Security Architecture for the Internet Protocol (original) (raw)
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PROPOSED STANDARD
Updated by: 6040, 7619 Errata Exist
Network Working Group S. Kent Request for Comments: 4301 K. Seo Obsoletes: 2401 BBN Technologies Category: Standards Track December 2005
Security Architecture for the Internet ProtocolStatus of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes an updated version of the "Security Architecture for IP", which is designed to provide security services for traffic at the IP layer. This document obsoletes RFC 2401 (November 1998).
Dedication
This document is dedicated to the memory of Charlie Lynn, a long-time senior colleague at BBN, who made very significant contributions to the IPsec documents.
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Table of Contents
1. Introduction ....................................................4 1.1. Summary of Contents of Document ............................4 1.2. Audience ...................................................4 1.3. Related Documents ..........................................5 2. Design Objectives ...............................................5 2.1. Goals/Objectives/Requirements/Problem Description ..........5 2.2. Caveats and Assumptions ....................................6 3. System Overview .................................................7 3.1. What IPsec Does ............................................7 3.2. How IPsec Works ............................................9 3.3. Where IPsec Can Be Implemented ............................10 4. Security Associations ..........................................11 4.1. Definition and Scope ......................................12 4.2. SA Functionality ..........................................16 4.3. Combining SAs .............................................17 4.4. Major IPsec Databases .....................................18 4.4.1. The Security Policy Database (SPD) .................19 4.4.1.1. Selectors .................................26 4.4.1.2. Structure of an SPD Entry .................30 4.4.1.3. More Regarding Fields Associated with Next Layer Protocols .................32 4.4.2. Security Association Database (SAD) ................34 4.4.2.1. Data Items in the SAD .....................36 4.4.2.2. Relationship between SPD, PFP flag, packet, and SAD .....................38 4.4.3. Peer Authorization Database (PAD) ..................43 4.4.3.1. PAD Entry IDs and Matching Rules ..........44 4.4.3.2. IKE Peer Authentication Data ..............45 4.4.3.3. Child SA Authorization Data ...............46 4.4.3.4. How the PAD Is Used .......................46 4.5. SA and Key Management .....................................47 4.5.1. Manual Techniques ..................................48 4.5.2. Automated SA and Key Management ....................48 4.5.3. Locating a Security Gateway ........................49 4.6. SAs and Multicast .........................................50 5. IP Traffic Processing ..........................................50 5.1. Outbound IP Traffic Processing (protected-to-unprotected) ................................52 5.1.1. Handling an Outbound Packet That Must Be Discarded ..........................................54 5.1.2. Header Construction for Tunnel Mode ................55 5.1.2.1. IPv4: Header Construction for Tunnel Mode ...............................57 5.1.2.2. IPv6: Header Construction for Tunnel Mode ...............................59 5.2. Processing Inbound IP Traffic (unprotected-to-protected) ..59
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6. ICMP Processing ................................................63 6.1. Processing ICMP Error Messages Directed to an IPsec Implementation ......................................63 6.1.1. ICMP Error Messages Received on the Unprotected Side of the Boundary ...................63 6.1.2. ICMP Error Messages Received on the Protected Side of the Boundary .....................64 6.2. Processing Protected, Transit ICMP Error Messages .........64 7. Handling Fragments (on the protected side of the IPsec boundary) ......................................................66 7.1. Tunnel Mode SAs that Carry Initial and Non-Initial Fragments .................................................67 7.2. Separate Tunnel Mode SAs for Non-Initial Fragments ........67 7.3. Stateful Fragment Checking ................................68 7.4. BYPASS/DISCARD Traffic ....................................69 8. Path MTU/DF Processing .........................................69 8.1. DF Bit ....................................................69 8.2. Path MTU (PMTU) Discovery .................................70 8.2.1. Propagation of PMTU ................................70 8.2.2. PMTU Aging .........................................71 9. Auditing .......................................................71 10. Conformance Requirements ......................................71 11. Security Considerations .......................................72 12. IANA Considerations ...........................................72 13. Differences from RFC 2401 .....................................72 14. Acknowledgements ..............................................75 Appendix A: Glossary ..............................................76 Appendix B: Decorrelation .........................................79 B.1. Decorrelation Algorithm ...................................79 Appendix C: ASN.1 for an SPD Entry ................................82 Appendix D: Fragment Handling Rationale ...........................88 D.1. Transport Mode and Fragments ..............................88 D.2. Tunnel Mode and Fragments .................................89 D.3. The Problem of Non-Initial Fragments ......................90 D.4. BYPASS/DISCARD Traffic ....................................93 D.5. Just say no to ports? .....................................94 D.6. Other Suggested Solutions..................................94 D.7. Consistency................................................95 D.8. Conclusions................................................95 Appendix E: Example of Supporting Nested SAs via SPD and Forwarding Table Entries...............................96 References.........................................................98 Normative References............................................98 Informative References..........................................99
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1. Introduction
1.1. Summary of Contents of Document
This document specifies the base architecture for IPsec-compliant systems. It describes how to provide a set of security services for traffic at the IP layer, in both the IPv4 [[Pos81a](#ref-Pos81a ""Internet Protocol"")] and IPv6 [[DH98](#ref-DH98 ""Internet Protocol, Version 6 (IPv6) Specification"")] environments. This document describes the requirements for systems that implement IPsec, the fundamental elements of such systems, and how the elements fit together and fit into the IP environment. It also describes the security services offered by the IPsec protocols, and how these services can be employed in the IP environment. This document does not address all aspects of the IPsec architecture. Other documents address additional architectural details in specialized environments, e.g., use of IPsec in Network Address Translation (NAT) environments and more comprehensive support for IP multicast. The fundamental components of the IPsec security architecture are discussed in terms of their underlying, required functionality. Additional RFCs (see Section 1.3 for pointers to other documents) define the protocols in (a), (c), and (d).
a. Security Protocols -- Authentication Header (AH) and
Encapsulating Security Payload (ESP)
b. Security Associations -- what they are and how they work,
how they are managed, associated processing
c. Key Management -- manual and automated (The Internet Key
Exchange (IKE))
d. Cryptographic algorithms for authentication and encryptionThis document is not a Security Architecture for the Internet; it addresses security only at the IP layer, provided through the use of a combination of cryptographic and protocol security mechanisms.
The spelling "IPsec" is preferred and used throughout this and all related IPsec standards. All other capitalizations of IPsec (e.g., IPSEC, IPSec, ipsec) are deprecated. However, any capitalization of the sequence of letters "IPsec" should be understood to refer to the IPsec protocols.
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in RFC 2119 [[Bra97](#ref-Bra97 ""Key words for use in RFCs to Indicate Requirement Level"")].
1.2. Audience
The target audience for this document is primarily individuals who implement this IP security technology or who architect systems that will use this technology. Technically adept users of this technology
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(end users or system administrators) also are part of the target audience. A glossary is provided in Appendix A to help fill in gaps in background/vocabulary. This document assumes that the reader is familiar with the Internet Protocol (IP), related networking technology, and general information system security terms and concepts.
1.3. Related Documents
As mentioned above, other documents provide detailed definitions of some of the components of IPsec and of their interrelationship. They include RFCs on the following topics:
a. security protocols -- RFCs describing the Authentication
Header (AH) [[Ken05b](#ref-Ken05b ""IP Authentication Header"")] and Encapsulating Security Payload
(ESP) [[Ken05a](#ref-Ken05a ""IP Encapsulating Security Payload (ESP)"")] protocols.
b. cryptographic algorithms for integrity and encryption -- one
RFC that defines the mandatory, default algorithms for use
with AH and ESP [[Eas05](#ref-Eas05 ""Cryptographic Algorithm Implementation Requirements For Encapsulating Security Payload (ESP) and Authentication Header (AH)"")], a similar RFC that defines the
mandatory algorithms for use with IKEv2 [[Sch05](#ref-Sch05 ""Cryptographic Algorithms for use in the Internet Key Exchange Version 2 (IKEv2)"")] plus a
separate RFC for each cryptographic algorithm.
c. automatic key management -- RFCs on "The Internet Key
Exchange (IKEv2) Protocol" [[Kau05](#ref-Kau05 ""The Internet Key Exchange (IKEv2) Protocol"")] and "Cryptographic
Algorithms for Use in the Internet Key Exchange Version 2
(IKEv2)" [[Sch05](#ref-Sch05 ""Cryptographic Algorithms for use in the Internet Key Exchange Version 2 (IKEv2)"")].2. Design Objectives
2.1. Goals/Objectives/Requirements/Problem Description
IPsec is designed to provide interoperable, high quality, cryptographically-based security for IPv4 and IPv6. The set of security services offered includes access control, connectionless integrity, data origin authentication, detection and rejection of replays (a form of partial sequence integrity), confidentiality (via encryption), and limited traffic flow confidentiality. These services are provided at the IP layer, offering protection in a standard fashion for all protocols that may be carried over IP (including IP itself).
IPsec includes a specification for minimal firewall functionality, since that is an essential aspect of access control at the IP layer. Implementations are free to provide more sophisticated firewall mechanisms, and to implement the IPsec-mandated functionality using those more sophisticated mechanisms. (Note that interoperability may suffer if additional firewall constraints on traffic flows are imposed by an IPsec implementation but cannot be negotiated based on the traffic selector features defined in this document and negotiated
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via IKEv2.) The IPsec firewall function makes use of the cryptographically-enforced authentication and integrity provided for all IPsec traffic to offer better access control than could be obtained through use of a firewall (one not privy to IPsec internal parameters) plus separate cryptographic protection.
Most of the security services are provided through use of two traffic security protocols, the Authentication Header (AH) and the Encapsulating Security Payload (ESP), and through the use of cryptographic key management procedures and protocols. The set of IPsec protocols employed in a context, and the ways in which they are employed, will be determined by the users/administrators in that context. It is the goal of the IPsec architecture to ensure that compliant implementations include the services and management interfaces needed to meet the security requirements of a broad user population.
When IPsec is correctly implemented and deployed, it ought not adversely affect users, hosts, and other Internet components that do not employ IPsec for traffic protection. IPsec security protocols (AH and ESP, and to a lesser extent, IKE) are designed to be cryptographic algorithm independent. This modularity permits selection of different sets of cryptographic algorithms as appropriate, without affecting the other parts of the implementation. For example, different user communities may select different sets of cryptographic algorithms (creating cryptographically-enforced cliques) if required.
To facilitate interoperability in the global Internet, a set of default cryptographic algorithms for use with AH and ESP is specified in [[Eas05](#ref-Eas05 ""Cryptographic Algorithm Implementation Requirements For Encapsulating Security Payload (ESP) and Authentication Header (AH)"")] and a set of mandatory-to-implement algorithms for IKEv2 is specified in [[Sch05](#ref-Sch05 ""Cryptographic Algorithms for use in the Internet Key Exchange Version 2 (IKEv2)"")]. [[Eas05](#ref-Eas05 ""Cryptographic Algorithm Implementation Requirements For Encapsulating Security Payload (ESP) and Authentication Header (AH)"")] and [[Sch05](#ref-Sch05 ""Cryptographic Algorithms for use in the Internet Key Exchange Version 2 (IKEv2)"")] will be periodically updated to keep pace with computational and cryptologic advances. By specifying these algorithms in documents that are separate from the AH, ESP, and IKEv2 specifications, these algorithms can be updated or replaced without affecting the standardization progress of the rest of the IPsec document suite. The use of these cryptographic algorithms, in conjunction with IPsec traffic protection and key management protocols, is intended to permit system and application developers to deploy high quality, Internet-layer, cryptographic security technology.
2.2. Caveats and Assumptions
The suite of IPsec protocols and associated default cryptographic algorithms are designed to provide high quality security for Internet traffic. However, the security offered by use of these protocols ultimately depends on the quality of their implementation, which is
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outside the scope of this set of standards. Moreover, the security of a computer system or network is a function of many factors, including personnel, physical, procedural, compromising emanations, and computer security practices. Thus, IPsec is only one part of an overall system security architecture.
Finally, the security afforded by the use of IPsec is critically dependent on many aspects of the operating environment in which the IPsec implementation executes. For example, defects in OS security, poor quality of random number sources, sloppy system management protocols and practices, etc., can all degrade the security provided by IPsec. As above, none of these environmental attributes are within the scope of this or other IPsec standards.
3. System Overview
This section provides a high level description of how IPsec works, the components of the system, and how they fit together to provide the security services noted above. The goal of this description is to enable the reader to "picture" the overall process/system, see how it fits into the IP environment, and to provide context for later sections of this document, which describe each of the components in more detail.
An IPsec implementation operates in a host, as a security gateway (SG), or as an independent device, affording protection to IP traffic. (A security gateway is an intermediate system implementing IPsec, e.g., a firewall or router that has been IPsec-enabled.) More detail on these classes of implementations is provided later, in Section 3.3. The protection offered by IPsec is based on requirements defined by a Security Policy Database (SPD) established and maintained by a user or system administrator, or by an application operating within constraints established by either of the above. In general, packets are selected for one of three processing actions based on IP and next layer header information ("Selectors", Section 4.4.1.1) matched against entries in the SPD. Each packet is either PROTECTed using IPsec security services, DISCARDed, or allowed to BYPASS IPsec protection, based on the applicable SPD policies identified by the Selectors.
3.1. What IPsec Does
IPsec creates a boundary between unprotected and protected interfaces, for a host or a network (see Figure 1 below). Traffic traversing the boundary is subject to the access controls specified by the user or administrator responsible for the IPsec configuration. These controls indicate whether packets cross the boundary unimpeded, are afforded security services via AH or ESP, or are discarded.
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IPsec security services are offered at the IP layer through selection of appropriate security protocols, cryptographic algorithms, and cryptographic keys. IPsec can be used to protect one or more "paths" (a) between a pair of hosts, (b) between a pair of security gateways, or (c) between a security gateway and a host. A compliant host implementation MUST support (a) and (c) and a compliant security gateway must support all three of these forms of connectivity, since under certain circumstances a security gateway acts as a host.
Unprotected
^ ^
| |
+-------------|-------|-------+
| +-------+ | | |
| |Discard|<--| V |
| +-------+ |B +--------+ |
................|y..| AH/ESP |..... IPsec Boundary
| +---+ |p +--------+ |
| |IKE|<----|a ^ |
| +---+ |s | |
| +-------+ |s | |
| |Discard|<--| | |
| +-------+ | | |
+-------------|-------|-------+
| |
V V
Protected
Figure 1. Top Level IPsec Processing ModelIn this diagram, "unprotected" refers to an interface that might also be described as "black" or "ciphertext". Here, "protected" refers to an interface that might also be described as "red" or "plaintext". The protected interface noted above may be internal, e.g., in a host implementation of IPsec, the protected interface may link to a socket layer interface presented by the OS. In this document, the term "inbound" refers to traffic entering an IPsec implementation via the unprotected interface or emitted by the implementation on the unprotected side of the boundary and directed towards the protected interface. The term "outbound" refers to traffic entering the implementation via the protected interface, or emitted by the implementation on the protected side of the boundary and directed toward the unprotected interface. An IPsec implementation may support more than one interface on either or both sides of the boundary.
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Note the facilities for discarding traffic on either side of the IPsec boundary, the BYPASS facility that allows traffic to transit the boundary without cryptographic protection, and the reference to IKE as a protected-side key and security management function.
IPsec optionally supports negotiation of IP compression [[SMPT01](#ref-SMPT01 ""IP Payload Compression Protocol (IPComp)"")], motivated in part by the observation that when encryption is employed within IPsec, it prevents effective compression by lower protocol layers.
3.2. How IPsec Works
IPsec uses two protocols to provide traffic security services -- Authentication Header (AH) and Encapsulating Security Payload (ESP). Both protocols are described in detail in their respective RFCs [[Ken05b](#ref-Ken05b ""IP Authentication Header""), [Ken05a](#ref-Ken05a ""IP Encapsulating Security Payload (ESP)"")]. IPsec implementations MUST support ESP and MAY support AH. (Support for AH has been downgraded to MAY because experience has shown that there are very few contexts in which ESP cannot provide the requisite security services. Note that ESP can be used to provide only integrity, without confidentiality, making it comparable to AH in most contexts.)
o The IP Authentication Header (AH) [[Ken05b](#ref-Ken05b ""IP Authentication Header"")] offers integrity and
data origin authentication, with optional (at the discretion of
the receiver) anti-replay features.
o The Encapsulating Security Payload (ESP) protocol [[Ken05a](#ref-Ken05a ""IP Encapsulating Security Payload (ESP)"")] offers
the same set of services, and also offers confidentiality. Use of
ESP to provide confidentiality without integrity is NOT
RECOMMENDED. When ESP is used with confidentiality enabled, there
are provisions for limited traffic flow confidentiality, i.e.,
provisions for concealing packet length, and for facilitating
efficient generation and discard of dummy packets. This
capability is likely to be effective primarily in virtual private
network (VPN) and overlay network contexts.
o Both AH and ESP offer access control, enforced through the
distribution of cryptographic keys and the management of traffic
flows as dictated by the Security Policy Database (SPD, [Section](#section-4.4.1)
[4.4.1](#section-4.4.1)).These protocols may be applied individually or in combination with each other to provide IPv4 and IPv6 security services. However, most security requirements can be met through the use of ESP by itself. Each protocol supports two modes of use: transport mode and tunnel mode. In transport mode, AH and ESP provide protection primarily for
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next layer protocols; in tunnel mode, AH and ESP are applied to tunneled IP packets. The differences between the two modes are discussed in Section 4.1.
IPsec allows the user (or system administrator) to control the granularity at which a security service is offered. For example, one can create a single encrypted tunnel to carry all the traffic between two security gateways, or a separate encrypted tunnel can be created for each TCP connection between each pair of hosts communicating across these gateways. IPsec, through the SPD management paradigm, incorporates facilities for specifying:
o which security protocol (AH or ESP) to employ, the mode (transport
or tunnel), security service options, what cryptographic
algorithms to use, and in what combinations to use the specified
protocols and services, and
o the granularity at which protection should be applied.Because most of the security services provided by IPsec require the use of cryptographic keys, IPsec relies on a separate set of mechanisms for putting these keys in place. This document requires support for both manual and automated distribution of keys. It specifies a specific public-key based approach (IKEv2 [[Kau05](#ref-Kau05 ""The Internet Key Exchange (IKEv2) Protocol"")]) for automated key management, but other automated key distribution techniques MAY be used.
Note: This document mandates support for several features for which support is available in IKEv2 but not in IKEv1, e.g., negotiation of an SA representing ranges of local and remote ports or negotiation of multiple SAs with the same selectors. Therefore, this document assumes use of IKEv2 or a key and security association management system with comparable features.
3.3. Where IPsec Can Be Implemented
There are many ways in which IPsec may be implemented in a host, or in conjunction with a router or firewall to create a security gateway, or as an independent security device.
a. IPsec may be integrated into the native IP stack. This requires access to the IP source code and is applicable to both hosts and security gateways, although native host implementations benefit the most from this strategy, as explained later (Section 4.4.1, paragraph 6; Section 4.4.1.1, last paragraph).
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b. In a "bump-in-the-stack" (BITS) implementation, IPsec is implemented "underneath" an existing implementation of an IP protocol stack, between the native IP and the local network drivers. Source code access for the IP stack is not required in this context, making this implementation approach appropriate for use with legacy systems. This approach, when it is adopted, is usually employed in hosts.
c. The use of a dedicated, inline security protocol processor is a common design feature of systems used by the military, and of some commercial systems as well. It is sometimes referred to as a "bump-in-the-wire" (BITW) implementation. Such implementations may be designed to serve either a host or a gateway. Usually, the BITW device is itself IP addressable. When supporting a single host, it may be quite analogous to a BITS implementation, but in supporting a router or firewall, it must operate like a security gateway.
This document often talks in terms of use of IPsec by a host or a security gateway, without regard to whether the implementation is native, BITS, or BITW. When the distinctions among these implementation options are significant, the document makes reference to specific implementation approaches.
A host implementation of IPsec may appear in devices that might not be viewed as "hosts". For example, a router might employ IPsec to protect routing protocols (e.g., BGP) and management functions (e.g., Telnet), without affecting subscriber traffic traversing the router. A security gateway might employ separate IPsec implementations to protect its management traffic and subscriber traffic. The architecture described in this document is very flexible. For example, a computer with a full-featured, compliant, native OS IPsec implementation should be capable of being configured to protect resident (host) applications and to provide security gateway protection for traffic traversing the computer. Such configuration would make use of the forwarding tables and the SPD selection function described in Sections 5.1 and 5.2.
4. Security Associations
This section defines Security Association management requirements for all IPv6 implementations and for those IPv4 implementations that implement AH, ESP, or both AH and ESP. The concept of a "Security Association" (SA) is fundamental to IPsec. Both AH and ESP make use of SAs, and a major function of IKE is the establishment and maintenance of SAs. All implementations of AH or ESP MUST support the concept of an SA as described below. The remainder of this
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section describes various aspects of SA management, defining required characteristics for SA policy management and SA management techniques.
4.1. Definition and Scope
An SA is a simplex "connection" that affords security services to the traffic carried by it. Security services are afforded to an SA by the use of AH, or ESP, but not both. If both AH and ESP protection are applied to a traffic stream, then two SAs must be created and coordinated to effect protection through iterated application of the security protocols. To secure typical, bi-directional communication between two IPsec-enabled systems, a pair of SAs (one in each direction) is required. IKE explicitly creates SA pairs in recognition of this common usage requirement.
For an SA used to carry unicast traffic, the Security Parameters Index (SPI) by itself suffices to specify an SA. (For information on the SPI, see Appendix A and the AH and ESP specifications [Ken05b, Ken05a].) However, as a local matter, an implementation may choose to use the SPI in conjunction with the IPsec protocol type (AH or ESP) for SA identification. If an IPsec implementation supports multicast, then it MUST support multicast SAs using the algorithm below for mapping inbound IPsec datagrams to SAs. Implementations that support only unicast traffic need not implement this de- multiplexing algorithm.
In many secure multicast architectures, e.g., [[RFC3740](./rfc3740 ""The Multicast Group Security Architecture"")], a central Group Controller/Key Server unilaterally assigns the Group Security Association's (GSA's) SPI. This SPI assignment is not negotiated or coordinated with the key management (e.g., IKE) subsystems that reside in the individual end systems that constitute the group. Consequently, it is possible that a GSA and a unicast SA can simultaneously use the same SPI. A multicast-capable IPsec implementation MUST correctly de-multiplex inbound traffic even in the context of SPI collisions.
Each entry in the SA Database (SAD) (Section 4.4.2) must indicate whether the SA lookup makes use of the destination IP address, or the destination and source IP addresses, in addition to the SPI. For multicast SAs, the protocol field is not employed for SA lookups. For each inbound, IPsec-protected packet, an implementation must conduct its search of the SAD such that it finds the entry that matches the "longest" SA identifier. In this context, if two or more SAD entries match based on the SPI value, then the entry that also matches based on destination address, or destination and source address (as indicated in the SAD entry) is the "longest" match. This implies a logical ordering of the SAD search as follows:
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1. Search the SAD for a match on the combination of SPI,
destination address, and source address. If an SAD entry
matches, then process the inbound packet with that
matching SAD entry. Otherwise, proceed to step 2.
