expected to have a much better cost/benefit ratio. Via The Via general-header field MUST be used by gateways and proxies to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses. It is analogous to the �Received� field of RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9] and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain. Via = "Via" ":" 1#( received-protocol received-by [ comment ] ) received-protocol = [ protocol-name "/" ] protocol-version protocol-name = token protocol-version = token received-by = ( host [ ":" port ] ) | pseudonym pseudonym = token The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients. The protocol-name is optional if and only if it would be �HTTP�. The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol. Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications. Comments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message. For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named �fred�, which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by forwarding it to the origin server at www.ics.uci.edu. The request received by www.ics.uci.edu would then have the following Via header field: Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1) Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports of hosts within the firewall region. This information SHOULD only be propagated if explicitly enabled. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host. For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example, Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy could be collapsed to Via: 1.0 ricky, 1.1 mertz, 1.0 lucy Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the hosts have already been replaced by pseudonyms. Applications MUST NOT combine entries which have different received-protocol values. Note: The Via header field replaces the Forwarded header field which was present in earlier drafts of this specification. Warning Warning headers are sent with responses using: Warning = "Warning" ":" warn-code SP warn-agent SP warn-text warn-code = 2DIGIT warn-agent = ( host [ ":" port ] ) | pseudonym ; the name or pseudonym of the server adding ; the Warning header, for use in debugging warn-text = quoted-string A response may carry more than one Warning header. The warn-text should be in a natural language and character set that is most likely to be intelligible to the human user receiving the response. This decision may be based on any available knowledge, such as the location of the cache or user, the Accept-Language field in a request, the Content-Language field in a response, etc. The default language is English and the default character set is ISO-8599-1. If a character set other than ISO-8599-1 is used, it must be encoded in the warn-text using the method described in RFC 1522 [14]. Any server or cache may add Warning headers to a response. New Warning headers should be added after any existing Warning headers. A cache MUST NOT delete any Warning header that it received with a response. However, if a cache successfully validates a cache entry, it SHOULD remove any Warning headers previously attached to that entry. It MUST then add any Warning headers received in the validating response. In other words, Warning headers are those that would be attached to the most recent relevant response. When multiple Warning headers are attached to a response, the user agent SHOULD display as many of them as possible, in the order that they appear in the response. If it is not possible to display all of the warnings, the user agent should follow these heuristics: � Warnings that appear early in the response take priority over those appearing later in the response. � Warnings in the user's preferred character set take priority over warnings in other character sets but with identical warn-codes and warn-agents. Systems that generate multiple Warning headers should order them with this user-agent behavior in mind. This is a list of the currently-defined warn-codes, each with a recommended warn-text in English, and a description of its meaning. 10 Response is stale MUST be included whenever the returned response is stale. A cache may add this warning to any response, but may never remove it until the response is known to be fresh. 11 Revalidation failed MUST be included if a cache returns a stale response because an attempt to revalidate the response failed, due to an inability to reach the server. A cache may add this warning to any response, but may never remove it until the response is successfully revalidated. 12 Disconnected operation SHOULD be included if the cache is intentionally disconnected from the rest of the network for a period of time. 99 Miscellaneous warning The warning text may include arbitrary information to be presented to a human user, or logged. A system receiving this warning MUST NOT take any automated action. WWW-Authenticate The WWW-Authenticate response-header field MUST be included in 401 (Unauthorized) response messages. The field value consists of at least one challenge that indicates the authentication scheme(s) and parameters applicable to the Request-URI. WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge The HTTP access authentication process is described in section ref AA \n 14. User agents MUST take special care in parsing the WWW-Authenticate field value if it contains more than one challenge, or if more than one WWW-Authenticate header field is provided, since the contents of a challenge may itself contain a comma-separated list of authentication parameters. Security Considerations This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.1 as described by this document. The discussion does not include definitive solutions to the problems revealed, though it does make some suggestions for reducing security risks. Authentication of Clients As mentioned in section ref AA \n 14, the Basic authentication scheme is not a secure method of user authentication, nor does it in any way protect the Entity-Body, which is transmitted in clear text across the physical network used as the carrier. HTTP does not prevent additional authentication schemes and encryption mechanisms from being employed to increase security or the addition of enhancements (such as schemes to use one-time passwords) to Basic authentication. The most serious flaw in Basic authentication is that it results in the essentially clear text transmission of the user's password over the physical network. It is this problem which Digest Authentication attempts to address. Because Basic authentication involves the clear text transmission of passwords it SHOULD never be used (without enhancements) to protect sensitive or valuable information. A common use of Basic authentication is for identification purposes -- requiring the user to provide a user name and password as a means of identification, for example, for purposes of gathering accurate usage statistics on a server. When used in this way it is tempting to think that there is no danger in its use if illicit access to the protected documents is not a major concern. This is only correct if the server issues both user name and password to the users and in particular does not allow the user to choose his or her own password. The danger arises because naive users frequently reuse a single password to avoid the task of maintaining multiple passwords. If a server permits users to select their own passwords, then the threat is not only illicit access to documents on the server but also illicit access to the accounts of all users who have chosen to use their account password. If users are allowed to choose their own password that also means the server must maintain files containing the (presumably encrypted) passwords. Many of these may be the account passwords of users perhaps at distant sites. The owner or administrator of such a system could conceivably incur liability if this information is not maintained in a secure fashion. Basic Authentication is also vulnerable to spoofing by counterfeit servers. If a user can be led to believe that he is connecting to a host containing information protected by basic authentication when in fact he is connecting to a hostile server or gateway then the attacker can request a password, store it for later use, and feign an error. This type of attack is not possible with Digest Authentication[26]. Server implementers SHOULD guard against the possibility of this sort of counterfeiting by gateways or CGI scripts. In particular it is very dangerous for a server to simply turn over a connection to a gateway since that gateway can then use the persistent connection mechanism to engage in multiple transactions with the client while impersonating the original server in a way that is not detectable by the client. Safe Methods The writers of client software should be aware that the software represents the user in their interactions over the Internet, and should be careful to allow the user to be aware of any actions they may take which may have an unexpected significance to themselves or others. In particular, the convention has been established that the GET and HEAD methods should never have the significance of taking an action other than retrieval. These methods should be considered �safe. � This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested. Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them. Abuse of Server Log Information A server is in the position to save personal data about a user's requests which may identify their reading patterns or subjects of interest. This information is clearly confidential in nature and its handling may be constrained by law in certain countries. People using the HTTP protocol to provide data are responsible for ensuring that such material is not distributed without the permission of any individuals that are identifiable by the published results. Transfer of Sensitive Information Like any generic data transfer protocol, HTTP cannot regulate the content of the data that is transferred, nor is there any a priori method of determining the sensitivity of any particular piece of information within the context of any given request. Therefore, applications SHOULD supply as much control over this information as possible to the provider of that information. Four header fields are worth special mention in this context: Server, Via, Referer and From. Revealing the specific software version of the server may allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementers SHOULD make the Server header field a configurable option. Proxies which serve as a portal through a network firewall SHOULD take special precautions regarding the transfer of header information that identifies the hosts behind the firewall. In particular, they SHOULD remove, or replace with sanitized versions, any Via fields generated behind the firewall. The Referer field allows reading patterns to be studied and reverse links drawn. Although it can be very useful, its power can be abused if user details are not separated from the information contained in the Referer. Even when the personal information has been removed, the Referer field may indicate a private document's URI whose publication would be inappropriate. The information sent in the From field might conflict with the user's privacy interests or their site's security policy, and hence it SHOULD NOT be transmitted without the user being able to disable, enable, and modify the contents of the field. The user MUST be able to set the contents of this field within a user preference or application defaults configuration. We suggest, though do not require, that a convenient toggle interface be provided for the user to enable or disable the sending of From and Referer information. Attacks Based On File and Path Names Implementations of HTTP origin servers SHOULD be careful to restrict the documents returned by HTTP requests to be only those that were intended by the server administrators. If an HTTP server translates HTTP URIs directly into file system calls, the server MUST take special care not to serve files that were not intended to be delivered to HTTP clients. For example, UNIX, Microsoft Windows, and other operating systems use �..� as a path component to indicate a directory level above the current one. On such a system, an HTTP server MUST disallow any such construct in the Request-URI if it would otherwise allow access to a resource outside those intended to be accessible via the HTTP server. Similarly, files intended for reference only internally to the server (such as access control files, configuration files, and script code) MUST be protected from inappropriate retrieval, since they might contain sensitive information. Experience has shown that minor bugs in such HTTP server implementations have turned into security risks. Personal Information HTTP clients are often privy to large amounts of personal information (e.g. the user's name, location, mail address, passwords, encryption keys, etc.), and SHOULD be very careful to prevent unintentional leakage of this information via the HTTP protocol to other sources. We very strongly recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers and implementers be particularly careful in this area. History shows that errors in this area are often both serious security and/or privacy problems, and often generate very adverse publicity for the implementer's company. Privacy Issues Connected to Accept headers Accept request headers can reveal information about the user to all servers which are accessed. The Accept-Language header in particular can reveal information the user would consider to be of a private nature, because the understanding of particular languages is often strongly correlated to the membership of a particular ethnic group. User agents which offer the option to configure the contents of an Accept-Language header to be sent in every request are strongly encouraged to let the configuration process include a message which makes the user aware of the loss of privacy involved. An approach that limits the loss of privacy would be for a user agent to omit the sending of Accept-Language headers by default, and to ask the user whether it should start sending Accept-Language headers to a server if it detects, by looking for any Vary or Alternates response headers generated by the server, that such sending could improve the quality of service. Elaborate user-customized accept header fields sent in every request, in particular if these include quality values, can be used by servers as relatively reliable and long-lived user identifiers. Such user identifiers would allow content providers to do click-trail tracking, and would allow collaborating content providers to match cross-server click-trails or form submissions of individual users. Note that for many users not behind a proxy, the network address of the host running the user agent will also serve as a long-lived user identifier. In environments where proxies are used to enhance privacy, user agents should be conservative in offering accept header configuration options to end users. As an extreme privacy measure, proxies could filter the accept headers in relayed requests. General purpose user agents which provide a high degree of header configurability should warn users about the loss of privacy which can be involved. DNS Spoofing Clients using HTTP rely heavily on the Domain Name Service, and are thus generally prone to security attacks based on the deliberate miss-association of IP addresses and DNS names. The deployment of DNSSECprivate HREF="#RefDNSSEC" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [27] should help this situation. In advance of this deployment, however, clients need to be cautious in assuming the continuing validity of an IP number/DNS name association. In particular, HTTP clients SHOULD rely on their name resolver for confirmation of an IP number/DNS name association, rather than caching the result of previous host name lookups. Many platforms already can cache host name lookups locally when appropriate, and they SHOULD be configured to do so. These lookups should be cached, however, only when the TTL (Time To Live) information reported by the name server makes it likely that the cached information will remain useful. If HTTP clients cache the results of host name lookups in order to achieve a performance improvement, they MUST observe the TTL information reported by DNS. If HTTP clients do not observe this rule, they could be spoofed when a previously-accessed server's IP address changes. As renumbering is expected to become increasingly commonprivate