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Network Working Group S. Kent Request for Comments: 2401 BBN Corp Obsoletes: 1825 R. Atkinson Category: Standards Track @Home Network

                                                         November 1998
          Security Architecture for the Internet Protocol

Status 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 (1998).  All Rights Reserved.

Table of Contents

1. Introduction………………………………………………..3

1.1 Summary of Contents of Document..................................3
1.2 Audience.........................................................3
1.3 Related Documents................................................4

2. Design Objectives……………………………………………4

2.1 Goals/Objectives/Requirements/Problem Description................4
2.2 Caveats and Assumptions..........................................5

3. System Overview……………………………………………..5

3.1 What IPsec Does..................................................6
3.2 How IPsec Works..................................................6
3.3 Where IPsec May Be Implemented...................................7

4. Security Associations………………………………………..8

4.1 Definition and Scope.............................................8
4.2 Security Association Functionality..............................10
4.3 Combining Security Associations.................................11
4.4 Security Association Databases..................................13
   4.4.1 The Security Policy Database (SPD).........................14
   4.4.2 Selectors..................................................17
   4.4.3 Security Association Database (SAD)........................21
4.5 Basic Combinations of Security Associations.....................24
4.6 SA and Key Management...........................................26
   4.6.1 Manual Techniques..........................................27
   4.6.2 Automated SA and Key Management............................27
   4.6.3 Locating a Security Gateway................................28
4.7 Security Associations and Multicast.............................29

Kent & Atkinson Standards Track [Page 1] RFC 2401 Security Architecture for IP November 1998

5. IP Traffic Processing……………………………………….30

5.1 Outbound IP Traffic Processing..................................30
   5.1.1 Selecting and Using an SA or SA Bundle.....................30
   5.1.2 Header Construction for Tunnel Mode........................31
      5.1.2.1 IPv4 -- Header Construction for Tunnel Mode...........31
      5.1.2.2 IPv6 -- Header Construction for Tunnel Mode...........32
5.2 Processing Inbound IP Traffic...................................33
   5.2.1 Selecting and Using an SA or SA Bundle.....................33
   5.2.2 Handling of AH and ESP tunnels.............................34

6. ICMP Processing (relevant to IPsec)…………………………..35

6.1 PMTU/DF Processing..............................................36
   6.1.1 DF Bit.....................................................36
   6.1.2 Path MTU Discovery (PMTU)..................................36
      6.1.2.1 Propagation of PMTU...................................36
      6.1.2.2 Calculation of PMTU...................................37
      6.1.2.3 Granularity of PMTU Processing........................37
      6.1.2.4 PMTU Aging............................................38

7. Auditing…………………………………………………..39 8. Use in Systems Supporting Information Flow Security…………….39

8.1 Relationship Between Security Associations and Data Sensitivity.40
8.2 Sensitivity Consistency Checking................................40
8.3 Additional MLS Attributes for Security Association Databases....41
8.4 Additional Inbound Processing Steps for MLS Networking..........41
8.5 Additional Outbound Processing Steps for MLS Networking.........41
8.6 Additional MLS Processing for Security Gateways.................42

9. Performance Issues………………………………………….42 10. Conformance Requirements……………………………………43 11. Security Considerations…………………………………….43 12. Differences from RFC 1825…………………………………..43 Acknowledgements………………………………………………44 Appendix A – Glossary…………………………………………45 Appendix B – Analysis/Discussion of PMTU/DF/Fragmentation Issues…..48

B.1 DF bit..........................................................48
B.2 Fragmentation...................................................48
B.3 Path MTU Discovery..............................................52
   B.3.1 Identifying the Originating Host(s)........................53
   B.3.2 Calculation of PMTU........................................55
   B.3.3 Granularity of Maintaining PMTU Data.......................56
   B.3.4 Per Socket Maintenance of PMTU Data........................57
   B.3.5 Delivery of PMTU Data to the Transport Layer...............57
   B.3.6 Aging of PMTU Data.........................................57

Appendix C – Sequence Space Window Code Example………………….58 Appendix D – Categorization of ICMP messages…………………….60 References……………………………………………………63 Disclaimer……………………………………………………64 Author Information…………………………………………….65 Full Copyright Statement……………………………………….66

Kent & Atkinson Standards Track [Page 2] RFC 2401 Security Architecture for IP November 1998

1. Introduction

1.1 Summary of Contents of Document

 This memo specifies the base architecture for IPsec compliant
 systems.  The goal of the architecture is to provide various security
 services for traffic at the IP layer, in both the IPv4 and IPv6
 environments.  This document describes the goals of such systems,
 their components and how they fit together with each other and 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 IPsec
 architecture.  Subsequent documents will address additional
 architectural details of a more advanced nature, e.g., use of IPsec
 in NAT environments and more complete support for IP multicast.  The
 following 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 automatic (The Internet Key
         Exchange (IKE))
      d. Algorithms for authentication and encryption
 This document is not an overall 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 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].

1.2 Audience

 The target audience for this document includes implementers of this
 IP security technology and others interested in gaining a general
 background understanding of this system.  In particular, prospective
 users of this technology (end users or system administrators) are
 part of the target audience.  A glossary is provided as an appendix

Kent & Atkinson Standards Track [Page 3] RFC 2401 Security Architecture for IP November 1998

 to help fill in gaps in background/vocabulary.  This document assumes
 that the reader is familiar with the Internet Protocol, related
 networking technology, and general 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 inter-relationship.
 They include RFCs on the following topics:
      a. "IP Security Document Roadmap" [TDG97] -- a document
         providing guidelines for specifications describing encryption
         and authentication algorithms used in this system.
      b. security protocols -- RFCs describing the Authentication
         Header (AH) [KA98a] and Encapsulating Security Payload (ESP)
         [KA98b] protocols.
      c. algorithms for authentication and encryption -- a separate
         RFC for each algorithm.
      d. automatic key management -- RFCs on "The Internet Key
         Exchange (IKE)" [HC98], "Internet Security Association and
         Key Management Protocol (ISAKMP)" [MSST97],"The OAKLEY Key
         Determination Protocol" [Orm97], and "The Internet IP
         Security Domain of Interpretation for ISAKMP" [Pip98].

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, protection against replays (a
 form of partial sequence integrity), confidentiality (encryption),
 and limited traffic flow confidentiality.  These services are
 provided at the IP layer, offering protection for IP and/or upper
 layer protocols.
 These objectives are met through the 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 any context, and the ways in which they are employed,
 will be determined by the security and system requirements of users,
 applications, and/or sites/organizations.
 When these mechanisms are correctly implemented and deployed, they
 ought not to adversely affect users, hosts, and other Internet
 components that do not employ these security mechanisms for

Kent & Atkinson Standards Track [Page 4] RFC 2401 Security Architecture for IP November 1998

 protection of their traffic.  These mechanisms also are designed to
 be algorithm-independent.  This modularity permits selection of
 different sets of algorithms without affecting the other parts of the
 implementation.  For example, different user communities may select
 different sets of algorithms (creating cliques) if required.
 A standard set of default algorithms is specified to facilitate
 interoperability in the global Internet.  The use of these
 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 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 the their implementation, which is 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 or a security gateway
 environment, affording protection to IP traffic.  The protection
 offered 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

Kent & Atkinson Standards Track [Page 5] RFC 2401 Security Architecture for IP November 1998

 established by either of the above.  In general, packets are selected
 for one of three processing modes based on IP and transport layer
 header information (Selectors, Section 4.4.2) matched against entries
 in the database (SPD).  Each packet is either afforded IPsec security
 services, discarded, or allowed to bypass IPsec, based on the
 applicable database policies identified by the Selectors.

3.1 What IPsec Does

 IPsec provides security services at the IP layer by enabling a system
 to select required security protocols, determine the algorithm(s) to
 use for the service(s), and put in place any cryptographic keys
 required to provide the requested services.  IPsec can be used to
 protect one or more "paths" between a pair of hosts, between a pair
 of security gateways, or between a security gateway and a host.  (The
 term "security gateway" is used throughout the IPsec documents to
 refer to an intermediate system that implements IPsec protocols.  For
 example, a router or a firewall implementing IPsec is a security
 gateway.)
 The set of security services that IPsec can provide includes access
 control, connectionless integrity, data origin authentication,
 rejection of replayed packets (a form of partial sequence integrity),
 confidentiality (encryption), and limited traffic flow
 confidentiality.  Because these services are provided at the IP
 layer, they can be used by any higher layer protocol, e.g., TCP, UDP,
 ICMP, BGP, etc.
 The IPsec DOI also supports negotiation of IP compression [SMPT98],
 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 --
 Authentication Header (AH) and Encapsulating Security Payload (ESP).
 Both protocols are described in more detail in their respective RFCs
 [KA98a, KA98b].
      o The IP Authentication Header (AH) [KA98a] provides
        connectionless integrity, data origin authentication, and an
        optional anti-replay service.
      o The Encapsulating Security Payload (ESP) protocol [KA98b] may
        provide confidentiality (encryption), and limited traffic flow
        confidentiality.  It also may provide connectionless

Kent & Atkinson Standards Track [Page 6] RFC 2401 Security Architecture for IP November 1998

        integrity, data origin authentication, and an anti-replay
        service.  (One or the other set of these security services
        must be applied whenever ESP is invoked.)
      o Both AH and ESP are vehicles for access control, based on the
        distribution of cryptographic keys and the management of
        traffic flows relative to these security protocols.
 These protocols may be applied alone or in combination with each
 other to provide a desired set of security services in IPv4 and IPv6.
 Each protocol supports two modes of use: transport mode and tunnel
 mode.  In transport mode the protocols provide protection primarily
 for upper layer protocols; in tunnel mode, the protocols are applied
 to tunneled IP packets.  The differences between the two modes are
 discussed in Section 4.
 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 management must incorporate facilities
 for specifying:
      o which security services to use and in what combinations
      o the granularity at which a given security protection should be
        applied
      o the algorithms used to effect cryptographic-based security
 Because these security services use shared secret values
 (cryptographic keys), IPsec relies on a separate set of mechanisms
 for putting these keys in place. (The keys are used for
 authentication/integrity and encryption services.)  This document
 requires support for both manual and automatic distribution of keys.
 It specifies a specific public-key based approach (IKE -- [MSST97,
 Orm97, HC98]) for automatic key management, but other automated key
 distribution techniques MAY be used.  For example, KDC-based systems
 such as Kerberos and other public-key systems such as SKIP could be
 employed.

3.3 Where IPsec May Be Implemented

 There are several ways in which IPsec may be implemented in a host or
 in conjunction with a router or firewall (to create a security
 gateway).  Several common examples are provided below:
      a. Integration of IPsec into the native IP implementation.  This
         requires access to the IP source code and is applicable to
         both hosts and security gateways.

Kent & Atkinson Standards Track [Page 7] RFC 2401 Security Architecture for IP November 1998

      b. "Bump-in-the-stack" (BITS) implementations, where 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 an outboard crypto processor is a common design
         feature of network security 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 (or both).  Usually the BITW device is 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.

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.  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
 Security Associations.  All implementations of AH or ESP MUST support
 the concept of a Security Association as described below.  The
 remainder of this section describes various aspects of Security
 Association management, defining required characteristics for SA
 policy management, traffic processing, and SA management techniques.