2. Search the SAD for a match on both SPI and destination address.
If the SAD entry matches, then process the inbound packet
with that matching SAD entry. Otherwise, proceed to step 3.
3. Search the SAD for a match on only SPI if the receiver has
chosen to maintain a single SPI space for AH and ESP, and on
both SPI and protocol, otherwise. If an SAD entry matches,
then process the inbound packet with that matching SAD entry.
Otherwise, discard the packet and log an auditable event.In practice, an implementation may choose any method (or none at all) to accelerate this search, although its externally visible behavior MUST be functionally equivalent to having searched the SAD in the above order. For example, a software-based implementation could index into a hash table by the SPI. The SAD entries in each hash table bucket's linked list could be kept sorted to have those SAD entries with the longest SA identifiers first in that linked list. Those SAD entries having the shortest SA identifiers could be sorted so that they are the last entries in the linked list. A hardware-based implementation may be able to effect the longest match search intrinsically, using commonly available Ternary Content-Addressable Memory (TCAM) features.
The indication of whether source and destination address matching is required to map inbound IPsec traffic to SAs MUST be set either as a side effect of manual SA configuration or via negotiation using an SA management protocol, e.g., IKE or Group Domain of Interpretation (GDOI) [[RFC3547](./rfc3547 ""The Group Domain of Interpretation"")]. Typically, Source-Specific Multicast (SSM) [HC03] groups use a 3-tuple SA identifier composed of an SPI, a destination multicast address, and source address. An Any-Source Multicast group SA requires only an SPI and a destination multicast address as an identifier.
If different classes of traffic (distinguished by Differentiated Services Code Point (DSCP) bits [[NiBlBaBL98](#ref-NiBlBaBL98 ""Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"")], [[Gro02](#ref-Gro02 ""New Terminology and Clarifications for Diffserv"")]) are sent on the same SA, and if the receiver is employing the optional anti-replay feature available in both AH and ESP, this could result in inappropriate discarding of lower priority packets due to the windowing mechanism used by this feature. Therefore, a sender SHOULD put traffic of different classes, but with the same selector values, on different SAs to support Quality of Service (QoS) appropriately. To permit this, the IPsec implementation MUST permit establishment and maintenance of multiple SAs between a given sender and receiver,
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with the same selectors. Distribution of traffic among these parallel SAs to support QoS is locally determined by the sender and is not negotiated by IKE. The receiver MUST process the packets from the different SAs without prejudice. These requirements apply to both transport and tunnel mode SAs. In the case of tunnel mode SAs, the DSCP values in question appear in the inner IP header. In transport mode, the DSCP value might change en route, but this should not cause problems with respect to IPsec processing since the value is not employed for SA selection and MUST NOT be checked as part of SA/packet validation. However, if significant re-ordering of packets occurs in an SA, e.g., as a result of changes to DSCP values en route, this may trigger packet discarding by a receiver due to application of the anti-replay mechanism.
DISCUSSION: Although the DSCP [[NiBlBaBL98](#ref-NiBlBaBL98 ""Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers""), [Gro02](#ref-Gro02 ""New Terminology and Clarifications for Diffserv"")] and Explicit Congestion Notification (ECN) [[RaFlBl01](#ref-RaFlBl01 ""The Addition of Explicit Congestion Notification (ECN) to IP"")] fields are not "selectors", as that term in used in this architecture, the sender will need a mechanism to direct packets with a given (set of) DSCP values to the appropriate SA. This mechanism might be termed a "classifier".
As noted above, two types of SAs are defined: transport mode and tunnel mode. IKE creates pairs of SAs, so for simplicity, we choose to require that both SAs in a pair be of the same mode, transport or tunnel.
A transport mode SA is an SA typically employed between a pair of hosts to provide end-to-end security services. When security is desired between two intermediate systems along a path (vs. end-to-end use of IPsec), transport mode MAY be used between security gateways or between a security gateway and a host. In the case where transport mode is used between security gateways or between a security gateway and a host, transport mode may be used to support in-IP tunneling (e.g., IP-in-IP [[Per96](#ref-Per96 ""IP Encapsulation within IP"")] or Generic Routing Encapsulation (GRE) tunneling [[FaLiHaMeTr00](#ref-FaLiHaMeTr00 ""Generic Routing Encapsulation (GRE)"")] or dynamic routing [[ToEgWa04](#ref-ToEgWa04 ""Use of IPsec Transport Mode for Dynamic Routing"")]) over transport mode SAs. To clarify, the use of transport mode by an intermediate system (e.g., a security gateway) is permitted only when applied to packets whose source address (for outbound packets) or destination address (for inbound packets) is an address belonging to the intermediate system itself. The access control functions that are an important part of IPsec are significantly limited in this context, as they cannot be applied to the end-to-end headers of the packets that traverse a transport mode SA used in this fashion. Thus, this way of using transport mode should be evaluated carefully before being employed in a specific context.
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In IPv4, a transport mode security protocol header appears immediately after the IP header and any options, and before any next layer protocols (e.g., TCP or UDP). In IPv6, the security protocol header appears after the base IP header and selected extension headers, but may appear before or after destination options; it MUST appear before next layer protocols (e.g., TCP, UDP, Stream Control Transmission Protocol (SCTP)). In the case of ESP, a transport mode SA provides security services only for these next layer protocols, not for the IP header or any extension headers preceding the ESP header. In the case of AH, the protection is also extended to selected portions of the IP header preceding it, selected portions of extension headers, and selected options (contained in the IPv4 header, IPv6 Hop-by-Hop extension header, or IPv6 Destination extension headers). For more details on the coverage afforded by AH, see the AH specification [[Ken05b](#ref-Ken05b ""IP Authentication Header"")].
A tunnel mode SA is essentially an SA applied to an IP tunnel, with the access controls applied to the headers of the traffic inside the tunnel. Two hosts MAY establish a tunnel mode SA between themselves. Aside from the two exceptions below, whenever either end of a security association is a security gateway, the SA MUST be tunnel mode. Thus, an SA between two security gateways is typically a tunnel mode SA, as is an SA between a host and a security gateway. The two exceptions are as follows.
o Where traffic is destined for a security gateway, e.g., Simple
Network Management Protocol (SNMP) commands, the security gateway
is acting as a host and transport mode is allowed. In this case,
the SA terminates at a host (management) function within a
security gateway and thus merits different treatment.
o As noted above, security gateways MAY support a transport mode SA
to provide security for IP traffic between two intermediate
systems along a path, e.g., between a host and a security gateway
or between two security gateways.Several concerns motivate the use of tunnel mode for an SA involving a security gateway. For example, if there are multiple paths (e.g., via different security gateways) to the same destination behind a security gateway, it is important that an IPsec packet be sent to the security gateway with which the SA was negotiated. Similarly, a packet that might be fragmented en route must have all the fragments delivered to the same IPsec instance for reassembly prior to cryptographic processing. Also, when a fragment is processed by IPsec and transmitted, then fragmented en route, it is critical that there be inner and outer headers to retain the fragmentation state data for the pre- and post-IPsec packet formats. Hence there are several reasons for employing tunnel mode when either end of an SA is
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a security gateway. (Use of an IP-in-IP tunnel in conjunction with transport mode can also address these fragmentation issues. However, this configuration limits the ability of IPsec to enforce access control policies on traffic.)
Note: AH and ESP cannot be applied using transport mode to IPv4 packets that are fragments. Only tunnel mode can be employed in such cases. For IPv6, it would be feasible to carry a plaintext fragment on a transport mode SA; however, for simplicity, this restriction also applies to IPv6 packets. See Section 7 for more details on handling plaintext fragments on the protected side of the IPsec barrier.
For a tunnel mode SA, there is an "outer" IP header that specifies the IPsec processing source and destination, plus an "inner" IP header that specifies the (apparently) ultimate source and destination for the packet. The security protocol header appears after the outer IP header, and before the inner IP header. If AH is employed in tunnel mode, portions of the outer IP header are afforded protection (as above), as well as all of the tunneled IP packet (i.e., all of the inner IP header is protected, as well as next layer protocols). If ESP is employed, the protection is afforded only to the tunneled packet, not to the outer header.
In summary,
a) A host implementation of IPsec MUST support both transport and tunnel mode. This is true for native, BITS, and BITW implementations for hosts.
b) A security gateway MUST support tunnel mode and MAY support transport mode. If it supports transport mode, that should be used only when the security gateway is acting as a host, e.g., for network management, or to provide security between two intermediate systems along a path.
4.2. SA Functionality
The set of security services offered by an SA depends on the security protocol selected, the SA mode, the endpoints of the SA, and the election of optional services within the protocol.
For example, both AH and ESP offer integrity and authentication services, but the coverage differs for each protocol and differs for transport vs. tunnel mode. If the integrity of an IPv4 option or IPv6 extension header must be protected en route between sender and receiver, AH can provide this service, except for IP or extension headers that may change in a fashion not predictable by the sender.
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However, the same security may be achieved in some contexts by applying ESP to a tunnel carrying a packet.
The granularity of access control provided is determined by the choice of the selectors that define each SA. Moreover, the authentication means employed by IPsec peers, e.g., during creation of an IKE (vs. child) SA also affects the granularity of the access control afforded.
If confidentiality is selected, then an ESP (tunnel mode) SA between two security gateways can offer partial traffic flow confidentiality. The use of tunnel mode allows the inner IP headers to be encrypted, concealing the identities of the (ultimate) traffic source and destination. Moreover, ESP payload padding also can be invoked to hide the size of the packets, further concealing the external characteristics of the traffic. Similar traffic flow confidentiality services may be offered when a mobile user is assigned a dynamic IP address in a dialup context, and establishes a (tunnel mode) ESP SA to a corporate firewall (acting as a security gateway). Note that fine-granularity SAs generally are more vulnerable to traffic analysis than coarse-granularity ones that are carrying traffic from many subscribers.
Note: A compliant implementation MUST NOT allow instantiation of an ESP SA that employs both NULL encryption and no integrity algorithm. An attempt to negotiate such an SA is an auditable event by both initiator and responder. The audit log entry for this event SHOULD include the current date/time, local IKE IP address, and remote IKE IP address. The initiator SHOULD record the relevant SPD entry.
4.3. Combining SAs
This document does not require support for nested security associations or for what RFC 2401 [[RFC2401](./rfc2401 ""Security Architecture for the Internet Protocol"")] called "SA bundles". These features still can be effected by appropriate configuration of both the SPD and the local forwarding functions (for inbound and outbound traffic), but this capability is outside of the IPsec module and thus the scope of this specification. As a result, management of nested/bundled SAs is potentially more complex and less assured than under the model implied by RFC 2401 [[RFC2401](./rfc2401 ""Security Architecture for the Internet Protocol"")]. An implementation that provides support for nested SAs SHOULD provide a management interface that enables a user or administrator to express the nesting requirement, and then create the appropriate SPD entries and forwarding table entries to effect the requisite processing. (See Appendix E for an example of how to configure nested SAs.)
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4.4. Major IPsec Databases
Many of the details associated with processing IP traffic in an IPsec implementation are largely a local matter, not subject to standardization. However, some external aspects of the processing must be standardized to ensure interoperability and to provide a minimum management capability that is essential for productive use of IPsec. This section describes a general model for processing IP traffic relative to IPsec functionality, in support of these interoperability and functionality goals. The model described below is nominal; implementations need not match details of this model as presented, but the external behavior of implementations MUST correspond to the externally observable characteristics of this model in order to be compliant.
There are three nominal databases in this model: the Security Policy Database (SPD), the Security Association Database (SAD), and the Peer Authorization Database (PAD). The first specifies the policies that determine the disposition of all IP traffic inbound or outbound from a host or security gateway (Section 4.4.1). The second database contains parameters that are associated with each established (keyed) SA (Section 4.4.2). The third database, the PAD, provides a link between an SA management protocol (such as IKE) and the SPD (Section 4.4.3).
Multiple Separate IPsec Contexts
If an IPsec implementation acts as a security gateway for multiple
subscribers, it MAY implement multiple separate IPsec contexts.
Each context MAY have and MAY use completely independent
identities, policies, key management SAs, and/or IPsec SAs. This
is for the most part a local implementation matter. However, a
means for associating inbound (SA) proposals with local contexts
is required. To this end, if supported by the key management
protocol in use, context identifiers MAY be conveyed from
initiator to responder in the signaling messages, with the result
that IPsec SAs are created with a binding to a particular context.
For example, a security gateway that provides VPN service to
multiple customers will be able to associate each customer's
traffic with the correct VPN.Forwarding vs Security Decisions
The IPsec model described here embodies a clear separation between
forwarding (routing) and security decisions, to accommodate a wide
range of contexts where IPsec may be employed. Forwarding may be
trivial, in the case where there are only two interfaces, or it
may be complex, e.g., if the context in which IPsec is implementedKent & Seo Standards Track [Page 18]
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employs a sophisticated forwarding function. IPsec assumes only
that outbound and inbound traffic that has passed through IPsec
processing is forwarded in a fashion consistent with the context
in which IPsec is implemented. Support for nested SAs is
optional; if required, it requires coordination between forwarding
tables and SPD entries to cause a packet to traverse the IPsec
boundary more than once."Local" vs "Remote"
In this document, with respect to IP addresses and ports, the
terms "Local" and "Remote" are used for policy rules. "Local"
refers to the entity being protected by an IPsec implementation,
i.e., the "source" address/port of outbound packets or the
"destination" address/port of inbound packets. "Remote" refers to
a peer entity or peer entities. The terms "source" and
"destination" are used for packet header fields."Non-initial" vs "Initial" Fragments
Throughout this document, the phrase "non-initial fragments" is
used to mean fragments that do not contain all of the selector
values that may be needed for access control (e.g., they might not
contain Next Layer Protocol, source and destination ports, ICMP
message type/code, Mobility Header type). And the phrase "initial
fragment" is used to mean a fragment that contains all the
selector values needed for access control. However, it should be
noted that for IPv6, which fragment contains the Next Layer
Protocol and ports (or ICMP message type/code or Mobility Header
type [[Mobip](#ref-Mobip ""Mobility Support in IPv6"")]) will depend on the kind and number of extension
headers present. The "initial fragment" might not be the first
fragment, in this context.4.4.1. The Security Policy Database (SPD)
An SA is a management construct used to enforce security policy for traffic crossing the IPsec boundary. Thus, an essential element of SA processing is an underlying Security Policy Database (SPD) that specifies what services are to be offered to IP datagrams and in what fashion. The form of the database and its interface are outside the scope of this specification. However, this section specifies minimum management functionality that must be provided, to allow a user or system administrator to control whether and how IPsec is applied to traffic transmitted or received by a host or transiting a security gateway. The SPD, or relevant caches, must be consulted during the processing of all traffic (inbound and outbound), including traffic not protected by IPsec, that traverses the IPsec boundary. This includes IPsec management traffic such as IKE. An IPsec
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implementation MUST have at least one SPD, and it MAY support multiple SPDs, if appropriate for the context in which the IPsec implementation operates. There is no requirement to maintain SPDs on a per-interface basis, as was specified in RFC 2401 [[RFC2401](./rfc2401 ""Security Architecture for the Internet Protocol"")]. However, if an implementation supports multiple SPDs, then it MUST include an explicit SPD selection function that is invoked to select the appropriate SPD for outbound traffic processing. The inputs to this function are the outbound packet and any local metadata (e.g., the interface via which the packet arrived) required to effect the SPD selection function. The output of the function is an SPD identifier (SPD-ID).
The SPD is an ordered database, consistent with the use of Access Control Lists (ACLs) or packet filters in firewalls, routers, etc. The ordering requirement arises because entries often will overlap due to the presence of (non-trivial) ranges as values for selectors. Thus, a user or administrator MUST be able to order the entries to express a desired access control policy. There is no way to impose a general, canonical order on SPD entries, because of the allowed use of wildcards for selector values and because the different types of selectors are not hierarchically related.
Processing Choices: DISCARD, BYPASS, PROTECT
An SPD must discriminate among traffic that is afforded IPsec
protection and traffic that is allowed to bypass IPsec. This
applies to the IPsec protection to be applied by a sender and to
the IPsec protection that must be present at the receiver. For
any outbound or inbound datagram, three processing choices are
possible: DISCARD, BYPASS IPsec, or PROTECT using IPsec. The
first choice refers to traffic that is not allowed to traverse the
IPsec boundary (in the specified direction). The second choice
refers to traffic that is allowed to cross the IPsec boundary
without IPsec protection. The third choice refers to traffic that
is afforded IPsec protection, and for such traffic the SPD must
specify the security protocols to be employed, their mode,
security service options, and the cryptographic algorithms to be
used.SPD-S, SPD-I, SPD-O
An SPD is logically divided into three pieces. The SPD-S (secure
traffic) contains entries for all traffic subject to IPsec
protection. SPD-O (outbound) contains entries for all outbound
traffic that is to be bypassed or discarded. SPD-I (inbound) is
applied to inbound traffic that will be bypassed or discarded.
All three of these can be decorrelated (with the exception noted
above for native host implementations) to facilitate caching. IfKent & Seo Standards Track [Page 20]
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an IPsec implementation supports only one SPD, then the SPD
consists of all three parts. If multiple SPDs are supported, some
of them may be partial, e.g., some SPDs might contain only SPD-I
entries, to control inbound bypassed traffic on a per-interface
basis. The split allows SPD-I to be consulted without having to
consult SPD-S, for such traffic. Since the SPD-I is just a part
of the SPD, if a packet that is looked up in the SPD-I cannot be
matched to an entry there, then the packet MUST be discarded.
Note that for outbound traffic, if a match is not found in SPD-S,
then SPD-O must be checked to see if the traffic should be
bypassed. Similarly, if SPD-O is checked first and no match is
found, then SPD-S must be checked. In an ordered,
non-decorrelated SPD, the entries for the SPD-S, SPD-I, and SPD-O
are interleaved. So there is one lookup in the SPD.SPD Entries
Each SPD entry specifies packet disposition as BYPASS, DISCARD, or
PROTECT. The entry is keyed by a list of one or more selectors.
The SPD contains an ordered list of these entries. The required
selector types are defined in [Section 4.4.1.1](#section-4.4.1.1). These selectors are
used to define the granularity of the SAs that are created in
response to an outbound packet or in response to a proposal from a
peer. The detailed structure of an SPD entry is described in
[Section 4.4.1.2](#section-4.4.1.2). Every SPD SHOULD have a nominal, final entry that
matches anything that is otherwise unmatched, and discards it.
The SPD MUST permit a user or administrator to specify policy
entries as follows:
- SPD-I: For inbound traffic that is to be bypassed or discarded,
the entry consists of the values of the selectors that apply to
the traffic to be bypassed or discarded.
- SPD-O: For outbound traffic that is to be bypassed or
discarded, the entry consists of the values of the selectors
that apply to the traffic to be bypassed or discarded.
- SPD-S: For traffic that is to be protected using IPsec, the
entry consists of the values of the selectors that apply to the
traffic to be protected via AH or ESP, controls on how to
create SAs based on these selectors, and the parameters needed
to effect this protection (e.g., algorithms, modes, etc.). Note
that an SPD-S entry also contains information such as "populate
from packet" (PFP) flag (see paragraphs below on "How To Derive
the Values for an SAD entry") and bits indicating whether theKent & Seo Standards Track [Page 21]
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SA lookup makes use of the local and remote IP addresses in
addition to the SPI (see AH [[Ken05b](#ref-Ken05b ""IP Authentication Header"")] or ESP [[Ken05a](#ref-Ken05a ""IP Encapsulating Security Payload (ESP)"")]
specifications).Representing Directionality in an SPD Entry
For traffic protected by IPsec, the Local and Remote address and
ports in an SPD entry are swapped to represent directionality,
consistent with IKE conventions. In general, the protocols that
IPsec deals with have the property of requiring symmetric SAs with
flipped Local/Remote IP addresses. However, for ICMP, there is
often no such bi-directional authorization requirement.
Nonetheless, for the sake of uniformity and simplicity, SPD
entries for ICMP are specified in the same way as for other
protocols. Note also that for ICMP, Mobility Header, and
non-initial fragments, there are no port fields in these packets.
ICMP has message type and code and Mobility Header has mobility
header type. Thus, SPD entries have provisions for expressing
access controls appropriate for these protocols, in lieu of the
normal port field controls. For bypassed or discarded traffic,
separate inbound and outbound entries are supported, e.g., to
permit unidirectional flows if required.OPAQUE and ANY
For each selector in an SPD entry, in addition to the literal
values that define a match, there are two special values: ANY and
OPAQUE. ANY is a wildcard that matches any value in the
corresponding field of the packet, or that matches packets where
that field is not present or is obscured. OPAQUE indicates that
the corresponding selector field is not available for examination
because it may not be present in a fragment, it does not exist for
the given Next Layer Protocol, or prior application of IPsec may
have encrypted the value. The ANY value encompasses the OPAQUE
value. Thus, OPAQUE need be used only when it is necessary to
distinguish between the case of any allowed value for a field, vs.
the absence or unavailability (e.g., due to encryption) of the
field.How to Derive the Values for an SAD Entry
For each selector in an SPD entry, the entry specifies how to
derive the corresponding values for a new SA Database (SAD, see
[Section 4.4.2](#section-4.4.2)) entry from those in the SPD and the packet. The
goal is to allow an SAD entry and an SPD cache entry to be created
based on specific selector values from the packet, or from the
matching SPD entry. For outbound traffic, there are SPD-S cache
entries and SPD-O cache entries. For inbound traffic notKent & Seo Standards Track [Page 22]
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protected by IPsec, there are SPD-I cache entries and there is the
SAD, which represents the cache for inbound IPsec-protected
traffic (see [Section 4.4.2](#section-4.4.2)). If IPsec processing is specified for
an entry, a "populate from packet" (PFP) flag may be asserted for
one or more of the selectors in the SPD entry (Local IP address;
Remote IP address; Next Layer Protocol; and, depending on Next
Layer Protocol, Local port and Remote port, or ICMP type/code, or
Mobility Header type). If asserted for a given selector X, the
flag indicates that the SA to be created should take its value for
X from the value in the packet. Otherwise, the SA should take its
value(s) for X from the value(s) in the SPD entry. Note: In the
non-PFP case, the selector values negotiated by the SA management
protocol (e.g., IKEv2) may be a subset of those in the SPD entry,
depending on the SPD policy of the peer. Also, whether a single
flag is used for, e.g., source port, ICMP type/code, and Mobility
Header (MH) type, or a separate flag is used for each, is a local
matter.
The following example illustrates the use of the PFP flag in the
context of a security gateway or a BITS/BITW implementation.
