HTTP/1.1, part 3: Message Payload and Content Negotiation (original) (raw)

Abstract

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. This document is Part 3 of the seven-part specification that defines the protocol referred to as "HTTP/1.1" and, taken together, obsoletes RFC 2616. Part 3 defines HTTP message content, metadata, and content negotiation.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on February 5, 2011.

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

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Editorial Note (To be removed by RFC Editor)

The changes in this draft are summarized in Appendix E.12.

Table of Contents

1. Introduction

This document defines HTTP/1.1 message payloads (a.k.a., content), the associated metadata header fields that define how the payload is intended to be interpreted by a recipient, the request header fields that might influence content selection, and the various selection algorithms that are collectively referred to as HTTP content negotiation.

This document is currently disorganized in order to minimize the changes between drafts and enable reviewers to see the smaller errata changes. The next draft will reorganize the sections to better reflect the content. In particular, the sections on entities will be renamed payload and moved to the first half of the document, while the sections on content negotiation and associated request header fields will be moved to the second half. The current mess reflects how widely dispersed these topics and associated requirements had become in [RFC2616].

1.1. Terminology

This specification uses a number of terms to refer to the roles played by participants in, and objects of, the HTTP communication.

content negotiation

1.2. Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

An implementation is not compliant if it fails to satisfy one or more of the "MUST" or "REQUIRED" level requirements for the protocols it implements. An implementation that satisfies all the "MUST" or "REQUIRED" level and all the "SHOULD" level requirements for its protocols is said to be "unconditionally compliant"; one that satisfies all the "MUST" level requirements but not all the "SHOULD" level requirements for its protocols is said to be "conditionally compliant".

1.3. Syntax Notation

This specification uses the ABNF syntax defined in Section 1.2 of [Part1] (which extends the syntax defined in [RFC5234] with a list rule). Appendix D shows the collected ABNF, with the list rule expanded.

The following core rules are included by reference, as defined in [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit sequence of data), SP (space), VCHAR (any visible USASCII character), and WSP (whitespace).

1.3.1. Core Rules

The core rules below are defined in Section 1.2.2 of [Part1]:

1.3.2. ABNF Rules defined in other Parts of the Specification

The ABNF rules below are defined in other parts:

absolute-URI = <absolute-URI, defined in [Part1], Section 2.6> Content-Length = <Content-Length, defined in [Part1], Section 9.2> header-field = <header-field, defined in [Part1], Section 3.2> partial-URI = <partial-URI, defined in [Part1], Section 2.6> qvalue = <qvalue, defined in [Part1], Section 6.4>

2. Protocol Parameters

2.1. 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 might be available in a given character set and a character set might provide more than one sequence of octets to represent a particular character. This definition is intended to allow various kinds of character encoding, 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.

HTTP character sets are identified by case-insensitive tokens. The complete set of tokens is defined by the IANA Character Set registry (<http://www.iana.org/assignments/character-sets>).

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 MUST represent the character set defined by that registry. Applications SHOULD limit their use of character sets to those defined by the IANA registry.

HTTP uses charset in two contexts: within an Accept-Charset request header (in which the charset value is an unquoted token) and as the value of a parameter in a Content-Type header (within a request or response), in which case the parameter value of the charset parameter can be quoted.

Implementors need to be aware of IETF character set requirements [RFC3629] [RFC2277].

2.1.1. Missing Charset

Some HTTP/1.0 software has interpreted a Content-Type header without charset parameter incorrectly to mean "recipient should guess". Senders wishing to defeat this behavior MAY include a charset parameter even when the charset is ISO-8859-1 ([ISO-8859-1]) and SHOULD do so when it is known that it will not confuse the recipient.

Unfortunately, some older HTTP/1.0 clients did not deal properly with an explicit charset parameter. HTTP/1.1 recipients MUST respect the charset label provided by the sender; and those user agents that have a provision to "guess" a charset MUST use the charset from the content-type field if they support that charset, rather than the recipient's preference, when initially displaying a document. See Section 2.3.1.