HREF="#Ref1900" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [24], the possibility of this form of attack will grow. Observing this requirement thus reduces this potential security vulnerability. This requirement also improves the load-balancing behavior of clients for replicated servers using the same DNS name and reduces the likelihood of a user's experiencing failure in accessing sites which use that strategy. Location Headers and Spoofing If a single server supports multiple organizations that do not trust one another, then it must check the values of Location and Content-Location headers in responses that are generated under control of said organizations to make sure that they do not attempt to invalidate resources over which they have no authority. Acknowledgments This specification makes heavy use of the augmented BNF and generic constructs defined by David H. Crocker for RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]. Similarly, it reuses many of the definitions provided by Nathaniel Borenstein and Ned Freed for MIME private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]. We hope that their inclusion in this specification will help reduce past confusion over the relationship between HTTP and Internet mail message formats. The HTTP protocol has evolved considerably over the past four years. It has benefited from a large and active developer community--the many people who have participated on the www-talk mailing list--and it is that community which has been most responsible for the success of HTTP and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau, Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M. Hallam-Baker, H�kon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli, Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special recognition for their efforts in defining early aspects of the protocol. This document has benefited greatly from the comments of all those participating in the HTTP-WG. In addition to those already mentioned, the following individuals have contributed to this specification: Gary Adams Harald Tveit Alvestrand Keith Ball Brian Behlendorf Paul Burchard Maurizio Codogno Mike Cowlishaw Roman Czyborra Michael A. Dolan Alan Freier Marc Hedlund Koen Holtman Alex Hopmann Bob Jernigan Shel Kaphan Rohit Khare Martijn Koster Alexei Kosut David M. Kristol Daniel LaLiberte Paul J. Leach Albert Lunde John C. Mallery Jean-Philippe Martin-Flatin Larry Masinter Mitra Gavin Nicol Scott Powers Bill Perry Jeffrey Perry Owen Rees Luigi Rizzo David Robinson Marc Salomon Rich Salz Jim Seidman Chuck Shotton Eric W. Sink Simon E. Spero Richard N. Taylor Robert S. Thau Fran�ois Yergeau Mary Ellen Zurko David Morris Greg Herlihy Bill (BearHeart) Weinman Allan M. Schiffman Much of the content and presentation of the caching design is due to suggestions and comments from individuals including: Shel Kaphan, Paul Leach, Koen Holtman, David Morris, Larry Masinter, and Roy Fielding. Most of the specification of ranges is based on work originally done by Ari Luotonen and John Franks, with additional input from Steve Zilles and Roy Fielding. XXX need acks for subgroup work. References [1] H. Alvestrand. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1766.txt" MACROBUTTON HtmlResAnchor Tags for the identification of languages.� RFC 1766, UNINETT, March 1995. [2] F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey, B. Alberti. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1436.txt" MACROBUTTON HtmlResAnchor The Internet Gopher Protocol: (a distributed document search and retrieval protocol)�, RFC 1436, University of Minnesota, March 1993. [3] T. Berners-Lee. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1630.txt" MACROBUTTON HtmlResAnchor Universal Resource Identifiers in WWW� A Unifying Syntax for the Expression of Names and Addresses of Objects on the Network as used in the World-Wide Web.� RFC 1630, CERN, June 1994. [4] T. Berners-Lee, L. Masinter, M. McCahill. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1738.txt" MACROBUTTON HtmlResAnchor Uniform Resource Locators (URL).� RFC 1738, CERN, Xerox PARC, University of Minnesota, December 1994. [5] T. Berners-Lee, D. Connolly. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1866.txt" MACROBUTTON HtmlResAnchor HyperText Markup Language Specification - 2.0.� RFC 1866, MIT/LCS, November 1995. [6] T. Berners-Lee, R. Fielding, H. Frystyk. PRIVATE HREF="http://www.ics.uci.edu/pub/ietf/http/"MACROBUTTON HtmlResAnchor "Hypertext Transfer Protocol - HTTP/1.0." Work in Progress (draft-ietf-http-v10-spec-04.txt), MIT/LCS, UC Irvine, September 1995. [7] N. Borenstein, N. Freed. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1521.ps" MACROBUTTON HtmlResAnchor MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies." RFC 1521, Bellcore, Innosoft, September 1993. [8] R. Braden. � PRIVATE HREF="http://ds.internic.net/std/std3.txt" MACROBUTTON HtmlResAnchor Requirements for Internet hosts - application and support.� STD 3, RFC 1123, IETF, October 1989. [9] D. H. Crocker. � PRIVATE HREF="http://ds.internic.net/std/std11.txt" MACROBUTTON HtmlResAnchor Standard for the Format of ARPA Internet Text Messages.� STD 11, RFC 822, UDEL, August 1982. [10] F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J. Sui, M. Grinbaum. �WAIS Interface Protocol Prototype Functional Specification.� (v1.5), Thinking Machines Corporation, April 1990. [11] R. Fielding. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1808.txt" MACROBUTTON HtmlResAnchor Relative Uniform Resource Locators.� RFC 1808, UC Irvine, June 1995. [12] M. Horton, R. Adams. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1036.txt" MACROBUTTON HtmlResAnchor Standard for interchange of USENET messages.� RFC 1036 (Obsoletes RFC 850), AT&T Bell Laboratories, Center for Seismic Studies, December 1987. [13] B. Kantor, P. Lapsley. � PRIVATE HREF="http://ds.internic.net/rfc/rfc977.txt" MACROBUTTON HtmlResAnchor Network News Transfer Protocol A Proposed Standard for the Stream-Based Transmission of News.� RFC 977, UC San Diego, UC Berkeley, February 1986. [14] K. Moore. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1522.txt" MACROBUTTON HtmlResAnchor MIME (Multipurpose Internet Mail Extensions) Part Two : Message Header Extensions for Non-ASCII Text.� RFC 1522, University of Tennessee, September 1993. [15] E. Nebel, L. Masinter. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1867.txt" MACROBUTTON HtmlResAnchor Form-based File Upload in HTML.� RFC 1867, Xerox Corporation, November 1995. [16] J. Postel. � PRIVATE HREF="http://ds.internic.net/std/std10.txt" MACROBUTTON HtmlResAnchor Simple Mail Transfer Protocol.� STD 10, RFC 821, USC/ISI, August 1982. [17] J. Postel. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1590.txt" MACROBUTTON HtmlResAnchor Media Type Registration Procedure.� RFC 1590, USC/ISI, March 1994. [18] J. Postel, J. K. Reynolds. � PRIVATE HREF="http://ds.internic.net/std/std9.txt" MACROBUTTON HtmlResAnchor File Transfer Protocol (FTP)� STD 9, RFC 959, USC/ISI, October 1985. [19] J. Reynolds, J. Postel. � PRIVATE HREF="http://ds.internic.net/std/std2.txt" MACROBUTTON HtmlResAnchor Assigned Numbers.� STD 2, RFC 1700, USC/ISI, October 1994. [20] K. Sollins, L. Masinter. � PRIVATE HREF="http://ds.internic.net/rfc/rfc1737.txt" MACROBUTTON HtmlResAnchor Functional Requirements for Uniform Resource Names.� RFC 1737, MIT/LCS, Xerox Corporation, December 1994. [21] US-ASCII. Coded Character Set - 7-Bit American Standard Code for Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986. [22] ISO-8859. International Standard -- Information Processing -- 8-bit Single-Byte Coded Graphic Character Sets -- Part 1: Latin alphabet No. 1, ISO 8859-1:1987. Part 2: Latin alphabet No. 2, ISO 8859-2, 1987. Part 3: Latin alphabet No. 3, ISO 8859-3, 1988. Part 4: Latin alphabet No. 4, ISO 8859-4, 1988. Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988. Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987. Part 7: Latin/Greek alphabet, ISO 8859-7, 1987. Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988. Part 9: Latin alphabet No. 5, ISO 8859-9, 1990. [23] Meyers, M. Rose � PRIVATE HREF="http://ds.internic.net/rfc/rfc1864.txt" MACROBUTTON HtmlResAnchor The Content-MD5 Header Field.