Consider an SPD entry where the allowed value for Remote address
is a range of IPv4 addresses: 192.0.2.1 to 192.0.2.10. Suppose an
outbound packet arrives with a destination address of 192.0.2.3,
and there is no extant SA to carry this packet. The value used
for the SA created to transmit this packet could be either of the
two values shown below, depending on what the SPD entry for this
selector says is the source of the selector value:
PFP flag value example of new
for the Remote SAD dest. address
addr. selector selector value
--------------- ------------
a. PFP TRUE 192.0.2.3 (one host)
b. PFP FALSE 192.0.2.1 to 192.0.2.10 (range of hosts)
Note that if the SPD entry above had a value of ANY for the Remote
address, then the SAD selector value would have to be ANY for case
(b), but would still be as illustrated for case (a). Thus, the
PFP flag can be used to prohibit sharing of an SA, even among
packets that match the same SPD entry.Management Interface
For every IPsec implementation, there MUST be a management
interface that allows a user or system administrator to manage the
SPD. The interface must allow the user (or administrator) to
specify the security processing to be applied to every packet that
traverses the IPsec boundary. (In a native host IPsecKent & Seo Standards Track [Page 23]
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implementation making use of a socket interface, the SPD may not
need to be consulted on a per-packet basis, as noted at the end of
[Section 4.4.1.1](#section-4.4.1.1) and in [Section 5](#section-5).) The management interface for
the SPD MUST allow creation of entries consistent with the
selectors defined in [Section 4.4.1.1](#section-4.4.1.1), and MUST support (total)
ordering of these entries, as seen via this interface. The SPD
entries' selectors are analogous to the ACL or packet filters
commonly found in a stateless firewall or packet filtering router
and which are currently managed this way.
In host systems, applications MAY be allowed to create SPD
entries. (The means of signaling such requests to the IPsec
implementation are outside the scope of this standard.) However,
the system administrator MUST be able to specify whether or not a
user or application can override (default) system policies. The
form of the management interface is not specified by this document
and may differ for hosts vs. security gateways, and within hosts
the interface may differ for socket-based vs. BITS
implementations. However, this document does specify a standard
set of SPD elements that all IPsec implementations MUST support.Decorrelation
The processing model described in this document assumes the
ability to decorrelate overlapping SPD entries to permit caching,
which enables more efficient processing of outbound traffic in
security gateways and BITS/BITW implementations. Decorrelation
[[CoSa04](#ref-CoSa04 ""On the Deterministic Enforcement of Un-ordered Security Policies"")] is only a means of improving performance and simplifying
the processing description. This RFC does not require a compliant
implementation to make use of decorrelation. For example, native
host implementations typically make use of caching implicitly
because they bind SAs to socket interfaces, and thus there is no
requirement to be able to decorrelate SPD entries in these
implementations.
Note: Unless otherwise qualified, the use of "SPD" refers to the
body of policy information in both ordered or decorrelated
(unordered) state. [Appendix B](#appendix-B) provides an algorithm that can be
used to decorrelate SPD entries, but any algorithm that produces
equivalent output may be used. Note that when an SPD entry is
decorrelated all the resulting entries MUST be linked together, so
that all members of the group derived from an individual, SPD
entry (prior to decorrelation) can all be placed into caches and
into the SAD at the same time. For example, suppose one starts
with an entry A (from an ordered SPD) that when decorrelated,
yields entries A1, A2, and A3. When a packet comes along that
matches, say A2, and triggers the creation of an SA, the SA
management protocol (e.g., IKEv2) negotiates A. And all 3Kent & Seo Standards Track [Page 24]
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decorrelated entries, A1, A2, and A3, are placed in the
appropriate SPD-S cache and linked to the SA. The intent is that
use of a decorrelated SPD ought not to create more SAs than would
have resulted from use of a not-decorrelated SPD.
If a decorrelated SPD is employed, there are three options for
what an initiator sends to a peer via an SA management protocol
(e.g., IKE). By sending the complete set of linked, decorrelated
entries that were selected from the SPD, a peer is given the best
possible information to enable selection of the appropriate SPD
entry at its end, especially if the peer has also decorrelated its
SPD. However, if a large number of decorrelated entries are
linked, this may create large packets for SA negotiation, and
hence fragmentation problems for the SA management protocol.
Alternatively, the original entry from the (correlated) SPD may be
retained and passed to the SA management protocol. Passing the
correlated SPD entry keeps the use of a decorrelated SPD a local
matter, not visible to peers, and avoids possible fragmentation
concerns, although it provides less precise information to a
responder for matching against the responder's SPD.
An intermediate approach is to send a subset of the complete set
of linked, decorrelated SPD entries. This approach can avoid the
fragmentation problems cited above yet provide better information
than the original, correlated entry. The major shortcoming of
this approach is that it may cause additional SAs to be created
later, since only a subset of the linked, decorrelated entries are
sent to a peer. Implementers are free to employ any of the
approaches cited above.
A responder uses the traffic selector proposals it receives via an
SA management protocol to select an appropriate entry in its SPD.
The intent of the matching is to select an SPD entry and create an
SA that most closely matches the intent of the initiator, so that
traffic traversing the resulting SA will be accepted at both ends.
If the responder employs a decorrelated SPD, it SHOULD use the
decorrelated SPD entries for matching, as this will generally
result in creation of SAs that are more likely to match the intent
of both peers. If the responder has a correlated SPD, then it
SHOULD match the proposals against the correlated entries. For
IKEv2, use of a decorrelated SPD offers the best opportunity for a
responder to generate a "narrowed" response.
In all cases, when a decorrelated SPD is available, the
decorrelated entries are used to populate the SPD-S cache. If the
SPD is not decorrelated, caching is not allowed and an orderedKent & Seo Standards Track [Page 25]
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search of SPD MUST be performed to verify that inbound traffic
arriving on an SA is consistent with the access control policy
expressed in the SPD.Handling Changes to the SPD While the System Is Running
If a change is made to the SPD while the system is running, a
check SHOULD be made of the effect of this change on extant SAs.
An implementation SHOULD check the impact of an SPD change on
extant SAs and SHOULD provide a user/administrator with a
mechanism for configuring what actions to take, e.g., delete an
affected SA, allow an affected SA to continue unchanged, etc.4.4.1.1. Selectors
An SA may be fine-grained or coarse-grained, depending on the selectors used to define the set of traffic for the SA. For example, all traffic between two hosts may be carried via a single SA, and afforded a uniform set of security services. Alternatively, traffic between a pair of hosts might be spread over multiple SAs, depending on the applications being used (as defined by the Next Layer Protocol and related fields, e.g., ports), with different security services offered by different SAs. Similarly, all traffic between a pair of security gateways could be carried on a single SA, or one SA could be assigned for each communicating host pair. The following selector parameters MUST be supported by all IPsec implementations to facilitate control of SA granularity. Note that both Local and Remote addresses should either be IPv4 or IPv6, but not a mix of address types. Also, note that the Local/Remote port selectors (and ICMP message type and code, and Mobility Header type) may be labeled as OPAQUE to accommodate situations where these fields are inaccessible due to packet fragmentation.
- Remote IP Address(es) (IPv4 or IPv6): This is a list of ranges
of IP addresses (unicast, broadcast (IPv4 only)). This
structure allows expression of a single IP address (via a
trivial range), or a list of addresses (each a trivial range),
or a range of addresses (low and high values, inclusive), as
well as the most generic form of a list of ranges. Address
ranges are used to support more than one remote system sharing
the same SA, e.g., behind a security gateway.
- Local IP Address(es) (IPv4 or IPv6): This is a list of ranges of
IP addresses (unicast, broadcast (IPv4 only)). This structure
allows expression of a single IP address (via a trivial range),
or a list of addresses (each a trivial range), or a range of
addresses (low and high values, inclusive), as well as the most
generic form of a list of ranges. Address ranges are used toKent & Seo Standards Track [Page 26]
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support more than one source system sharing the same SA, e.g.,
behind a security gateway. Local refers to the address(es)
being protected by this implementation (or policy entry).
Note: The SPD does not include support for multicast address
entries. To support multicast SAs, an implementation should
make use of a Group SPD (GSPD) as defined in [[RFC3740](./rfc3740 ""The Multicast Group Security Architecture"")]. GSPD
entries require a different structure, i.e., one cannot use the
symmetric relationship associated with local and remote address
values for unicast SAs in a multicast context. Specifically,
outbound traffic directed to a multicast address on an SA would
not be received on a companion, inbound SA with the multicast
address as the source.
- Next Layer Protocol: Obtained from the IPv4 "Protocol" or the
IPv6 "Next Header" fields. This is an individual protocol
number, ANY, or for IPv6 only, OPAQUE. The Next Layer Protocol
is whatever comes after any IP extension headers that are
present. To simplify locating the Next Layer Protocol, there
SHOULD be a mechanism for configuring which IPv6 extension
headers to skip. The default configuration for which protocols
to skip SHOULD include the following protocols: 0 (Hop-by-hop
options), 43 (Routing Header), 44 (Fragmentation Header), and 60
(Destination Options). Note: The default list does NOT include
51 (AH) or 50 (ESP). From a selector lookup point of view,
IPsec treats AH and ESP as Next Layer Protocols.
Several additional selectors depend on the Next Layer Protocol
value:
* If the Next Layer Protocol uses two ports (as do TCP, UDP,
SCTP, and others), then there are selectors for Local and
Remote Ports. Each of these selectors has a list of ranges
of values. Note that the Local and Remote ports may not be
available in the case of receipt of a fragmented packet or if
the port fields have been protected by IPsec (encrypted);
thus, a value of OPAQUE also MUST be supported. Note: In a
non-initial fragment, port values will not be available. If
a port selector specifies a value other than ANY or OPAQUE,
it cannot match packets that are non-initial fragments. If
the SA requires a port value other than ANY or OPAQUE, an
arriving fragment without ports MUST be discarded. (See
[Section 7](#section-7), "Handling Fragments".)
* If the Next Layer Protocol is a Mobility Header, then there
is a selector for IPv6 Mobility Header message type (MH type)
[[Mobip](#ref-Mobip ""Mobility Support in IPv6"")]. This is an 8-bit value that identifies a particular
mobility message. Note that the MH type may not be availableKent & Seo Standards Track [Page 27]
RFC 4301 Security Architecture for IP December 2005
in the case of receipt of a fragmented packet. (See [Section](#section-7)
[7](#section-7), "Handling Fragments".) For IKE, the IPv6 Mobility Header
message type (MH type) is placed in the most significant
eight bits of the 16-bit local "port" selector.
* If the Next Layer Protocol value is ICMP, then there is a
16-bit selector for the ICMP message type and code. The
message type is a single 8-bit value, which defines the type
of an ICMP message, or ANY. The ICMP code is a single 8-bit
value that defines a specific subtype for an ICMP message.
For IKE, the message type is placed in the most significant 8
bits of the 16-bit selector and the code is placed in the
least significant 8 bits. This 16-bit selector can contain a
single type and a range of codes, a single type and ANY code,
and ANY type and ANY code. Given a policy entry with a range
of Types (T-start to T-end) and a range of Codes (C-start to
C-end), and an ICMP packet with Type t and Code c, an
implementation MUST test for a match using
(T-start*256) + C-start <= (t*256) + c <= (T-end*256) +
C-end
Note that the ICMP message type and code may not be available
in the case of receipt of a fragmented packet. (See [Section](#section-7)
[7](#section-7), "Handling Fragments".)
- Name: This is not a selector like the others above. It is not
acquired from a packet. A name may be used as a symbolic
identifier for an IPsec Local or Remote address. Named SPD
entries are used in two ways:
1. A named SPD entry is used by a responder (not an initiator)
in support of access control when an IP address would not be
appropriate for the Remote IP address selector, e.g., for
"road warriors". The name used to match this field is
communicated during the IKE negotiation in the ID payload.
In this context, the initiator's Source IP address (inner IP
header in tunnel mode) is bound to the Remote IP address in
the SAD entry created by the IKE negotiation. This address
overrides the Remote IP address value in the SPD, when the
SPD entry is selected in this fashion. All IPsec
implementations MUST support this use of names.
2. A named SPD entry may be used by an initiator to identify a
user for whom an IPsec SA will be created (or for whom
traffic may be bypassed). The initiator's IP source address
(from inner IP header in tunnel mode) is used to replace the
following if and when they are created:Kent & Seo Standards Track [Page 28]
RFC 4301 Security Architecture for IP December 2005
- local address in the SPD cache entry
- local address in the outbound SAD entry
- remote address in the inbound SAD entry
Support for this use is optional for multi-user, native host
implementations and not applicable to other implementations.
Note that this name is used only locally; it is not
communicated by the key management protocol. Also, name
forms other than those used for case 1 above (responder) are
applicable in the initiator context (see below).
An SPD entry can contain both a name (or a list of names) and
also values for the Local or Remote IP address.
For case 1, responder, the identifiers employed in named SPD
entries are one of the following four types:
a. a fully qualified user name string (email), e.g.,
mozart@foo.example.com
(this corresponds to ID_RFC822_ADDR in IKEv2)
b. a fully qualified DNS name, e.g.,
foo.example.com
(this corresponds to ID_FQDN in IKEv2)
c. X.500 distinguished name, e.g., [[WaKiHo97](#ref-WaKiHo97 ""Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names"")],
CN = Stephen T. Kent, O = BBN Technologies,
SP = MA, C = US
(this corresponds to ID_DER_ASN1_DN in IKEv2, after
decoding)
d. a byte string
(this corresponds to Key_ID in IKEv2)
For case 2, initiator, the identifiers employed in named SPD
entries are of type byte string. They are likely to be Unix
UIDs, Windows security IDs, or something similar, but could
also be a user name or account name. In all cases, this
identifier is only of local concern and is not transmitted.The IPsec implementation context determines how selectors are used. For example, a native host implementation typically makes use of a socket interface. When a new connection is established, the SPD can be consulted and an SA bound to the socket. Thus, traffic sent via that socket need not result in additional lookups to the SPD (SPD-O and SPD-S) cache. In contrast, a BITS, BITW, or security gateway implementation needs to look at each packet and perform an SPD-O/SPD-S cache lookup based on the selectors.
Kent & Seo Standards Track [Page 29]
RFC 4301 Security Architecture for IP December 2005
4.4.1.2. Structure of an SPD Entry
This section contains a prose description of an SPD entry. Also, Appendix C provides an example of an ASN.1 definition of an SPD entry.
This text describes the SPD in a fashion that is intended to map directly into IKE payloads to ensure that the policy required by SPD entries can be negotiated through IKE. Unfortunately, the semantics of the version of IKEv2 published concurrently with this document [[Kau05](#ref-Kau05 ""The Internet Key Exchange (IKEv2) Protocol"")] do not align precisely with those defined for the SPD. Specifically, IKEv2 does not enable negotiation of a single SA that binds multiple pairs of local and remote addresses and ports to a single SA. Instead, when multiple local and remote addresses and ports are negotiated for an SA, IKEv2 treats these not as pairs, but as (unordered) sets of local and remote values that can be arbitrarily paired. Until IKE provides a facility that conveys the semantics that are expressed in the SPD via selector sets (as described below), users MUST NOT include multiple selector sets in a single SPD entry unless the access control intent aligns with the IKE "mix and match" semantics. An implementation MAY warn users, to alert them to this problem if users create SPD entries with multiple selector sets, the syntax of which indicates possible conflicts with current IKE semantics.
The management GUI can offer the user other forms of data entry and display, e.g., the option of using address prefixes as well as ranges, and symbolic names for protocols, ports, etc. (Do not confuse the use of symbolic names in a management interface with the SPD selector "Name".) Note that Remote/Local apply only to IP addresses and ports, not to ICMP message type/code or Mobility Header type. Also, if the reserved, symbolic selector value OPAQUE or ANY is employed for a given selector type, only that value may appear in the list for that selector, and it must appear only once in the list for that selector. Note that ANY and OPAQUE are local syntax conventions -- IKEv2 negotiates these values via the ranges indicated below:
ANY: start = 0 end = <max>
OPAQUE: start = <max> end = 0An SPD is an ordered list of entries each of which contains the following fields.
o Name -- a list of IDs. This quasi-selector is optional.
The forms that MUST be supported are described above in
[Section 4.4.1.1](#section-4.4.1.1) under "Name".Kent & Seo Standards Track [Page 30]
RFC 4301 Security Architecture for IP December 2005
o PFP flags -- one per traffic selector. A given flag, e.g.,
for Next Layer Protocol, applies to the relevant selector
across all "selector sets" (see below) contained in an SPD
entry. When creating an SA, each flag specifies for the
corresponding traffic selector whether to instantiate the
selector from the corresponding field in the packet that
triggered the creation of the SA or from the value(s) in
the corresponding SPD entry (see [Section 4.4.1](#section-4.4.1), "How to
Derive the Values for an SAD Entry"). Whether a single
flag is used for, e.g., source port, ICMP type/code, and
MH type, or a separate flag is used for each, is a local
matter. There are PFP flags for:
- Local Address
- Remote Address
- Next Layer Protocol
- Local Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
- Remote Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
o One to N selector sets that correspond to the "condition"
for applying a particular IPsec action. Each selector set
contains:
- Local Address
- Remote Address
- Next Layer Protocol
- Local Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
- Remote Port, or ICMP message type/code or Mobility
Header type (depending on the next layer protocol)
Note: The "next protocol" selector is an individual value
(unlike the local and remote IP addresses) in a selector
set entry. This is consistent with how IKEv2 negotiates
the Traffic Selector (TS) values for an SA. It also makes
sense because one may need to associate different port
fields with different protocols. It is possible to
associate multiple protocols (and ports) with a single SA
by specifying multiple selector sets for that SA.
o Processing info -- which action is required -- PROTECT,
BYPASS, or DISCARD. There is just one action that goes
with all the selector sets, not a separate action for each
set. If the required processing is PROTECT, the entry
contains the following information.
- IPsec mode -- tunnel or transportKent & Seo Standards Track [Page 31]
RFC 4301 Security Architecture for IP December 2005
- (if tunnel mode) local tunnel address -- For a
non-mobile host, if there is just one interface, this
is straightforward; if there are multiple
interfaces, this must be statically configured. For a
mobile host, the specification of the local address
is handled externally to IPsec.
- (if tunnel mode) remote tunnel address -- There is no
standard way to determine this. See 4.5.3, "Locating
a Security Gateway".
- Extended Sequence Number -- Is this SA using extended
sequence numbers?
- stateful fragment checking -- Is this SA using
stateful fragment checking? (See [Section 7](#section-7) for more
details.)
- Bypass DF bit (T/F) -- applicable to tunnel mode SAs
- Bypass DSCP (T/F) or map to unprotected DSCP values
(array) if needed to restrict bypass of DSCP values --
applicable to tunnel mode SAs
- IPsec protocol -- AH or ESP
- algorithms -- which ones to use for AH, which ones to
use for ESP, which ones to use for combined mode,
ordered by decreasing priorityIt is a local matter as to what information is kept with regard to handling extant SAs when the SPD is changed.
4.4.1.3. More Regarding Fields Associated with Next Layer Protocols
Additional selectors are often associated with fields in the Next Layer Protocol header. A particular Next Layer Protocol can have zero, one, or two selectors. There may be situations where there aren't both local and remote selectors for the fields that are dependent on the Next Layer Protocol. The IPv6 Mobility Header has only a Mobility Header message type. AH and ESP have no further selector fields. A system may be willing to send an ICMP message type and code that it does not want to receive. In the descriptions below, "port" is used to mean a field that is dependent on the Next Layer Protocol.
A. If a Next Layer Protocol has no "port" selectors, then
the Local and Remote "port" selectors are set to OPAQUE in
the relevant SPD entry, e.g.,
Local's
next layer protocol = AH
"port" selector = OPAQUEKent & Seo Standards Track [Page 32]
RFC 4301 Security Architecture for IP December 2005
Remote's
next layer protocol = AH
"port" selector = OPAQUE
B. Even if a Next Layer Protocol has only one selector, e.g.,
Mobility Header type, then the Local and Remote "port"
selectors are used to indicate whether a system is
willing to send and/or receive traffic with the specified
"port" values. For example, if Mobility Headers of a
specified type are allowed to be sent and received via an
SA, then the relevant SPD entry would be set as follows:
Local's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
Remote's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
If Mobility Headers of a specified type are allowed to be
sent but NOT received via an SA, then the relevant SPD
entry would be set as follows:
Local's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
Remote's
next layer protocol = Mobility Header
"port" selector = OPAQUE
If Mobility Headers of a specified type are allowed to be
received but NOT sent via an SA, then the relevant SPD
entry would be set as follows:
Local's
next layer protocol = Mobility Header
"port" selector = OPAQUE
Remote's
next layer protocol = Mobility Header
"port" selector = Mobility Header message type
C. If a system is willing to send traffic with a particular
"port" value but NOT receive traffic with that kind of
port value, the system's traffic selectors are set as
follows in the relevant SPD entry:Kent & Seo Standards Track [Page 33]
RFC 4301 Security Architecture for IP December 2005
Local's
next layer protocol = ICMP
"port" selector = <specific ICMP type & code>
Remote's
next layer protocol = ICMP
"port" selector = OPAQUE
D. To indicate that a system is willing to receive traffic
with a particular "port" value but NOT send that kind of
traffic, the system's traffic selectors are set as follows
in the relevant SPD entry:
Local's
next layer protocol = ICMP
"port" selector = OPAQUE
Remote's
next layer protocol = ICMP
"port" selector = <specific ICMP type & code>
For example, if a security gateway is willing to allow
systems behind it to send ICMP traceroutes, but is not
willing to let outside systems run ICMP traceroutes to
systems behind it, then the security gateway's traffic
selectors are set as follows in the relevant SPD entry:
Local's
next layer protocol = 1 (ICMPv4)
"port" selector = 30 (traceroute)
Remote's
next layer protocol = 1 (ICMPv4)
"port" selector = OPAQUE4.4.2. Security Association Database (SAD)
In each IPsec implementation, there is a nominal Security Association Database (SAD), in which each entry defines the parameters associated with one SA. Each SA has an entry in the SAD. For outbound processing, each SAD entry is pointed to by entries in the SPD-S part of the SPD cache. For inbound processing, for unicast SAs, the SPI is used either alone to look up an SA or in conjunction with the IPsec protocol type. If an IPsec implementation supports multicast, the SPI plus destination address, or SPI plus destination and source addresses are used to look up the SA. (See Section 4.1 for details on the algorithm that MUST be used for mapping inbound IPsec datagrams to SAs.) The following parameters are associated with each entry in
Kent & Seo Standards Track [Page 34]
RFC 4301 Security Architecture for IP December 2005
the SAD. They should all be present except where otherwise noted, e.g., AH Authentication algorithm. This description does not purport to be a MIB, only a specification of the minimal data items required to support an SA in an IPsec implementation.