2.2. Content Codings

Content coding values indicate an encoding transformation that has been or can be applied to a representation. Content codings are primarily used to allow a representation to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Frequently, the representation is stored in coded form, transmitted directly, and only decoded by the recipient.

All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (Section 6.3) and Content-Encoding (Section 6.5) 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.

identity

2.2.1. Content Coding Registry

The HTTP Content Coding Registry defines the name space for the content coding names.

Registrations MUST include the following fields:

Names of content codings MUST NOT overlap with names of transfer codings (Section 6.2 of [Part1]), unless the encoding transformation is identical (as it is the case for the compression codings defined in Section 6.2.2 of [Part1]).

Values to be added to this name space require a specification (see "Specification Required" in Section 4.1 of [RFC5226]), and MUST conform to the purpose of content coding defined in this section.

2.3. Media Types

HTTP uses Internet Media Types [RFC2046] in the Content-Type (Section 6.9) and Accept (Section 6.1) header fields in order to provide open and extensible data typing and type negotiation.

Parameters MAY follow the type/subtype in the form of attribute/value pairs.

The type, subtype, and parameter attribute names are case-insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. The presence or absence of a parameter might be significant to the processing of a media-type, depending on its definition within the media type registry.

A parameter value that matches the token production can be transmitted as either a token or within a quoted-string. The quoted and unquoted values are equivalent.

Note that some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations SHOULD only use media type parameters when they are required by that type/subtype definition.

Media-type values are registered with the Internet Assigned Number Authority (IANA). The media type registration process is outlined in [RFC4288]. Use of non-registered media types is discouraged.

2.3.1. Canonicalization and Text Defaults

Internet media types are registered with a canonical form. A representation transferred via HTTP messages MUST be in the appropriate canonical form prior to its transmission except for "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. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire representation. HTTP applications MUST accept CRLF, bare CR, and bare LF as indicating a line break in text media received via HTTP. In addition, if the text is in a character encoding that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character encodings, HTTP allows the use of whatever octet sequences are defined by that character encoding to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the payload 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 a representation is encoded with a content-coding, 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 encoding (Section 2.1) 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 encodings other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value. See Section 2.1.1 for compatibility problems.

2.3.2. Multipart Types

MIME provides for a number of "multipart" types -- encapsulations of one or more representations within a single message-body. All multipart types share a common syntax, as defined in Section 5.1.1 of [RFC2046], 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.

In general, HTTP treats a multipart message-body no differently than any other media type: strictly as payload. HTTP does not use the multipart boundary as an indicator of message-body length. In all other respects, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within each body-part of a multipart message-body do not have any significance to HTTP beyond that defined by their MIME semantics.

If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".

2.4. Language Tags

A language tag, as defined in [RFC5646], 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.

In summary, a language tag is composed of one or more parts: A primary language subtag followed by a possibly empty series of subtags:

Example tags include:

en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

See [RFC5646] for further information.

3. Payload

HTTP messages MAY transfer a payload if not otherwise restricted by the request method or response status code. The payload consists of metadata, in the form of header fields, and data, in the form of the sequence of octets in the message-body after any transfer-coding has been decoded.

A "payload" in HTTP is always a partial or complete representation of some resource. We use separate terms for payload and representation because some messages contain only the associated representation's header fields (e.g., responses to HEAD) or only some part(s) of the representation (e.g., the 206 status code).

3.2. Payload Body

A payload body is only present in a message when a message-body is present, as described in Section 3.3 of [Part1]. The payload body is obtained from the message-body by decoding any Transfer-Encoding that might have been applied to ensure safe and proper transfer of the message.

4. Representation

A "representation" is information in a format that can be readily communicated from one party to another. A resource representation is information that reflects the state of that resource, as observed at some point in the past (e.g., in a response to GET) or to be desired at some point in the future (e.g., in a PUT request).