4.1 Definition and Scope

 A Security Association (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 is applied to a traffic stream, then two
 (or more) SAs are created to afford protection to the traffic stream.
 To secure typical, bi-directional communication between two hosts, or
 between two security gateways, two Security Associations (one in each
 direction) are required.
 A security association is uniquely identified by a triple consisting
 of a Security Parameter Index (SPI), an IP Destination Address, and a
 security protocol (AH or ESP) identifier.  In principle, the
 Destination Address may be a unicast address, an IP broadcast
 address, or a multicast group address.  However, IPsec SA management
 mechanisms currently are defined only for unicast SAs.  Hence, in the

Kent & Atkinson Standards Track [Page 8] RFC 2401 Security Architecture for IP November 1998

 discussions that follow, SAs will be described in the context of
 point-to-point communication, even though the concept is applicable
 in the point-to-multipoint case as well.
 As noted above, two types of SAs are defined: transport mode and
 tunnel mode.  A transport mode SA is a security association between
 two hosts.  In IPv4, a transport mode security protocol header
 appears immediately after the IP header and any options, and before
 any higher layer protocols (e.g., TCP or UDP).  In IPv6, the security
 protocol header appears after the base IP header and extensions, but
 may appear before or after destination options, and before higher
 layer protocols.  In the case of ESP, a transport mode SA provides
 security services only for these higher 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, 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 [KA98a].
 A tunnel mode SA is essentially an SA applied to an IP tunnel.
 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 always a tunnel mode SA, as is an SA between a host and a security
 gateway.  Note that for the case where traffic is destined for a
 security gateway, e.g., SNMP commands, the security gateway is acting
 as a host and transport mode is allowed.  But in that case, the
 security gateway is not acting as a gateway, i.e., not transiting
 traffic.  Two hosts MAY establish a tunnel mode SA between
 themselves.  The requirement for any (transit traffic) SA involving a
 security gateway to be a tunnel SA arises due to the need to avoid
 potential problems with regard to fragmentation and reassembly of
 IPsec packets, and in circumstances where multiple paths (e.g., via
 different security gateways) exist to the same destination behind the
 security gateways.
 For a tunnel mode SA, there is an "outer" IP header that specifies
 the IPsec processing destination, plus an "inner" IP header that
 specifies the (apparently) ultimate 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 higher layer protocols).  If ESP is
 employed, the protection is afforded only to the tunneled packet, not
 to the outer header.

Kent & Atkinson Standards Track [Page 9] RFC 2401 Security Architecture for IP November 1998

 In summary,
         a) A host MUST support both transport and tunnel mode.
         b) A security gateway is required to support only tunnel
            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.

4.2 Security Association 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 on the
 election of optional services within the protocol.  For example, AH
 provides data origin authentication and connectionless integrity for
 IP datagrams (hereafter referred to as just "authentication").  The
 "precision" of the authentication service is a function of the
 granularity of the security association with which AH is employed, as
 discussed in Section 4.4.2, "Selectors".
 AH also offers an anti-replay (partial sequence integrity) service at
 the discretion of the receiver, to help counter denial of service
 attacks.  AH is an appropriate protocol to employ when
 confidentiality is not required (or is not permitted, e.g , due to
 government restrictions on use of encryption).  AH also provides
 authentication for selected portions of the IP header, which may be
 necessary in some contexts.  For example, 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 the
 non-predictable but mutable parts of the IP header.)
 ESP optionally provides confidentiality for traffic.  (The strength
 of the confidentiality service depends in part, on the encryption
 algorithm employed.)  ESP also may optionally provide authentication
 (as defined above).  If authentication is negotiated for an ESP SA,
 the receiver also may elect to enforce an anti-replay service with
 the same features as the AH anti-replay service.  The scope of the
 authentication offered by ESP is narrower than for AH, i.e., the IP
 header(s) "outside" the ESP header is(are) not protected.  If only
 the upper layer protocols need to be authenticated, then ESP
 authentication is an appropriate choice and is more space efficient
 than use of AH encapsulating ESP.  Note that although both
 confidentiality and authentication are optional, they cannot both be
 omitted. At least one of them MUST be selected.
 If confidentiality service 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

Kent & Atkinson Standards Track [Page 10] RFC 2401 Security Architecture for IP November 1998

 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 which are
 carrying traffic from many subscribers.

4.3 Combining Security Associations

 The IP datagrams transmitted over an individual SA are afforded
 protection by exactly one security protocol, either AH or ESP, but
 not both.  Sometimes a security policy may call for a combination of
 services for a particular traffic flow that is not achievable with a
 single SA.  In such instances it will be necessary to employ multiple
 SAs to implement the required security policy.  The term "security
 association bundle" or "SA bundle" is applied to a sequence of SAs
 through which traffic must be processed to satisfy a security policy.
 The order of the sequence is defined by the policy.  (Note that the
 SAs that comprise a bundle may terminate at different endpoints. For
 example, one SA may extend between a mobile host and a security
 gateway and a second, nested SA may extend to a host behind the
 gateway.)
 Security associations may be combined into bundles in two ways:
 transport adjacency and iterated tunneling.
         o Transport adjacency refers to applying more than one
           security protocol to the same IP datagram, without invoking
           tunneling.  This approach to combining AH and ESP allows
           for only one level of combination; further nesting yields
           no added benefit (assuming use of adequately strong
           algorithms in each protocol) since the processing is
           performed at one IPsec instance at the (ultimate)
           destination.
           Host 1 --- Security ---- Internet -- Security --- Host 2
            | |        Gwy 1                      Gwy 2        | |
            | |                                                | |
            | -----Security Association 1 (ESP transport)------- |
            |                                                    |
            -------Security Association 2 (AH transport)----------
         o Iterated tunneling refers to the application of multiple
           layers of security protocols effected through IP tunneling.
           This approach allows for multiple levels of nesting, since
           each tunnel can originate or terminate at a different IPsec

Kent & Atkinson Standards Track [Page 11] RFC 2401 Security Architecture for IP November 1998

           site along the path.  No special treatment is expected for
           ISAKMP traffic at intermediate security gateways other than
           what can be specified through appropriate SPD entries (See
           Case 3 in Section 4.5)
           There are 3 basic cases of iterated tunneling -- support is
           required only for cases 2 and 3.:
           1. both endpoints for the SAs are the same -- The inner and
              outer tunnels could each be either AH or ESP, though it
              is unlikely that Host 1 would specify both to be the
              same, i.e., AH inside of AH or ESP inside of ESP.
              Host 1 --- Security ---- Internet -- Security --- Host 2
               | |        Gwy 1                      Gwy 2        | |
               | |                                                | |
               | -------Security Association 1 (tunnel)---------- | |
               |                                                    |
               ---------Security Association 2 (tunnel)--------------
           2. one endpoint of the SAs is the same -- The inner and
              uter tunnels could each be either AH or ESP.
              Host 1 --- Security ---- Internet -- Security --- Host 2
               | |        Gwy 1                      Gwy 2         |
               | |                                     |           |
               | ----Security Association 1 (tunnel)----           |
               |                                                   |
               ---------Security Association 2 (tunnel)-------------
           3. neither endpoint is the same -- The inner and outer
              tunnels could each be either AH or ESP.
              Host 1 --- Security ---- Internet -- Security --- Host 2
               |          Gwy 1                      Gwy 2         |
               |            |                          |           |
               |            --Security Assoc 1 (tunnel)-           |
               |                                                   |
               -----------Security Association 2 (tunnel)-----------
 These two approaches also can be combined, e.g., an SA bundle could
 be constructed from one tunnel mode SA and one or two transport mode
 SAs, applied in sequence.  (See Section 4.5 "Basic Combinations of
 Security Associations.") Note that nested tunnels can also occur
 where neither the source nor the destination endpoints of any of the
 tunnels are the same.  In that case, there would be no host or
 security gateway with a bundle corresponding to the nested tunnels.

Kent & Atkinson Standards Track [Page 12] RFC 2401 Security Architecture for IP November 1998

 For transport mode SAs, only one ordering of security protocols seems
 appropriate.  AH is applied to both the upper layer protocols and
 (parts of) the IP header.  Thus if AH is used in a transport mode, in
 conjunction with ESP, AH SHOULD appear as the first header after IP,
 prior to the appearance of ESP.  In that context, AH is applied to
 the ciphertext output of ESP.  In contrast, for tunnel mode SAs, one
 can imagine uses for various orderings of AH and ESP.  The required
 set of SA bundle types that MUST be supported by a compliant IPsec
 implementation is described in Section 4.5.

4.4 Security Association 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 security associations, in support of these
 interoperability and functionality goals.  The model described below
 is nominal; compliant implementations need not match details of this
 model as presented, but the external behavior of such implementations
 must be mappable to the externally observable characteristics of this
 model.
 There are two nominal databases in this model: the Security Policy
 Database and the Security Association Database.  The former specifies
 the policies that determine the disposition of all IP traffic inbound
 or outbound from a host, security gateway, or BITS or BITW IPsec
 implementation.  The latter database contains parameters that are
 associated with each (active) security association.  This section
 also defines the concept of a Selector, a set of IP and upper layer
 protocol field values that is used by the Security Policy Database to
 map traffic to a policy, i.e., an SA (or SA bundle).
 Each interface for which IPsec is enabled requires nominally separate
 inbound vs. outbound databases (SAD and SPD), because of the
 directionality of many of the fields that are used as selectors.
 Typically there is just one such interface, for a host or security
 gateway (SG).  Note that an SG would always have at least 2
 interfaces, but the "internal" one to the corporate net, usually
 would not have IPsec enabled and so only one pair of SADs and one
 pair of SPDs would be needed.  On the other hand, if a host had
 multiple interfaces or an SG had multiple external interfaces, it
 might be necessary to have separate SAD and SPD pairs for each
 interface.