� RFC 1864, Carnegie Mellon, Dover Beach Consulting, October, 1995. [24] B. Carpenter, Y. Rekhter, � PRIVATE HREF="http://ds.internic.net/rfc/rfc1900.txt" MACROBUTTON HtmlResAnchor Renumbering Needs Work�. RFC 1900, IAB, February 1996. [25] Gzip is available from the GNU project at <URL: PRIVATE HREF="ftp://prep.ai.mit.edu/pub/gnu/" MACROBUTTON HtmlResAnchor ftp://prep.ai.mit.edu/pub/gnu/>. A more formal specification is currently a work in progress. [26] Work In Progress for Digest authentication of the IETF HTTP working group. [27] TBS, Work in progress (XXX should put RFC in here� ) [28] Mills, D, � PRIVATE HREF="http://ds.internic.net/rfc/rfc1305.txt" MACROBUTTON HtmlResAnchor Network Time Protocol, Version 3�, Specification, Implementation and Analysis RFC 1305, University of Delaware, March, 1992. [29] Work in progress of the HTTP working group (XXX is this correct reference for incomplete work?). [30] S. Spero. �Analysis of HTTP Performance Problems� URL:http://sunsite.unc.edu/mdma-release/http-prob.html [31] E. Rescorla, A. Schiffman �The Secure HyperText Transfer Protocol� Internet-Draft (work in progress). [32] A. Freier, P Karlton, P. Kocher. �SSL Version 3.0" Internet-Draft� (work in progress). [33] Jeffrey C. Mogul. �The Case for Persistent-Connection HTTP�. In Proc.SIGCOMM '95 Symposium on Communications Architectures and Protocols, pages 299-313. Cambridge, MA, August, 1995. [34] Jeffrey C. Mogul. �The Case for Persistent-Connection HTTP�. Research, Report 95/4, Digital Equipment Corporation Western Research Laboratory, May, 1995., <URL: PRIVATE HREF="http://www.research.digital.com/wrl/techreports/abstracts/95.4.html" MACROBUTTON HtmlResAnchor http://www.research.digital.com/wrl/techreports/abstracts/95.4.html> [35] Work in progress of the HTTP working group on state management. Authors' Addresses private HREF="http://www.ics.uci.edu/\~fielding/"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor Roy T. Fielding PRIVATE HREF="http://www.ics.uci.edu/\~fielding/"MACROBUTTON HtmlResAnchor Roy T. Fielding Department of Information and Computer Science University of California Irvine, CA 92717-3425, USA Fax: +1 (714) 824-4056 Email: fielding@ics.uci.edu PRIVATE HREF="http://www.w3.org/pub/WWW/People/Frystyk/"MACROBUTTON HtmlResAnchor Henrik Frystyk Nielsen W3 Consortium MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, USA Fax: +1 (617) 258 8682 Email: frystyk@w3.org PRIVATE HREF="http://www.w3.org/pub/WWW/People/Berners-Lee/"MACROBUTTON HtmlResAnchor Tim Berners-Lee Director, W3 Consortium MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, USA Fax: +1 (617) 258 8682 Email: timbl@w3.org PRIVATE HREF="http://www.w3.org/pub/WWW/People/Gettys/" MACROBUTTON HtmlResAnchor Jim Gettys MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139, USA Fax: +1 (617) 258 8682 Email: jg@w3.org PRIVATE HREF="http://www.research.digital.com/wrl/people/mogul/bio.html" MACROBUTTON HtmlResAnchor Jeffrey C. Mogul Western Research Laboratory Digital Equipment Corporation 250 University Avenue Palo Alto, California, 94305, U.S.A. Email: mogul@wrl.dec.com Appendices These appendices are provided for informational reasons only -- they do not form a part of the HTTP/1.1 specification. Internet Media Type message/http In addition to defining the HTTP/1.1 protocol, this document serves as the specification for the Internet media type �message/http�. The following is to be registered with IANA private HREF="#RefMediaType"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [17]. Media Type name: message Media subtype name: http Required parameters: none Optional parameters: version, msgtype version: The HTTP-Version number of the enclosed message (e.g., "1.1"). If not present, the version can be determined from the first line of the body. msgtype: The message type -- "request" or "response". If not present, the type can be determined from the first line of the body. Encoding considerations: only "7bit", "8bit", or "binary" are permitted Security considerations: none Tolerant Applications Although this document specifies the requirements for the generation of HTTP/1.1 messages, not all applications will be correct in their implementation. We therefore recommend that operational applications be tolerant of deviations whenever those deviations can be interpreted unambiguously. Clients SHOULD be tolerant in parsing the Status-Line and servers tolerant when parsing the Request-Line. In particular, they SHOULD accept any amount of SP or HT characters between fields, even though only a single SP is required. The line terminator for HTTP-header fields is the sequence CRLF. However, we recommend that applications, when parsing such headers, recognize a single LF as a line terminator and ignore the leading CR. Differences Between HTTP Bodies and RFC 1521 Internet Message Bodies HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822 private HREF="#RefSTD11"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [9]) and the Multipurpose Internet Mail Extensions (MIME private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]) to allow entities to be transmitted in an open variety of representations and with extensible mechanisms. However, RFC 1521 discusses mail, and HTTP has a few features that are different than those described in RFC 1521. These differences were carefully chosen to optimize performance over binary connections, to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients. At the time of this writing, it is expected that RFC 1521 will be revised. The revisions may include some of the practices found in HTTP/1.1 but not in RFC 1521. This appendix describes specific areas where HTTP differs from RFC 1521. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions may be required. Conversion to Canonical Form RFC 1521 requires that an Internet mail entity be converted to canonical form prior to being transferred, as described in Appendix G of RFC 1521 private HREF="#RefMIME1" Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]. Section ref Canonical_Text_Defau\n 7.7.1 of this document describes the forms allowed for subtypes of the �text� media type when transmitted over HTTP. RFC 1521 requires that content with a typeof �text� represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content when a message is transmitted over HTTP. Where it is possible, a proxy or gateway from HTTP to a strict RFC 1521 environment SHOULD translate all line breaks within the text media types described in section ref Canonical_Text_Defau\n 7.7.1 of this document to the RFC 1521 canonical form of CRLF. Note, however, that this may be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some character sets which do not use octets 13 and 10 to represent CR and LF, as is the case for some multi-byte character sets. Conversion of Date Formats HTTP/1.1 uses a restricted set of date formats (section ref HTTP_Date \n 7.3.1) to simplify the process of date comparison. Proxies and gateways from other protocols SHOULD ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary. Introduction of Content-Encoding RFC 1521 does not include any concept equivalent to HTTP/1.1's Content-Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols MUST either change the value of the Content-Type header field or decode the Entity-Body before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of �;conversions=� to perform an equivalent function as Content-Encoding. However, this parameter is not part of RFC 1521.) No Content-Transfer-Encoding HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC 1521. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any non-identity CTE (�quoted-printable� or �base64�) encoding prior to delivering the response message to an HTTP client. Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where �safe transport� is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol. HTTP Header Fields in Multipart Body-Parts In RFC 1521, most header fields in multipart body-parts are generally ignored unless the field name begins with �Content-�. In HTTP/1.1, multipart body-parts may contain any HTTP header fields which are significant to the meaning of that part. Introduction of Transfer-Encoding HTTP/1.1 introduces the Transfer-Encoding header field (section ref Transfer_Encoding \n 18.43). Proxies/gateways MUST remove any transfer coding prior to forwarding a message via a MIME-compliant protocol. The process for decoding the �chunked� transfer coding (section ref Transfer_Codings \n 7.6) can be represented in pseudo-code as: length := 0 read chunk-size and CRLF while (chunk-size > 0) { read chunk-data and CRLF append chunk-data to Entity-Body length := length + chunk-size read chunk-size and CRLF } read entity-header while (entity-header not empty) { append entity-header to existing header fields read entity-header } Content-Length := length Remove "chunked" from Transfer-Encoding MIME-Version HTTP is not a MIME-compliant protocol (see Appendix ref MIME \n 23.3). However, HTTP/1.1 messages may include a single MIME-Version general-header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field indicates that the message is in full compliance with the MIME protocol (as defined in private HREF="#RefMIME1"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [7]). Proxies/gateways are responsible for ensuring full compliance (where possible) when exporting HTTP messages to strict MIME environments. MIME-Version = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT MIME version �1.0� is the default for use in HTTP/1.1. However, HTTP/1.1 message parsing and semantics are defined by this document and not the MIME specification. Changes from HTTP/1.0 This section will summarize major differences between versions HTTP/1.0 and HTTP/1.1. Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses The requirements that clients and servers support the Host request-header, report an error if the Host request-header (section ref Host \n 18.24) is missing from an HTTP/1.1 request, and accept absolute URIs (Section ref Request_URI \n 9.1.2) are among the most important changes from HTTP/1.0. In HTTP/1.0 there is a one-to-one relationship of IP addresses and servers. There is no other way to distinguish the intended server of a request than the IP address to which that request is directed. The HTTP/1.1 change will allow the Internet, once HTTP/1.0 clients and servers are no longer common, to support multiple Web sites from a single IP address, greatly simplifying large operational Web servers, where allocation of many IP addresses to a single host has created serious problems. The Internet will also be able to recover the IP addresses that have been used for the sole purpose of allowing root-level domain names to be used in HTTP URLs. Given the rate of growth of the Web, and the number of servers already deployed, it is extremely important that implementations of HTTP/1.1 correctly implement these new requirements: � both clients and servers MUST support the Host request-header � Host request-headers are required in HTTP/1.1 requests. � servers MUST report an error if an HTTP/1.1 request does not include a Host request-header � servers MUST accept absolute URIs Additional Features This appendix documents protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementers should be aware of these features, but cannot rely upon their presence in, or interoperability with, other HTTP/1.1 applications. Some of these describe proposed experimental features, and some describe features that experimental deployment found lacking that are now addressed in the base HTTP/1.1 specification. Additional Request Methods PATCH The PATCH method is similar to PUT except that the entity contains a list of differences between the original version of the resource identified by the Request-URI and the desired content of the resource entity after the PATCH action has been applied. The list of differences is in a format defined by the media type of the entity (e.g., �application/diff�) and MUST include sufficient information to allow the server to recreate the changes necessary to convert the original version of the resource entity to the desired version. If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable. For compatibility with HTTP/1.0 applications, all PATCH requests MUST include a valid Content-Length header field unless the server is known to be HTTP/1.1 compliant. When sending a PATCH request to an HTTP/1.1 server, a client MUST use a valid Content-Length or the �chunked� Transfer-Encoding. The server SHOULD respond with a 400 (Bad Request) message if it cannot determine the length of the request message's content, or with 411 (Length Required) if it wishes to insist on receiving a valid Content-Length. The actual method for determining how the patched resource is placed, and what happens to its predecessor, is defined entirely by the origin server. If the original version of the resource being patched included a Content-Version header field, the request entity MUST include a Derived-From header field corresponding to the value of the original Content-Version header field. Applications are encouraged to use these fields for constructing versioning relationships and resolving version conflicts. PATCH requests must obey the entity transmission requirements set out in section ref Entity_Transmission_\n 13.4.1. Caches that implement PATCH should invalidate cached responses as defined in section ref Invalidation_After_U\n 16.10 for PUT. LINK The LINK method establishes one or more Link relationships between the existing resource identified by the Request-URI and other existing resources. The difference between LINK and other methods allowing links to be established between resources is that the LINK method does not allow any Entity-Body to be sent in the request and does not directly result in the creation of new resources. If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable. Caches that implement LINK should invalidate cached responses as defined in section ref Invalidation_After_U\n 16.10 for PUT. UNLINK The UNLINK method removes one or more Link relationships from the existing resource identified by the Request-URI. These relationships may have been established using the LINK method or by any other method supporting the Link header. The removal of a link to a resource does not imply that the resource ceases to exist or becomes inaccessible for future references. If the request passes through a cache and the Request-URI identifies a currently cached entity, that entity MUST be removed from the cache. Responses to this method are not cachable. Caches that implement UNLINK should invalidate cached responses as defined in section ref Invalidation_After_U\n 16.10 for PUT. PUT To support the PATCH method, if the entity being PUT was derived from an existing resource which included a Content-Version header field, the new entity MUST include a Derived-From header field corresponding to the value of the original Content-Version header field. Multiple Derived-From values may be included if the entity was derived from multiple resources with Content-Version information. Applications are encouraged to use these fields for constructing versioning relationships and resolving version conflicts. Additional Header Field Definitions Content-Version The Content-Version entity-header field defines the version tag associated with a rendition of an evolving entity. Together with the Derived-From field described in section ref Derived_From \n 23.5.2.2, it allows a group of people to work simultaneously on the creation of a work as an iterative process. The field SHOULD be used to allow evolution of a particular work along a single path. It SHOULD NOT be used to indicate derived works or renditions in different representations. It MAY also me used as an opaque value for comparing a cached entity's version with that of the current resource entity. Content-Version = "Content-Version" ":" quoted-string Examples of the Content-Version field include: Content-Version: "2.1.2" Content-Version: "Fred 19950116-12:26:48" Content-Version: "2.5a4-omega7" The value of the Content-Version field SHOULD be considered opaque to all parties but the origin server. A user agent MAY suggest a value for the version of an entity transferred via a PUT request; however, only the origin server can reliably assign that value. Derived-From The Derived-From entity-header field can be used to indicate the version tag of the resource from which the enclosed entity was derived before modifications were made by the sender. This field is used to help manage the process of merging successive changes to a resource, particularly when such