Most, but not all, representations transferred via HTTP are intended to be a representation of the target resource (the resource identified by the effective request URI). The precise semantics of a representation are determined by the type of message (request or response), the request method, the response status code, and the representation metadata. For example, the above semantic is true for the representation in any 200 (OK) response to GET and for the representation in any PUT request. A 200 response to PUT, in contrast, contains either a representation that describes the successful action or a representation of the target resource, with the latter indicated by a Content-Location header field with the same value as the effective request URI. Likewise, response messages with an error status code usually contain a representation that describes the error and what next steps are suggested for resolving it.

4.2. Representation Data

The representation body associated with an HTTP message is either provided as the payload body of the message or referred to by the message semantics and the effective request URI. The representation data is in a format and encoding defined by the representation metadata header fields.

The data type of the representation data is determined via the header fields Content-Type and Content-Encoding. These define a two-layer, ordered encoding model:

representation-data := Content-Encoding( Content-Type( bits ) )

Content-Type specifies the media type of the underlying data, which defines both the data format and how that data SHOULD be processed by the recipient (within the scope of the request method semantics). Any HTTP/1.1 message containing a payload body SHOULD include a Content-Type header field defining the media type of the associated representation unless that metadata is unknown to the sender. If the Content-Type header field is not present, it indicates that the sender does not know the media type of the representation; recipients MAY either assume that the media type is "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the content to determine its type.

In practice, resource owners do not always properly configure their origin server to provide the correct Content-Type for a given representation, with the result that some clients will examine a response body's content and override the specified type. Clients that do so risk drawing incorrect conclusions, which might expose additional security risks (e.g., "privilege escalation"). Furthermore, it is impossible to determine the sender's intent by examining the data format: many data formats match multiple media types that differ only in processing semantics. Implementers are encouraged to provide a means of disabling such "content sniffing" when it is used.

Content-Encoding is used to indicate any additional content codings applied to the data, usually for the purpose of data compression, that are a property of the representation. If Content-Encoding is not present, then there is no additional encoding beyond that defined by the Content-Type.

5. Content Negotiation

HTTP responses include a representation which contains information for interpretation, whether by a human user or for further processing. Often, the server has different ways of representing the same information; for example, in different formats, languages, or using different character encodings.

HTTP clients and their users might have different or variable capabilities, characteristics or preferences which would influence which representation, among those available from the server, would be best for the server to deliver. For this reason, HTTP provides mechanisms for "content negotiation" -- a process of allowing selection of a representation of a given resource, when more than one is available.

This specification defines two patterns of content negotiation; "server-driven", where the server selects the representation based upon the client's stated preferences, and "agent-driven" negotiation, where the server provides a list of representations for the client to choose from, based upon their metadata. In addition, there are other patterns: some applications use an "active content" pattern, where the server returns active content which runs on the client and, based on client available parameters, selects additional resources to invoke. "Transparent Content Negotiation" ([RFC2295]) has also been proposed.

These patterns are all widely used, and have trade-offs in applicability and practicality. In particular, when the number of preferences or capabilities to be expressed by a client are large (such as when many different formats are supported by a user-agent), server-driven negotiation becomes unwieldy, and might not be appropriate. Conversely, when the number of representations to choose from is very large, agent-driven negotiation might not be appropriate.

Note that in all cases, the supplier of representations has the responsibility for determining which representations might be considered to be the "same information".

5.1. Server-driven Negotiation

If the selection of the best representation for a response is made by an algorithm located at the server, it is called server-driven negotiation. Selection is based on the available representations of the response (the dimensions over which it can vary; e.g., language, content-coding, etc.) and the contents of particular header fields in the request message or on other information pertaining to the request (such as the network address of the client).

Server-driven negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent MAY include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.

Server-driven negotiation has disadvantages:

  1. It is impossible for the server to accurately determine what might be "best" for any given user, since that would require complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want to view it on screen or print it on paper?).
  2. Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage of responses have multiple representations) and a potential violation of the user's privacy.
  3. It complicates the implementation of an origin server and the algorithms for generating responses to a request.
  4. It might limit a public cache's ability to use the same response for multiple user's requests.