For each of the selectors defined in Section 4.4.1.1, the entry for an inbound SA in the SAD MUST be initially populated with the value or values negotiated at the time the SA was created. (See the paragraph in Section 4.4.1 under "Handling Changes to the SPD while the System is Running" for guidance on the effect of SPD changes on extant SAs.) For a receiver, these values are used to check that the header fields of an inbound packet (after IPsec processing) match the selector values negotiated for the SA. Thus, the SAD acts as a cache for checking the selectors of inbound traffic arriving on SAs. For the receiver, this is part of verifying that a packet arriving on an SA is consistent with the policy for the SA. (See Section 6 for rules for ICMP messages.) These fields can have the form of specific values, ranges, ANY, or OPAQUE, as described in Section 4.4.1.1, "Selectors". Note also that there are a couple of situations in which the SAD can have entries for SAs that do not have corresponding entries in the SPD. Since this document does not mandate that the SAD be selectively cleared when the SPD is changed, SAD entries can remain when the SPD entries that created them are changed or deleted. Also, if a manually keyed SA is created, there could be an SAD entry for this SA that does not correspond to any SPD entry.
Note: The SAD can support multicast SAs, if manually configured. An outbound multicast SA has the same structure as a unicast SA. The source address is that of the sender, and the destination address is the multicast group address. An inbound, multicast SA must be configured with the source addresses of each peer authorized to transmit to the multicast SA in question. The SPI value for a multicast SA is provided by a multicast group controller, not by the receiver, as for a unicast SA. Because an SAD entry may be required to accommodate multiple, individual IP source addresses that were part of an SPD entry (for unicast SAs), the required facility for inbound, multicast SAs is a feature already present in an IPsec implementation. However, because the SPD has no provisions for accommodating multicast entries, this document does not specify an automated way to create an SAD entry for a multicast, inbound SA. Only manually configured SAD entries can be created to accommodate inbound, multicast traffic.
Implementation Guidance: This document does not specify how an SPD-S entry refers to the corresponding SAD entry, as this is an implementation-specific detail. However, some implementations (based on experience from RFC 2401) are known to have problems in this regard. In particular, simply storing the (remote tunnel header IP
Kent & Seo Standards Track [Page 35]
RFC 4301 Security Architecture for IP December 2005
address, remote SPI) pair in the SPD cache is not sufficient, since the pair does not always uniquely identify a single SAD entry. For instance, two hosts behind the same NAT could choose the same SPI value. The situation also may arise if a host is assigned an IP address (e.g., via DHCP) previously used by some other host, and the SAs associated with the old host have not yet been deleted via dead peer detection mechanisms. This may lead to packets being sent over the wrong SA or, if key management ensures the pair is unique, denying the creation of otherwise valid SAs. Thus, implementors should implement links between the SPD cache and the SAD in a way that does not engender such problems.
4.4.2.1. Data Items in the SAD
The following data items MUST be in the SAD:
o Security Parameter Index (SPI): a 32-bit value selected by the
receiving end of an SA to uniquely identify the SA. In an SAD
entry for an outbound SA, the SPI is used to construct the
packet's AH or ESP header. In an SAD entry for an inbound SA, the
SPI is used to map traffic to the appropriate SA (see text on
unicast/multicast in [Section 4.1](#section-4.1)).
o Sequence Number Counter: a 64-bit counter used to generate the
Sequence Number field in AH or ESP headers. 64-bit sequence
numbers are the default, but 32-bit sequence numbers are also
supported if negotiated.
o Sequence Counter Overflow: a flag indicating whether overflow of
the sequence number counter should generate an auditable event and
prevent transmission of additional packets on the SA, or whether
rollover is permitted. The audit log entry for this event SHOULD
include the SPI value, current date/time, Local Address, Remote
Address, and the selectors from the relevant SAD entry.
o Anti-Replay Window: a 64-bit counter and a bit-map (or equivalent)
used to determine whether an inbound AH or ESP packet is a replay.
Note: If anti-replay has been disabled by the receiver for an SA,
e.g., in the case of a manually keyed SA, then the Anti-Replay
Window is ignored for the SA in question. 64-bit sequence numbers
are the default, but this counter size accommodates 32-bit
sequence numbers as well.
o AH Authentication algorithm, key, etc. This is required only if
AH is supported.Kent & Seo Standards Track [Page 36]
RFC 4301 Security Architecture for IP December 2005
o ESP Encryption algorithm, key, mode, IV, etc. If a combined mode
algorithm is used, these fields will not be applicable.
o ESP integrity algorithm, keys, etc. If the integrity service is
not selected, these fields will not be applicable. If a combined
mode algorithm is used, these fields will not be applicable.
o ESP combined mode algorithms, key(s), etc. This data is used when
a combined mode (encryption and integrity) algorithm is used with
ESP. If a combined mode algorithm is not used, these fields are
not applicable.
o Lifetime of this SA: a time interval after which an SA must be
replaced with a new SA (and new SPI) or terminated, plus an
indication of which of these actions should occur. This may be
expressed as a time or byte count, or a simultaneous use of both
with the first lifetime to expire taking precedence. A compliant
implementation MUST support both types of lifetimes, and MUST
support a simultaneous use of both. If time is employed, and if
IKE employs X.509 certificates for SA establishment, the SA
lifetime must be constrained by the validity intervals of the
certificates, and the NextIssueDate of the Certificate Revocation
Lists (CRLs) used in the IKE exchange for the SA. Both initiator
and responder are responsible for constraining the SA lifetime in
this fashion. Note: The details of how to handle the refreshing
of keys when SAs expire is a local matter. However, one
reasonable approach is:
(a) If byte count is used, then the implementation SHOULD count the
number of bytes to which the IPsec cryptographic algorithm is
applied. For ESP, this is the encryption algorithm (including
Null encryption) and for AH, this is the authentication
algorithm. This includes pad bytes, etc. Note that
implementations MUST be able to handle having the counters at
the ends of an SA get out of synch, e.g., because of packet
loss or because the implementations at each end of the SA
aren't doing things the same way.
(b) There SHOULD be two kinds of lifetime -- a soft lifetime that
warns the implementation to initiate action such as setting up
a replacement SA, and a hard lifetime when the current SA ends
and is destroyed.
(c) If the entire packet does not get delivered during the SA's
lifetime, the packet SHOULD be discarded.
o IPsec protocol mode: tunnel or transport. Indicates which mode of
AH or ESP is applied to traffic on this SA.Kent & Seo Standards Track [Page 37]
RFC 4301 Security Architecture for IP December 2005
o Stateful fragment checking flag. Indicates whether or not
stateful fragment checking applies to this SA.
o Bypass DF bit (T/F) -- applicable to tunnel mode SAs where both
inner and outer headers are IPv4.
o DSCP values -- the set of DSCP values allowed for packets carried
over this SA. If no values are specified, no DSCP-specific
filtering is applied. If one or more values are specified, these
are used to select one SA among several that match the traffic
selectors for an outbound packet. Note that these values are NOT
checked against inbound traffic arriving on the SA.
o Bypass DSCP (T/F) or map to unprotected DSCP values (array) if
needed to restrict bypass of DSCP values -- applicable to tunnel
mode SAs. This feature maps DSCP values from an inner header to
values in an outer header, e.g., to address covert channel
signaling concerns.
o Path MTU: any observed path MTU and aging variables.
o Tunnel header IP source and destination address -- both addresses
must be either IPv4 or IPv6 addresses. The version implies the
type of IP header to be used. Only used when the IPsec protocol
mode is tunnel.4.4.2.2. Relationship between SPD, PFP flag, packet, and SAD
For each selector, the following tables show the relationship
between the value in the SPD, the PFP flag, the value in the
triggering packet, and the resulting value in the SAD. Note that
the administrative interface for IPsec can use various syntactic
options to make it easier for the administrator to enter rules.
For example, although a list of ranges is what IKEv2 sends, it
might be clearer and less error prone for the user to enter a
single IP address or IP address prefix.Kent & Seo Standards Track [Page 38]
RFC 4301 Security Architecture for IP December 2005
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
loc addr list of ranges 0 IP addr "S" list of ranges
ANY 0 IP addr "S" ANY
list of ranges 1 IP addr "S" "S"
ANY 1 IP addr "S" "S"
rem addr list of ranges 0 IP addr "D" list of ranges
ANY 0 IP addr "D" ANY
list of ranges 1 IP addr "D" "D"
ANY 1 IP addr "D" "D"
protocol list of prot's* 0 prot. "P" list of prot's*
ANY** 0 prot. "P" ANY
OPAQUE**** 0 prot. "P" OPAQUE
list of prot's* 0 not avail. discard packet
ANY** 0 not avail. ANY
OPAQUE**** 0 not avail. OPAQUE
list of prot's* 1 prot. "P" "P"
ANY** 1 prot. "P" "P"
OPAQUE**** 1 prot. "P" ***
list of prot's* 1 not avail. discard packet
ANY** 1 not avail. discard packet
OPAQUE**** 1 not avail. ***Kent & Seo Standards Track [Page 39]
RFC 4301 Security Architecture for IP December 2005
If the protocol is one that has two ports, then there will be
selectors for both Local and Remote ports.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
loc port list of ranges 0 src port "s" list of ranges
ANY 0 src port "s" ANY
OPAQUE 0 src port "s" OPAQUE
list of ranges 0 not avail. discard packet
ANY 0 not avail. ANY
OPAQUE 0 not avail. OPAQUE
list of ranges 1 src port "s" "s"
ANY 1 src port "s" "s"
OPAQUE 1 src port "s" ***
list of ranges 1 not avail. discard packet
ANY 1 not avail. discard packet
OPAQUE 1 not avail. ***
rem port list of ranges 0 dst port "d" list of ranges
ANY 0 dst port "d" ANY
OPAQUE 0 dst port "d" OPAQUE
list of ranges 0 not avail. discard packet
ANY 0 not avail. ANY
OPAQUE 0 not avail. OPAQUE
list of ranges 1 dst port "d" "d"
ANY 1 dst port "d" "d"
OPAQUE 1 dst port "d" ***
list of ranges 1 not avail. discard packet
ANY 1 not avail. discard packet
OPAQUE 1 not avail. ***Kent & Seo Standards Track [Page 40]
RFC 4301 Security Architecture for IP December 2005
If the protocol is mobility header, then there will be a selector
for mh type.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
-------- ---------------- --- ------------ --------------
mh type list of ranges 0 mh type "T" list of ranges
ANY 0 mh type "T" ANY
OPAQUE 0 mh type "T" OPAQUE
list of ranges 0 not avail. discard packet
ANY 0 not avail. ANY
OPAQUE 0 not avail. OPAQUE
list of ranges 1 mh type "T" "T"
ANY 1 mh type "T" "T"
OPAQUE 1 mh type "T" ***
list of ranges 1 not avail. discard packet
ANY 1 not avail. discard packet
OPAQUE 1 not avail. ***Kent & Seo Standards Track [Page 41]
RFC 4301 Security Architecture for IP December 2005
If the protocol is ICMP, then there will be a 16-bit selector for
ICMP type and ICMP code. Note that the type and code are bound to
each other, i.e., the codes apply to the particular type. This
16-bit selector can contain a single type and a range of codes, a
single type and ANY code, and ANY type and ANY code.
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
--------- ---------------- --- ------------ --------------
ICMP type a single type & 0 type "t" & single type &
and code range of codes code "c" range of codes
a single type & 0 type "t" & single type &
ANY code code "c" ANY code
ANY type & ANY 0 type "t" & ANY type &
code code "c" ANY code
OPAQUE 0 type "t" & OPAQUE
code "c"
a single type & 0 not avail. discard packet
range of codes
a single type & 0 not avail. discard packet
ANY code
ANY type & 0 not avail. ANY type &
ANY code ANY code
OPAQUE 0 not avail. OPAQUE
a single type & 1 type "t" & "t" and "c"
range of codes code "c"
a single type & 1 type "t" & "t" and "c"
ANY code code "c"
ANY type & 1 type "t" & "t" and "c"
ANY code code "c"
OPAQUE 1 type "t" & ***
code "c"
a single type & 1 not avail. discard packet
range of codes
a single type & 1 not avail. discard packet
ANY code
ANY type & 1 not avail. discard packet
ANY code
OPAQUE 1 not avail. ***Kent & Seo Standards Track [Page 42]
RFC 4301 Security Architecture for IP December 2005
If the name selector is used:
Value in
Triggering Resulting SAD
Selector SPD Entry PFP Packet Entry
--------- ---------------- --- ------------ --------------
name list of user or N/A N/A N/A
system names
* "List of protocols" is the information, not the way
that the SPD or SAD or IKEv2 have to represent this
information.
** 0 (zero) is used by IKE to indicate ANY for
protocol.
*** Use of PFP=1 with an OPAQUE value is an error and
SHOULD be prohibited by an IPsec implementation.
**** The protocol field cannot be OPAQUE in IPv4. This
table entry applies only to IPv6.4.4.3. Peer Authorization Database (PAD)
The Peer Authorization Database (PAD) provides the link between the SPD and a security association management protocol such as IKE. It embodies several critical functions:
o identifies the peers or groups of peers that are authorized
to communicate with this IPsec entity
o specifies the protocol and method used to authenticate each
peer
o provides the authentication data for each peer
o constrains the types and values of IDs that can be asserted
by a peer with regard to child SA creation, to ensure that the
peer does not assert identities for lookup in the SPD that it
is not authorized to represent, when child SAs are created
o peer gateway location info, e.g., IP address(es) or DNS names,
MAY be included for peers that are known to be "behind" a
security gatewayThe PAD provides these functions for an IKE peer when the peer acts as either the initiator or the responder.
To perform these functions, the PAD contains an entry for each peer or group of peers with which the IPsec entity will communicate. An entry names an individual peer (a user, end system or security gateway) or specifies a group of peers (using ID matching rules defined below). The entry specifies the authentication protocol (e.g., IKEv1, IKEv2, KINK) method used (e.g., certificates or pre- shared secrets) and the authentication data (e.g., the pre-shared
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secret or the trust anchor relative to which the peer's certificate will be validated). For certificate-based authentication, the entry also may provide information to assist in verifying the revocation status of the peer, e.g., a pointer to a CRL repository or the name of an Online Certificate Status Protocol (OCSP) server associated with the peer or with the trust anchor associated with the peer.
Each entry also specifies whether the IKE ID payload will be used as a symbolic name for SPD lookup, or whether the remote IP address provided in traffic selector payloads will be used for SPD lookups when child SAs are created.
Note that the PAD information MAY be used to support creation of more than one tunnel mode SA at a time between two peers, e.g., two tunnels to protect the same addresses/hosts, but with different tunnel endpoints.
4.4.3.1. PAD Entry IDs and Matching Rules
The PAD is an ordered database, where the order is defined by an administrator (or a user in the case of a single-user end system). Usually, the same administrator will be responsible for both the PAD and SPD, since the two databases must be coordinated. The ordering requirement for the PAD arises for the same reason as for the SPD, i.e., because use of "star name" entries allows for overlaps in the set of IKE IDs that could match a specific entry.
Six types of IDs are supported for entries in the PAD, consistent with the symbolic name types and IP addresses used to identify SPD entries. The ID for each entry acts as the index for the PAD, i.e., it is the value used to select an entry. All of these ID types can be used to match IKE ID payload types. The six types are:
o DNS name (specific or partial)
o Distinguished Name (complete or sub-tree constrained)
o [RFC 822](./rfc822) email address (complete or partially qualified)
o IPv4 address (range)
o IPv6 address (range)
o Key ID (exact match only)The first three name types can accommodate sub-tree matching as well as exact matches. A DNS name may be fully qualified and thus match exactly one name, e.g., foo.example.com. Alternatively, the name may encompass a group of peers by being partially specified, e.g., the string ".example.com" could be used to match any DNS name ending in these two domain name components.
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Similarly, a Distinguished Name may specify a complete Distinguished Name to match exactly one entry, e.g., CN = Stephen, O = BBN Technologies, SP = MA, C = US. Alternatively, an entry may encompass a group of peers by specifying a sub-tree, e.g., an entry of the form "C = US, SP = MA" might be used to match all DNs that contain these two attributes as the top two Relative Distinguished Names (RDNs).
For an RFC 822 e-mail addresses, the same options exist. A complete address such as foo@example.com matches one entity, but a sub-tree name such as "@example.com" could be used to match all the entities with names ending in those two domain names to the right of the @.
The specific syntax used by an implementation to accommodate sub-tree matching for distinguished names, domain names or RFC 822 e-mail addresses is a local matter. But, at a minimum, sub-tree matching of the sort described above MUST be supported. (Substring matching within a DN, DNS name, or RFC 822 address MAY be supported, but is not required.)
For IPv4 and IPv6 addresses, the same address range syntax used for SPD entries MUST be supported. This allows specification of an individual address (via a trivial range), an address prefix (by choosing a range that adheres to Classless Inter-Domain Routing (CIDR)-style prefixes), or an arbitrary address range.
The Key ID field is defined as an OCTET string in IKE. For this name type, only exact-match syntax MUST be supported (since there is no explicit structure for this ID type). Additional matching functions MAY be supported for this ID type.
4.4.3.2. IKE Peer Authentication Data
Once an entry is located based on an ordered search of the PAD based on ID field matching, it is necessary to verify the asserted identity, i.e., to authenticate the asserted ID. For each PAD entry, there is an indication of the type of authentication to be performed. This document requires support for two required authentication data types:
- X.509 certificate
- pre-shared secretFor authentication based on an X.509 certificate, the PAD entry contains a trust anchor via which the end entity (EE) certificate for the peer must be verifiable, either directly or via a certificate path. See RFC 3280 for the definition of a trust anchor. An entry used with certificate-based authentication MAY include additional data to facilitate certificate revocation status, e.g., a list of
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appropriate OCSP responders or CRL repositories, and associated authentication data. For authentication based on a pre-shared secret, the PAD contains the pre-shared secret to be used by IKE.
This document does not require that the IKE ID asserted by a peer be syntactically related to a specific field in an end entity certificate that is employed to authenticate the identity of that peer. However, it often will be appropriate to impose such a requirement, e.g., when a single entry represents a set of peers each of whom may have a distinct SPD entry. Thus, implementations MUST provide a means for an administrator to require a match between an asserted IKE ID and the subject name or subject alt name in a certificate. The former is applicable to IKE IDs expressed as distinguished names; the latter is appropriate for DNS names, RFC 822 e-mail addresses, and IP addresses. Since KEY ID is intended for identifying a peer authenticated via a pre-shared secret, there is no requirement to match this ID type to a certificate field.
See IKEv1 [[HarCar98](#ref-HarCar98 ""The Internet Key Exchange (IKE)"")] and IKEv2 [[Kau05](#ref-Kau05 ""The Internet Key Exchange (IKEv2) Protocol"")] for details of how IKE performs peer authentication using certificates or pre-shared secrets.
This document does not mandate support for any other authentication methods, although such methods MAY be employed.
4.4.3.3. Child SA Authorization Data
Once an IKE peer is authenticated, child SAs may be created. Each PAD entry contains data to constrain the set of IDs that can be asserted by an IKE peer, for matching against the SPD. Each PAD entry indicates whether the IKE ID is to be used as a symbolic name for SPD matching, or whether an IP address asserted in a traffic selector payload is to be used.
If the entry indicates that the IKE ID is to be used, then the PAD entry ID field defines the authorized set of IDs. If the entry indicates that child SAs traffic selectors are to be used, then an additional data element is required, in the form of IPv4 and/or IPv6 address ranges. (A peer may be authorized for both address types, so there MUST be provision for both a v4 and a v6 address range.)
4.4.3.4. How the PAD Is Used
During the initial IKE exchange, the initiator and responder each assert their identity via the IKE ID payload and send an AUTH payload to verify the asserted identity. One or more CERT payloads may be transmitted to facilitate the verification of each asserted identity.
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When an IKE entity receives an IKE ID payload, it uses the asserted ID to locate an entry in the PAD, using the matching rules described above. The PAD entry specifies the authentication method to be employed for the identified peer. This ensures that the right method is used for each peer and that different methods can be used for different peers. The entry also specifies the authentication data that will be used to verify the asserted identity. This data is employed in conjunction with the specified method to authenticate the peer, before any CHILD SAs are created.
Child SAs are created based on the exchange of traffic selector payloads, either at the end of the initial IKE exchange or in subsequent CREATE_CHILD_SA exchanges. The PAD entry for the (now authenticated) IKE peer is used to constrain creation of child SAs; specifically, the PAD entry specifies how the SPD is searched using a traffic selector proposal from a peer. There are two choices: either the IKE ID asserted by the peer is used to find an SPD entry via its symbolic name, or peer IP addresses asserted in traffic selector payloads are used for SPD lookups based on the remote IP address field portion of an SPD entry. It is necessary to impose these constraints on creation of child SAs to prevent an authenticated peer from spoofing IDs associated with other, legitimate peers.
Note that because the PAD is checked before searching for an SPD entry, this safeguard protects an initiator against spoofing attacks. For example, assume that IKE A receives an outbound packet destined for IP address X, a host served by a security gateway. RFC 2401 [[RFC2401](./rfc2401 ""Security Architecture for the Internet Protocol"")] and this document do not specify how A determines the address of the IKE peer serving X. However, any peer contacted by A as the presumed representative for X must be registered in the PAD in order to allow the IKE exchange to be authenticated. Moreover, when the authenticated peer asserts that it represents X in its traffic selector exchange, the PAD will be consulted to determine if the peer in question is authorized to represent X. Thus, the PAD provides a binding of address ranges (or name sub-spaces) to peers, to counter such attacks.
4.5. SA and Key Management
All IPsec implementations MUST support both manual and automated SA and cryptographic key management. The IPsec protocols, AH and ESP, are largely independent of the associated SA management techniques, although the techniques involved do affect some of the security services offered by the protocols. For example, the optional anti-replay service available for AH and ESP requires automated SA management. Moreover, the granularity of key distribution employed with IPsec determines the granularity of authentication provided. In general, data origin authentication in AH and ESP is limited by the
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extent to which secrets used with the integrity algorithm (or with a key management protocol that creates such secrets) are shared among multiple possible sources.
The following text describes the minimum requirements for both types of SA management.
4.5.1. Manual Techniques
The simplest form of management is manual management, in which a person manually configures each system with keying material and SA management data relevant to secure communication with other systems. Manual techniques are practical in small, static environments but they do not scale well. For example, a company could create a virtual private network (VPN) using IPsec in security gateways at several sites. If the number of sites is small, and since all the sites come under the purview of a single administrative domain, this might be a feasible context for manual management techniques. In this case, the security gateway might selectively protect traffic to and from other sites within the organization using a manually configured key, while not protecting traffic for other destinations. It also might be appropriate when only selected communications need to be secured. A similar argument might apply to use of IPsec entirely within an organization for a small number of hosts and/or gateways. Manual management techniques often employ statically configured, symmetric keys, though other options also exist.
4.5.2. Automated SA and Key Management
Widespread deployment and use of IPsec requires an Internet-standard, scalable, automated, SA management protocol. Such support is required to facilitate use of the anti-replay features of AH and ESP, and to accommodate on-demand creation of SAs, e.g., for user- and session-oriented keying. (Note that the notion of "rekeying" an SA actually implies creation of a new SA with a new SPI, a process that generally implies use of an automated SA/key management protocol.)
The default automated key management protocol selected for use with IPsec is IKEv2 [[Kau05](#ref-Kau05 ""The Internet Key Exchange (IKEv2) Protocol"")]. This document assumes the availability of certain functions from the key management protocol that are not supported by IKEv1. Other automated SA management protocols MAY be employed.