Kent & Atkinson Standards Track [Page 13] RFC 2401 Security Architecture for IP November 1998

4.4.1 The Security Policy Database (SPD)

 Ultimately, a security association is a management construct used to
 enforce a security policy in the IPsec environment.  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 does specify certain minimum management functionality that
 must be provided, to allow a user or system administrator to control
 how IPsec is applied to traffic transmitted or received by a host or
 transiting a security gateway.
 The SPD must be consulted during the processing of all traffic
 (INBOUND and OUTBOUND), including non-IPsec traffic.  In order to
 support this, the SPD requires distinct entries for inbound and
 outbound traffic.  One can think of this as separate SPDs (inbound
 vs.  outbound).  In addition, a nominally separate SPD must be
 provided for each IPsec-enabled interface.
 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 apply IPsec.  The first choice refers to traffic
 that is not allowed to exit the host, traverse the security gateway,
 or be delivered to an application at all.  The second choice refers
 to traffic that is allowed to pass without additional IPsec
 protection.  The third choice refers to traffic that is afforded
 IPsec protection, and for such traffic the SPD must specify the
 security services to be provided, protocols to be employed,
 algorithms to be used, etc.
 For every IPsec implementation, there MUST be an administrative
 interface that allows a user or system administrator to manage the
 SPD.  Specifically, every inbound or outbound packet is subject to
 processing by IPsec and the SPD must specify what action will be
 taken in each case.  Thus the administrative interface must allow the
 user (or system administrator) to specify the security processing to
 be applied to any packet entering or exiting the system, on a packet
 by packet basis.  (In a host IPsec implementation making use of a
 socket interface, the SPD may not need to be consulted on a per
 packet basis, but the effect is still the same.)  The management
 interface for the SPD MUST allow creation of entries consistent with
 the selectors defined in Section 4.4.2, and MUST support (total)
 ordering of these entries.  It is expected that through the use of
 wildcards in various selector fields, and because all packets on a

Kent & Atkinson Standards Track [Page 14] RFC 2401 Security Architecture for IP November 1998

 single UDP or TCP connection will tend to match a single SPD entry,
 this requirement will not impose an unreasonably detailed level of
 SPD specification.  The selectors are analogous to what are found in
 a stateless firewall or filtering router and which are currently
 manageable this way.
 In host systems, applications MAY be allowed to select what security
 processing is to be applied to the traffic they generate and consume.
 (Means of signalling 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.  Note that
 application specified policies may satisfy system requirements, so
 that the system may not need to do additional IPsec processing beyond
 that needed to meet an application's requirements.  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.
 The SPD contains an ordered list of policy entries.  Each policy
 entry is keyed by one or more selectors that define the set of IP
 traffic encompassed by this policy entry.  (The required selector
 types are defined in Section 4.4.2.)  These define the granularity of
 policies or SAs.  Each entry includes an indication of whether
 traffic matching this policy will be bypassed, discarded, or subject
 to IPsec processing.  If IPsec processing is to be applied, the entry
 includes an SA (or SA bundle) specification, listing the IPsec
 protocols, modes, and algorithms to be employed, including any
 nesting requirements.  For example, an entry may call for all
 matching traffic to be protected by ESP in transport mode using
 3DES-CBC with an explicit IV, nested inside of AH in tunnel mode
 using HMAC/SHA-1.  For each selector, the policy entry specifies how
 to derive the corresponding values for a new Security Association
 Database (SAD, see Section 4.4.3) entry from those in the SPD and the
 packet (Note that at present, ranges are only supported for IP
 addresses; but wildcarding can be expressed for all selectors):
         a. use the value in the packet itself -- This will limit use
            of the SA to those packets which have this packet's value
            for the selector even if the selector for the policy entry
            has a range of allowed values or a wildcard for this
            selector.
         b. use the value associated with the policy entry -- If this
            were to be just a single value, then there would be no
            difference between (b) and (a).  However, if the allowed
            values for the selector are a range (for IP addresses) or

Kent & Atkinson Standards Track [Page 15] RFC 2401 Security Architecture for IP November 1998

            wildcard, then in the case of a range,(b) would enable use
            of the SA by any packet with a selector value within the
            range not just by packets with the selector value of the
            packet that triggered the creation of the SA.  In the case
            of a wildcard, (b) would allow use of the SA by packets
            with any value for this selector.
 For example, suppose there is an SPD entry where the allowed value
 for source address is any of a range of hosts (192.168.2.1 to
 192.168.2.10).  And suppose that a packet is to be sent that has a
 source address of 192.168.2.3.  The value to be used for the SA could
 be any of the sample values below depending on what the policy entry
 for this selector says is the source of the selector value:
         source for the  example of
         value to be     new SAD
         used in the SA  selector value
         --------------- ------------
         a. packet       192.168.2.3 (one host)
         b. SPD entry    192.168.2.1 to 192.168.2.10 (range of hosts)
 Note that if the SPD entry had an allowed value of wildcard for the
 source address, then the SAD selector value could be wildcard (any
 host).  Case (a) can be used to prohibit sharing, even among packets
 that match the same SPD entry.
 As described below in Section 4.4.3, selectors may include "wildcard"
 entries and hence the selectors for two entries may overlap.  (This
 is analogous to the overlap that arises with ACLs or filter entries
 in routers or packet filtering firewalls.)  Thus, to ensure
 consistent, predictable processing, SPD entries MUST be ordered and
 the SPD MUST always be searched in the same order, so that the first
 matching entry is consistently selected.  (This requirement is
 necessary as the effect of processing traffic against SPD entries
 must be deterministic, but there is no way to canonicalize SPD
 entries given the use of wildcards for some selectors.)  More detail
 on matching of packets against SPD entries is provided in Section 5.
 Note that if ESP is specified, either (but not both) authentication
 or encryption can be omitted.  So it MUST be possible to configure
 the SPD value for the authentication or encryption algorithms to be
 "NULL".  However, at least one of these services MUST be selected,
 i.e., it MUST NOT be possible to configure both of them as "NULL".
 The SPD can be used to map traffic to specific SAs or SA bundles.
 Thus it can function both as the reference database for security
 policy and as the map to existing SAs (or SA bundles).  (To
 accommodate the bypass and discard policies cited above, the SPD also

Kent & Atkinson Standards Track [Page 16] RFC 2401 Security Architecture for IP November 1998

 MUST provide a means of mapping traffic to these functions, even
 though they are not, per se, IPsec processing.)  The way in which the
 SPD operates is different for inbound vs. outbound traffic and it
 also may differ for host vs.  security gateway, BITS, and BITW
 implementations.  Sections 5.1 and 5.2 describe the use of the SPD
 for outbound and inbound processing, respectively.
 Because a security policy may require that more than one SA be
 applied to a specified set of traffic, in a specific order, the
 policy entry in the SPD must preserve these ordering requirements,
 when present.  Thus, it must be possible for an IPsec implementation
 to determine that an outbound or inbound packet must be processed
 thorough a sequence of SAs.  Conceptually, for outbound processing,
 one might imagine links (to the SAD) from an SPD entry for which
 there are active SAs, and each entry would consist of either a single
 SA or an ordered list of SAs that comprise an SA bundle.  When a
 packet is matched against an SPD entry and there is an existing SA or
 SA bundle that can be used to carry the traffic, the processing of
 the packet is controlled by the SA or SA bundle entry on the list.
 For an inbound IPsec packet for which multiple IPsec SAs are to be
 applied, the lookup based on destination address, IPsec protocol, and
 SPI should identify a single SA.
 The SPD is used to control the flow of ALL traffic through an IPsec
 system, including security and key management traffic (e.g., ISAKMP)
 from/to entities behind a security gateway.  This means that ISAKMP
 traffic must be explicitly accounted for in the SPD, else it will be
 discarded.  Note that a security gateway could prohibit traversal of
 encrypted packets in various ways, e.g., having a DISCARD entry in
 the SPD for ESP packets or providing proxy key exchange.  In the
 latter case, the traffic would be internally routed to the key
 management module in the security gateway.

4.4.2 Selectors

 An SA (or SA bundle) 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
 Protocol and Port fields), 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 for SA management to facilitate control of SA
 granularity.  Note that in the case of receipt of a packet with an
 ESP header, e.g., at an encapsulating security gateway or BITW

Kent & Atkinson Standards Track [Page 17] RFC 2401 Security Architecture for IP November 1998

 implementation, the transport layer protocol, source/destination
 ports, and Name (if present) may be "OPAQUE", i.e., inaccessible
 because of encryption or fragmentation.  Note also that both Source
 and Destination addresses should either be IPv4 or IPv6.
  1. Destination IP Address (IPv4 or IPv6): this may be a single IP

address (unicast, anycast, broadcast (IPv4 only), or multicast

      group), a range of addresses (high and low values (inclusive),
      address + mask, or a wildcard address.  The last three are used
      to support more than one destination system sharing the same SA
      (e.g., behind a security gateway). Note that this selector is
      conceptually different from the "Destination IP Address" field
      in the <Destination IP Address, IPsec Protocol, SPI> tuple used
      to uniquely identify an SA.  When a tunneled packet arrives at
      the tunnel endpoint, its SPI/Destination address/Protocol are
      used to look up the SA for this packet in the SAD.  This
      destination address comes from the encapsulating IP header.
      Once the packet has been processed according to the tunnel SA
      and has come out of the tunnel, its selectors are "looked up" in
      the Inbound SPD.  The Inbound SPD has a selector called
      destination address.  This IP destination address is the one in
      the inner (encapsulated) IP header.  In the case of a
      transport'd packet, there will be only one IP header and this
      ambiguity does not exist.  [REQUIRED for all implementations]
  1. Source IP Address(es) (IPv4 or IPv6): this may be a single IP

address (unicast, anycast, broadcast (IPv4 only), or multicast

      group), range of addresses (high and low values inclusive),
      address + mask, or a wildcard address.  The last three are used
      to support more than one source system sharing the same SA
      (e.g., behind a security gateway or in a multihomed host).
      [REQUIRED for all implementations]
  1. Name: There are 2 cases (Note that these name forms are

supported in the IPsec DOI.)

              1. User ID
                  a. a fully qualified user name string (DNS), e.g.,
                     mozart@foo.bar.com
                  b. X.500 distinguished name, e.g., C = US, SP = MA,
                     O = GTE Internetworking, CN = Stephen T. Kent.
              2. System name (host, security gateway, etc.)
                  a. a fully qualified DNS name, e.g., foo.bar.com
                  b. X.500 distinguished name
                  c. X.500 general name
      NOTE: One of the possible values of this selector is "OPAQUE".

Kent & Atkinson Standards Track [Page 18] RFC 2401 Security Architecture for IP November 1998

      [REQUIRED for the following cases.  Note that support for name
      forms other than addresses is not required for manually keyed
      SAs.
              o User ID
                  - native host implementations
                  - BITW and BITS implementations acting as HOSTS
                    with only one user
                  - security gateway implementations for INBOUND
                    processing.
              o System names -- all implementations]
  1. Data sensitivity level: (IPSO/CIPSO labels)

[REQUIRED for all systems providing information flow security as

      per Section 8, OPTIONAL for all other systems.]
  1. Transport Layer Protocol: Obtained from the IPv4 "Protocol" or

the IPv6 "Next Header" fields. This may be an individual

      protocol number.  These packet fields may not contain the
      Transport Protocol due to the presence of IP extension headers,
      e.g., a Routing Header, AH, ESP, Fragmentation Header,
      Destination Options, Hop-by-hop options, etc.  Note that the
      Transport Protocol may not be available in the case of receipt
      of a packet with an ESP header, thus a value of "OPAQUE" SHOULD
      be supported.
      [REQUIRED for all implementations]
      NOTE: To locate the transport protocol, a system has to chain
      through the packet headers checking the "Protocol" or "Next
      Header" field until it encounters either one it recognizes as a
      transport protocol, or until it reaches one that isn't on its
      list of extension headers, or until it encounters an ESP header
      that renders the transport protocol opaque.
  1. Source and Destination (e.g., TCP/UDP) Ports: These may be

individual UDP or TCP port values or a wildcard port. (The use

      of the Next Protocol field and the Source and/or Destination
      Port fields (in conjunction with the Source and/or Destination
      Address fields), as an SA selector is sometimes referred to as
      "session-oriented keying.").  Note that the source and
      destination ports may not be available in the case of receipt of
      a packet with an ESP header, thus a value of "OPAQUE" SHOULD be
      supported.
      The following table summarizes the relationship between the
      "Next Header" value in the packet and SPD and the derived Port
      Selector value for the SPD and SAD.