HTTP/1.1 includes the following request-header fields for enabling server-driven negotiation through description of user agent capabilities and user preferences: Accept (Section 6.1), Accept-Charset (Section 6.2), Accept-Encoding (Section 6.3), Accept-Language (Section 6.4), and User-Agent (Section 9.9 of [Part2]). However, an origin server is not limited to these dimensions and MAY vary the response based on any aspect of the request, including information outside the request-header fields or within extension header fields not defined by this specification.

The Vary header field (Section 3.5 of [Part6]) can be used to express the parameters the server uses to select a representation that is subject to server-driven negotiation.

5.2. Agent-driven Negotiation

With agent-driven negotiation, selection of the best representation for a response is performed by the user agent after receiving an initial response from the origin server. Selection is based on a list of the available representations of the response included within the header fields or body of the initial response, with each representation identified by its own URI. Selection from among the representations can be performed automatically (if the user agent is capable of doing so) or manually by the user selecting from a generated (possibly hypertext) menu.

Agent-driven negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally when public caches are used to distribute server load and reduce network usage.

Agent-driven negotiation suffers from the disadvantage of needing a second request to obtain the best alternate representation. This second request is only efficient when caching is used. In addition, this specification does not define any mechanism for supporting automatic selection, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1.

This specification defines the 300 (Multiple Choices) and 406 (Not Acceptable) status codes for enabling agent-driven negotiation when the server is unwilling or unable to provide a varying response using server-driven negotiation.

7. IANA Considerations

7.2. Content Coding Registry

The registration procedure for HTTP Content Codings is now defined by Section 2.2.1 of this document.

8. 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.

8.2. Content-Disposition Issues

[RFC2183], from which the often implemented Content-Disposition (see Appendix B.1) header in HTTP is derived, has a number of very serious security considerations. Content-Disposition is not part of the HTTP standard, but since it is widely implemented, we are documenting its use and risks for implementors. See Section 5 of [RFC2183] for details.

9. Acknowledgments

10. References

10.1. Normative References

[ISO-8859-1]

International Organization for Standardization, “Information technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1”, ISO/IEC 8859-1:1998, 1998.

[Part1]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: URIs, Connections, and Message Parsing”, Internet-Draft draft-ietf-httpbis-p1-messaging-11 (work in progress), August 2010.

[Part2]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 2: Message Semantics”, Internet-Draft draft-ietf-httpbis-p2-semantics-11 (work in progress), August 2010.

[Part4]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 4: Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-11 (work in progress), August 2010.

[Part5]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests and Partial Responses”, Internet-Draft draft-ietf-httpbis-p5-range-11 (work in progress), August 2010.

[Part6]

Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-11 (work in progress), August 2010.

[RFC1864]

Myers, J. and M. Rose, “The Content-MD5 Header Field”, RFC 1864, October 1995.

[RFC1950]

Deutsch, L. and J-L. Gailly, “ZLIB Compressed Data Format Specification version 3.3”, RFC 1950, May 1996.
RFC 1950 is an Informational RFC, thus it might be less stable than this specification. On the other hand, this downward reference was present since the publication of RFC 2068 in 1997 ([RFC2068]), therefore it is unlikely to cause problems in practice. See also [BCP97].

[RFC1951]

Deutsch, P., “DEFLATE Compressed Data Format Specification version 1.3”, RFC 1951, May 1996.
RFC 1951 is an Informational RFC, thus it might be less stable than this specification. On the other hand, this downward reference was present since the publication of RFC 2068 in 1997 ([RFC2068]), therefore it is unlikely to cause problems in practice. See also [BCP97].

[RFC1952]

Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. Randers-Pehrson, “GZIP file format specification version 4.3”, RFC 1952, May 1996.
RFC 1952 is an Informational RFC, thus it might be less stable than this specification. On the other hand, this downward reference was present since the publication of RFC 2068 in 1997 ([RFC2068]), therefore it is unlikely to cause problems in practice. See also [BCP97].

[RFC2045]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996.

[RFC2046]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types”, RFC 2046, November 1996.

[RFC2119]

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.

[RFC4647]

Phillips, A., Ed. and M. Davis, Ed., “Matching of Language Tags”, BCP 47, RFC 4647, September 2006.