When an automated SA/key management protocol is employed, the output from this protocol is used to generate multiple keys for a single SA. This also occurs because distinct keys are used for each of the two
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SAs created by IKE. If both integrity and confidentiality are employed, then a minimum of four keys are required. Additionally, some cryptographic algorithms may require multiple keys, e.g., 3DES.
The Key Management System may provide a separate string of bits for each key or it may generate one string of bits from which all keys are extracted. If a single string of bits is provided, care needs to be taken to ensure that the parts of the system that map the string of bits to the required keys do so in the same fashion at both ends of the SA. To ensure that the IPsec implementations at each end of the SA use the same bits for the same keys, and irrespective of which part of the system divides the string of bits into individual keys, the encryption keys MUST be taken from the first (left-most, high-order) bits and the integrity keys MUST be taken from the remaining bits. The number of bits for each key is defined in the relevant cryptographic algorithm specification RFC. In the case of multiple encryption keys or multiple integrity keys, the specification for the cryptographic algorithm must specify the order in which they are to be selected from a single string of bits provided to the cryptographic algorithm.
4.5.3. Locating a Security Gateway
This section discusses issues relating to how a host learns about the existence of relevant security gateways and, once a host has contacted these security gateways, how it knows that these are the correct security gateways. The details of where the required information is stored is a local matter, but the Peer Authorization Database (PAD) described in Section 4.4 is the most likely candidate. (Note: S* indicates a system that is running IPsec, e.g., SH1 and SG2 below.)
Consider a situation in which a remote host (SH1) is using the Internet to gain access to a server or other machine (H2) and there is a security gateway (SG2), e.g., a firewall, through which H1's traffic must pass. An example of this situation would be a mobile host crossing the Internet to his home organization's firewall (SG2). This situation raises several issues:
How does SH1 know/learn about the existence of the security gateway SG2?
How does it authenticate SG2, and once it has authenticated SG2, how does it confirm that SG2 has been authorized to represent H2?
How does SG2 authenticate SH1 and verify that SH1 is authorized to contact H2?
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- How does SH1 know/learn about any additional gateways that provide alternate paths to H2?
To address these problems, an IPsec-supporting host or security gateway MUST have an administrative interface that allows the user/administrator to configure the address of one or more security gateways for ranges of destination addresses that require its use. This includes the ability to configure information for locating and authenticating one or more security gateways and verifying the authorization of these gateways to represent the destination host. (The authorization function is implied in the PAD.) This document does not address the issue of how to automate the discovery/verification of security gateways.
4.6. SAs and Multicast
The receiver-orientation of the SA implies that, in the case of unicast traffic, the destination system will select the SPI value. By having the destination select the SPI value, there is no potential for manually configured SAs to conflict with automatically configured (e.g., via a key management protocol) SAs or for SAs from multiple sources to conflict with each other. For multicast traffic, there are multiple destination systems associated with a single SA. So some system or person will need to coordinate among all multicast groups to select an SPI or SPIs on behalf of each multicast group and then communicate the group's IPsec information to all of the legitimate members of that multicast group via mechanisms not defined here.
Multiple senders to a multicast group SHOULD use a single Security Association (and hence SPI) for all traffic to that group when a symmetric key encryption or integrity algorithm is employed. In such circumstances, the receiver knows only that the message came from a system possessing the key for that multicast group. In such circumstances, a receiver generally will not be able to authenticate which system sent the multicast traffic. Specifications for other, more general multicast approaches are deferred to the IETF Multicast Security Working Group.
5. IP Traffic Processing
As mentioned in Section 4.4.1, "The Security Policy Database (SPD)", the SPD (or associated caches) MUST be consulted during the processing of all traffic that crosses the IPsec protection boundary, including IPsec management traffic. If no policy is found in the SPD that matches a packet (for either inbound or outbound traffic), the packet MUST be discarded. To simplify processing, and to allow for very fast SA lookups (for SG/BITS/BITW), this document introduces the
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notion of an SPD cache for all outbound traffic (SPD-O plus SPD-S), and a cache for inbound, non-IPsec-protected traffic (SPD-I). (As mentioned earlier, the SAD acts as a cache for checking the selectors of inbound IPsec-protected traffic arriving on SAs.) There is nominally one cache per SPD. For the purposes of this specification, it is assumed that each cached entry will map to exactly one SA. Note, however, exceptions arise when one uses multiple SAs to carry traffic of different priorities (e.g., as indicated by distinct DSCP values) but the same selectors. Note also, that there are a couple of situations in which the SAD can have entries for SAs that do not have corresponding entries in the SPD. Since this document does not mandate that the SAD be selectively cleared when the SPD is changed, SAD entries can remain when the SPD entries that created them are changed or deleted. Also, if a manually keyed SA is created, there could be an SAD entry for this SA that does not correspond to any SPD entry.
Since SPD entries may overlap, one cannot safely cache these entries in general. Simple caching might result in a match against a cache entry, whereas an ordered search of the SPD would have resulted in a match against a different entry. But, if the SPD entries are first decorrelated, then the resulting entries can safely be cached. Each cached entry will indicate that matching traffic should be bypassed or discarded, appropriately. (Note: The original SPD entry might result in multiple SAs, e.g., because of PFP.) Unless otherwise noted, all references below to the "SPD" or "SPD cache" or "cache" are to a decorrelated SPD (SPD-I, SPD-O, SPD-S) or the SPD cache containing entries from the decorrelated SPD.
Note: In a host IPsec implementation based on sockets, the SPD will be consulted whenever a new socket is created to determine what, if any, IPsec processing will be applied to the traffic that will flow on that socket. This provides an implicit caching mechanism, and the portions of the preceding discussion that address caching can be ignored in such implementations.
Note: It is assumed that one starts with a correlated SPD because that is how users and administrators are accustomed to managing these sorts of access control lists or firewall filter rules. Then the decorrelation algorithm is applied to build a list of cache-able SPD entries. The decorrelation is invisible at the management interface.
For inbound IPsec traffic, the SAD entry selected by the SPI serves as the cache for the selectors to be matched against arriving IPsec packets, after AH or ESP processing has been performed.
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5.1. Outbound IP Traffic Processing (protected-to-unprotected)
First consider the path for traffic entering the implementation via a protected interface and exiting via an unprotected interface.
Unprotected Interface
^
|
(nested SAs) +----------+
-------------------|Forwarding|<-----+
| +----------+ |
| ^ |
| | BYPASS |
V +-----+ |
+-------+ | SPD | +--------+
...| SPD-I |.................|Cache|.....|PROCESS |...IPsec
| (*) | | (*) |---->|(AH/ESP)| boundary
+-------+ +-----+ +--------+
| +-------+ / ^
| |DISCARD| <--/ |
| +-------+ |
| |
| +-------------+
|---------------->|SPD Selection|
+-------------+
^
| +------+
| -->| ICMP |
| / +------+
|/
|
|
Protected Interface
Figure 2. Processing Model for Outbound Traffic
(*) = The SPD caches are shown here. If there
is a cache miss, then the SPD is checked.
There is no requirement that an
implementation buffer the packet if
there is a cache miss.Kent & Seo Standards Track [Page 52]
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IPsec MUST perform the following steps when processing outbound packets:
When a packet arrives from the subscriber (protected) interface, invoke the SPD selection function to obtain the SPD-ID needed to choose the appropriate SPD. (If the implementation uses only one SPD, this step is a no-op.)
Match the packet headers against the cache for the SPD specified by the SPD-ID from step 1. Note that this cache contains entries from SPD-O and SPD-S.
3a. If there is a match, then process the packet as specified by the matching cache entry, i.e., BYPASS, DISCARD, or PROTECT using AH or ESP. If IPsec processing is applied, there is a link from the SPD cache entry to the relevant SAD entry (specifying the mode, cryptographic algorithms, keys, SPI, PMTU, etc.). IPsec processing is as previously defined, for tunnel or transport modes and for AH or ESP, as specified in their respective RFCs [[Ken05b](#ref-Ken05b ""IP Authentication Header""), [Ken05a](#ref-Ken05a ""IP Encapsulating Security Payload (ESP)"")]. Note that the SA PMTU value, plus the value of the stateful fragment checking flag (and the DF bit in the IP header of the outbound packet) determine whether the packet can (must) be fragmented prior to or after IPsec processing, or if it must be discarded and an ICMP PMTU message is sent.
3b. If no match is found in the cache, search the SPD (SPD-S and SPD-O parts) specified by SPD-ID. If the SPD entry calls for BYPASS or DISCARD, create one or more new outbound SPD cache entries and if BYPASS, create one or more new inbound SPD cache entries. (More than one cache entry may be created since a decorrelated SPD entry may be linked to other such entries that were created as a side effect of the decorrelation process.) If the SPD entry calls for PROTECT, i.e., creation of an SA, the key management mechanism (e.g., IKEv2) is invoked to create the SA. If SA creation succeeds, a new outbound (SPD-S) cache entry is created, along with outbound and inbound SAD entries, otherwise the packet is discarded. (A packet that triggers an SPD lookup MAY be discarded by the implementation, or it MAY be processed against the newly created cache entry, if one is created.) Since SAs are created in pairs, an SAD entry for the corresponding inbound SA also is created, and it contains the selector values derived from the SPD entry (and packet, if any PFP flags were "true") used to create the inbound SA, for use in checking inbound traffic delivered via the SA.
- The packet is passed to the outbound forwarding function (operating outside of the IPsec implementation), to select the interface to which the packet will be directed. This function
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may cause the packet to be passed back across the IPsec boundary,
for additional IPsec processing, e.g., in support of nested SAs.
If so, there MUST be an entry in SPD-I database that permits
inbound bypassing of the packet, otherwise the packet will be
discarded. If necessary, i.e., if there is more than one SPD-I,
the traffic being looped back MAY be tagged as coming from this
internal interface. This would allow the use of a different
SPD-I for "real" external traffic vs. looped traffic, if needed.Note: With the exception of IPv4 and IPv6 transport mode, an SG, BITS, or BITW implementation MAY fragment packets before applying IPsec. (This applies only to IPv4. For IPv6 packets, only the originator is allowed to fragment them.) The device SHOULD have a configuration setting to disable this. The resulting fragments are evaluated against the SPD in the normal manner. Thus, fragments not containing port numbers (or ICMP message type and code, or Mobility Header type) will only match rules having port (or ICMP message type and code, or MH type) selectors of OPAQUE or ANY. (See Section 7 for more details.)
Note: With regard to determining and enforcing the PMTU of an SA, the IPsec system MUST follow the steps described in Section 8.2.
5.1.1. Handling an Outbound Packet That Must Be Discarded
If an IPsec system receives an outbound packet that it finds it must discard, it SHOULD be capable of generating and sending an ICMP message to indicate to the sender of the outbound packet that the packet was discarded. The type and code of the ICMP message will depend on the reason for discarding the packet, as specified below. The reason SHOULD be recorded in the audit log. The audit log entry for this event SHOULD include the reason, current date/time, and the selector values from the packet.
a. The selectors of the packet matched an SPD entry requiring the packet to be discarded.
IPv4 Type = 3 (destination unreachable) Code = 13
(Communication Administratively Prohibited)
IPv6 Type = 1 (destination unreachable) Code = 1
(Communication with destination administratively
prohibited)b1. The IPsec system successfully reached the remote peer but was unable to negotiate the SA required by the SPD entry matching the packet because, for example, the remote peer is administratively prohibited from communicating with the initiator, the initiating
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peer was unable to authenticate itself to the remote peer, the
remote peer was unable to authenticate itself to the initiating
peer, or the SPD at the remote peer did not have a suitable
entry.
IPv4 Type = 3 (destination unreachable) Code = 13
(Communication Administratively Prohibited)
IPv6 Type = 1 (destination unreachable) Code = 1
(Communication with destination administratively
prohibited)b2. The IPsec system was unable to set up the SA required by the SPD entry matching the packet because the IPsec peer at the other end of the exchange could not be contacted.
IPv4 Type = 3 (destination unreachable) Code = 1 (host
unreachable)
IPv6 Type = 1 (destination unreachable) Code = 3 (address
unreachable)Note that an attacker behind a security gateway could send packets with a spoofed source address, W.X.Y.Z, to an IPsec entity causing it to send ICMP messages to W.X.Y.Z. This creates an opportunity for a denial of service (DoS) attack among hosts behind a security gateway. To address this, a security gateway SHOULD include a management control to allow an administrator to configure an IPsec implementation to send or not send the ICMP messages under these circumstances, and if this facility is selected, to rate limit the transmission of such ICMP responses.
5.1.2. Header Construction for Tunnel Mode
This section describes the handling of the inner and outer IP headers, extension headers, and options for AH and ESP tunnels, with regard to outbound traffic processing. This includes how to construct the encapsulating (outer) IP header, how to process fields in the inner IP header, and what other actions should be taken for outbound, tunnel mode traffic. The general processing described here is modeled after RFC 2003, "IP Encapsulation within IP" [[Per96](#ref-Per96 ""IP Encapsulation within IP"")]:
o The outer IP header Source Address and Destination Address
identify the "endpoints" of the tunnel (the encapsulator and
decapsulator). The inner IP header Source Address and Destination
Addresses identify the original sender and recipient of the
datagram (from the perspective of this tunnel), respectively.Kent & Seo Standards Track [Page 55]
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(See footnote 3 after the table in 5.1.2.1 for more details on the
encapsulating source IP address.)
o The inner IP header is not changed except as noted below for TTL
(or Hop Limit) and the DS/ECN Fields. The inner IP header
otherwise remains unchanged during its delivery to the tunnel exit
point.
o No change to IP options or extension headers in the inner header
occurs during delivery of the encapsulated datagram through the
tunnel.Note: IPsec tunnel mode is different from IP-in-IP tunneling (RFC 2003 [[Per96](#ref-Per96 ""IP Encapsulation within IP"")]) in several ways:
o IPsec offers certain controls to a security administrator to
manage covert channels (which would not normally be a concern for
tunneling) and to ensure that the receiver examines the right
portions of the received packet with respect to application of
access controls. An IPsec implementation MAY be configurable with
regard to how it processes the outer DS field for tunnel mode for
transmitted packets. For outbound traffic, one configuration
setting for the outer DS field will operate as described in the
following sections on IPv4 and IPv6 header processing for IPsec
tunnels. Another will allow the outer DS field to be mapped to a
fixed value, which MAY be configured on a per-SA basis. (The value
might really be fixed for all traffic outbound from a device, but
per-SA granularity allows that as well.) This configuration option
allows a local administrator to decide whether the covert channel
provided by copying these bits outweighs the benefits of copying.
o IPsec describes how to handle ECN or DS and provides the ability
to control propagation of changes in these fields between
unprotected and protected domains. In general, propagation from a
protected to an unprotected domain is a covert channel and thus
controls are provided to manage the bandwidth of this channel.
Propagation of ECN values in the other direction are controlled so
that only legitimate ECN changes (indicating occurrence of
congestion between the tunnel endpoints) are propagated. By
default, DS propagation from an unprotected domain to a protected
domain is not permitted. However, if the sender and receiver do
not share the same DS code space, and the receiver has no way of
learning how to map between the two spaces, then it may be
appropriate to deviate from the default. Specifically, an IPsec
implementation MAY be configurable in terms of how it processes
the outer DS field for tunnel mode for received packets. It may
be configured to either discard the outer DS value (the default)
OR to overwrite the inner DS field with the outer DS field. IfKent & Seo Standards Track [Page 56]
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offered, the discard vs. overwrite behavior MAY be configured on a
per-SA basis. This configuration option allows a local
administrator to decide whether the vulnerabilities created by
copying these bits outweigh the benefits of copying. See
[[RFC2983](./rfc2983 ""Differentiated Services and Tunnels"")] for further information on when each of these behaviors
may be useful, and also for the possible need for diffserv traffic
conditioning prior or subsequent to IPsec processing (including
tunnel decapsulation).
o IPsec allows the IP version of the encapsulating header to be
different from that of the inner header.The tables in the following sub-sections show the handling for the different header/option fields ("constructed" means that the value in the outer field is constructed independently of the value in the inner).
5.1.2.1. IPv4: Header Construction for Tunnel Mode
<-- How Outer Hdr Relates to Inner Hdr -->
Outer Hdr at Inner Hdr at
IPv4 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 4 (1) no change
header length constructed no change
DS Field copied from inner hdr (5) no change
ECN Field copied from inner hdr constructed (6)
total length constructed no change
ID constructed no change
flags (DF,MF) constructed, DF (4) no change
fragment offset constructed no change
TTL constructed (2) decrement (2)
protocol AH, ESP no change
checksum constructed constructed (2)(6)
src address constructed (3) no change
dest address constructed (3) no change
Options never copied no change
Notes:
(1) The IP version in the encapsulating header can be different
from the value in the inner header.
(2) The TTL in the inner header is decremented by the encapsulator
prior to forwarding and by the decapsulator if it forwards the
packet. (The IPv4 checksum changes when the TTL changes.)Kent & Seo Standards Track [Page 57]
RFC 4301 Security Architecture for IP December 2005
Note: Decrementing the TTL value is a normal part of
forwarding a packet. Thus, a packet originating from the same
node as the encapsulator does not have its TTL decremented,
since the sending node is originating the packet rather than
forwarding it. This applies to BITS and native IPsec
implementations in hosts and routers. However, the IPsec
processing model includes an external forwarding capability.
TTL processing can be used to prevent looping of packets,
e.g., due to configuration errors, within the context of this
processing model.
(3) Local and Remote addresses depend on the SA, which is used to
determine the Remote address, which in turn determines which
Local address (net interface) is used to forward the packet.
Note: For multicast traffic, the destination address, or
source and destination addresses, may be required for
demuxing. In that case, it is important to ensure consistency
over the lifetime of the SA by ensuring that the source
address that appears in the encapsulating tunnel header is the
same as the one that was negotiated during the SA
establishment process. There is an exception to this general
rule, i.e., a mobile IPsec implementation will update its
source address as it moves.
(4) Configuration determines whether to copy from the inner header
(IPv4 only), clear, or set the DF.
(5) If the packet will immediately enter a domain for which the
DSCP value in the outer header is not appropriate, that value
MUST be mapped to an appropriate value for the domain
[[NiBlBaBL98](#ref-NiBlBaBL98 ""Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"")]. See [RFC 2475](./rfc2475) [[BBCDWW98](#ref-BBCDWW98 ""An Architecture for Differentiated Service"")] for further
information.
(6) If the ECN field in the inner header is set to ECT(0) or
ECT(1), where ECT is ECN-Capable Transport (ECT), and if the
ECN field in the outer header is set to Congestion Experienced
(CE), then set the ECN field in the inner header to CE;
otherwise, make no change to the ECN field in the inner
header. (The IPv4 checksum changes when the ECN changes.)Note: IPsec does not copy the options from the inner header into the outer header, nor does IPsec construct the options in the outer header. However, post-IPsec code MAY insert/construct options for the outer header.
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5.1.2.2. IPv6: Header Construction for Tunnel Mode
<-- How Outer Hdr Relates Inner Hdr --->
Outer Hdr at Inner Hdr at
IPv6 Encapsulator Decapsulator
Header fields: -------------------- ------------
version 6 (1) no change
DS Field copied from inner hdr (5) no change (9)
ECN Field copied from inner hdr constructed (6)
flow label copied or configured (8) no change
payload length constructed no change
next header AH,ESP,routing hdr no change
hop limit constructed (2) decrement (2)
src address constructed (3) no change
dest address constructed (3) no change
Extension headers never copied (7) no change
Notes:
(1) - (6) See [Section 5.1.2.1](#section-5.1.2.1).
(7) IPsec does not copy the extension headers from the inner
packet into outer headers, nor does IPsec construct extension
headers in the outer header. However, post-IPsec code MAY
insert/construct extension headers for the outer header.
(8) See [[RaCoCaDe04](#ref-RaCoCaDe04 ""IPv6 Flow Label Specification"")]. Copying is acceptable only for end systems,
not SGs. If an SG copied flow labels from the inner header to
the outer header, collisions might result.
(9) An implementation MAY choose to provide a facility to pass the
DS value from the outer header to the inner header, on a per-
SA basis, for received tunnel mode packets. The motivation
for providing this feature is to accommodate situations in
which the DS code space at the receiver is different from that
of the sender and the receiver has no way of knowing how to
translate from the sender's space. There is a danger in
copying this value from the outer header to the inner header,
since it enables an attacker to modify the outer DSCP value in
a fashion that may adversely affect other traffic at the
receiver. Hence the default behavior for IPsec
implementations is NOT to permit such copying.5.2. Processing Inbound IP Traffic (unprotected-to-protected)
Inbound processing is somewhat different from outbound processing, because of the use of SPIs to map IPsec-protected traffic to SAs. The inbound SPD cache (SPD-I) is applied only to bypassed or
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discarded traffic. If an arriving packet appears to be an IPsec fragment from an unprotected interface, reassembly is performed prior to IPsec processing. The intent for any SPD cache is that a packet that fails to match any entry is then referred to the corresponding SPD. Every SPD SHOULD have a nominal, final entry that catches anything that is otherwise unmatched, and discards it. This ensures that non-IPsec-protected traffic that arrives and does not match any SPD-I entry will be discarded.
Unprotected Interface
|
V
+-----+ IPsec protected
------------------->|Demux|-------------------+
| +-----+ |
| | |
| Not IPsec | |
| | |
| V |
| +-------+ +---------+ |
| |DISCARD|<---|SPD-I (*)| |
| +-------+ +---------+ |
| | |
| |-----+ |
| | | |
| | V |
| | +------+ |
| | | ICMP | |
| | +------+ |
| | V
+---------+ | +-----------+....|SPD-O ()|............|...................|PROCESS()|...IPsec +---------+ | | (AH/ESP) | Boundary ^ | +-----------+ | | +---+ | | BYPASS | +-->|IKE| | | | | +---+ | | V | V | +----------+ +---------+ +----+ |--------<------|Forwarding|<---------|SAD Check|-->|ICMP| nested SAs +----------+ | () | +----+ | +---------+ V Protected Interface
Figure 3. Processing Model for Inbound TrafficKent & Seo Standards Track [Page 60]
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(*) = The caches are shown here. If there is
a cache miss, then the SPD is checked.
There is no requirement that an
implementation buffer the packet if
there is a cache miss.
(**) = This processing includes using the
packet's SPI, etc., to look up the SA
in the SAD, which forms a cache of the
SPD for inbound packets (except for
cases noted in Sections [4.4.2](#section-4.4.2) and [5](#section-5)).
See step 3a below.
(***) = This SAD check refers to step 4 below.Prior to performing AH or ESP processing, any IP fragments that arrive via the unprotected interface are reassembled (by IP). Each inbound IP datagram to which IPsec processing will be applied is identified by the appearance of the AH or ESP values in the IP Next Protocol field (or of AH or ESP as a next layer protocol in the IPv6 context).