Kent & Atkinson Standards Track [Page 19] RFC 2401 Security Architecture for IP November 1998

        Next Hdr        Transport Layer   Derived Port Selector Field
        in Packet       Protocol in SPD   Value in SPD and SAD
        --------        ---------------   ---------------------------
        ESP             ESP or ANY        ANY (i.e., don't look at it)
        -don't care-    ANY               ANY (i.e., don't look at it)
        specific value  specific value    NOT ANY (i.e., drop packet)
           fragment
        specific value  specific value    actual port selector field
           not fragment
      If the packet has been fragmented, then the port information may
      not be available in the current fragment.  If so, discard the
      fragment.  An ICMP PMTU should be sent for the first fragment,
      which will have the port information.  [MAY be supported]
 The IPsec implementation context determines how selectors are used.
 For example, a host implementation integrated into the stack may make
 use of a socket interface.  When a new connection is established the
 SPD can be consulted and an SA (or SA bundle) bound to the socket.
 Thus traffic sent via that socket need not result in additional
 lookups to the SPD/SAD.  In contrast, a BITS, BITW, or security
 gateway implementation needs to look at each packet and perform an
 SPD/SAD lookup based on the selectors. The allowable values for the
 selector fields differ between the traffic flow, the security
 association, and the security policy.
 The following table summarizes the kinds of entries that one needs to
 be able to express in the SPD and SAD.  It shows how they relate to
 the fields in data traffic being subjected to IPsec screening.
 (Note: the "wild" or "wildcard" entry for src and dst addresses
 includes a mask, range, etc.)

Field Traffic Value SAD Entry SPD Entry ——– ————- —————- ——————– src addr single IP addr single,range,wild single,range,wildcard dst addr single IP addr single,range,wild single,range,wildcard xpt protocol* xpt protocol single,wildcard single,wildcard src port* single src port single,wildcard single,wildcard dst port* single dst port single,wildcard single,wildcard user id* single user id single,wildcard single,wildcard sec. labels single value single,wildcard single,wildcard

  • The SAD and SPD entries for these fields could be "OPAQUE"

because the traffic value is encrypted.

 NOTE: In principle, one could have selectors and/or selector values
 in the SPD which cannot be negotiated for an SA or SA bundle.
 Examples might include selector values used to select traffic for

Kent & Atkinson Standards Track [Page 20] RFC 2401 Security Architecture for IP November 1998

 discarding or enumerated lists which cause a separate SA to be
 created for each item on the list.  For now, this is left for future
 versions of this document and the list of required selectors and
 selector values is the same for the SPD and the SAD.  However, it is
 acceptable to have an administrative interface that supports use of
 selector values which cannot be negotiated provided that it does not
 mislead the user into believing it is creating an SA with these
 selector values.  For example, the interface may allow the user to
 specify an enumerated list of values but would result in the creation
 of a separate policy and SA for each item on the list.  A vendor
 might support such an interface to make it easier for its customers
 to specify clear and concise policy specifications.

4.4.3 Security Association Database (SAD)

 In each IPsec implementation there is a nominal Security Association
 Database, in which each entry defines the parameters associated with
 one SA.  Each SA has an entry in the SAD.  For outbound processing,
 entries are pointed to by entries in the SPD.  Note that if an SPD
 entry does not currently point to an SA that is appropriate for the
 packet, the implementation creates an appropriate SA (or SA Bundle)
 and links the SPD entry to the SAD entry (see Section 5.1.1).  For
 inbound processing, each entry in the SAD is indexed by a destination
 IP address, IPsec protocol type, and SPI.  The following parameters
 are associated with each entry in the SAD.  This description does not
 purport to be a MIB, but only a specification of the minimal data
 items required to support an SA in an IPsec implementation.
 For inbound processing: The following packet fields are used to look
 up the SA in the SAD:
       o Outer Header's Destination IP address: the IPv4 or IPv6
         Destination address.
         [REQUIRED for all implementations]
       o IPsec Protocol: AH or ESP, used as an index for SA lookup
         in this database.  Specifies the IPsec protocol to be
         applied to the traffic on this SA.
         [REQUIRED for all implementations]
       o SPI: the 32-bit value used to distinguish among different
         SAs terminating at the same destination and using the same
         IPsec protocol.
         [REQUIRED for all implementations]
 For each of the selectors defined in Section 4.4.2, the SA entry in
 the SAD MUST contain the value or values which were negotiated at the
 time the SA was created.  For the sender, these values are used to
 decide whether a given SA is appropriate for use with an outbound
 packet.  This is part of checking to see if there is an existing SA

Kent & Atkinson Standards Track [Page 21] RFC 2401 Security Architecture for IP November 1998

 that can be used.  For the receiver, these values are used to check
 that the selector values in an inbound packet match those for the SA
 (and thus indirectly those for the matching policy).  For the
 receiver, this is part of verifying that the SA was appropriate for
 this packet.  (See Section 6 for rules for ICMP messages.)  These
 fields can have the form of specific values, ranges, wildcards, or
 "OPAQUE" as described in section 4.4.2, "Selectors".  Note that for
 an ESP SA, the encryption algorithm or the authentication algorithm
 could be "NULL".  However they MUST not both be "NULL".
 The following SAD fields are used in doing IPsec processing:
       o Sequence Number Counter: a 32-bit value used to generate the
         Sequence Number field in AH or ESP headers.
         [REQUIRED for all implementations, but used only for outbound
         traffic.]
       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.
         [REQUIRED for all implementations, but used only for outbound
         traffic.]
       o Anti-Replay Window: a 32-bit counter and a bit-map (or
         equivalent) used to determine whether an inbound AH or ESP
         packet is a replay.
         [REQUIRED for all implementations but used only for inbound
         traffic. NOTE: If anti-replay has been disabled by the
         receiver, e.g., in the case of a manually keyed SA, then the
         Anti-Replay Window is not used.]
       o AH Authentication algorithm, keys, etc.
         [REQUIRED for AH implementations]
       o ESP Encryption algorithm, keys, IV mode, IV, etc.
         [REQUIRED for ESP implementations]
       o ESP authentication algorithm, keys, etc. If the
         authentication service is not selected, this field will be
         null.
         [REQUIRED for ESP implementations]
       o Lifetime of this Security Association: 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, 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 CRLs used in the IKE exchange

Kent & Atkinson Standards Track [Page 22] RFC 2401 Security Architecture for IP November 1998

         for the SA.  Both initiator and responder are responsible for
         constraining SA lifetime in this fashion.
         [REQUIRED for all implementations]
         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
               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 SHOULD 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 which warns the implementation to initiate
               action such as setting up a replacement SA and a
               hard lifetime when the current SA ends.
           (c) If the entire packet does not get delivered during
               the SAs lifetime, the packet SHOULD be discarded.
       o IPsec protocol mode: tunnel, transport or wildcard.
         Indicates which mode of AH or ESP is applied to traffic on
         this SA.  Note that if this field is "wildcard" at the
         sending end of the SA, then the application has to specify
         the mode to the IPsec implementation.  This use of wildcard
         allows the same SA to be used for either tunnel or transport
         mode traffic on a per packet basis, e.g., by different
         sockets.  The receiver does not need to know the mode in
         order to properly process the packet's IPsec headers.
         [REQUIRED as follows, unless implicitly defined by context:
                 - host implementations must support all modes
                 - gateway implementations must support tunnel mode]
         NOTE: The use of wildcard for the protocol mode of an inbound
         SA may add complexity to the situation in the receiver (host
         only).  Since the packets on such an SA could be delivered in
         either tunnel or transport mode, the security of an incoming
         packet could depend in part on which mode had been used to
         deliver it.  If, as a result, an application cared about the
         SA mode of a given packet, then the application would need a
         mechanism to obtain this mode information.

Kent & Atkinson Standards Track [Page 23] RFC 2401 Security Architecture for IP November 1998

       o Path MTU: any observed path MTU and aging variables.  See
         Section 6.1.2.4
         [REQUIRED for all implementations but used only for outbound
         traffic]

4.5 Basic Combinations of Security Associations

 This section describes four examples of combinations of security
 associations that MUST be supported by compliant IPsec hosts or
 security gateways.  Additional combinations of AH and/or ESP in
 tunnel and/or transport modes MAY be supported at the discretion of
 the implementor.  Compliant implementations MUST be capable of
 generating these four combinations and on receipt, of processing
 them, but SHOULD be able to receive and process any combination.  The
 diagrams and text below describe the basic cases.  The legend for the
 diagrams is:
      ==== = one or more security associations (AH or ESP, transport
             or tunnel)
      ---- = connectivity (or if so labelled, administrative boundary)
      Hx   = host x
      SGx  = security gateway x
      X*   = X supports IPsec
 NOTE: The security associations below can be either AH or ESP.  The
 mode (tunnel vs transport) is determined by the nature of the
 endpoints.  For host-to-host SAs, the mode can be either transport or
 tunnel.
 Case 1.  The case of providing end-to-end security between 2 hosts
      across the Internet (or an Intranet).
               ====================================
               |                                  |
              H1* ------ (Inter/Intranet) ------ H2*
      Note that either transport or tunnel mode can be selected by the
      hosts.  So the headers in a packet between H1 and H2 could look
      like any of the following:
                Transport                  Tunnel
           -----------------          ---------------------
           1. [IP1][AH][upper]        4. [IP2][AH][IP1][upper]
           2. [IP1][ESP][upper]       5. [IP2][ESP][IP1][upper]
           3. [IP1][AH][ESP][upper]

Kent & Atkinson Standards Track [Page 24] RFC 2401 Security Architecture for IP November 1998

      Note that there is no requirement to support general nesting,
      but in transport mode, both AH and ESP can be applied to the
      packet.  In this event, the SA establishment procedure MUST
      ensure that first ESP, then AH are applied to the packet.
 Case 2.  This case illustrates simple virtual private networks
      support.
                     ===========================
                     |                         |
---------------------|----                  ---|-----------------------
|                    |   |                  |  |                      |
|  H1 -- (Local --- SG1* |--- (Internet) ---| SG2* --- (Local --- H2  |
|        Intranet)       |                  |          Intranet)      |
--------------------------                  ---------------------------
    admin. boundary                               admin. boundary
      Only tunnel mode is required here.  So the headers in a packet
      between SG1 and SG2 could look like either of the following:
                      Tunnel
              ---------------------
              4. [IP2][AH][IP1][upper]
              5. [IP2][ESP][IP1][upper]
 Case 3.  This case combines cases 1 and 2, adding end-to-end security
      between the sending and receiving hosts.  It imposes no new
      requirements on the hosts or security gateways, other than a
      requirement for a security gateway to be configurable to pass
      IPsec traffic (including ISAKMP traffic) for hosts behind it.
   ===============================================================
   |                                                             |
   |                 =========================                   |
   |                 |                       |                   |
---|-----------------|----                ---|-------------------|---
|  |                 |   |                |  |                   |  |
| H1* -- (Local --- SG1* |-- (Internet) --| SG2* --- (Local --- H2* |
|        Intranet)       |                |          Intranet)      |
--------------------------                ---------------------------
     admin. boundary                            admin. boundary
 Case 4.  This covers the situation where a remote host (H1) uses the
      Internet to reach an organization's firewall (SG2) and to then
      gain access to some server or other machine (H2).  The remote
      host could be a mobile host (H1) dialing up to a local PPP/ARA
      server (not shown) on the Internet and then crossing the
      Internet to the home organization's firewall (SG2), etc.  The

Kent & Atkinson Standards Track [Page 25] RFC 2401 Security Architecture for IP November 1998

      details of support for this case, (how H1 locates SG2,
      authenticates it, and verifies its authorization to represent
      H2) are discussed in Section 4.6.3, "Locating a Security
      Gateway".
      ======================================================
      |                                                    |
      |==============================                      |
      ||                            |                      |
      ||                         ---|----------------------|---
      ||                         |  |                      |  |
      H1* ----- (Internet) ------| SG2* ---- (Local ----- H2* |
            ^                    |           Intranet)        |
            |                    ------------------------------
      could be dialup              admin. boundary (optional)
      to PPP/ARA server
      Only tunnel mode is required between H1 and SG2.  So the choices
      for the SA between H1 and SG2 would be one of the ones in case
      2.  The choices for the SA between H1 and H2 would be one of the
      ones in case 1.
      Note that in this case, the sender MUST apply the transport
      header before the tunnel header.  Therefore the management
      interface to the IPsec implementation MUST support configuration
      of the SPD and SAD to ensure this ordering of IPsec header
      application.
 As noted above, support for additional combinations of AH and ESP is
 optional.  Use of other, optional combinations may adversely affect
 interoperability.