[RFC5234]

Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.

[RFC5646]

Phillips, A., Ed. and M. Davis, Ed., “Tags for Identifying Languages”, BCP 47, RFC 5646, September 2009.

10.2. Informative References

[BCP97]

Klensin, J. and S. Hartman, “Handling Normative References to Standards-Track Documents”, BCP 97, RFC 4897, June 2007.

[RFC1945]

Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.

[RFC2049]

Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples”, RFC 2049, November 1996.

[RFC2068]

Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.

[RFC2076]

Palme, J., “Common Internet Message Headers”, RFC 2076, February 1997.

[RFC2183]

Troost, R., Dorner, S., and K. Moore, “Communicating Presentation Information in Internet Messages: The Content-Disposition Header Field”, RFC 2183, August 1997.

[RFC2277]

Alvestrand, H., “IETF Policy on Character Sets and Languages”, BCP 18, RFC 2277, January 1998.

[RFC2295]

Holtman, K. and A. Mutz, “Transparent Content Negotiation in HTTP”, RFC 2295, March 1998.

[RFC2388]

Masinter, L., “Returning Values from Forms: multipart/form-data”, RFC 2388, August 1998.

[RFC2557]

Palme, F., Hopmann, A., Shelness, N., and E. Stefferud, “MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)”, RFC 2557, March 1999.

[RFC2616]

Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.

[RFC3629]

Yergeau, F., “UTF-8, a transformation format of ISO 10646”, RFC 3629, STD 63, November 2003.

[RFC3864]

Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.

[RFC4288]

Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.

[RFC5226]

Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.

[RFC5322]

Resnick, P., “Internet Message Format”, RFC 5322, October 2008.

Appendix A. Differences between HTTP and MIME

HTTP/1.1 uses many of the constructs defined for Internet Mail ([RFC5322]) and the Multipurpose Internet Mail Extensions (MIME [RFC2045]) to allow a message-body to be transmitted in an open variety of representations and with extensible mechanisms. However, RFC 2045 discusses mail, and HTTP has a few features that are different from those described in MIME. 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.

This appendix describes specific areas where HTTP differs from MIME. 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 might be required.

A.1. MIME-Version

HTTP is not a MIME-compliant protocol. 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 [RFC2045]). Proxies/gateways are responsible for ensuring full compliance (where possible) when exporting HTTP messages to strict MIME environments.

MIME-Version = "MIME-Version" ":" OWS MIME-Version-v MIME-Version-v = 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.

A.2. Conversion to Canonical Form

MIME requires that an Internet mail body-part be converted to canonical form prior to being transferred, as described in Section 4 of [RFC2049]. Section 2.3.1 of this document describes the forms allowed for subtypes of the "text" media type when transmitted over HTTP. [RFC2046] requires that content with a type of "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 MIME environment SHOULD translate all line breaks within the text media types described in Section 2.3.1 of this document to the RFC 2049 canonical form of CRLF. Note, however, that this might 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 will break any cryptographic checksums applied to the original content unless the original content is already in canonical form. Therefore, the canonical form is recommended for any content that uses such checksums in HTTP.

A.3. Conversion of Date Formats

HTTP/1.1 uses a restricted set of date formats (Section 6.1 of [Part1]) 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.

A.4. Introduction of Content-Encoding

MIME 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 representation before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=" to perform a function equivalent to Content-Encoding. However, this parameter is not part of the MIME standards).

A.5. No Content-Transfer-Encoding

HTTP does not use the Content-Transfer-Encoding field of MIME. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any Content-Transfer-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.

A.6. Introduction of Transfer-Encoding

HTTP/1.1 introduces the Transfer-Encoding header field (Section 9.7 of [Part1]). Proxies/gateways MUST remove any transfer-coding prior to forwarding a message via a MIME-compliant protocol.

A.7. MHTML and Line Length Limitations

HTTP implementations which share code with MHTML [RFC2557] implementations need to be aware of MIME line length limitations. Since HTTP does not have this limitation, HTTP does not fold long lines. MHTML messages being transported by HTTP follow all conventions of MHTML, including line length limitations and folding, canonicalization, etc., since HTTP transports all message-bodies as payload (see Section 2.3.2) and does not interpret the content or any MIME header lines that might be contained therein.