changes are being made in parallel and from multiple sources. Derived-From = "Derived-From" ":" quoted-string An example use of the field is: Derived-From: "2.1.1" The Derived-From field is required for PUT and PATCH requests if the entity being sent was previously retrieved from the same URI and a Content-Version header was included with the entity when it was last retrieved. Link The Link entity-header field provides a means for describing a relationship between two resources, generally between the requested resource and some other resource. An entity MAY include multiple Link values. Links at the metainformation level typically indicate relationships like hierarchical structure and navigation paths. The Link field is semantically equivalent to the element in HTML private HREF="#RefHTML"Error! Bookmark not defined.MACROBUTTON HtmlResAnchor [5]. Link = "Link" ":" #("<" URI ">" *( ";" link-param ) link-param = ( ( "rel" "=" relationship ) | ( "rev" "=" relationship ) | ( "title" "=" quoted-string ) | ( "anchor" "=" <"> URI <"> ) | ( link-extension ) ) link-extension = token [ "=" ( token | quoted-string ) ] relationship = sgml-name | ( <"> sgml-name *( SP sgml-name) <"> ) sgml-name = ALPHA *( ALPHA | DIGIT | "." | "-" ) Relationship values are case-insensitive and MAY be extended within the constraints of the sgml-name syntax. The title parameter MAY be used to label the destination of a link such that it can be used as identification within a human-readable menu. The anchor parameter MAY be used to indicate a source anchor other than the entire current resource, such as a fragment of this resource or a third resource. Examples of usage include: Link: http://www.cern.ch/TheBook/chapter2; rel="Previous" Link: mailto:timbl@w3.org; rev="Made"; title="Tim Berners-Lee" The first example indicates that chapter2 is previous to this resource in a logical navigation path. The second indicates that the person responsible for making the resource available is identified by the given e-mail address. URI The URI header field has, in past versions of this specification, been used as a combination of the existing Location, Content-Location, and Alternates header fields. Its primary purpose has been to include a list of additional URIs for the resource, including names and mirror locations. However, it has become clear that the combination of many different functions within this single field has been a barrier to consistently and correctly implementing any of those functions. Furthermore, we believe that the identification of names and mirror locations would be better performed via the Link header field. The URI header field is therefore deprecated in favor of those other fields. URI-header = "URI" ":" 1#( "<" URI ">" ) Compatibility with HTTP/1.0 Persistent Connections Some clients and servers may wish to be compatible with some previous implementations of persistent connections in HTTP/1.0 clients and servers. These implementations are faulty, and the new facilities in HTTP/1.1 are designed to rectify these problems. The fear was that some existing 1.0 clients may be sending Keep-Alive to a proxy server that doesn't understand Connection, which would then erroneously forward it to the next inbound server, which would establish the Keep-Alive connection and result in a dead 1.0 proxy waiting for the close on the response. The result is that 1.0 clients must be prevented from using Keep-Alive when talking to proxies. However, talking to proxies is the most important use of persistent connections, so that is clearly unacceptable. Therefore, we need some other mechanism for indicating a persistent connection is desired, which is safe to use even when talking to an old proxy that ignores Connection. As it turns out, there are two ways to accomplish that: Introduce a new keyword (persist) which is declared to be valid only when received from an HTTP/1.1 message. Declare persistence to be the default for HTTP/1.1 messages and introduce a new keyword (close) for declaring non-persistence. The following describes the original, buggy form of persistent connections. When connecting to an origin server an HTTP client MAY send the Keep-Alive connection-token in addition to the Persist connection-token: Connection: Keep-Alive,Persist An HTTP/1.0 server would then respond with the Keep-Alive connection token and the client may proceed with an HTTP/1.0 (or Keep-Alive) persistent connection. An HTTP/1.1 server may also establish persistent connections with HTTP/1.0 clients upon receipt of a Keep-Alive connection token. However, a persistent connection with an HTTP/1.0 client cannot make use of the chunked transfer-coding, and therefore MUST use a Content-Length for marking the ending boundary of each Entity-Body. A client MUST NOT send the Keep-Alive connection token to a proxy server as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing the Connection header field. The Keep-Alive Header When the Keep-Alive connection-token has been transmitted with a request or a response a Keep-Alive header field MAY also be included. The Keep-Alive header field takes the following form: Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param keepalive-param = param-name "=" value The Keep-Alive header itself is optional, and is used only if a parameter is being sent. HTTP/1.1 does not define any parameters. If the Keep-Alive header is sent, the corresponding connection token MUST be transmitted. The Keep-Alive header MUST be ignored if received without the connection token. Compatibility with Previous Versions It is beyond the scope of a protocol specification to mandate compliance with previous versions. HTTP/1.1 was deliberately designed, however, to make supporting previous versions easy. While we are contemplating a separate document containing advice to implementers, we feel it worth noting that at the time of composing this specification, we would expect commercial HTTP/1.1 servers to: � recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1 requests; � understand any valid request in the format of HTTP/0.9, 1.0, or 1.1; � respond appropriately with a message in the same major version used by the client. And we would expect HTTP/1.1 clients to: � recognize the format of the Status-Line for HTTP/1.0 and 1.1 responses; � understand any valid response in the format of HTTP/0.9, 1.0, or 1.1. For most implementations of HTTP/1.0, each connection is established by the client prior to the request and closed by the server after sending the response. A few implementations implement the Keep-Alive version of persistent connections described in section ref Keep_Alive_Header \n 23.5.2.5.1. INTERNET-DRAFT Hypertext Transfer Protocol - HTTP/1.1 time \@ "dddd, MMMM dd, yyyy" Thursday, May 02, 1996 Fielding, Frystyk, Berners-Lee, Gettys, and Mogul [Page page 64] Fielding, Frystyk, Berners-Lee, Gettys and Mogul [Page page 9] page \# "'Page: '#' '" Page: 5 CONTENT page \# "'Page: '#' '" Page: 5 CHUNKED page \# "'Page: '#' '" Page: 5 MEDIATYPES page \# "'Page: '#' '" Page: 5 FULLURL page \# "'Page: '#' '" Page: 5 FULLURL, HOST page \# "'Page: '#' '" Page: 5 REWRITE issue page \# "'Page: '#' '" Page: 5 Placeholder for Range proposal. page \# "'Page: '#' '" Page: 5 CODES page \# "'Page: '#' '" Page: 5 Version not supported page \# "'Page: '#' '" Page: 5 TRACE page \# "'Page: '#' '" Page: 5 CONNEG page \# "'Page: '#' '" Page: 5 INTEGOK, CONTENT-MD5. Updated to draft of 4/2/96 page \# "'Page: '#' '" Page: 5 HOST, FULLURL page \# "'Page: '#' '" Page: 5 TRACE page \# "'Page: '#' '" Page: 5 UPGRADE page \# "'Page: '#' '" Page: 5 FORWARDED page \# "'Page: '#' '" Page: 5 Basic authentication Warning, John Franks. page \# "'Page: '#' '" Page: 5 SECFILE page \# "'Page: '#' '" Page: 5 PRIVACY page \# "'Page: '#' '" Page: 5 ACCEPT page \# "'Page: '#' '" Page: 5 DNS PAGE \# "'Page: '#' '" Page: 5 CONTENT XXX check if an RFC has been issued by publication. page \# "'Page: '#' '" Page: 5 MEDIATYPES page \# "'Page: '#' '" Page: 5 FULLURL, HOST %�������*������/��=���������%�������������/��=���������%�������������/��=�����������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������/��>��S��^�������� ��F ��� ��� 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Variant_IDs _Toc355696055 _Toc355709897 _Toc355710118 _Toc355711044 _Toc355712015 Variant_Sets _Toc355696056 _Toc355709898 _Toc355710119 _Toc355711045 _Toc355712016Range_Protocol_Param _Toc355696057 _Toc355709899 _Toc355710120 _Toc355711046 _Toc355712017 _Toc355696058 _Toc355710121 Range_Units _Toc355709900 _Toc355711047 _Toc355712018 _Toc355696059 _Toc355710122 Byte_Ranges _Toc355709901 _Toc355711048 _Toc355712019 _Toc355696060 _Toc355710123Content_Ranges _Toc355709902 _Toc355711049 _Toc355712020Message _Toc355696061 _Toc355709903 _Toc355710124 _Toc355711050 _Toc355712021 Message_Types _Toc355696062 _Toc355709904 _Toc355710125 _Toc355711051 _Toc355712022Message_Headers _Toc355696063 _Toc355709905 _Toc355710126 _Toc355711052 _Toc355712023General_Header _Toc355696064 _Toc355709906 _Toc355710127 _Toc355711053 _Toc355712024Request _Toc355696065 _Toc355709907 _Toc355710128 _Toc355711054 _Toc355712025 Request_Line _Toc355696066 _Toc355709908 _Toc355710129 _Toc355711055 _Toc355712026Method _Toc355696067 _Toc355709909 _Toc355710130 _Toc355711056 _Toc355712027 Request_URI _Toc355696068 _Toc355709910 _Toc355710131 _Toc355711057 _Toc355712028Request_Header _Toc355696069 _Toc355709911 _Toc355710132 _Toc355711058 _Toc355712029 _Toc355696070 _Toc355709912 _Toc355710133 _Toc355711059 _Toc355712030Response _Toc355696071 _Toc355709913 _Toc355710134 _Toc355711060 _Toc355712031 Status_Line _Toc355696072 _Toc355709914 _Toc355710135 _Toc355711061 _Toc355712032 Status_Code _Toc355696073 _Toc355709915 _Toc355710136 _Toc355711062 _Toc355712033Response_Header _Toc355696074 _Toc355709916 _Toc355710137 _Toc355711063 _Toc355712034Entity _Toc355696075 _Toc355709917 _Toc355710138 _Toc355711064 _Toc355712035 Entity_Header _Toc355696076 _Toc355709918 _Toc355710139 _Toc355711065 _Toc355712036 Entity_Body _Toc355696077 _Toc355709919 _Toc355710140 _Toc355711066 _Toc355712037BodyType _Toc355696078 _Toc355709920 _Toc355710141 _Toc355711067 _Toc355712038 BodyLength _Toc355696079 _Toc355709921 _Toc355710142 _Toc355711068 _Toc355712039 Status_Codes _Toc355696080 _Toc355709922 _Toc355710143 _Toc355711069 _Toc355712040Code1xx _Toc355696081 _Toc355709923 _Toc355710144 _Toc355711070 _Toc355712041Code100Code101Code2xx _Toc355696082 _Toc355709924 _Toc355710145 _Toc355711071 _Toc355712042Code200Code201Code202Code203Code204Code205Code206Code3xx _Toc355696083 _Toc355709925 _Toc355710146 _Toc355711072 _Toc355712043Code300Code301Code302Code303Code304Code305Code4xx _Toc355696084 _Toc355709926 _Toc355710147 _Toc355711073 _Toc355712044Code400Code401Code402Code403Code404Code405Code406Code407Code408Code409Code410Code411Code412Code413Code414Code415Code5xx _Toc355696085 _Toc355709927 _Toc355710148 _Toc355711074 _Toc355712045Code500Code501Code502Code503Code504Code505Methods _Toc355696086 _Toc355709928 _Toc355710149 _Toc355711075 _Toc355712046OPTIONS _Toc355696087 _Toc355709929 _Toc355710150 _Toc355711076 _Toc355712047GET _Toc355696088 _Toc355709930 _Toc355710151 _Toc355711077 _Toc355712048HEAD _Toc355696089 _Toc355709931 _Toc355710152 _Toc355711078 _Toc355712049POST _Toc355696090 _Toc355709932 _Toc355710153 _Toc355711079 _Toc355712050 _Toc355696091 _Toc355710154 _Toc355709933 _Toc355711080 _Toc355712051Entity_Transmission_PUT _Toc355696092 _Toc355709934 _Toc355710155 _Toc355711081 _Toc355712052DELETE _Toc355696093 _Toc355709935 _Toc355710156 _Toc355711082 _Toc355712053TRACE _Toc355696094 _Toc355709936 _Toc355710157 _Toc355711083 _Toc355712054AA _Toc355696095 _Toc355709937 _Toc355710158 _Toc355711084 _Toc355712055BasicAA _Toc355696096 _Toc355709938 _Toc355710159 _Toc355711085 _Toc355712056DigestAA _Toc355696097 _Toc355709939 _Toc355710160 _Toc355711086 _Toc355712057Content_Negotiation _Toc355696098 _Toc355709940 _Toc355710161 _Toc355711087 _Toc355712058 _Toc355696099Negotiation_Faciliti _Toc355709941 _Toc355710162 _Toc355711088 _Toc355712059Caching_In_HTTP _Toc355696100 _Toc355709942 _Toc355710163 _Toc355711089 _Toc355712060Semantic_Transparenc _Toc355696101 _Toc355709943 _Toc355710164 _Toc355711090 _Toc355712061 _Toc355696102Cache_Correctness _Toc355709944 _Toc355710165 _Toc355711091 _Toc355712062Expiration_ModelCache_Control_Mechan _Toc355696103 _Toc355709945 _Toc355710166 _Toc355711092 _Toc355712063Warnings _Toc355696104 _Toc355709946 _Toc355710167 _Toc355711093 _Toc355712064Explicit_User_Agent_ _Toc355696105 _Toc355709947 _Toc355710168 _Toc355711094 _Toc355712065Exceptions_to_the_Ru _Toc355696106 _Toc355709948 _Toc355710169 _Toc355711095 _Toc355712066Client_Controlled_Be _Toc355696107 _Toc355709949 _Toc355710170 _Toc355711096 _Toc355712067 _Toc355696108 _Toc355709950 _Toc355710171 _Toc355711097 _Toc355712068Server_Specified_Exp _Toc355696109 _Toc355709951 _Toc355710172 _Toc355711098 _Toc355712069Limitations_On_Expir _Toc355696110 _Toc355709952 _Toc355710173 _Toc355711099 _Toc355712070Heuristic_Expiration _Toc355696111 _Toc355709953 _Toc355710174 _Toc355711100 _Toc355712071Age_Calculations _Toc355696112 _Toc355709954 _Toc355710175 _Toc355711101 _Toc355712072Expiration_Calculati _Toc355696113 _Toc355709955 _Toc355710176 _Toc355711102 _Toc355712073Disambiguating_Expir _Toc355696114 _Toc355710177Scope_Of_Expiration _Toc355709956 _Toc355711103 _Toc355712074 _Toc355696115 _Toc355709957 _Toc355710178 _Toc355711104 _Toc355712075Disambiguating_Multi _Toc355696116 _Toc355709958 _Toc355710179 _Toc355711105 _Toc355712076Validation_Model _Toc355696117 _Toc355709959 _Toc355710180 _Toc355711106 _Toc355712077Last_Modified_Dates _Toc355696118 _Toc355709960 _Toc355710181 _Toc355711107 _Toc355712078Tags _Toc355696119 _Toc355709961 _Toc355710182 _Toc355711108 _Toc355712079Weak_and_Strong_Tags _Toc355696120 _Toc355709962 _Toc355710183 _Toc355711109 _Toc355712080Rules_For_Tags_and_L _Toc355696121 _Toc355709963 _Toc355710184 _Toc355711110 _Toc355712081 _Toc355696122 _Toc355710185Nonvalidating_Condit _Toc355709964 _Toc355711111 _Toc355712082Constructing_Respons _Toc355696123 _Toc355709965 _Toc355710186 _Toc355711112 _Toc355712083EtoE_and_HbyH_Header _Toc355696124 _Toc355709966 _Toc355710187 _Toc355711113 _Toc355712084Non_Modifiable_Headr _Toc355696125 _Toc355709967 _Toc355710188 _Toc355711114 _Toc355712085Combining_Headers _Toc355696126 _Toc355709968 _Toc355710189 _Toc355711115 _Toc355712086Combining_Byte_Range _Toc355696127 _Toc355709969 _Toc355710190 _Toc355711116 _Toc355712087Caching_and_Varying_ _Toc355696128 _Toc355709970 _Toc355710191 _Toc355711117 _Toc355712088Vary_Header_Use _Toc355696129 _Toc355709971 _Toc355710192 _Toc355711118 _Toc355712089 _Toc355696130 _Toc355710193Alternates_Header_Us _Toc355709972 _Toc355711119 _Toc355712090Variant_ID_Use _Toc355696131 _Toc355709973 _Toc355710194 _Toc355711120 _Toc355712091Shared_and_Non_Share _Toc355696132 _Toc355709974 _Toc355710195 _Toc355711121 _Toc355712092 _Toc355711122 _Toc355709975 _Toc355696134 _Toc355709976 _Toc355710197 _Toc355711123 _Toc355712093 _Toc355709977 _Toc355710198 _Toc355711124 _Toc355712094 _Toc355709978 _Toc355710199 _Toc355711125 _Toc355712095Errors_Or_Incomplete _Toc355696138 _Toc355709979 _Toc355710200 _Toc355711126 _Toc355712096Caching_and_Status_C _Toc355696139 _Toc355709980 _Toc355710201 _Toc355711127 _Toc355712097 _Toc355696140 _Toc355709981 _Toc355710202 _Toc355711128 _Toc355712098 _Toc355696141 _Toc355710203Side_Effects_of_Get_ _Toc355709982 _Toc355711129 _Toc355712099 _Toc355696142 _Toc355710204Invalidation_After_U _Toc355709983 _Toc355711130 _Toc355712100Write_Through_Mandan _Toc355696143 _Toc355709984 _Toc355710205 _Toc355711131 _Toc355712101 _Toc355696144Varying_Resources_An _Toc355709985 _Toc355710206 _Toc355711132 _Toc355712102Varying_Resources_an _Toc355696145 _Toc355710207 _Toc355709986 _Toc355711133 _Toc355712103 _Toc355696146 _Toc355710208Caching_of_Negative_ _Toc355709987 _Toc355711134 _Toc355712104 History_Lists _Toc355696147 _Toc355709988 _Toc355710209 _Toc355711135 _Toc355712105Persistent_Connectio _Toc355696148 _Toc355709989 _Toc355710210 _Toc355711136 _Toc355712106Persist_Purpose _Toc355696149 _Toc355709990 _Toc355710211 _Toc355711137 _Toc355712107Persist_Overall_Oper _Toc355696150 _Toc355709991 _Toc355710212 _Toc355711138 _Toc355712108Persist_Negotiation _Toc355696151 _Toc355709992 _Toc355710213 _Toc355711139 _Toc355712109Persist_Pipe_Lining _Toc355696152 _Toc355709993 _Toc355710214 _Toc355711140 _Toc355712110Persist_Entity_Bodie _Toc355696153 _Toc355709994 _Toc355710215 _Toc355711141 _Toc355712111Persist_Proxy_Server _Toc355696154 _Toc355709995 _Toc355710216 _Toc355711142 _Toc355712112Persist_Interactions _Toc355696155 _Toc355709996 _Toc355710217 _Toc355711143 _Toc355712113Persist_Practical_Co _Toc355696156 _Toc355709997 _Toc355710218 _Toc355711144 _Toc355712114 HeaderFields _Toc355696157 _Toc355709998 _Toc355710219 _Toc355711145 _Toc355712115Accept _Toc355696158 _Toc355709999 _Toc355710220 _Toc355711146 _Toc355712116 OLE_LINK5Accept_Charset _Toc355696159 _Toc355710000 _Toc355710221 _Toc355711147 _Toc355712117Accept_Encoding _Toc355696160 _Toc355710001 _Toc355710222 _Toc355711148 _Toc355712118Accept_Language _Toc355696161 _Toc355710002 _Toc355710223 _Toc355711149 _Toc355712119 OLE_LINK8 _Toc355696162 _Toc355710224 Accept_Ranges _Toc355710003 _Toc355711150 _Toc355712120Age _Toc355696163 _Toc355710004 _Toc355710225 _Toc355711151 _Toc355712121Allow _Toc355696164 _Toc355710005 _Toc355710226 _Toc355711152 _Toc355712122 Authorization Alternates _Toc355696165 _Toc355710006 _Toc355710227 _Toc355711153 _Toc355712123 _Toc355696166 _Toc355710007 _Toc355710228 _Toc355711154 _Toc355712124 Cache_Control _Toc355696167 _Toc355710008 _Toc355710229 _Toc355711155 _Toc355712125 _Toc355696168 _Toc355710230What_is_Cachable _Toc355710009 _Toc355711156 _Toc355712126What_May_be_Stored_b _Toc355696169 _Toc355710010 _Toc355710231 _Toc355711157 _Toc355712127Modifications_of_Exp _Toc355696170 _Toc355710011 _Toc355710232 _Toc355711158 _Toc355712128 _Toc355696171 _Toc355710233Cache_Revalidation_a _Toc355710012 _Toc355711159 _Toc355712129Cache_Miscellaneous_ _Toc355696172 _Toc355710013 _Toc355710234 _Toc355711160 _Toc355712130 Connection _Toc355696173 _Toc355710014 _Toc355710235 _Toc355711161 _Toc355712131 Content_Base _Toc355696174 _Toc355710015 _Toc355710236 _Toc355711162 _Toc355712132Content_Encoding _Toc355696175 _Toc355710016 _Toc355710237 _Toc355711163 _Toc355712133Content_Language _Toc355696176 _Toc355710017 _Toc355710238 _Toc355711164 _Toc355712134Content_Length _Toc355696177 _Toc355710018 _Toc355710239 _Toc355711165 _Toc355712135Content_Location _Toc355696178 _Toc355710019 _Toc355710240 _Toc355711166 _Toc355712136 _Toc355711167 _Toc355696179 Content_MD5 _Toc355710020 _Toc355710241 _Toc355712137 _Toc355696180 _Toc355710242 Content_Range _Toc355710021 _Toc355711168 _Toc355712138MIME_byteranges _Toc355696181 _Toc355710022 _Toc355710243 _Toc355711169 _Toc355712139Additional_Rules_For _Toc355696182 _Toc355710023 _Toc355710244 _Toc355711170 _Toc355712140 Content_Type _Toc355696183 _Toc355710024 _Toc355710245 _Toc355711171 _Toc355712141Date _Toc355696184 _Toc355710025 _Toc355710246 _Toc355711172 _Toc355712142 _Toc355696185ETag _Toc355710026 _Toc355710247 _Toc355711173 _Toc355712143 _Toc355696186 _Toc355710248Expires _Toc355710027 _Toc355711174 _Toc355712144From _Toc355696187 _Toc355710028 _Toc355710249 _Toc355711175 _Toc355712145Host _Toc355696188 _Toc355710029 _Toc355710250 _Toc355711176 _Toc355712146If_Modified_Since _Toc355696189 _Toc355710030 _Toc355710251 _Toc355711177 _Toc355712147If_ValidIf_Match _Toc355696190 _Toc355710031 _Toc355710252 _Toc355711178 _Toc355712148 _Toc355696191 _Toc355710032 _Toc355710253 _Toc355711179 _Toc355712149 _Toc355696192 _Toc355710254Range_If _Toc355710033 _Toc355711180 _Toc355712150Unless_Modified_Sinc _Toc355696193 _Toc355710034 _Toc355710255 _Toc355711181 _Toc355712151 Last_Modified _Toc355696194 _Toc355710035 _Toc355710256 _Toc355711182 _Toc355712152Location _Toc355696195 _Toc355710036 _Toc355710257 _Toc355711183 _Toc355712153 Max_Forwards _Toc355696196 _Toc355710037 _Toc355710258 _Toc355711184 _Toc355712154Persist _Toc355696197 _Toc355710038 _Toc355710259 _Toc355711185 _Toc355712155Pragma _Toc355696198 _Toc355710039 _Toc355710260 _Toc355711186 _Toc355712156Proxy_Authenticate _Toc355696199 _Toc355710040 _Toc355710261 _Toc355711187 _Toc355712157Proxy_Authorization _Toc355696200 _Toc355710041 _Toc355710262 _Toc355711188 _Toc355712158Public _Toc355696201 _Toc355710042 _Toc355710263 _Toc355711189 _Toc355712159Range _Toc355696202 _Toc355710043 _Toc355710264 _Toc355711190 _Toc355712160Referer _Toc355696203 _Toc355710044 _Toc355710265 _Toc355711191 _Toc355712161 Retry_After _Toc355696204 _Toc355710045 _Toc355710266 _Toc355711192 _Toc355712162Server _Toc355696205 _Toc355710046 _Toc355710267 _Toc355711193 _Toc355712163Title _Toc355696206 _Toc355710047 _Toc355710268 _Toc355711194 _Toc355712164Transfer_Encoding _Toc355696207 _Toc355710048 _Toc355710269 _Toc355711195 _Toc355712165Upgrade _Toc355696208 _Toc355710049 _Toc355710270 _Toc355711196 _Toc355712166 User_Agent _Toc355696209 _Toc355710050 _Toc355710271 _Toc355711197 _Toc355712167WWW_AuthenticateVary _Toc355696210 _Toc355710051 _Toc355710272 _Toc355711198 _Toc355712168 ForwardedVia _Toc355696211 _Toc355710052 _Toc355710273 _Toc355711199 _Toc355712169 _Toc355696212Warning _Toc355710053 _Toc355710274 _Toc355711200 _Toc355712170 _Toc355696213 _Toc355710054 _Toc355710275 _Toc355711201 _Toc355712171Security _Toc355696214 _Toc355710055 _Toc355710276 _Toc355711202 _Toc355712172 AuthSecurity _Toc355696215 _Toc355710056 _Toc355710277 _Toc355711203 _Toc355712173 SafeMethods _Toc355696216 _Toc355710057 _Toc355710278 _Toc355711204 _Toc355712174LogAbuse _Toc355696217 _Toc355710058 _Toc355710279 _Toc355711205 _Toc355712175 Sensitive _Toc355696218 _Toc355710059 _Toc355710280 _Toc355711206 _Toc355712176PathNameSecurity _Toc355696219 _Toc355710060 _Toc355710281 _Toc355711207 _Toc355712177 SecPersonal _Toc355696220 _Toc355710061 _Toc355710282 _Toc355711208 _Toc355712178SecPrivacyAccept _Toc355696221 _Toc355710062 _Toc355710283 _Toc355711209 _Toc355712179SecDNS _Toc355696222 _Toc355710063 _Toc355710284 _Toc355711210 _Toc355712180 _Toc355696223 _Toc355710285Location_Headers_and _Toc355710064 _Toc355711211 _Toc355712181Acknowledgments _Toc355696224 _Toc355710065 _Toc355710286 _Toc355711212 _Toc355712182 References _Toc355696225 _Toc355710066 _Toc355710287 _Toc355711213 _Toc355712183Authors _Toc355696226 _Toc355710067 _Toc355710288 _Toc355711214 RefLangTags RefGopherRefURIRefURLRefHTML RefHTTP10RefMIME1RefSTD3RefSTD11RefWAIS RefRelURL RefUSENETRefNNTPRefMIME2 RefFileUploadRefSMTP RefMediaTypeRefFTPRefIANARefURNRefASCII RefISO8859Ref1864Ref1900GZIP DigestRef RefDNSSECNTPRefSHTTPRefSSL _Toc355712184 Appendices _Toc355696227 _Toc355710068 _Toc355710289 _Toc355711215 _Toc355712185 message_http _Toc355696228 _Toc355710069 _Toc355710290 _Toc355711216 _Toc355712186Tolerant _Toc355696229 _Toc355710070 _Toc355710291 _Toc355711217 _Toc355712187MIME _Toc355696230 _Toc355710071 _Toc355710292 _Toc355711218 _Toc355712188Conversion_To_Canoni _Toc355696231 _Toc355710072 _Toc355710293 _Toc355711219 _Toc355712189Conversion_of_Date_F _Toc355696232 _Toc355710073 _Toc355710294 _Toc355711220 _Toc355712190MIME_CE _Toc355696233 _Toc355710074 _Toc355710295 _Toc355711221 _Toc355712191No_Content_Transfer_ _Toc355696234 _Toc355710075 _Toc355710296 _Toc355711222 _Toc355712192 MIME_parts _Toc355696235 _Toc355710076 _Toc355710297 _Toc355711223 _Toc355712193MIME_TE _Toc355696236 _Toc355710077 _Toc355710298 _Toc355711224 _Toc355712194 MIME_Version _Toc355696237 _Toc355710078 _Toc355710299 _Toc355711225 _Toc355712195Changes _Toc355696238 _Toc355710079 _Toc355710300 _Toc355711226 _Toc355712196 OLE_LINK4 _Toc355696239 _Toc355710080 _Toc355710301 _Toc355711227 _Toc355712197AppHost Additional _Toc355696240 _Toc355710081 _Toc355710302 _Toc355711228 _Toc355712198Additional_Methods _Toc355696241 _Toc355710082 _Toc355710303 _Toc355711229 _Toc355712199PATCHLINKUNLINKAdditional_Headers _Toc355696242 _Toc355710083 _Toc355710304 _Toc355711230 _Toc355712200Content_Version Derived_From Link_Header URI_HeaderCompatibility_With_1Keep_Alive_HeaderCompatibility_With_P _Toc355696243 _Toc355710084 _Toc355710305 _Toc355711231 _Toc355712201����������B��B��B��B��B��B�������������������� �� �� �� �� �� �������������V���V���V���V���V���V��W��W��W��W��W��W���b���b���b���b���b���b���g���g���g���g���g���g��������������������������5���5���5���5���5���5���������������������������������������������������^���^���^���^���^���^���s���s���s���s���s���s���������������������������m���m���m���m���m���m���������������������������������������������������������������������������������������������������O���O���O���O���O���O���������������������������j���j���j���j���j���j���������������������������'�'�'�'�'�'�������������.�.�.�.�.�.�c�c�c�c�c�c�0 �0 �0 �0 �0 �0 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