4.6 SA and Key Management

 IPsec mandates support for 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 services available for AH and ESP require automated SA
 management.  Moreover, the granularity of key distribution employed
 with IPsec determines the granularity of authentication provided.
 (See also a discussion of this issue in Section 4.7.)  In general,
 data origin authentication in AH and ESP is limited by the extent to
 which secrets used with the authentication algorithm (or with a key
 management protocol that creates such secrets) are shared among
 multiple possible sources.

Kent & Atkinson Standards Track [Page 26] RFC 2401 Security Architecture for IP November 1998

 The following text describes the minimum requirements for both types
 of SA management.

4.6.1 Manual Techniques

 The simplest form of management is manual management, in which a
 person manually configures each system with keying material and
 security association 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 is likely to 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.

4.6.2 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 IKE [MSST97, Orm97, HC98] under the IPsec domain of
 interpretation [Pip98].  Other automated SA management protocols MAY
 be employed.
 When an automated SA/key management protocol is employed, the output
 from this protocol may be used to generate multiple keys, e.g., for a
 single ESP SA.  This may arise because:
     o the encryption algorithm uses multiple keys (e.g., triple DES)
     o the authentication algorithm uses multiple keys
     o both encryption and authentication algorithms are employed

Kent & Atkinson Standards Track [Page 27] RFC 2401 Security Architecture for IP November 1998

 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 of them
 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 key(s) MUST be taken from the first (left-most, high-
 order) bits and the authentication key(s) MUST be taken from the
 remaining bits.  The number of bits for each key is defined in the
 relevant algorithm specification RFC.  In the case of multiple
 encryption keys or multiple authentication keys, the specification
 for the algorithm must specify the order in which they are to be
 selected from a single string of bits provided to the algorithm.

4.6.3 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.
 Consider a situation in which a remote host (H1) 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 (Road Warrior) crossing the Internet to the home organization's
 firewall (SG2).  (See Case 4 in the section 4.5 Basic Combinations of
 Security Associations.) This situation raises several issues:
      1. How does H1 know/learn about the existence of the security
         gateway SG2?
      2. How does it authenticate SG2, and once it has authenticated
         SG2, how does it confirm that SG2 has been authorized to
         represent H2?
      3. How does SG2 authenticate H1 and verify that H1 is authorized
         to contact H2?
      4. How does H1 know/learn about backup gateways which provide
         alternate paths to H2?
 To address these problems, a host or security gateway MUST have an
 administrative interface that allows the user/administrator to
 configure the address of a security gateway for any sets of
 destination addresses that require its use. This includes the ability
 to configure:

Kent & Atkinson Standards Track [Page 28] RFC 2401 Security Architecture for IP November 1998

      o the requisite information for locating and authenticating the
        security gateway and verifying its authorization to represent
        the destination host.
      o the requisite information for locating and authenticating any
        backup gateways and verifying their authorization to represent
        the destination host.
 It is assumed that the SPD is also configured with policy information
 that covers any other IPsec requirements for the path to the security
 gateway and the destination host.
 This document does not address the issue of how to automate the
 discovery/verification of security gateways.

4.7 Security Associations and Multicast

 The receiver-orientation of the Security Association implies that, in
 the case of unicast traffic, the destination system will normally
 select the SPI value.  By having the destination select the SPI
 value, there is no potential for manually configured Security
 Associations to conflict with automatically configured (e.g., via a
 key management protocol) Security Associations or for Security
 Associations from multiple sources to conflict with each other.  For
 multicast traffic, there are multiple destination systems per
 multicast group.  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 Security Parameter Index) for all traffic to
 that group when a symmetric key encryption or authentication
 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 cases are deferred
 to later IPsec documents.
 At the time this specification was published, automated protocols for
 multicast key distribution were not considered adequately mature for
 standardization.  For multicast groups having relatively few members,
 manual key distribution or multiple use of existing unicast key
 distribution algorithms such as modified Diffie-Hellman appears
 feasible.  For very large groups, new scalable techniques will be
 needed.  An example of current work in this area is the Group Key
 Management Protocol (GKMP) [HM97].

Kent & Atkinson Standards Track [Page 29] RFC 2401 Security Architecture for IP November 1998

5. IP Traffic Processing

 As mentioned in Section 4.4.1 "The Security Policy Database (SPD)",
 the SPD must be consulted during the processing of all traffic
 (INBOUND and OUTBOUND), including non-IPsec traffic.  If no policy is
 found in the SPD that matches the packet (for either inbound or
 outbound traffic), the packet MUST be discarded.
 NOTE: All of the cryptographic algorithms used in IPsec expect their
 input in canonical network byte order (see Appendix in RFC 791) and
 generate their output in canonical network byte order.  IP packets
 are also transmitted in network byte order.

5.1 Outbound IP Traffic Processing

5.1.1 Selecting and Using an SA or SA Bundle

 In a security gateway or BITW implementation (and in many BITS
 implementations), each outbound packet is compared against the SPD to
 determine what processing is required for the packet.  If the packet
 is to be discarded, this is an auditable event.  If the traffic is
 allowed to bypass IPsec processing, the packet continues through
 "normal" processing for the environment in which the IPsec processing
 is taking place.  If IPsec processing is required, the packet is
 either mapped to an existing SA (or SA bundle), or a new SA (or SA
 bundle) is created for the packet.  Since a packet's selectors might
 match multiple policies or multiple extant SAs and since the SPD is
 ordered, but the SAD is not, IPsec MUST:
         1. Match the packet's selector fields against the outbound
            policies in the SPD to locate the first appropriate
            policy, which will point to zero or more SA bundles in the
            SAD.
         2. Match the packet's selector fields against those in the SA
            bundles found in (1) to locate the first SA bundle that
            matches.  If no SAs were found or none match, create an
            appropriate SA bundle and link the SPD entry to the SAD
            entry.  If no key management entity is found, drop the
            packet.
         3. Use the SA bundle found/created in (2) to do the required
            IPsec processing, e.g., authenticate and encrypt.
 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.

Kent & Atkinson Standards Track [Page 30] RFC 2401 Security Architecture for IP November 1998

 NOTE: A compliant implementation MUST not allow instantiation of an
 ESP SA that employs both a NULL encryption and a NULL authentication
 algorithm.  An attempt to negotiate such an SA is an auditable event.

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.  This
 includes how to construct the encapsulating (outer) IP header, how to
 handle fields in the inner IP header, and what other actions should
 be taken.  The general idea is modeled after the one used in RFC
 2003, "IP Encapsulation with 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.  (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 to decrement the TTL
        as noted below, and 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.
      o If need be, other protocol headers such as the IP
        Authentication header may be inserted between the outer IP
        header and the inner IP header.
 The tables in the following sub-sections show the handling for the
 different header/option fields (constructed = 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
     TOS              copied from inner hdr (5)    no change
     total length     constructed                  no change
     ID               constructed                  no change
     flags (DF,MF)    constructed, DF (4)          no change
     fragmt offset    constructed                  no change

Kent & Atkinson Standards Track [Page 31] RFC 2401 Security Architecture for IP November 1998

     TTL              constructed (2)              decrement (2)
     protocol         AH, ESP, routing hdr         no change
     checksum         constructed                  constructed (2)
     src address      constructed (3)              no change
     dest address     constructed (3)              no change
 Options            never copied                 no change
      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 checksum changes when the TTL
         changes.)
         Note: The decrementing of the TTL is one of the usual actions
         that takes place when forwarding a packet.  Packets
         originating from the same node as the encapsulator do not
         have their TTL's decremented, as the sending node is
         originating the packet rather than forwarding it.
      3. src and dest addresses depend on the SA, which is used to
         determine the dest address which in turn determines which src
         address (net interface) is used to forward the packet.
         NOTE: In principle, the encapsulating IP source address can
         be any of the encapsulator's interface addresses or even an
         address different from any of the encapsulator's IP
         addresses, (e.g., if it's acting as a NAT box) so long as the
         address is reachable through the encapsulator from the
         environment into which the packet is sent.  This does not
         cause a problem because IPsec does not currently have any
         INBOUND processing requirement that involves the Source
         Address of the encapsulating IP header.  So while the
         receiving tunnel endpoint looks at the Destination Address in
         the encapsulating IP header, it only looks at the Source
         Address in the inner (encapsulated) IP header.
      4. configuration determines whether to copy from the inner
         header (IPv4 only), clear or set the DF.
      5. If Inner Hdr is IPv4 (Protocol = 4), copy the TOS.  If Inner
         Hdr is IPv6 (Protocol = 41), map the Class to TOS.

5.1.2.2 IPv6 – Header Construction for Tunnel Mode

 See previous section 5.1.2 for notes 1-5 indicated by (footnote
 number).

Kent & Atkinson Standards Track [Page 32] RFC 2401 Security Architecture for IP November 1998

                      <-- How Outer Hdr  Relates Inner Hdr --->
                      Outer Hdr at                 Inner Hdr at
 IPv6                 Encapsulator                 Decapsulator
   Header fields:     --------------------         ------------
     version          6 (1)                        no change
     class            copied or configured (6)     no change
     flow id          copied or configured         no change
     len              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                 no change
      6. If Inner Hdr is IPv6 (Next Header = 41), copy the Class.  If
         Inner Hdr is IPv4 (Next Header = 4), map the TOS to Class.

5.2 Processing Inbound IP Traffic

 Prior to performing AH or ESP processing, any IP fragments are
 reassembled.  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 an extension header in
 the IPv6 context).
 Note: Appendix C contains sample code for a bitmask check for a 32
 packet window that can be used for implementing anti-replay service.