Appendix B. Additional Features

[RFC1945] and [RFC2068] document protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementors are advised to 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.

A number of other headers, such as Content-Disposition and Title, from SMTP and MIME are also often implemented (see [RFC2076]).

B.1. Content-Disposition

The "Content-Disposition" response-header field has been proposed as a means for the origin server to suggest a default filename if the user requests that the content is saved to a file. This usage is derived from the definition of Content-Disposition in [RFC2183].

An example is

Content-Disposition: attachment; filename="fname.ext"

The receiving user agent SHOULD NOT respect any directory path information present in the filename-parm parameter, which is the only parameter believed to apply to HTTP implementations at this time. The filename SHOULD be treated as a terminal component only.

If this header is used in a response with the application/octet-stream content-type, the implied suggestion is that the user agent should not display the response, but directly enter a "save response as..." dialog.

See Section 8.2 for Content-Disposition security issues.

Appendix C. Changes from RFC 2616

Clarify contexts that charset is used in. (Section 2.1)

Remove base URI setting semantics for Content-Location due to poor implementation support, which was caused by too many broken servers emitting bogus Content-Location headers, and also the potentially undesirable effect of potentially breaking relative links in content-negotiated resources. (Section 6.7)

Remove reference to non-existant identity transfer-coding value tokens. (Appendix A.5)

Appendix D. Collected ABNF

Accept = "Accept:" OWS Accept-v Accept-Charset = "Accept-Charset:" OWS Accept-Charset-v Accept-Charset-v = ( "," OWS ) ( charset / "" ) [ OWS ";" OWS "q=" qvalue ] ( OWS "," [ OWS ( charset / "" ) [ OWS ";" OWS "q=" qvalue ] ] ) Accept-Encoding = "Accept-Encoding:" OWS Accept-Encoding-v Accept-Encoding-v = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] ) ) *( OWS "," [ OWS codings [ OWS ";" OWS "q=" qvalue ] ] ) ] Accept-Language = "Accept-Language:" OWS Accept-Language-v Accept-Language-v = *( "," OWS ) language-range [ OWS ";" OWS "q=" qvalue ] *( OWS "," [ OWS language-range [ OWS ";" OWS "q=" qvalue ] ] ) Accept-v = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [ OWS media-range [ accept-params ] ] ) ]

Content-Encoding = "Content-Encoding:" OWS Content-Encoding-v Content-Encoding-v = *( "," OWS ) content-coding *( OWS "," [ OWS content-coding ] ) Content-Language = "Content-Language:" OWS Content-Language-v Content-Language-v = *( "," OWS ) language-tag *( OWS "," [ OWS language-tag ] ) Content-Length = <Content-Length, defined in [Part1], Section 9.2> Content-Location = "Content-Location:" OWS Content-Location-v Content-Location-v = absolute-URI / partial-URI Content-MD5 = "Content-MD5:" OWS Content-MD5-v Content-MD5-v = <base64 of 128 bit MD5 digest as per [RFC1864]> Content-Range = <Content-Range, defined in [Part5], Section 5.2> Content-Type = "Content-Type:" OWS Content-Type-v Content-Type-v = media-type

Expires = <Expires, defined in [Part6], Section 3.3>

Last-Modified = <Last-Modified, defined in [Part4], Section 6.6>

MIME-Version = "MIME-Version:" OWS MIME-Version-v MIME-Version-v = 1DIGIT "." 1DIGIT