IPsec MUST perform the following steps:
When a packet arrives, it may be tagged with the ID of the interface (physical or virtual) via which it arrived, if necessary, to support multiple SPDs and associated SPD-I caches. (The interface ID is mapped to a corresponding SPD-ID.)
The packet is examined and demuxed into one of two categories:
- If the packet appears to be IPsec protected and it is addressed to this device, an attempt is made to map it to an active SA via the SAD. Note that the device may have multiple IP addresses that may be used in the SAD lookup, e.g., in the case of protocols such as SCTP.
- Traffic not addressed to this device, or addressed to this device and not AH or ESP, is directed to SPD-I lookup. (This implies that IKE traffic MUST have an explicit BYPASS entry in the SPD.) If multiple SPDs are employed, the tag assigned to the packet in step 1 is used to select the appropriate SPD-I (and cache) to search. SPD-I lookup determines whether the action is DISCARD or BYPASS.
3a. If the packet is addressed to the IPsec device and AH or ESP is specified as the protocol, the packet is looked up in the SAD. For unicast traffic, use only the SPI (or SPI plus protocol). For multicast traffic, use the SPI plus the destination or SPI plus destination and source addresses, as specified in Section 4.1. In either case (unicast or multicast), if there is no match, discard the traffic. This is an auditable event. The audit log
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entry for this event SHOULD include the current date/time, SPI,
source and destination of the packet, IPsec protocol, and any
other selector values of the packet that are available. If the
packet is found in the SAD, process it accordingly (see step 4).3b. If the packet is not addressed to the device or is addressed to this device and is not AH or ESP, look up the packet header in the (appropriate) SPD-I cache. If there is a match and the packet is to be discarded or bypassed, do so. If there is no cache match, look up the packet in the corresponding SPD-I and create a cache entry as appropriate. (No SAs are created in response to receipt of a packet that requires IPsec protection; only BYPASS or DISCARD cache entries can be created this way.) If there is no match, discard the traffic. This is an auditable event. The audit log entry for this event SHOULD include the current date/time, SPI if available, IPsec protocol if available, source and destination of the packet, and any other selector values of the packet that are available.
3c. Processing of ICMP messages is assumed to take place on the unprotected side of the IPsec boundary. Unprotected ICMP messages are examined and local policy is applied to determine whether to accept or reject these messages and, if accepted, what action to take as a result. For example, if an ICMP unreachable message is received, the implementation must decide whether to act on it, reject it, or act on it with constraints. (See Section 6.)
Apply AH or ESP processing as specified, using the SAD entry selected in step 3a above. Then match the packet against the inbound selectors identified by the SAD entry to verify that the received packet is appropriate for the SA via which it was received.
If an IPsec system receives an inbound packet on an SA and the packet's header fields are not consistent with the selectors for the SA, it MUST discard the packet. This is an auditable event. The audit log entry for this event SHOULD include the current date/time, SPI, IPsec protocol(s), source and destination of the packet, any other selector values of the packet that are available, and the selector values from the relevant SAD entry. The system SHOULD also be capable of generating and sending an IKE notification of INVALID_SELECTORS to the sender (IPsec peer), indicating that the received packet was discarded because of failure to pass selector checks.
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To minimize the impact of a DoS attack, or a mis-configured peer, the IPsec system SHOULD include a management control to allow an administrator to configure the IPsec implementation to send or not send this IKE notification, and if this facility is selected, to rate limit the transmission of such notifications.
After traffic is bypassed or processed through IPsec, it is handed to the inbound forwarding function for disposition. This function may cause the packet to be sent (outbound) across the IPsec boundary for additional inbound IPsec processing, e.g., in support of nested SAs. If so, then as with ALL outbound traffic that is to be bypassed, the packet MUST be matched against an SPD-O entry. Ultimately, the packet should be forwarded to the destination host or process for disposition.
6. ICMP Processing
This section describes IPsec handling of ICMP traffic. There are two categories of ICMP traffic: error messages (e.g., type = destination unreachable) and non-error messages (e.g., type = echo). This section applies exclusively to error messages. Disposition of non-error, ICMP messages (that are not addressed to the IPsec implementation itself) MUST be explicitly accounted for using SPD entries.
The discussion in this section applies to ICMPv6 as well as to ICMPv4. Also, a mechanism SHOULD be provided to allow an administrator to cause ICMP error messages (selected, all, or none) to be logged as an aid to problem diagnosis.
6.1. Processing ICMP Error Messages Directed to an IPsec Implementation
6.1.1. ICMP Error Messages Received on the Unprotected Side of the Boundary
Figure 3 in Section 5.2 shows a distinct ICMP processing module on the unprotected side of the IPsec boundary, for processing ICMP messages (error or otherwise) that are addressed to the IPsec device and that are not protected via AH or ESP. An ICMP message of this sort is unauthenticated, and its processing may result in denial or degradation of service. This suggests that, in general, it would be desirable to ignore such messages. However, many ICMP messages will be received by hosts or security gateways from unauthenticated sources, e.g., routers in the public Internet. Ignoring these ICMP messages can degrade service, e.g., because of a failure to process PMTU message and redirection messages. Thus, there is also a motivation for accepting and acting upon unauthenticated ICMP messages.
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To accommodate both ends of this spectrum, a compliant IPsec implementation MUST permit a local administrator to configure an IPsec implementation to accept or reject unauthenticated ICMP traffic. This control MUST be at the granularity of ICMP type and MAY be at the granularity of ICMP type and code. Additionally, an implementation SHOULD incorporate mechanisms and parameters for dealing with such traffic. For example, there could be the ability to establish a minimum PMTU for traffic (on a per destination basis), to prevent receipt of an unauthenticated ICMP from setting the PMTU to a trivial size.
If an ICMP PMTU message passes the checks above and the system is configured to accept it, then there are two possibilities. If the implementation applies fragmentation on the ciphertext side of the boundary, then the accepted PMTU information is passed to the forwarding module (outside of the IPsec implementation), which uses it to manage outbound packet fragmentation. If the implementation is configured to effect plaintext side fragmentation, then the PMTU information is passed to the plaintext side and processed as described in Section 8.2.
6.1.2. ICMP Error Messages Received on the Protected Side of the Boundary
These ICMP messages are not authenticated, but they do come from sources on the protected side of the IPsec boundary. Thus, these messages generally are viewed as more "trustworthy" than their counterparts arriving from sources on the unprotected side of the boundary. The major security concern here is that a compromised host or router might emit erroneous ICMP error messages that could degrade service for other devices "behind" the security gateway, or that could even result in violations of confidentiality. For example, if a bogus ICMP redirect were consumed by a security gateway, it could cause the forwarding table on the protected side of the boundary to be modified so as to deliver traffic to an inappropriate destination "behind" the gateway. Thus, implementers MUST provide controls to allow local administrators to constrain the processing of ICMP error messages received on the protected side of the boundary, and directed to the IPsec implementation. These controls are of the same type as those employed on the unprotected side, described above in Section 6.1.1.
6.2. Processing Protected, Transit ICMP Error Messages
When an ICMP error message is transmitted via an SA to a device "behind" an IPsec implementation, both the payload and the header of the ICMP message require checking from an access control perspective. If one of these messages is forwarded to a host behind a security
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gateway, the receiving host IP implementation will make decisions based on the payload, i.e., the header of the packet that purportedly triggered the error response. Thus, an IPsec implementation MUST be configurable to check that this payload header information is consistent with the SA via which it arrives. (This means that the payload header, with source and destination address and port fields reversed, matches the traffic selectors for the SA.) If this sort of check is not performed, then, for example, anyone with whom the receiving IPsec system (A) has an active SA could send an ICMP Destination Unreachable message that refers to any host/net with which A is currently communicating, and thus effect a highly efficient DoS attack regarding communication with other peers of A. Normal IPsec receiver processing of traffic is not sufficient to protect against such attacks. However, not all contexts may require such checks, so it is also necessary to allow a local administrator to configure an implementation to NOT perform such checks.
To accommodate both policies, the following convention is adopted. If an administrator wants to allow ICMP error messages to be carried by an SA without inspection of the payload, then configure an SPD entry that explicitly allows for carriage of such traffic. If an administrator wants IPsec to check the payload of ICMP error messages for consistency, then do not create any SPD entries that accommodate carriage of such traffic based on the ICMP packet header. This convention motivates the following processing description.
IPsec senders and receivers MUST support the following processing for ICMP error messages that are sent and received via SAs.
If an SA exists that accommodates an outbound ICMP error message, then the message is mapped to the SA and only the IP and ICMP headers are checked upon receipt, just as would be the case for other traffic. If no SA exists that matches the traffic selectors associated with an ICMP error message, then the SPD is searched to determine if such an SA can be created. If so, the SA is created and the ICMP error message is transmitted via that SA. Upon receipt, this message is subject to the usual traffic selector checks at the receiver. This processing is exactly what would happen for traffic in general, and thus does not represent any special processing for ICMP error messages.
If no SA exists that would carry the outbound ICMP message in question, and if no SPD entry would allow carriage of this outbound ICMP error message, then an IPsec implementation MUST map the message to the SA that would carry the return traffic associated with the packet that triggered the ICMP error message. This requires an IPsec implementation to detect outbound ICMP error messages that map to no extant SA or SPD entry, and treat them specially with regard to SA
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creation and lookup. The implementation extracts the header for the packet that triggered the error (from the ICMP message payload), reverses the source and destination IP address fields, extracts the protocol field, and reverses the port fields (if accessible). It then uses this extracted information to locate an appropriate, active outbound SA, and transmits the error message via this SA. If no such SA exists, no SA will be created, and this is an auditable event.
If an IPsec implementation receives an inbound ICMP error message on an SA, and the IP and ICMP headers of the message do not match the traffic selectors for the SA, the receiver MUST process the received message in a special fashion. Specifically, the receiver must extract the header of the triggering packet from the ICMP payload, and reverse fields as described above to determine if the packet is consistent with the selectors for the SA via which the ICMP error message was received. If the packet fails this check, the IPsec implementation MUST NOT forwarded the ICMP message to the destination. This is an auditable event.
7. Handling Fragments (on the protected side of the IPsec boundary)
Earlier sections of this document describe mechanisms for (a) fragmenting an outbound packet after IPsec processing has been applied and reassembling it at the receiver before IPsec processing and (b) handling inbound fragments received from the unprotected side of the IPsec boundary. This section describes how an implementation should handle the processing of outbound plaintext fragments on the protected side of the IPsec boundary. (See Appendix D, "Fragment Handling Rationale".) In particular, it addresses:
o mapping an outbound non-initial fragment to the right SA
(or finding the right SPD entry)
o verifying that a received non-initial fragment is
authorized for the SA via which it was received
o mapping outbound and inbound non-initial fragments to the
right SPD-O/SPD-I entry or the relevant cache entry, for
BYPASS/DISCARD trafficNote: In Section 4.1, transport mode SAs have been defined to not carry fragments (IPv4 or IPv6). Note also that in Section 4.4.1, two special values, ANY and OPAQUE, were defined for selectors and that ANY includes OPAQUE. The term "non-trivial" is used to mean that the selector has a value other than OPAQUE or ANY.
Note: The term "non-initial fragment" is used here to indicate a fragment that does not contain all the selector values that may be needed for access control. As observed in Section 4.4.1, depending on the Next Layer Protocol, in addition to Ports, the ICMP message
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type/code or Mobility Header type could be missing from non-initial fragments. Also, for IPv6, even the first fragment might NOT contain the Next Layer Protocol or Ports (or ICMP message type/code, or Mobility Header type) depending on the kind and number of extension headers present. If a non-initial fragment contains the Port (or ICMP type and code or Mobility Header type) but not the Next Layer Protocol, then unless there is an SPD entry for the relevant Local/Remote addresses with ANY for Next Layer Protocol and Port (or ICMP type and code or Mobility Header type), the fragment would not contain all the selector information needed for access control.
To address the above issues, three approaches have been defined:
o Tunnel mode SAs that carry initial and non-initial fragments
(See [Section 7.1](#section-7.1).)
o Separate tunnel mode SAs for non-initial fragments (See
[Section 7.2](#section-7.2).)
o Stateful fragment checking (See [Section 7.3](#section-7.3).)7.1. Tunnel Mode SAs that Carry Initial and Non-Initial Fragments
All implementations MUST support tunnel mode SAs that are configured to pass traffic without regard to port field (or ICMP type/code or Mobility Header type) values. If the SA will carry traffic for specified protocols, the selector set for the SA MUST specify the port fields (or ICMP type/code or Mobility Header type) as ANY. An SA defined in this fashion will carry all traffic including initial and non-initial fragments for the indicated Local/Remote addresses and specified Next Layer protocol(s). If the SA will carry traffic without regard to a specific protocol value (i.e., ANY is specified as the (Next Layer) protocol selector value), then the port field values are undefined and MUST be set to ANY as well. (As noted in 4.4.1, ANY includes OPAQUE as well as all specific values.)
7.2. Separate Tunnel Mode SAs for Non-Initial Fragments
An implementation MAY support tunnel mode SAs that will carry only non-initial fragments, separate from non-fragmented packets and initial fragments. The OPAQUE value will be used to specify port (or ICMP type/code or Mobility Header type) field selectors for an SA to carry such fragments. Receivers MUST perform a minimum offset check on IPv4 (non-initial) fragments to protect against overlapping fragment attacks when SAs of this type are employed. Because such checks cannot be performed on IPv6 non-initial fragments, users and administrators are advised that carriage of such fragments may be dangerous, and implementers may choose to NOT support such SAs for IPv6 traffic. Also, an SA of this sort will carry all non-initial fragments that match a specified Local/Remote address pair and
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protocol value, i.e., the fragments carried on this SA belong to packets that if not fragmented, might have gone on separate SAs of differing security. Therefore, users and administrators are advised to protect such traffic using ESP (with integrity) and the "strongest" integrity and encryption algorithms in use between both peers. (Determination of the "strongest" algorithms requires imposing an ordering of the available algorithms, a local determination at the discretion of the initiator of the SA.)
Specific port (or ICMP type/code or Mobility Header type) selector values will be used to define SAs to carry initial fragments and non-fragmented packets. This approach can be used if a user or administrator wants to create one or more tunnel mode SAs between the same Local/Remote addresses that discriminate based on port (or ICMP type/code or Mobility Header type) fields. These SAs MUST have non-trivial protocol selector values, otherwise approach #1 above MUST be used.
Note: In general, for the approach described in this section, one needs only a single SA between two implementations to carry all non-initial fragments. However, if one chooses to have multiple SAs between the two implementations for QoS differentiation, then one might also want multiple SAs to carry fragments-without-ports, one for each supported QoS class. Since support for QoS via distinct SAs is a local matter, not mandated by this document, the choice to have multiple SAs to carry non-initial fragments should also be local.
7.3. Stateful Fragment Checking
An implementation MAY support some form of stateful fragment checking for a tunnel mode SA with non-trivial port (or ICMP type/code or MH type) field values (not ANY or OPAQUE). Implementations that will transmit non-initial fragments on a tunnel mode SA that makes use of non-trivial port (or ICMP type/code or MH type) selectors MUST notify a peer via the IKE NOTIFY NON_FIRST_FRAGMENTS_ALSO payload.
The peer MUST reject this proposal if it will not accept non-initial fragments in this context. If an implementation does not successfully negotiate transmission of non-initial fragments for such an SA, it MUST NOT send such fragments over the SA. This standard does not specify how peers will deal with such fragments, e.g., via reassembly or other means, at either sender or receiver. However, a receiver MUST discard non-initial fragments that arrive on an SA with non-trivial port (or ICMP type/code or MH type) selector values unless this feature has been negotiated. Also, the receiver MUST discard non-initial fragments that do not comply with the security policy applied to the overall packet. Discarding such packets is an auditable event. Note that in network configurations where fragments
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of a packet might be sent or received via different security gateways or BITW implementations, stateful strategies for tracking fragments may fail.
7.4. BYPASS/DISCARD Traffic
All implementations MUST support DISCARDing of fragments using the normal SPD packet classification mechanisms. All implementations MUST support stateful fragment checking to accommodate BYPASS traffic for which a non-trivial port range is specified. The concern is that BYPASS of a cleartext, non-initial fragment arriving at an IPsec implementation could undermine the security afforded IPsec-protected traffic directed to the same destination. For example, consider an IPsec implementation configured with an SPD entry that calls for IPsec protection of traffic between a specific source/destination address pair, and for a specific protocol and destination port, e.g., TCP traffic on port 23 (Telnet). Assume that the implementation also allows BYPASS of traffic from the same source/destination address pair and protocol, but for a different destination port, e.g., port 119 (NNTP). An attacker could send a non-initial fragment (with a forged source address) that, if bypassed, could overlap with IPsec-protected traffic from the same source and thus violate the integrity of the IPsec-protected traffic. Requiring stateful fragment checking for BYPASS entries with non-trivial port ranges prevents attacks of this sort. As noted above, in network configurations where fragments of a packet might be sent or received via different security gateways or BITW implementations, stateful strategies for tracking fragments may fail.
8. Path MTU/DF Processing
The application of AH or ESP to an outbound packet increases the size of a packet and thus may cause a packet to exceed the PMTU for the SA via which the packet will travel. An IPsec implementation also may receive an unprotected ICMP PMTU message and, if it chooses to act upon the message, the result will affect outbound traffic processing. This section describes the processing required of an IPsec implementation to deal with these two PMTU issues.
8.1. DF Bit
All IPsec implementations MUST support the option of copying the DF bit from an outbound packet to the tunnel mode header that it emits, when traffic is carried via a tunnel mode SA. This means that it MUST be possible to configure the implementation's treatment of the DF bit (set, clear, copy from inner header) for each SA. This applies to SAs where both inner and outer headers are IPv4.
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8.2. Path MTU (PMTU) Discovery
This section discusses IPsec handling for unprotected Path MTU Discovery messages. ICMP PMTU is used here to refer to an ICMP message for:
IPv4 ([RFC 792](./rfc792) [[Pos81b](#ref-Pos81b ""Internet Control Message Protocol"")]):
- Type = 3 (Destination Unreachable)
- Code = 4 (Fragmentation needed and DF set)
- Next-Hop MTU in the low-order 16 bits of the
second word of the ICMP header (labeled "unused"
in [RFC 792](./rfc792)), with high-order 16 bits set to zero)
IPv6 ([RFC 2463](./rfc2463) [[CD98](#ref-CD98 ""Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification"")]):
- Type = 2 (Packet Too Big)
- Code = 0 (Fragmentation needed)
- Next-Hop MTU in the 32-bit MTU field of the ICMP6
message8.2.1. Propagation of PMTU
When an IPsec implementation receives an unauthenticated PMTU message, and it is configured to process (vs. ignore) such messages, it maps the message to the SA to which it corresponds. This mapping is effected by extracting the header information from the payload of the PMTU message and applying the procedure described in Section 5.2. The PMTU determined by this message is used to update the SAD PMTU field, taking into account the size of the AH or ESP header that will be applied, any crypto synchronization data, and the overhead imposed by an additional IP header, in the case of a tunnel mode SA.
In a native host implementation, it is possible to maintain PMTU data at the same granularity as for unprotected communication, so there is no loss of functionality. Signaling of the PMTU information is internal to the host. For all other IPsec implementation options, the PMTU data must be propagated via a synthesized ICMP PMTU. In these cases, the IPsec implementation SHOULD wait for outbound traffic to be mapped to the SAD entry. When such traffic arrives, if the traffic would exceed the updated PMTU value the traffic MUST be handled as follows:
Case 1: Original (cleartext) packet is IPv4 and has the DF
bit set. The implementation SHOULD discard the packet
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Case 2: Original (cleartext) packet is IPv4 and has the DF
bit clear. The implementation SHOULD fragment (before or
after encryption per its configuration) and then forward
the fragments. It SHOULD NOT send a PMTU ICMP message.
Case 3: Original (cleartext) packet is IPv6. The implementation
SHOULD discard the packet and send a PMTU ICMP message.8.2.2. PMTU Aging
In all IPsec implementations, the PMTU associated with an SA MUST be "aged" and some mechanism is required to update the PMTU in a timely manner, especially for discovering if the PMTU is smaller than required by current network conditions. A given PMTU has to remain in place long enough for a packet to get from the source of the SA to the peer, and to propagate an ICMP error message if the current PMTU is too big.
Implementations SHOULD use the approach described in the Path MTU Discovery document (RFC 1191 [[MD90](#ref-MD90 ""Path MTU discovery"")], Section 6.3), which suggests periodically resetting the PMTU to the first-hop data-link MTU and then letting the normal PMTU Discovery processes update the PMTU as necessary. The period SHOULD be configurable.
9. Auditing
IPsec implementations are not required to support auditing. For the most part, the granularity of auditing is a local matter. However, several auditable events are identified in this document, and for each of these events a minimum set of information that SHOULD be included in an audit log is defined. Additional information also MAY be included in the audit log for each of these events, and additional events, not explicitly called out in this specification, also MAY result in audit log entries. There is no requirement for the receiver to transmit any message to the purported transmitter in response to the detection of an auditable event, because of the potential to induce denial of service via such action.
10. Conformance Requirements
All IPv4 IPsec implementations MUST comply with all requirements of this document. All IPv6 implementations MUST comply with all requirements of this document.
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11. Security Considerations
The focus of this document is security; hence security considerations permeate this specification.
IPsec imposes stringent constraints on bypass of IP header data in both directions, across the IPsec barrier, especially when tunnel mode SAs are employed. Some constraints are absolute, while others are subject to local administrative controls, often on a per-SA basis. For outbound traffic, these constraints are designed to limit covert channel bandwidth. For inbound traffic, the constraints are designed to prevent an adversary who has the ability to tamper with one data stream (on the unprotected side of the IPsec barrier) from adversely affecting other data streams (on the protected side of the barrier). The discussion in Section 5 dealing with processing DSCP values for tunnel mode SAs illustrates this concern.
If an IPsec implementation is configured to pass ICMP error messages over SAs based on the ICMP header values, without checking the header information from the ICMP message payload, serious vulnerabilities may arise. Consider a scenario in which several sites (A, B, and C) are connected to one another via ESP-protected tunnels: A-B, A-C, and B-C. Also assume that the traffic selectors for each tunnel specify ANY for protocol and port fields and IP source/destination address ranges that encompass the address range for the systems behind the security gateways serving each site. This would allow a host at site B to send an ICMP Destination Unreachable message to any host at site A, that declares all hosts on the net at site C to be unreachable. This is a very efficient DoS attack that could have been prevented if the ICMP error messages were subjected to the checks that IPsec provides, if the SPD is suitably configured, as described in Section 6.2.
12. IANA Considerations
The IANA has assigned the value (3) for the asn1-modules registry and has assigned the object identifier 1.3.6.1.5.8.3.1 for the SPD module. See Appendix C, "ASN.1 for an SPD Entry".
This architecture document differs substantially from RFC 2401 [[RFC2401](./rfc2401 ""Security Architecture for the Internet Protocol"")] in detail and in organization, but the fundamental notions are unchanged.
o The processing model has been revised to address new IPsec scenarios, improve performance, and simplify implementation. This includes a separation between forwarding (routing) and SPD
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selection, several SPD changes, and the addition of an outbound SPD
cache and an inbound SPD cache for bypassed or discarded traffic.