5.2.1 Selecting and Using an SA or SA Bundle

 Mapping the IP datagram to the appropriate SA is simplified because
 of the presence of the SPI in the AH or ESP header.  Note that the
 selector checks are made on the inner headers not the outer (tunnel)
 headers.  The steps followed are:
         1. Use the packet's destination address (outer IP header),
            IPsec protocol, and SPI to look up the SA in the SAD.  If
            the SA lookup fails, drop the packet and log/report the
            error.
         2. Use the SA found in (1) to do the IPsec processing, e.g.,
            authenticate and decrypt.  This step includes matching the
            packet's (Inner Header if tunneled) selectors to the
            selectors in the SA.  Local policy determines the
            specificity of the SA selectors (single value, list,
            range, wildcard).  In general, a packet's source address
            MUST match the SA selector value.  However, an ICMP packet
            received on a tunnel mode SA may have a source address

Kent & Atkinson Standards Track [Page 33] RFC 2401 Security Architecture for IP November 1998

            other than that bound to the SA and thus such packets
            should be permitted as exceptions to this check.  For an
            ICMP packet, the selectors from the enclosed problem
            packet (the source and destination addresses and ports
            should be swapped) should be checked against the selectors
            for the SA.  Note that some or all of these selectors may
            be inaccessible because of limitations on how many bits of
            the problem packet the ICMP packet is allowed to carry or
            due to encryption.  See Section 6.
            Do (1) and (2) for every IPsec header until a Transport
            Protocol Header or an IP header that is NOT for this
            system is encountered.  Keep track of what SAs have been
            used and their order of application.
         3. Find an incoming policy in the SPD that matches the
            packet.  This could be done, for example, by use of
            backpointers from the SAs to the SPD or by matching the
            packet's selectors (Inner Header if tunneled) against
            those of the policy entries in the SPD.
         4. Check whether the required IPsec processing has been
            applied, i.e., verify that the SA's found in (1) and (2)
            match the kind and order of SAs required by the policy
            found in (3).
            NOTE: The correct "matching" policy will not necessarily
            be the first inbound policy found.  If the check in (4)
            fails, steps (3) and (4) are repeated until all policy
            entries have been checked or until the check succeeds.
 At the end of these steps, pass the resulting packet to the Transport
 Layer or forward the packet.  Note that any IPsec headers processed
 in these steps may have been removed, but that this information,
 i.e., what SAs were used and the order of their application, may be
 needed for subsequent IPsec or firewall processing.
 Note that in the case of a security gateway, if forwarding causes a
 packet to exit via an IPsec-enabled interface, then additional IPsec
 processing may be applied.

5.2.2 Handling of AH and ESP tunnels

 The handling of the inner and outer IP headers, extension headers,
 and options for AH and ESP tunnels should be performed as described
 in the tables in Section 5.1.

Kent & Atkinson Standards Track [Page 34] RFC 2401 Security Architecture for IP November 1998

6. ICMP Processing (relevant to IPsec)

 The focus of this section is on the handling of ICMP error messages.
 Other ICMP traffic, e.g., Echo/Reply, should be treated like other
 traffic and can be protected on an end-to-end basis using SAs in the
 usual fashion.
 An ICMP error message protected by AH or ESP and generated by a
 router SHOULD be processed and forwarded in a tunnel mode SA.  Local
 policy determines whether or not it is subjected to source address
 checks by the router at the destination end of the tunnel.  Note that
 if the router at the originating end of the tunnel is forwarding an
 ICMP error message from another router, the source address check
 would fail.  An ICMP message protected by AH or ESP and generated by
 a router MUST NOT be forwarded on a transport mode SA (unless the SA
 has been established to the router acting as a host, e.g., a Telnet
 connection used to manage a router).  An ICMP message generated by a
 host SHOULD be checked against the source IP address selectors bound
 to the SA in which the message arrives.  Note that even if the source
 of an ICMP error message is authenticated, the returned IP header
 could be invalid. Accordingly, the selector values in the IP header
 SHOULD also be checked to be sure that they are consistent with the
 selectors for the SA over which the ICMP message was received.
 The table in Appendix D characterize ICMP messages as being either
 host generated, router generated, both, unknown/unassigned.  ICMP
 messages falling into the last two categories should be handled as
 determined by the receiver's policy.
 An ICMP message not protected by AH or ESP is unauthenticated and its
 processing and/or forwarding may result in denial of service.  This
 suggests that, in general, it would be desirable to ignore such
 messages.  However, it is expected that many routers (vs. security
 gateways) will not implement IPsec for transit traffic and thus
 strict adherence to this rule would cause many ICMP messages to be
 discarded.  The result is that some critical IP functions would be
 lost, e.g., redirection and PMTU processing.  Thus it MUST be
 possible to configure an IPsec implementation to accept or reject
 (router) ICMP traffic as per local security policy.
 The remainder of this section addresses how PMTU processing MUST be
 performed at hosts and security gateways.  It addresses processing of
 both authenticated and unauthenticated ICMP PMTU messages.  However,
 as noted above, unauthenticated ICMP messages MAY be discarded based
 on local policy.

Kent & Atkinson Standards Track [Page 35] RFC 2401 Security Architecture for IP November 1998

6.1 PMTU/DF Processing

6.1.1 DF Bit

 In cases where a system (host or gateway) adds an encapsulating
 header (ESP tunnel or AH tunnel), it MUST support the option of
 copying the DF bit from the original packet to the encapsulating
 header (and processing ICMP PMTU messages).  This means that it MUST
 be possible to configure the system's treatment of the DF bit (set,
 clear, copy from encapsulated header) for each interface.  (See
 Appendix B for rationale.)

6.1.2 Path MTU Discovery (PMTU)

 This section discusses IPsec handling for Path MTU Discovery
 messages.  ICMP PMTU is used here to refer to an ICMP message for:
         IPv4 (RFC 792):
                 - 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 (labelled "unused" in RFC
                   792), with high-order 16 bits set to zero
         IPv6 (RFC 1885):
                 - Type = 2 (Packet Too Big)
                 - Code = 0 (Fragmentation needed)
                 - Next-Hop MTU in the 32 bit MTU field of the ICMP6
                   message

6.1.2.1 Propagation of PMTU

 The amount of information returned with the ICMP PMTU message (IPv4
 or IPv6) is limited and this affects what selectors are available for
 use in further propagating the PMTU information.  (See Appendix B for
 more detailed discussion of this topic.)
 o PMTU message with 64 bits of IPsec header -- If the ICMP PMTU
   message contains only 64 bits of the IPsec header (minimum for
   IPv4), then a security gateway MUST support the following options
   on a per SPI/SA basis:
      a. if the originating host can be determined (or the possible
         sources narrowed down to a manageable number), send the PM
         information to all the possible originating hosts.
      b. if the originating host cannot be determined, store the PMTU
         with the SA and wait until the next packet(s) arrive from the
         originating host for the relevant security association.  If

Kent & Atkinson Standards Track [Page 36] RFC 2401 Security Architecture for IP November 1998

         the packet(s) are bigger than the PMTU, drop the packet(s),
         and compose ICMP PMTU message(s) with the new packet(s) and
         the updated PMTU, and send the ICMP message(s) about the
         problem to the originating host. Retain the PMTU information
         for any message that might arrive subsequently (see Section
         6.1.2.4, "PMTU Aging").
 o PMTU message with >64 bits of IPsec header -- If the ICMP message
   contains more information from the original packet then there may
   be enough non-opaque information to immediately determine to which
   host to propagate the ICMP/PMTU message and to provide that system
   with the 5 fields (source address, destination address, source
   port, destination port, transport protocol) needed to determine
   where to store/update the PMTU.  Under such circumstances, a
   security gateway MUST generate an ICMP PMTU message immediately
   upon receipt of an ICMP PMTU from further down the path.
 o Distributing the PMTU to the Transport Layer -- The host mechanism
   for getting the updated PMTU to the transport layer is unchanged,
   as specified in RFC 1191 (Path MTU Discovery).

6.1.2.2 Calculation of PMTU

 The calculation of PMTU from an ICMP PMTU MUST take into account the
 addition of any IPsec header -- AH transport, ESP transport, AH/ESP
 transport, ESP tunnel, AH tunnel.  (See Appendix B for discussion of
 implementation issues.)
 Note: In some situations the addition of IPsec headers could result
 in an effective PMTU (as seen by the host or application) that is
 unacceptably small.  To avoid this problem, the implementation may
 establish a threshold below which it will not report a reduced PMTU.
 In such cases, the implementation would apply IPsec and then fragment
 the resulting packet according to the PMTU.  This would result in a
 more efficient use of the available bandwidth.

6.1.2.3 Granularity of PMTU Processing

 In hosts, the granularity with which ICMP PMTU processing can be done
 differs depending on the implementation situation.  Looking at a
 host, there are 3 situations that are of interest with respect to
 PMTU issues (See Appendix B for additional details on this topic.):
      a. Integration of IPsec into the native IP implementation
      b. Bump-in-the-stack implementations, where IPsec is implemented
         "underneath" an existing implementation of a TCP/IP protocol
         stack, between the native IP and the local network drivers

Kent & Atkinson Standards Track [Page 37] RFC 2401 Security Architecture for IP November 1998

      c. No IPsec implementation -- This case is included because it
         is relevant in cases where a security gateway is sending PMTU
         information back to a host.
 Only in case (a) can the PMTU data be maintained at the same
 granularity as communication associations.  In (b) and (c), the IP
 layer will only be able to maintain PMTU data at the granularity of
 source and destination IP addresses (and optionally TOS), as
 described in RFC 1191.  This is an important difference, because more
 than one communication association may map to the same source and
 destination IP addresses, and each communication association may have
 a different amount of IPsec header overhead (e.g., due to use of
 different transforms or different algorithms).
 Implementation of the calculation of PMTU and support for PMTUs at
 the granularity of individual communication associations is a local
 matter.  However, a socket-based implementation of IPsec in a host
 SHOULD maintain the information on a per socket basis.  Bump in the
 stack systems MUST pass an ICMP PMTU to the host IP implementation,
 after adjusting it for any IPsec header overhead added by these
 systems.  The calculation of the overhead SHOULD be determined by
 analysis of the SPI and any other selector information present in a
 returned ICMP PMTU message.

6.1.2.4 PMTU Aging

 In all systems (host or gateway) implementing IPsec and maintaining
 PMTU information, the PMTU associated with a security association
 (transport or tunnel) MUST be "aged" and some mechanism put in place
 for updating the PMTU in a timely manner, especially for discovering
 if the PMTU is smaller than it needs to be.  A given PMTU has to
 remain in place long enough for a packet to get from the source end
 of the security association to the system at the other end of the
 security association and propagate back an ICMP error message if the
 current PMTU is too big.  Note that if there are nested tunnels,
 multiple packets and round trip times might be required to get an
 ICMP message back to an encapsulator or originating host.
 Systems SHOULD use the approach described in the Path MTU Discovery
 document (RFC 1191, 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.

Kent & Atkinson Standards Track [Page 38] RFC 2401 Security Architecture for IP November 1998

7. Auditing

 Not all systems that implement IPsec will implement auditing.  For
 the most part, the granularity of auditing is a local matter.
 However, several auditable events are identified in the AH and ESP
 specifications 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.

8. Use in Systems Supporting Information Flow Security

 Information of various sensitivity levels may be carried over a
 single network.  Information labels (e.g., Unclassified, Company
 Proprietary, Secret) [DoD85, DoD87] are often employed to distinguish
 such information.  The use of labels facilitates segregation of
 information, in support of information flow security models, e.g.,
 the Bell-LaPadula model [BL73].  Such models, and corresponding
 supporting technology, are designed to prevent the unauthorized flow
 of sensitive information, even in the face of Trojan Horse attacks.
 Conventional, discretionary access control (DAC) mechanisms, e.g.,
 based on access control lists, generally are not sufficient to
 support such policies, and thus facilities such as the SPD do not
 suffice in such environments.
 In the military context, technology that supports such models is
 often referred to as multi-level security (MLS).  Computers and
 networks often are designated "multi-level secure" if they support
 the separation of labelled data in conjunction with information flow
 security policies.  Although such technology is more broadly
 applicable than just military applications, this document uses the
 acronym "MLS" to designate the technology, consistent with much
 extant literature.
 IPsec mechanisms can easily support MLS networking.  MLS networking
 requires the use of strong Mandatory Access Controls (MAC), which
 unprivileged users or unprivileged processes are incapable of
 controlling or violating.  This section pertains only to the use of
 these IP security mechanisms in MLS (information flow security
 policy) environments.  Nothing in this section applies to systems not
 claiming to provide MLS.