OWS = <OWS, defined in [Part1], Section 1.2.2>

absolute-URI = <absolute-URI, defined in [Part1], Section 2.6> accept-ext = OWS ";" OWS token [ "=" word ] accept-params = OWS ";" OWS "q=" qvalue *accept-ext attribute = token

charset = token codings = ( content-coding / "*" ) content-coding = token content-disposition = "Content-Disposition:" OWS content-disposition-v content-disposition-v = disposition-type *( OWS ";" OWS disposition-parm )

disp-extension-parm = token "=" word disp-extension-token = token disposition-parm = filename-parm / disp-extension-parm disposition-type = "attachment" / disp-extension-token

filename-parm = "filename=" quoted-string

header-field = <header-field, defined in [Part1], Section 3.2>

language-range = <language-range, defined in [RFC4647], Section 2.1> language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

media-range = ( "/" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS ";" OWS parameter ) media-type = type "/" subtype *( OWS ";" OWS parameter )

parameter = attribute "=" value partial-URI = <partial-URI, defined in [Part1], Section 2.6>

quoted-string = <quoted-string, defined in [Part1], Section 1.2.2> qvalue = <qvalue, defined in [Part1], Section 6.4>

subtype = token

token = <token, defined in [Part1], Section 1.2.2> type = token

value = word

word = <word, defined in [Part1], Section 1.2.2>

ABNF diagnostics:

; Accept defined but not used ; Accept-Charset defined but not used ; Accept-Encoding defined but not used ; Accept-Language defined but not used ; Content-Encoding defined but not used ; Content-Language defined but not used ; Content-Length defined but not used ; Content-Location defined but not used ; Content-MD5 defined but not used ; Content-Range defined but not used ; Content-Type defined but not used ; Expires defined but not used ; Last-Modified defined but not used ; MIME-Version defined but not used ; content-disposition defined but not used ; header-field defined but not used

Appendix E. Change Log (to be removed by RFC Editor before publication)

E.1. Since RFC2616

Extracted relevant partitions from [RFC2616].

E.2. Since draft-ietf-httpbis-p3-payload-00

Closed issues:

E.3. Since draft-ietf-httpbis-p3-payload-01

E.4. Since draft-ietf-httpbis-p3-payload-02

Closed issues:

E.5. Since draft-ietf-httpbis-p3-payload-03

Closed issues:

E.6. Since draft-ietf-httpbis-p3-payload-04

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

E.7. Since draft-ietf-httpbis-p3-payload-05

Other changes:

E.8. Since draft-ietf-httpbis-p3-payload-06

Closed issues:

E.9. Since draft-ietf-httpbis-p3-payload-07

Closed issues:

Partly resolved issues:

E.10. Since draft-ietf-httpbis-p3-payload-08

Closed issues:

E.11. Since draft-ietf-httpbis-p3-payload-09

Closed issues:

E.12. Since draft-ietf-httpbis-p3-payload-10

Closed issues:

Index

A B C D G H I M P R

Authors' Addresses

Roy T. Fielding (editor)
Day Software
23 Corporate Plaza DR, Suite 280
Newport Beach, CA 92660
USA
Phone: +1-949-706-5300
Fax: +1-949-706-5305
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/

Jim Gettys
Alcatel-Lucent Bell Labs
21 Oak Knoll Road
Carlisle, MA 01741
USA
Email: jg@freedesktop.org
URI: http://gettys.wordpress.com/

Jeffrey C. Mogul
Hewlett-Packard Company
HP Labs, Large Scale Systems Group
1501 Page Mill Road, MS 1177
Palo Alto, CA 94304
USA
Email: JeffMogul@acm.org

Henrik Frystyk Nielsen
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
USA
Email: henrikn@microsoft.com

Larry Masinter
Adobe Systems, Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: LMM@acm.org
URI: http://larry.masinter.net/

Paul J. Leach
Microsoft Corporation
1 Microsoft Way
Redmond, WA 98052
Email: paulle@microsoft.com

Tim Berners-Lee
World Wide Web Consortium
MIT Computer Science and Artificial Intelligence Laboratory
The Stata Center, Building 32
32 Vassar Street
Cambridge, MA 02139
USA
Email: timbl@w3.org
URI: http://www.w3.org/People/Berners-Lee/

Yves Lafon (editor)
World Wide Web Consortium
W3C / ERCIM
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
France
Email: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/

Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Phone: +49 251 2807760
Fax: +49 251 2807761
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/