There is also a new database, the Peer Authorization Database
(PAD). This provides a link between an SA management protocol
(such as IKE) and the SPD.o There is no longer a requirement to support nested SAs or "SA bundles". Instead this functionality can be achieved through SPD and forwarding table configuration. An example of a configuration has been added in Appendix E.
o SPD entries were redefined to provide more flexibility. Each SPD entry now consists of 1 to N sets of selectors, where each selector set contains one protocol and a "list of ranges" can now be specified for the Local IP address, Remote IP address, and whatever fields (if any) are associated with the Next Layer Protocol (Local Port, Remote Port, ICMP message type and code, and Mobility Header type). An individual value for a selector is represented via a trivial range and ANY is represented via a range than spans all values for the selector. An example of an ASN.1 description is included in Appendix C.
o TOS (IPv4) and Traffic Class (IPv6) have been replaced by DSCP and ECN. The tunnel section has been updated to explain how to handle DSCP and ECN bits.
o For tunnel mode SAs, an SG, BITS, or BITW implementation is now allowed to fragment packets before applying IPsec. This applies only to IPv4. For IPv6 packets, only the originator is allowed to fragment them.
o When security is desired between two intermediate systems along a path or between an intermediate system and an end system, transport mode may now be used between security gateways and between a security gateway and a host.
o This document clarifies that for all traffic that crosses the IPsec boundary, including IPsec management traffic, the SPD or associated caches must be consulted.
o This document defines how to handle the situation of a security gateway with multiple subscribers requiring separate IPsec contexts.
o A definition of reserved SPIs has been added.
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o Text has been added explaining why ALL IP packets must be checked -- IPsec includes minimal firewall functionality to support access control at the IP layer.
o The tunnel section has been updated to clarify how to handle the IP options field and IPv6 extension headers when constructing the outer header.
o SA mapping for inbound traffic has been updated to be consistent with the changes made in AH and ESP for support of unicast and multicast SAs.
o Guidance has been added regarding how to handle the covert channel created in tunnel mode by copying the DSCP value to outer header.
o Support for AH in both IPv4 and IPv6 is no longer required.
o PMTU handling has been updated. The appendix on PMTU/DF/Fragmentation has been deleted.
o Three approaches have been added for handling plaintext fragments on the protected side of the IPsec boundary. Appendix D documents the rationale behind them.
o Added revised text describing how to derive selector values for SAs (from the SPD entry or from the packet, etc.)
o Added a new table describing the relationship between selector values in an SPD entry, the PFP flag, and resulting selector values in the corresponding SAD entry.
o Added Appendix B to describe decorrelation.
o Added text describing how to handle an outbound packet that must be discarded.
o Added text describing how to handle a DISCARDED inbound packet, i.e., one that does not match the SA upon which it arrived.
o IPv6 mobility header has been added as a possible Next Layer Protocol. IPv6 Mobility Header message type has been added as a selector.
o ICMP message type and code have been added as selectors.
o The selector "data sensitivity level" has been removed to simplify things.
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o Updated text describing handling ICMP error messages. The appendix on "Categorization of ICMP Messages" has been deleted.
o The text for the selector name has been updated and clarified.
o The "Next Layer Protocol" has been further explained and a default list of protocols to skip when looking for the Next Layer Protocol has been added.
o The text has been amended to say that this document assumes use of IKEv2 or an SA management protocol with comparable features.
o Text has been added clarifying the algorithm for mapping inbound IPsec datagrams to SAs in the presence of multicast SAs.
o The appendix "Sequence Space Window Code Example" has been removed.
o With respect to IP addresses and ports, the terms "Local" and "Remote" are used for policy rules (replacing source and destination). "Local" refers to the entity being protected by an IPsec implementation, i.e., the "source" address/port of outbound packets or the "destination" address/port of inbound packets. "Remote" refers to a peer entity or peer entities. The terms "source" and "destination" are still used for packet header fields.
14. Acknowledgements
The authors would like to acknowledge the contributions of Ran Atkinson, who played a critical role in initial IPsec activities, and who authored the first series of IPsec standards: RFCs 1825-1827; and Charlie Lynn, who made significant contributions to the second series of IPsec standards (RFCs 2401, 2402, and 2406) and to the current versions, especially with regard to IPv6 issues. The authors also would like to thank the members of the IPsec and MSEC working groups who have contributed to the development of this protocol specification.
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Appendix A: Glossary
This section provides definitions for several key terms that are employed in this document. Other documents provide additional definitions and background information relevant to this technology, e.g., [[Shi00](#ref-Shi00 ""Internet Security Glossary"")], [[VK83](#ref-VK83 ""Security Mechanisms in High-level Networks"")], and [[HA94](#ref-HA94 ""On Internet Authentication"")]. Included in this glossary are generic security service and security mechanism terms, plus IPsec-specific terms.
Access Control A security service that prevents unauthorized use of a resource, including the prevention of use of a resource in an unauthorized manner. In the IPsec context, the resource to which access is being controlled is often:
o for a host, computing cycles or data
o for a security gateway, a network behind the gateway
or bandwidth on that network.Anti-replay See "Integrity" below.
Authentication Used informally to refer to the combination of two nominally distinct security services, data origin authentication and connectionless integrity. See the definitions below for each of these services.
Availability When viewed as a security service, addresses the security concerns engendered by attacks against networks that deny or degrade service. For example, in the IPsec context, the use of anti-replay mechanisms in AH and ESP support availability.
Confidentiality The security service that protects data from unauthorized disclosure. The primary confidentiality concern in most instances is unauthorized disclosure of application-level data, but disclosure of the external characteristics of communication also can be a concern in some circumstances. Traffic flow confidentiality is the service that addresses this latter concern by concealing source and destination addresses, message length, or frequency of communication. In the IPsec context, using ESP in tunnel mode, especially at a security gateway, can provide some level of traffic flow confidentiality. (See also "Traffic Analysis" below.)
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Data Origin Authentication A security service that verifies the identity of the claimed source of data. This service is usually bundled with connectionless integrity service.
Encryption A security mechanism used to transform data from an intelligible form (plaintext) into an unintelligible form (ciphertext), to provide confidentiality. The inverse transformation process is designated "decryption". Often the term "encryption" is used to generically refer to both processes.
Integrity A security service that ensures that modifications to data are detectable. Integrity comes in various flavors to match application requirements. IPsec supports two forms of integrity: connectionless and a form of partial sequence integrity. Connectionless integrity is a service that detects modification of an individual IP datagram, without regard to the ordering of the datagram in a stream of traffic. The form of partial sequence integrity offered in IPsec is referred to as anti-replay integrity, and it detects arrival of duplicate IP datagrams (within a constrained window). This is in contrast to connection-oriented integrity, which imposes more stringent sequencing requirements on traffic, e.g., to be able to detect lost or re-ordered messages. Although authentication and integrity services often are cited separately, in practice they are intimately connected and almost always offered in tandem.
Protected vs. Unprotected "Protected" refers to the systems or interfaces that are inside the IPsec protection boundary, and "unprotected" refers to the systems or interfaces that are outside the IPsec protection boundary. IPsec provides a boundary through which traffic passes. There is an asymmetry to this barrier, which is reflected in the processing model. Outbound data, if not discarded or bypassed, is protected via the application of AH or ESP and the addition of the corresponding headers. Inbound data, if not discarded or bypassed, is processed via the removal of AH or ESP headers. In this document, inbound traffic enters an IPsec implementation from the "unprotected" interface. Outbound traffic enters the implementation via the "protected" interface, or is internally generated by the implementation on the "protected" side of the boundary and directed toward the "unprotected" interface. An IPsec implementation may support more than one interface on either or both sides of the boundary. The protected interface may be
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internal, e.g., in a host implementation of IPsec. The protected
interface may link to a socket layer interface presented by the
OS.Security Association (SA) A simplex (uni-directional) logical connection, created for security purposes. All traffic traversing an SA is provided the same security processing. In IPsec, an SA is an Internet-layer abstraction implemented through the use of AH or ESP. State data associated with an SA is represented in the SA Database (SAD).
Security Gateway An intermediate system that acts as the communications interface between two networks. The set of hosts (and networks) on the external side of the security gateway is termed unprotected (they are generally at least less protected than those "behind" the SG), while the networks and hosts on the internal side are viewed as protected. The internal subnets and hosts served by a security gateway are presumed to be trusted by virtue of sharing a common, local, security administration. In the IPsec context, a security gateway is a point at which AH and/or ESP is implemented in order to serve a set of internal hosts, providing security services for these hosts when they communicate with external hosts also employing IPsec (either directly or via another security gateway).
Security Parameters Index (SPI) An arbitrary 32-bit value that is used by a receiver to identify the SA to which an incoming packet should be bound. For a unicast SA, the SPI can be used by itself to specify an SA, or it may be used in conjunction with the IPsec protocol type. Additional IP address information is used to identify multicast SAs. The SPI is carried in AH and ESP protocols to enable the receiving system to select the SA under which a received packet will be processed. An SPI has only local significance, as defined by the creator of the SA (usually the receiver of the packet carrying the SPI); thus an SPI is generally viewed as an opaque bit string. However, the creator of an SA may choose to interpret the bits in an SPI to facilitate local processing.
Traffic Analysis The analysis of network traffic flow for the purpose of deducing information that is useful to an adversary. Examples of such information are frequency of transmission, the identities of the conversing parties, sizes of packets, and flow identifiers [Sch94].
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Appendix B: Decorrelation
This appendix is based on work done for caching of policies in the IP Security Policy Working Group by Luis Sanchez, Matt Condell, and John Zao.
Two SPD entries are correlated if there is a non-null intersection between the values of corresponding selectors in each entry. Caching correlated SPD entries can lead to incorrect policy enforcement. A solution to this problem, which still allows for caching, is to remove the ambiguities by decorrelating the entries. That is, the SPD entries must be rewritten so that for every pair of entries there exists a selector for which there is a null intersection between the values in both of the entries. Once the entries are decorrelated, there is no longer any ordering requirement on them, since only one entry will match any lookup. The next section describes decorrelation in more detail and presents an algorithm that may be used to implement decorrelation.
B.1. Decorrelation Algorithm
The basic decorrelation algorithm takes each entry in a correlated SPD and divides it into a set of entries using a tree structure. The nodes of the tree are the selectors that may overlap between the policies. At each node, the algorithm creates a branch for each of the values of the selector. It also creates one branch for the complement of the union of all selector values. Policies are then formed by traversing the tree from the root to each leaf. The policies at the leaves are compared to the set of already decorrelated policy rules. Each policy at a leaf is either completely overridden by a policy in the already decorrelated set and is discarded or is decorrelated with all the policies in the decorrelated set and is added to it.
The basic algorithm does not guarantee an optimal set of decorrelated entries. That is, the entries may be broken up into smaller sets than is necessary, though they will still provide all the necessary policy information. Some extensions to the basic algorithm are described later to improve this and improve the performance of the algorithm.
C A set of ordered, correlated entries (a correlated SPD).
Ci The ith entry in C.
U The set of decorrelated entries being built from C.
Ui The ith entry in U.
Sik The kth selection for policy Ci.
Ai The action for policy Ci.Kent & Seo Standards Track [Page 79]
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A policy (SPD entry) P may be expressed as a sequence of selector values and an action (BYPASS, DISCARD, or PROTECT):
Ci = Si1 x Si2 x ... x Sik -> Ai- Put C1 in set U as U1
For each policy Cj (j > 1) in C
If Cj is decorrelated with every entry in U, then add it to U.
If Cj is correlated with one or more entries in U, create a tree rooted at the policy Cj that partitions Cj into a set of decorrelated entries. The algorithm starts with a root node where no selectors have yet been chosen.
A) Choose a selector in Cj, Sjn, that has not yet been chosen when
traversing the tree from the root to this node. If there are no
selectors not yet used, continue to the next unfinished branch
until all branches have been completed. When the tree is
completed, go to step D.
T is the set of entries in U that are correlated with the entry
at this node.
The entry at this node is the entry formed by the selector
values of each of the branches between the root and this node.
Any selector values that are not yet represented by branches
assume the corresponding selector value in Cj, since the values
in Cj represent the maximum value for each selector.
B) Add a branch to the tree for each value of the selector Sjn that
appears in any of the entries in T. (If the value is a superset
of the value of Sjn in Cj, then use the value in Cj, since that
value represents the universal set.) Also add a branch for the
complement of the union of all the values of the selector Sjn
in T. When taking the complement, remember that the universal
set is the value of Sjn in Cj. A branch need not be created
for the null set.
C) Repeat A and B until the tree is completed.
D) The entry to each leaf now represents an entry that is a subset
of Cj. The entries at the leaves completely partition Cj in
such a way that each entry is either completely overridden by
an entry in U, or is decorrelated with the entries in U.
Add all the decorrelated entries at the leaves of the tree to U.Kent & Seo Standards Track [Page 80]
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Get next Cj and go to 2.
When all entries in C have been processed, then U will contain an decorrelated version of C.
There are several optimizations that can be made to this algorithm. A few of them are presented here.
It is possible to optimize, or at least improve, the amount of branching that occurs by carefully choosing the order of the selectors used for the next branch. For example, if a selector Sjn can be chosen so that all the values for that selector in T are equal to or a superset of the value of Sjn in Cj, then only a single branch needs to be created (since the complement will be null).
Branches of the tree do not have to proceed with the entire decorrelation algorithm. For example, if a node represents an entry that is decorrelated with all the entries in U, then there is no reason to continue decorrelating that branch. Also, if a branch is completely overridden by an entry in U, then there is no reason to continue decorrelating the branch.
An additional optimization is to check to see if a branch is overridden by one of the CORRELATED entries in set C that has already been decorrelated. That is, if the branch is part of decorrelating Cj, then check to see if it was overridden by an entry Cm, m < j. This is a valid check, since all the entries Cm are already expressed in U.
Along with checking if an entry is already decorrelated in step 2, check if Cj is overridden by any entry in U. If it is, skip it since it is not relevant. An entry x is overridden by another entry y if every selector in x is equal to or a subset of the corresponding selector in entry y.
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Appendix C: ASN.1 for an SPD Entry
This appendix is included as an additional way to describe SPD entries, as defined in Section 4.4.1. It uses ASN.1 syntax that has been successfully compiled. This syntax is merely illustrative and need not be employed in an implementation to achieve compliance. The SPD description in Section 4.4.1 is normative.
SPDModule
{iso(1) org (3) dod (6) internet (1) security (5) mechanisms (5)
ipsec (8) asn1-modules (3) spd-module (1) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
RDNSequence FROM PKIX1Explicit88
{ iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-pkix1-explicit(18) } ;
-- An SPD is a list of policies in decreasing order of preference
SPD ::= SEQUENCE OF SPDEntry
SPDEntry ::= CHOICE {
iPsecEntry IPsecEntry, -- PROTECT traffic
bypassOrDiscard [0] BypassOrDiscardEntry } -- DISCARD/BYPASS
IPsecEntry ::= SEQUENCE { -- Each entry consists of
name NameSets OPTIONAL,
pFPs PacketFlags, -- Populate from packet flags
-- Applies to ALL of the corresponding
-- traffic selectors in the SelectorLists
condition SelectorLists, -- Policy "condition"
processing Processing -- Policy "action"
}
BypassOrDiscardEntry ::= SEQUENCE {
bypass BOOLEAN, -- TRUE BYPASS, FALSE DISCARD
condition InOutBound }
InOutBound ::= CHOICE {
outbound [0] SelectorLists,
inbound [1] SelectorLists,
bothways [2] BothWays }Kent & Seo Standards Track [Page 82]
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BothWays ::= SEQUENCE {
inbound SelectorLists,
outbound SelectorLists }
NameSets ::= SEQUENCE {
passed SET OF Names-R, -- Matched to IKE ID by
-- responder
local SET OF Names-I } -- Used internally by IKE
-- initiator
Names-R ::= CHOICE { -- IKEv2 IDs
dName RDNSequence, -- ID_DER_ASN1_DN
fqdn FQDN, -- ID_FQDN
[rfc822](./rfc822) [0] RFC822Name, -- ID_RFC822_ADDR
keyID OCTET STRING } -- KEY_ID
Names-I ::= OCTET STRING -- Used internally by IKE
-- initiator
FQDN ::= IA5String
RFC822Name ::= IA5String
PacketFlags ::= BIT STRING {
-- if set, take selector value from packet
-- establishing SA
-- else use value in SPD entry
localAddr (0),
remoteAddr (1),
protocol (2),
localPort (3),
remotePort (4) }
SelectorLists ::= SET OF SelectorList
SelectorList ::= SEQUENCE {
localAddr AddrList,
remoteAddr AddrList,
protocol ProtocolChoice }
Processing ::= SEQUENCE {
extSeqNum BOOLEAN, -- TRUE 64 bit counter, FALSE 32 bit
seqOverflow BOOLEAN, -- TRUE rekey, FALSE terminate & audit
fragCheck BOOLEAN, -- TRUE stateful fragment checking,
-- FALSE no stateful fragment checking
lifetime SALifetime,
spi ManualSPI,
algorithms ProcessingAlgs,Kent & Seo Standards Track [Page 83]
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tunnel TunnelOptions OPTIONAL } -- if absent, use
-- transport mode
SALifetime ::= SEQUENCE {
seconds [0] INTEGER OPTIONAL,
bytes [1] INTEGER OPTIONAL }
ManualSPI ::= SEQUENCE {
spi INTEGER,
keys KeyIDs }
KeyIDs ::= SEQUENCE OF OCTET STRING
ProcessingAlgs ::= CHOICE {
ah [0] IntegrityAlgs, -- AH
esp [1] ESPAlgs} -- ESP
ESPAlgs ::= CHOICE {
integrity [0] IntegrityAlgs, -- integrity only
confidentiality [1] ConfidentialityAlgs, -- confidentiality
-- only
both [2] IntegrityConfidentialityAlgs,
combined [3] CombinedModeAlgs }
IntegrityConfidentialityAlgs ::= SEQUENCE {
integrity IntegrityAlgs,
confidentiality ConfidentialityAlgs }
-- Integrity Algorithms, ordered by decreasing preference
IntegrityAlgs ::= SEQUENCE OF IntegrityAlg
-- Confidentiality Algorithms, ordered by decreasing preference
ConfidentialityAlgs ::= SEQUENCE OF ConfidentialityAlg
-- Integrity Algorithms
IntegrityAlg ::= SEQUENCE {
algorithm IntegrityAlgType,
parameters ANY -- DEFINED BY algorithm -- OPTIONAL }
IntegrityAlgType ::= INTEGER {
none (0),
auth-HMAC-MD5-96 (1),
auth-HMAC-SHA1-96 (2),
auth-DES-MAC (3),
auth-KPDK-MD5 (4),
auth-AES-XCBC-96 (5)
-- tbd (6..65535)
}Kent & Seo Standards Track [Page 84]
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-- Confidentiality Algorithms
ConfidentialityAlg ::= SEQUENCE {
algorithm ConfidentialityAlgType,
parameters ANY -- DEFINED BY algorithm -- OPTIONAL }
ConfidentialityAlgType ::= INTEGER {
encr-DES-IV64 (1),
encr-DES (2),
encr-3DES (3),
encr-RC5 (4),
encr-IDEA (5),
encr-CAST (6),
encr-BLOWFISH (7),
encr-3IDEA (8),
encr-DES-IV32 (9),
encr-RC4 (10),
encr-NULL (11),
encr-AES-CBC (12),
encr-AES-CTR (13)
-- tbd (14..65535)
}
CombinedModeAlgs ::= SEQUENCE OF CombinedModeAlg
CombinedModeAlg ::= SEQUENCE {
algorithm CombinedModeType,
parameters ANY -- DEFINED BY algorithm} -- defined outside
-- of this document for AES modes.
CombinedModeType ::= INTEGER {
comb-AES-CCM (1),
comb-AES-GCM (2)
-- tbd (3..65535)
}
TunnelOptions ::= SEQUENCE {
dscp DSCP,
ecn BOOLEAN, -- TRUE Copy CE to inner header
df DF,
addresses TunnelAddresses }
TunnelAddresses ::= CHOICE {
ipv4 IPv4Pair,
ipv6 [0] IPv6Pair }
IPv4Pair ::= SEQUENCE {
local OCTET STRING (SIZE(4)),
remote OCTET STRING (SIZE(4)) }Kent & Seo Standards Track [Page 85]
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IPv6Pair ::= SEQUENCE {
local OCTET STRING (SIZE(16)),
remote OCTET STRING (SIZE(16)) }
DSCP ::= SEQUENCE {
copy BOOLEAN, -- TRUE copy from inner header
-- FALSE do not copy
mapping OCTET STRING OPTIONAL} -- points to table
-- if no copy
DF ::= INTEGER {
clear (0),
set (1),
copy (2) }
ProtocolChoice::= CHOICE {
anyProt AnyProtocol, -- for ANY protocol
noNext [0] NoNextLayerProtocol, -- has no next layer
-- items
oneNext [1] OneNextLayerProtocol, -- has one next layer
-- item
twoNext [2] TwoNextLayerProtocol, -- has two next layer
-- items
fragment FragmentNoNext } -- has no next layer
-- info
AnyProtocol ::= SEQUENCE {
id INTEGER (0), -- ANY protocol
nextLayer AnyNextLayers }
AnyNextLayers ::= SEQUENCE { -- with either
first AnyNextLayer, -- ANY next layer selector
second AnyNextLayer } -- ANY next layer selector
NoNextLayerProtocol ::= INTEGER (2..254)
FragmentNoNext ::= INTEGER (44) -- Fragment identifier
OneNextLayerProtocol ::= SEQUENCE {
id INTEGER (1..254), -- ICMP, MH, ICMPv6
nextLayer NextLayerChoice } -- ICMP Type*256+Code
-- MH Type*256
TwoNextLayerProtocol ::= SEQUENCE {
id INTEGER (2..254), -- Protocol
local NextLayerChoice, -- Local and
remote NextLayerChoice } -- Remote portsKent & Seo Standards Track [Page 86]
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NextLayerChoice ::= CHOICE {
any AnyNextLayer,
opaque [0] OpaqueNextLayer,
range [1] NextLayerRange }
-- Representation of ANY in next layer field
AnyNextLayer ::= SEQUENCE {
start INTEGER (0),
end INTEGER (65535) }
-- Representation of OPAQUE in next layer field.