Kent & Atkinson Standards Track [Page 39] RFC 2401 Security Architecture for IP November 1998

 As used in this section, "sensitivity information" might include
 implementation-defined hierarchic levels, categories, and/or
 releasability information.
 AH can be used to provide strong authentication in support of
 mandatory access control decisions in MLS environments.  If explicit
 IP sensitivity information (e.g., IPSO [Ken91]) is used and
 confidentiality is not considered necessary within the particular
 operational environment, AH can be used to authenticate the binding
 between sensitivity labels in the IP header and the IP payload
 (including user data).  This is a significant improvement over
 labeled IPv4 networks where the sensitivity information is trusted
 even though there is no authentication or cryptographic binding of
 the information to the IP header and user data.  IPv4 networks might
 or might not use explicit labelling.  IPv6 will normally use implicit
 sensitivity information that is part of the IPsec Security
 Association but not transmitted with each packet instead of using
 explicit sensitivity information.  All explicit IP sensitivity
 information MUST be authenticated using either ESP, AH, or both.
 Encryption is useful and can be desirable even when all of the hosts
 are within a protected environment, for example, behind a firewall or
 disjoint from any external connectivity.  ESP can be used, in
 conjunction with appropriate key management and encryption
 algorithms, in support of both DAC and MAC.  (The choice of
 encryption and authentication algorithms, and the assurance level of
 an IPsec implementation will determine the environments in which an
 implementation may be deemed sufficient to satisfy MLS requirements.)
 Key management can make use of sensitivity information to provide
 MAC.  IPsec implementations on systems claiming to provide MLS SHOULD
 be capable of using IPsec to provide MAC for IP-based communications.

8.1 Relationship Between Security Associations and Data Sensitivity

 Both the Encapsulating Security Payload and the Authentication Header
 can be combined with appropriate Security Association policies to
 provide multi-level secure networking.  In this case each SA (or SA
 bundle) is normally used for only a single instance of sensitivity
 information.  For example, "PROPRIETARY - Internet Engineering" must
 be associated with a different SA (or SA bundle) from "PROPRIETARY -
 Finance".

8.2 Sensitivity Consistency Checking

 An MLS implementation (both host and router) MAY associate
 sensitivity information, or a range of sensitivity information with
 an interface, or a configured IP address with its associated prefix
 (the latter is sometimes referred to as a logical interface, or an

Kent & Atkinson Standards Track [Page 40] RFC 2401 Security Architecture for IP November 1998

 interface alias).  If such properties exist, an implementation SHOULD
 compare the sensitivity information associated with the packet
 against the sensitivity information associated with the interface or
 address/prefix from which the packet arrived, or through which the
 packet will depart.  This check will either verify that the
 sensitivities match, or that the packet's sensitivity falls within
 the range of the interface or address/prefix.
 The checking SHOULD be done on both inbound and outbound processing.

8.3 Additional MLS Attributes for Security Association Databases

 Section 4.4 discussed two Security Association databases (the
 Security Policy Database (SPD) and the Security Association Database
 (SAD)) and the associated policy selectors and SA attributes.  MLS
 networking introduces an additional selector/attribute:
  1. Sensitivity information.
 The Sensitivity information aids in selecting the appropriate
 algorithms and key strength, so that the traffic gets a level of
 protection appropriate to its importance or sensitivity as described
 in section 8.1.  The exact syntax of the sensitivity information is
 implementation defined.

8.4 Additional Inbound Processing Steps for MLS Networking

 After an inbound packet has passed through IPsec processing, an MLS
 implementation SHOULD first check the packet's sensitivity (as
 defined by the SA (or SA bundle) used for the packet) with the
 interface or address/prefix as described in section 8.2 before
 delivering the datagram to an upper-layer protocol or forwarding it.
 The MLS system MUST retain the binding between the data received in
 an IPsec protected packet and the sensitivity information in the SA
 or SAs used for processing, so appropriate policy decisions can be
 made when delivering the datagram to an application or forwarding
 engine.  The means for maintaining this binding are implementation
 specific.

8.5 Additional Outbound Processing Steps for MLS Networking

 An MLS implementation of IPsec MUST perform two additional checks
 besides the normal steps detailed in section 5.1.1.  When consulting
 the SPD or the SAD to find an outbound security association, the MLS
 implementation MUST use the sensitivity of the data to select an

Kent & Atkinson Standards Track [Page 41] RFC 2401 Security Architecture for IP November 1998

 appropriate outbound SA or SA bundle.  The second check comes before
 forwarding the packet out to its destination, and is the sensitivity
 consistency checking described in section 8.2.

8.6 Additional MLS Processing for Security Gateways

 An MLS security gateway MUST follow the previously mentioned inbound
 and outbound processing rules as well as perform some additional
 processing specific to the intermediate protection of packets in an
 MLS environment.
 A security gateway MAY act as an outbound proxy, creating SAs for MLS
 systems that originate packets forwarded by the gateway.  These MLS
 systems may explicitly label the packets to be forwarded, or the
 whole originating network may have sensitivity characteristics
 associated with it.  The security gateway MUST create and use
 appropriate SAs for AH, ESP, or both, to protect such traffic it
 forwards.
 Similarly such a gateway SHOULD accept and process inbound AH and/or
 ESP packets and forward appropriately, using explicit packet
 labeling, or relying on the sensitivity characteristics of the
 destination network.

9. Performance Issues

 The use of IPsec imposes computational performance costs on the hosts
 or security gateways that implement these protocols.  These costs are
 associated with the memory needed for IPsec code and data structures,
 and the computation of integrity check values, encryption and
 decryption, and added per-packet handling.  The per-packet
 computational costs will be manifested by increased latency and,
 possibly, reduced throughout.  Use of SA/key management protocols,
 especially ones that employ public key cryptography, also adds
 computational performance costs to use of IPsec.  These per-
 association computational costs will be manifested in terms of
 increased latency in association establishment.  For many hosts, it
 is anticipated that software-based cryptography will not appreciably
 reduce throughput, but hardware may be required for security gateways
 (since they represent aggregation points), and for some hosts.
 The use of IPsec also imposes bandwidth utilization costs on
 transmission, switching, and routing components of the Internet
 infrastructure, components not implementing IPsec.  This is due to
 the increase in the packet size resulting from the addition of AH
 and/or ESP headers, AH and ESP tunneling (which adds a second IP
 header), and the increased packet traffic associated with key
 management protocols.  It is anticipated that, in most instances,

Kent & Atkinson Standards Track [Page 42] RFC 2401 Security Architecture for IP November 1998

 this increased bandwidth demand will not noticeably affect the
 Internet infrastructure.  However, in some instances, the effects may
 be significant, e.g., transmission of ESP encrypted traffic over a
 dialup link that otherwise would have compressed the traffic.
 Note: The initial SA establishment overhead will be felt in the first
 packet.  This delay could impact the transport layer and application.
 For example, it could cause TCP to retransmit the SYN before the
 ISAKMP exchange is done.  The effect of the delay would be different
 on UDP than TCP because TCP shouldn't transmit anything other than
 the SYN until the connection is set up whereas UDP will go ahead and
 transmit data beyond the first packet.
 Note: As discussed earlier, compression can still be employed at
 layers above IP.  There is an IETF working group (IP Payload
 Compression Protocol (ippcp)) working on "protocol specifications
 that make it possible to perform lossless compression on individual
 payloads before the payload is processed by a protocol that encrypts
 it. These specifications will allow for compression operations to be
 performed prior to the encryption of a payload by IPsec protocols."

10. Conformance Requirements

 All IPv4 systems that claim to implement IPsec MUST comply with all
 requirements of the Security Architecture document.  All IPv6 systems
 MUST comply with all requirements of the Security Architecture
 document.

11. Security Considerations

 The focus of this document is security; hence security considerations
 permeate this specification.

12. Differences from RFC 1825

 This architecture document differs substantially from RFC 1825 in
 detail and in organization, but the fundamental notions are
 unchanged.  This document provides considerable additional detail in
 terms of compliance specifications.  It introduces the SPD and SAD,
 and the notion of SA selectors.  It is aligned with the new versions
 of AH and ESP, which also differ from their predecessors.  Specific
 requirements for supported combinations of AH and ESP are newly
 added, as are details of PMTU management.

Kent & Atkinson Standards Track [Page 43] RFC 2401 Security Architecture for IP November 1998

Acknowledgements

 Many of the concepts embodied in this specification were derived from
 or influenced by the US Government's SP3 security protocol, ISO/IEC's
 NLSP, the proposed swIPe security protocol [SDNS, ISO, IB93, IBK93],
 and the work done for SNMP Security and SNMPv2 Security.
 For over 3 years (although it sometimes seems *much* longer), this
 document has evolved through multiple versions and iterations.
 During this time, many people have contributed significant ideas and
 energy to the process and the documents themselves.  The authors
 would like to thank Karen Seo for providing extensive help in the
 review, editing, background research, and coordination for this
 version of the specification.  The authors would also like to thank
 the members of the IPsec and IPng working groups, with special
 mention of the efforts of (in alphabetic order): Steve Bellovin,
 Steve Deering, James Hughes, Phil Karn, Frank Kastenholz, Perry
 Metzger, David Mihelcic, Hilarie Orman, Norman Shulman, William
 Simpson, Harry Varnis, and Nina Yuan.

Kent & Atkinson Standards Track [Page 44] RFC 2401 Security Architecture for IP November 1998

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., [VK83, HA94].  Included in this glossary are generic security
 service and security mechanism terms, plus IPsec-specific terms.
   Access Control
      Access control is 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
      This term is 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
      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
      Confidentiality is 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.)

Kent & Atkinson Standards Track [Page 45] RFC 2401 Security Architecture for IP November 1998

   Encryption
      Encryption is 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".  Oftimes the
      term "encryption" is used to generically refer to both
      processes.
   Data Origin Authentication
      Data origin authentication is a security service that verifies
      the identity of the claimed source of data.  This service is
      usually bundled with connectionless integrity service.
   Integrity
      Integrity is 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.
   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.
   Security Gateway
      A security gateway is 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
      viewed as untrusted (or less trusted), while the networks and
      hosts and on the internal side are viewed as trusted (or more
      trusted).  The internal subnets and hosts served by a security
      gateway are presumed to be trusted by virtue of sharing a
      common, local, security administration.  (See "Trusted
      Subnetwork" below.) In the IPsec context, a security gateway is
      a point at which AH and/or ESP is implemented in order to serve

Kent & Atkinson Standards Track [Page 46] RFC 2401 Security Architecture for IP November 1998

      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).
   SPI
      Acronym for "Security Parameters Index".  The combination of a
      destination address, a security protocol, and an SPI uniquely
      identifies a security association (SA, see above).  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, flow identifiers, etc.
      [Sch94]
   Trusted Subnetwork
      A subnetwork containing hosts and routers that trust each other
      not to engage in active or passive attacks.  There also is an
      assumption that the underlying communications channel (e.g., a
      LAN or CAN) isn't being attacked by other means.