-- Matches IKE convention
OpaqueNextLayer ::= SEQUENCE {
start INTEGER (65535),
end INTEGER (0) }
-- Range for a next layer field
NextLayerRange ::= SEQUENCE {
start INTEGER (0..65535),
end INTEGER (0..65535) }
-- List of IP addresses
AddrList ::= SEQUENCE {
v4List IPv4List OPTIONAL,
v6List [0] IPv6List OPTIONAL }
-- IPv4 address representations
IPv4List ::= SEQUENCE OF IPv4Range
IPv4Range ::= SEQUENCE { -- close, but not quite right ...
ipv4Start OCTET STRING (SIZE (4)),
ipv4End OCTET STRING (SIZE (4)) }
-- IPv6 address representations
IPv6List ::= SEQUENCE OF IPv6Range
IPv6Range ::= SEQUENCE { -- close, but not quite right ...
ipv6Start OCTET STRING (SIZE (16)),
ipv6End OCTET STRING (SIZE (16)) }
ENDKent & Seo Standards Track [Page 87]
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Appendix D: Fragment Handling Rationale
There are three issues that must be resolved regarding processing of (plaintext) fragments in IPsec:
- mapping a non-initial, outbound fragment to the right SA
(or finding the right SPD entry)
- verifying that a received, non-initial fragment is authorized
for the SA via which it is received
- mapping outbound and inbound non-initial fragments to the
right SPD/cache entry, for BYPASS/DISCARD trafficThe first and third issues arise because we need a deterministic algorithm for mapping traffic to SAs (and SPD/cache entries). All three issues are important because we want to make sure that non-initial fragments that cross the IPsec boundary do not cause the access control policies in place at the receiver (or transmitter) to be violated.
D.1. Transport Mode and Fragments
First, we note that transport mode SAs have been defined to not carry fragments. This is a carryover from RFC 2401, where transport mode SAs always terminated at endpoints. This is a fundamental requirement because, in the worst case, an IPv4 fragment to which IPsec was applied might then be fragmented (as a ciphertext packet), en route to the destination. IP fragment reassembly procedures at the IPsec receiver would not be able to distinguish between pre-IPsec fragments and fragments created after IPsec processing.
For IPv6, only the sender is allowed to fragment a packet. As for IPv4, an IPsec implementation is allowed to fragment tunnel mode packets after IPsec processing, because it is the sender relative to the (outer) tunnel header. However, unlike IPv4, it would be feasible to carry a plaintext fragment on a transport mode SA, because the fragment header in IPv6 would appear after the AH or ESP header, and thus would not cause confusion at the receiver with respect to reassembly. Specifically, the receiver would not attempt reassembly for the fragment until after IPsec processing. To keep things simple, this specification prohibits carriage of fragments on transport mode SAs for IPv6 traffic.
When only end systems used transport mode SAs, the prohibition on carriage of fragments was not a problem, since we assumed that the end system could be configured to not offer a fragment to IPsec. For a native host implementation, this seems reasonable, and, as someone already noted, RFC 2401 warned that a BITS implementation might have to reassemble fragments before performing an SA lookup. (It would
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then apply AH or ESP and could re-fragment the packet after IPsec processing.) Because a BITS implementation is assumed to be able to have access to all traffic emanating from its host, even if the host has multiple interfaces, this was deemed a reasonable mandate.
In this specification, it is acceptable to use transport mode in cases where the IPsec implementation is not the ultimate destination, e.g., between two SGs. In principle, this creates a new opportunity for outbound, plaintext fragments to be mapped to a transport mode SA for IPsec processing. However, in these new contexts in which a transport mode SA is now approved for use, it seems likely that we can continue to prohibit transmission of fragments, as seen by IPsec, i.e., packets that have an "outer header" with a non-zero fragment offset field. For example, in an IP overlay network, packets being sent over transport mode SAs are IP-in-IP tunneled and thus have the necessary inner header to accommodate fragmentation prior to IPsec processing. When carried via a transport mode SA, IPsec would not examine the inner IP header for such traffic, and thus would not consider the packet to be a fragment.
D.2. Tunnel Mode and Fragments
For tunnel mode SAs, it has always been the case that outbound fragments might arrive for processing at an IPsec implementation. The need to accommodate fragmented outbound packets can pose a problem because a non-initial fragment generally will not contain the port fields associated with a next layer protocol such as TCP, UDP, or SCTP. Thus, depending on the SPD configuration for a given IPsec implementation, plaintext fragments might or might not pose a problem.
For example, if the SPD requires that all traffic between two address ranges is offered IPsec protection (no BYPASS or DISCARD SPD entries apply to this address range), then it should be easy to carry non-initial fragments on the SA defined for this address range, since the SPD entry implies an intent to carry ALL traffic between the address ranges. But, if there are multiple SPD entries that could match a fragment, and if these entries reference different subsets of port fields (vs. ANY), then it is not possible to map an outbound non-initial fragment to the right entry, unambiguously. (If we choose to allow carriage of fragments on transport mode SAs for IPv6, the problems arises in that context as well.)
This problem largely, though not exclusively, motivated the definition of OPAQUE as a selector value for port fields in RFC 2401. The other motivation for OPAQUE is the observation that port fields might not be accessible due to the prior application of IPsec. For example, if a host applied IPsec to its traffic and that traffic
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arrived at an SG, these fields would be encrypted. The algorithm specified for locating the "next layer protocol" described in RFC 2401 also motivated use of OPAQUE to accommodate an encrypted next layer protocol field in such circumstances. Nonetheless, the primary use of the OPAQUE value was to match traffic selector fields in packets that did not contain port fields (non-initial fragments), or packets in which the port fields were already encrypted (as a result of nested application of IPsec). RFC 2401 was ambiguous in discussing the use of OPAQUE vs. ANY, suggesting in some places that ANY might be an alternative to OPAQUE.
We gain additional access control capability by defining both ANY and OPAQUE values. OPAQUE can be defined to match only fields that are not accessible. We could define ANY as the complement of OPAQUE, i.e., it would match all values but only for accessible port fields. We have therefore simplified the procedure employed to locate the next layer protocol in this document, so that we treat ESP and AH as next layer protocols. As a result, the notion of an encrypted next layer protocol field has vanished, and there is also no need to worry about encrypted port fields either. And accordingly, OPAQUE will be applicable only to non-initial fragments.
Since we have adopted the definitions above for ANY and OPAQUE, we need to clarify how these values work when the specified protocol does not have port fields, and when ANY is used for the protocol selector. Accordingly, if a specific protocol value is used as a selector, and if that protocol has no port fields, then the port field selectors are to be ignored and ANY MUST be specified as the value for the port fields. (In this context, ICMP TYPE and CODE values are lumped together as a single port field (for IKEv2 negotiation), as is the IPv6 Mobility Header TYPE value.) If the protocol selector is ANY, then this should be treated as equivalent to specifying a protocol for which no port fields are defined, and thus the port selectors should be ignored, and MUST be set to ANY.
D.3. The Problem of Non-Initial Fragments
For an SG implementation, it is obvious that fragments might arrive from end systems behind the SG. A BITW implementation also may encounter fragments from a host or gateway behind it. (As noted earlier, native host implementations and BITS implementations probably can avoid the problems described below.) In the worst case, fragments from a packet might arrive at distinct BITW or SG instantiations and thus preclude reassembly as a solution option. Hence, in RFC 2401 we adopted a general requirement that fragments must be accommodated in tunnel mode for all implementations. However,
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RFC 2401 did not provide a perfect solution. The use of OPAQUE as a selector value for port fields (a SHOULD in RFC 2401) allowed an SA to carry non-initial fragments.
Using the features defined in RFC 2401, if one defined an SA between two IPsec (SG or BITW) implementations using the OPAQUE value for both port fields, then all non-initial fragments matching the source/destination (S/D) address and protocol values for the SA would be mapped to that SA. Initial fragments would NOT map to this SA, if we adopt a strict definition of OPAQUE. However, RFC 2401 did not provide detailed guidance on this and thus it may not have been apparent that use of this feature would essentially create a "non-initial fragment only" SA.
In the course of discussing the "fragment-only" SA approach, it was noted that some subtle problems, problems not considered in RFC 2401, would have to be avoided. For example, an SA of this sort must be configured to offer the "highest quality" security services for any traffic between the indicated S/D addresses (for the specified protocol). This is necessary to ensure that any traffic captured by the fragment-only SA is not offered degraded security relative to what it would have been offered if the packet were not fragmented. A possible problem here is that we may not be able to identify the "highest quality" security services defined for use between two IPsec implementation, since the choice of security protocols, options, and algorithms is a lattice, not a totally ordered set. (We might safely say that BYPASS < AH < ESP w/integrity, but it gets complicated if we have multiple ESP encryption or integrity algorithm options.) So, one has to impose a total ordering on these security parameters to make this work, but this can be done locally.
However, this conservative strategy has a possible performance downside. If most traffic traversing an IPsec implementation for a given S/D address pair (and specified protocol) is bypassed, then a fragment-only SA for that address pair might cause a dramatic increase in the volume of traffic afforded crypto processing. If the crypto implementation cannot support high traffic rates, this could cause problems. (An IPsec implementation that is capable of line rate or near line rate crypto performance would not be adversely affected by this SA configuration approach. Nonetheless, the performance impact is a potential concern, specific to implementation capabilities.)
Another concern is that non-initial fragments sent over a dedicated SA might be used to effect overlapping reassembly attacks, when combined with an apparently acceptable initial fragment. (This sort of attack assumes creation of bogus fragments and is not a side effect of normal fragmentation.) This concern is easily addressed in
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IPv4, by checking the fragment offset value to ensure that no non-initial fragments have a small enough offset to overlap port fields that should be contained in the initial fragment. Recall that the IPv4 MTU minimum is 576 bytes, and the max IP header length is 60 bytes, so any ports should be present in the initial fragment. If we require all non-initial fragments to have an offset of, say, 128 or greater, just to be on the safe side, this should prevent successful attacks of this sort. If the intent is only to protect against this sort of reassembly attack, this check need be implemented only by a receiver.
IPv6 also has a fragment offset, carried in the fragmentation extension header. However, IPv6 extension headers are variable in length and there is no analogous max header length value that we can use to check non-initial fragments, to reject ones that might be used for an attack of the sort noted above. A receiver would need to maintain state analogous to reassembly state, to provide equivalent protection. So, only for IPv4 is it feasible to impose a fragment offset check that would reject attacks designed to circumvent port field checks by IPsec (or firewalls) when passing non-initial fragments.
Another possible concern is that in some topologies and SPD configurations this approach might result in an access control surprise. The notion is that if we create an SA to carry ALL (non-initial) fragments, then that SA would carry some traffic that might otherwise arrive as plaintext via a separate path, e.g., a path monitored by a proxy firewall. But, this concern arises only if the other path allows initial fragments to traverse it without requiring reassembly, presumably a bad idea for a proxy firewall. Nonetheless, this does represent a potential problem in some topologies and under certain assumptions with respect to SPD and (other) firewall rule sets, and administrators need to be warned of this possibility.
A less serious concern is that non-initial fragments sent over a non-initial fragment-only SA might represent a DoS opportunity, in that they could be sent when no valid, initial fragment will ever arrive. This might be used to attack hosts behind an SG or BITW device. However, the incremental risk posed by this sort of attack, which can be mounted only by hosts behind an SG or BITW device, seems small.
If we interpret the ANY selector value as encompassing OPAQUE, then a single SA with ANY values for both port fields would be able to accommodate all traffic matching the S/D address and protocol traffic selectors, an alternative to using the OPAQUE value. But, using ANY
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here precludes multiple, distinct SAs between the same IPsec implementations for the same address pairs and protocol. So, it is not an exactly equivalent alternative.
Fundamentally, fragment handling problems arise only when more than one SA is defined with the same S/D address and protocol selector values, but with different port field selector values.
D.4. BYPASS/DISCARD Traffic
We also have to address the non-initial fragment processing issue for BYPASS/DISCARD entries, independent of SA processing. This is largely a local matter for two reasons:
1) We have no means for coordinating SPD entries for such
traffic between IPsec implementations since IKE is not
invoked.
2) Many of these entries refer to traffic that is NOT
directed to or received from a location that is using
IPsec. So there is no peer IPsec implementation with
which to coordinate via any means.However, this document should provide guidance here, consistent with our goal of offering a well-defined, access control function for all traffic, relative to the IPsec boundary. To that end, this document says that implementations MUST support fragment reassembly for BYPASS/DISCARD traffic when port fields are specified. An implementation also MUST permit a user or administrator to accept such traffic or reject such traffic using the SPD conventions described in Section 4.4.1. The concern is that BYPASS of a cleartext, non-initial fragment arriving at an IPsec implementation could undermine the security afforded IPsec-protected traffic directed to the same destination. For example, consider an IPsec implementation configured with an SPD entry that calls for IPsec-protection of traffic between a specific source/destination address pair, and for a specific protocol and destination port, e.g., TCP traffic on port 23 (Telnet). Assume that the implementation also allows BYPASS of traffic from the same source/destination address pair and protocol, but for a different destination port, e.g., port 119 (NNTP). An attacker could send a non-initial fragment (with a forged source address) that, if bypassed, could overlap with IPsec-protected traffic from the same source and thus violate the integrity of the IPsec-protected traffic. Requiring stateful fragment checking for BYPASS entries with non-trivial port ranges prevents attacks of this sort.
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D.5. Just say no to ports?
It has been suggested that we could avoid the problems described above by not allowing port field selectors to be used in tunnel mode. But the discussion above shows this to be an unnecessarily stringent approach, i.e., since no problems arise for the native OS and BITS implementations. Moreover, some WG members have described scenarios where use of tunnel mode SAs with (non-trivial) port field selectors is appropriate. So the challenge is defining a strategy that can deal with this problem in BITW and SG contexts. Also note that BYPASS/DISCARD entries in the SPD that make use of ports pose the same problems, irrespective of tunnel vs. transport mode notions.
Some folks have suggested that a firewall behind an SG or BITW should be left to enforce port-level access controls and the effects of fragmentation. However, this seems to be an incongruous suggestion in that elsewhere in IPsec (e.g., in IKE payloads) we are concerned about firewalls that always discard fragments. If many firewalls don't pass fragments in general, why should we expect them to deal with fragments in this case? So, this analysis rejects the suggestion of disallowing use of port field selectors with tunnel mode SAs.
D.6. Other Suggested Solutions
One suggestion is to reassemble fragments at the sending IPsec implementation, and thus avoid the problem entirely. This approach is invisible to a receiver and thus could be adopted as a purely local implementation option.
A more sophisticated version of this suggestion calls for establishing and maintaining minimal state from each initial fragment encountered, to allow non-initial fragments to be matched to the right SAs or SPD/cache entries. This implies an extension to the current processing model (and the old one). The IPsec implementation would intercept all fragments; capture Source/Destination IP addresses, protocol, packet ID, and port fields from initial fragments; and then use this data to map non-initial fragments to SAs that require port fields. If this approach is employed, the receiver needs to employ an equivalent scheme, as it too must verify that received fragments are consistent with SA selector values. A non-initial fragment that arrives prior to an initial fragment could be cached or discarded, awaiting arrival of the corresponding initial fragment.
A downside of both approaches noted above is that they will not always work. When a BITW device or SG is configured in a topology that might allow some fragments for a packet to be processed at different SGs or BITW devices, then there is no guarantee that all
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fragments will ever arrive at the same IPsec device. This approach also raises possible processing problems. If the sender caches non-initial fragments until the corresponding initial fragment arrives, buffering problems might arise, especially at high speeds. If the non-initial fragments are discarded rather than cached, there is no guarantee that traffic will ever pass, e.g., retransmission will result in different packet IDs that cannot be matched with prior transmissions. In any case, housekeeping procedures will be needed to decide when to delete the fragment state data, adding some complexity to the system. Nonetheless, this is a viable solution in some topologies, and these are likely to be common topologies.
The Working Group rejected an earlier version of the convention of creating an SA to carry only non-initial fragments, something that was supported implicitly under the RFC 2401 model via use of OPAQUE port fields, but never clearly articulated in RFC 2401. The (rejected) text called for each non-initial fragment to be treated as protocol 44 (the IPv6 fragment header protocol ID) by the sender and receiver. This approach has the potential to make IPv4 and IPv6 fragment handling more uniform, but it does not fundamentally change the problem, nor does it address the issue of fragment handling for BYPASS/DISCARD traffic. Given the fragment overlap attack problem that IPv6 poses, it does not seem that it is worth the effort to adopt this strategy.
D.7. Consistency
Earlier, the WG agreed to allow an IPsec BITS, BITW, or SG to perform fragmentation prior to IPsec processing. If this fragmentation is performed after SA lookup at the sender, there is no "mapping to the right SA" problem. But, the receiver still needs to be able to verify that the non-initial fragments are consistent with the SA via which they are received. Since the initial fragment might be lost en route, the receiver encounters all of the potential problems noted above. Thus, if we are to be consistent in our decisions, we need to say how a receiver will deal with the non-initial fragments that arrive.
D.8. Conclusions
There is no simple, uniform way to handle fragments in all contexts. Different approaches work better in different contexts. Thus, this document offers 3 choices -- one MUST and two MAYs. At some point in the future, if the community gains experience with the two MAYs, they may become SHOULDs or MUSTs or other approaches may be proposed.
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Appendix E: Example of Supporting Nested SAs via SPD and Forwarding Table Entries
This appendix provides an example of how to configure the SPD and forwarding tables to support a nested pair of SAs, consistent with the new processing model. For simplicity, this example assumes just one SPD-I.
The goal in this example is to support a transport mode SA from A to C, carried over a tunnel mode SA from A to B. For example, A might be a laptop connected to the public Internet, B might be a firewall that protects a corporate network, and C might be a server on the corporate network that demands end-to-end authentication of A's traffic.
+---+ +---+ +---+
| A |=====| B | | C |
| |------------| |
| |=====| | | |
+---+ +---+ +---+A's SPD contains entries of the form:
Next Layer
Rule Local Remote Protocol Action
---- ----- ------ ---------- -----------------------
1 C A ESP BYPASS
2 A C ICMP,ESP PROTECT(ESP,tunnel,integr+conf)
3 A C ANY PROTECT(ESP,transport,integr-only)
4 A B ICMP,IKE BYPASSA's unprotected-side forwarding table is set so that outbound packets destined for C are looped back to the protected side. A's protected-side forwarding table is set so that inbound ESP packets are looped back to the unprotected side. A's forwarding tables contain entries of the form:
Unprotected-side forwarding table
Rule Local Remote Protocol Action
---- ----- ------ -------- ---------------------------
1 A C ANY loop back to protected side
2 A B ANY forward to BKent & Seo Standards Track [Page 96]
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Protected-side forwarding table
Rule Local Remote Protocol Action
---- ----- ------ -------- -----------------------------
1 A C ESP loop back to unprotected sideAn outbound TCP packet from A to C would match SPD rule 3 and have transport mode ESP applied to it. The unprotected-side forwarding table would then loop back the packet. The packet is compared against SPD-I (see Figure 2), matches SPD rule 1, and so it is BYPASSed. The packet is treated as an outbound packet and compared against the SPD for a third time. This time it matches SPD rule 2, so ESP is applied in tunnel mode. This time the forwarding table doesn't loop back the packet, because the outer destination address is B, so the packet goes out onto the wire.
An inbound TCP packet from C to A is wrapped in two ESP headers; the outer header (ESP in tunnel mode) shows B as the source, whereas the inner header (ESP transport mode) shows C as the source. Upon arrival at A, the packet would be mapped to an SA based on the SPI, have the outer header removed, and be decrypted and integrity-checked. Then it would be matched against the SAD selectors for this SA, which would specify C as the source and A as the destination, derived from SPD rule 2. The protected-side forwarding function would then send it back to the unprotected side based on the addresses and the next layer protocol (ESP), indicative of nesting. It is compared against SPD-O (see Figure 3) and found to match SPD rule 1, so it is BYPASSed. The packet is mapped to an SA based on the SPI, integrity-checked, and compared against the SAD selectors derived from SPD rule 3. The forwarding function then passes it up to the next layer, because it isn't an ESP packet.
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References
Normative References
[BBCDWW98] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Service", RFC 2475, December 1998.
[Bra97] Bradner, S., "Key words for use in RFCs to Indicate Requirement Level", BCP 14, RFC 2119, March 1997.
[CD98] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998.
[DH98] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
[Eas05] 3rd Eastlake, D., "Cryptographic Algorithm Implementation Requirements For Encapsulating Security Payload (ESP) and Authentication Header (AH)", RFC 4305, December 2005.
[HarCar98] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, November 1998.
[Kau05] Kaufman, C., Ed., "The Internet Key Exchange (IKEv2) Protocol", RFC 4306, December 2005.
[Ken05a] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.
[Ken05b] Kent, S., "IP Authentication Header", RFC 4302, December 2005.
[MD90] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990.
[Mobip] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004.
[Pos81a] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[Pos81b] Postel, J., "Internet Control Message Protocol", RFC 792, September 1981.
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[Sch05] Schiller, J., "Cryptographic Algorithms for use in the Internet Key Exchange Version 2 (IKEv2)", RFC 4307, December 2005.
[WaKiHo97] Wahl, M., Kille, S., and T. Howes, "Lightweight Directory Access Protocol (v3): UTF-8 String Representation of Distinguished Names", RFC 2253, December 1997.
Informative References
[CoSa04] Condell, M., and L. Sanchez, "On the Deterministic Enforcement of Un-ordered Security Policies", BBN Technical Memo 1346, March 2004.
[FaLiHaMeTr00] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P. Traina, "Generic Routing Encapsulation (GRE)", RFC 2784, March 2000.
[Gro02] Grossman, D., "New Terminology and Clarifications for Diffserv", RFC 3260, April 2002. [HC03] Holbrook, H. and B. Cain, "Source Specific Multicast for IP", Work in Progress, November 3, 2002.
[HA94] Haller, N. and R. Atkinson, "On Internet Authentication", RFC 1704, October 1994.
[NiBlBaBL98] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
[Per96] Perkins, C., "IP Encapsulation within IP", RFC 2003, October 1996.
[RaFlBl01] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001.
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401, November 1998.
[RFC2983] Black, D., "Differentiated Services and Tunnels", RFC 2983, October 2000.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The Group Domain of Interpretation", RFC 3547, July 2003.
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[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security Architecture", RFC 3740, March 2004.
[RaCoCaDe04] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6 Flow Label Specification", RFC 3697, March 2004.
[Sch94] Schneier, B., Applied Cryptography, Section 8.6, John Wiley & Sons, New York, NY, 1994.
[Shi00] Shirey, R., "Internet Security Glossary", RFC 2828, May 2000.
[SMPT01] Shacham, A., Monsour, B., Pereira, R., and M. Thomas, "IP Payload Compression Protocol (IPComp)", RFC 3173, September 2001.
[ToEgWa04] Touch, J., Eggert, L., and Y. Wang, "Use of IPsec Transport Mode for Dynamic Routing", RFC 3884, September 2004.
[VK83] V.L. Voydock & S.T. Kent, "Security Mechanisms in High-level Networks", ACM Computing Surveys, Vol. 15, No. 2, June 1983.
Authors' Addresses
Stephen Kent BBN Technologies 10 Moulton Street Cambridge, MA 02138 USA
Phone: +1 (617) 873-3988 EMail: kent@bbn.com
Karen Seo BBN Technologies 10 Moulton Street Cambridge, MA 02138 USA
Phone: +1 (617) 873-3152 EMail: kseo@bbn.com
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Full Copyright Statement
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