Kent & Atkinson Standards Track [Page 47] RFC 2401 Security Architecture for IP November 1998

Appendix B – Analysis/Discussion of PMTU/DF/Fragmentation Issues

B.1 DF bit

 In cases where a system (host or gateway) adds an encapsulating
 header (e.g., ESP tunnel), should/must the DF bit in the original
 packet be copied to the encapsulating header?
 Fragmenting seems correct for some situations, e.g., it might be
 appropriate to fragment packets over a network with a very small MTU,
 e.g., a packet radio network, or a cellular phone hop to mobile node,
 rather than propagate back a very small PMTU for use over the rest of
 the path.  In other situations, it might be appropriate to set the DF
 bit in order to get feedback from later routers about PMTU
 constraints which require fragmentation.  The existence of both of
 these situations argues for enabling a system to decide whether or
 not to fragment over a particular network "link", i.e., for requiring
 an implementation to be able to copy the DF bit (and to process ICMP
 PMTU messages), but making it an option to be selected on a per
 interface basis.  In other words, an administrator should be able to
 configure the router's treatment of the DF bit (set, clear, copy from
 encapsulated header) for each interface.
 Note: If a bump-in-the-stack implementation of IPsec attempts to
 apply different IPsec algorithms based on source/destination ports,
 it will be difficult to apply Path MTU adjustments.

B.2 Fragmentation

 If required, IP fragmentation occurs after IPsec processing within an
 IPsec implementation.  Thus, transport mode AH or ESP is applied only
 to whole IP datagrams (not to IP fragments).  An IP packet to which
 AH or ESP has been applied may itself be fragmented by routers en
 route, and such fragments MUST be reassembled prior to IPsec
 processing at a receiver.  In tunnel mode, AH or ESP is applied to an
 IP packet, the payload of which may be a fragmented IP packet.  For
 example, a security gateway, "bump-in-the-stack" (BITS), or "bump-
 in-the-wire" (BITW) IPsec implementation may apply tunnel mode AH to
 such fragments.  Note that BITS or BITW implementations are examples
 of where a host IPsec implementation might receive fragments to which
 tunnel mode is to be applied.  However, if transport mode is to be
 applied, then these implementations MUST reassemble the fragments
 prior to applying IPsec.

Kent & Atkinson Standards Track [Page 48] RFC 2401 Security Architecture for IP November 1998

 NOTE: IPsec always has to figure out what the encapsulating IP header
 fields are.  This is independent of where you insert IPsec and is
 intrinsic to the definition of IPsec.  Therefore any IPsec
 implementation that is not integrated into an IP implementation must
 include code to construct the necessary IP headers (e.g., IP2):
      o AH-tunnel --> IP2-AH-IP1-Transport-Data
      o ESP-tunnel -->  IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer
 Overall, the fragmentation/reassembly approach described above works
 for all cases examined.
                            AH Xport   AH Tunnel  ESP Xport  ESP Tunnel

Implementation approach IPv4 IPv6 IPv4 IPv6 IPv4 IPv6 IPv4 IPv6 ———————– —- —- —- —- —- —- —- —- Hosts (integr w/ IP stack) Y Y Y Y Y Y Y Y Hosts (betw/ IP and drivers) Y Y Y Y Y Y Y Y S. Gwy (integr w/ IP stack) Y Y Y Y Outboard crypto processor *

  • If the crypto processor system has its own IP address, then it

is covered by the security gateway case. This box receives

        the packet from the host and performs IPsec processing.  It
        has to be able to handle the same AH, ESP, and related
        IPv4/IPv6 tunnel processing that a security gateway would have
        to handle.  If it doesn't have it's own address, then it is
        similar to the bump-in-the stack implementation between IP and
        the network drivers.
 The following analysis assumes that:
      1. There is only one IPsec module in a given system's stack.
         There isn't an IPsec module A (adding ESP/encryption and
         thus) hiding the transport protocol, SRC port, and DEST port
         from IPsec module B.
      2. There are several places where IPsec could be implemented (as
         shown in the table above).
              a. Hosts with integration of IPsec into the native IP
                 implementation.  Implementer has access to the source
                 for the stack.
              b. Hosts with bump-in-the-stack implementations, where
                 IPsec is implemented between IP and the local network
                 drivers.  Source access for stack is not available;
                 but there are well-defined interfaces that allows the
                 IPsec code to be incorporated into the system.

Kent & Atkinson Standards Track [Page 49] RFC 2401 Security Architecture for IP November 1998

              c. Security gateways and outboard crypto processors with
                 integration of IPsec into the stack.
      3. Not all of the above approaches are feasible in all hosts.
         But it was assumed that for each approach, there are some
         hosts for whom the approach is feasible.
 For each of the above 3 categories, there are IPv4 and IPv6, AH
 transport and tunnel modes, and ESP transport and tunnel modes -- for
 a total of 24 cases (3 x 2 x 4).
 Some header fields and interface fields are listed here for ease of
 reference -- they're not in the header order, but instead listed to
 allow comparison between the columns.  (* = not covered by AH
 authentication.  ESP authentication doesn't cover any headers that
 precede it.)
                                           IP/Transport Interface
           IPv4            IPv6            (RFC 1122 -- Sec 3.4)
           ----            ----            ----------------------
           Version = 4     Version = 6
           Header Len
           *TOS            Class,Flow Lbl  TOS
           Packet Len      Payload Len     Len
           ID                              ID (optional)
           *Flags                          DF
           *Offset
           *TTL            *Hop Limit      TTL
           Protocol        Next Header
           *Checksum
           Src Address     Src Address     Src Address
           Dst Address     Dst Address     Dst Address
           Options?        Options?        Opt
           ? = AH covers Option-Type and Option-Length, but
               might not cover Option-Data.
 The results for each of the 20 cases is shown below ("works" = will
 work if system fragments after outbound IPsec processing, reassembles
 before inbound IPsec processing).  Notes indicate implementation
 issues.
  a. Hosts (integrated into IP stack)
        o AH-transport  --> (IP1-AH-Transport-Data)
                  - IPv4 -- works
                  - IPv6 -- works
        o AH-tunnel --> (IP2-AH-IP1-Transport-Data)
                  - IPv4 -- works
                  - IPv6 -- works

Kent & Atkinson Standards Track [Page 50] RFC 2401 Security Architecture for IP November 1998

        o ESP-transport --> (IP1-ESP_hdr-Transport-Data-ESP_trailer)
                  - IPv4 -- works
                  - IPv6 -- works
        o ESP-tunnel -->  (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer)
                  - IPv4 -- works
                  - IPv6 -- works
  b. Hosts (Bump-in-the-stack) -- put IPsec between IP layer and
     network drivers.  In this case, the IPsec module would have to do
     something like one of the following for fragmentation and
     reassembly.
          - do the fragmentation/reassembly work itself and
            send/receive the packet directly to/from the network
            layer.  In AH or ESP transport mode, this is fine.  In AH
            or ESP tunnel mode where the tunnel end is at the ultimate
            destination, this is fine.  But in AH or ESP tunnel modes
            where the tunnel end is different from the ultimate
            destination and where the source host is multi-homed, this
            approach could result in sub-optimal routing because the
            IPsec module may be unable to obtain the information
            needed (LAN interface and next-hop gateway) to direct the
            packet to the appropriate network interface.  This is not
            a problem if the interface and next-hop gateway are the
            same for the ultimate destination and for the tunnel end.
            But if they are different, then IPsec would need to know
            the LAN interface and the next-hop gateway for the tunnel
            end.  (Note: The tunnel end (security gateway) is highly
            likely to be on the regular path to the ultimate
            destination.  But there could also be more than one path
            to the destination, e.g., the host could be at an
            organization with 2 firewalls.  And the path being used
            could involve the less commonly chosen firewall.)  OR
          - pass the IPsec'd packet back to the IP layer where an
            extra IP header would end up being pre-pended and the
            IPsec module would have to check and let IPsec'd fragments
            go by.
                                  OR
          - pass the packet contents to the IP layer in a form such
            that the IP layer recreates an appropriate IP header
     At the network layer, the IPsec module will have access to the
     following selectors from the packet -- SRC address, DST address,
     Next Protocol, and if there's a transport layer header --> SRC
     port and DST port.  One cannot assume IPsec has access to the
     Name.  It is assumed that the available selector information is
     sufficient to figure out the relevant Security Policy entry and
     Security Association(s).

Kent & Atkinson Standards Track [Page 51] RFC 2401 Security Architecture for IP November 1998

        o AH-transport  --> (IP1-AH-Transport-Data)
                  - IPv4 -- works
                  - IPv6 -- works
        o AH-tunnel --> (IP2-AH-IP1-Transport-Data)
                  - IPv4 -- works
                  - IPv6 -- works
        o ESP-transport --> (IP1-ESP_hdr-Transport-Data-ESP_trailer)
                  - IPv4 -- works
                  - IPv6 -- works
        o ESP-tunnel -->  (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer)
                  - IPv4 -- works
                  - IPv6 -- works
  c. Security gateways -- integrate IPsec into the IP stack
     NOTE: The IPsec module will have access to the following
     selectors from the packet -- SRC address, DST address, Next
     Protocol, and if there's a transport layer header --> SRC port
     and DST port.  It won't have access to the User ID (only Hosts
     have access to User ID information.)  Unlike some Bump-in-the-
     stack implementations, security gateways may be able to look up
     the Source Address in the DNS to provide a System Name, e.g., in
     situations involving use of dynamically assigned IP addresses in
     conjunction with dynamically updated DNS entries.  It also won't
     have access to the transport layer information if there is an ESP
     header, or if it's not the first fragment of a fragmented
     message.  It is assumed that the available selector information
     is sufficient to figure out the relevant Security Policy entry
     and Security Association(s).
        o AH-tunnel --> (IP2-AH-IP1-Transport-Data)
                  - IPv4 -- works
                  - IPv6 -- works
        o ESP-tunnel -->  (IP2-ESP_hdr-IP1-Transport-Data-ESP_trailer)
                  - IPv4 -- works
                  - IPv6 -- works
  • *

B.3 Path MTU Discovery

 As mentioned earlier, "ICMP PMTU" refers to an ICMP message used for
 Path MTU Discovery.
 The legend for the diagrams below in B.3.1 and B.3.3 (but not B.3.2)
 is:
      ==== = security association (AH or ESP, transport or tunnel)

Kent & Atkinson Standards Track [Page 52] RFC 2401 Security Architecture for IP November 1998

  1. — = connectivity (or if so labelled, administrative boundary)

…. = ICMP message (hereafter referred to as ICMP PMTU) for

              IPv4:
              - 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 (labelled unused in RFC 792),
                with high-order 16 bits set to zero
              IPv6 (RFC 1885):
              - Type = 2 (Packet Too Big)
              - Code = 0 (Fragmentation needed and DF set)
              - Next-Hop MTU in the 32 bit MTU field of the ICMP6
      Hx   = host x
      Rx   = router x
      SGx  = security gateway x
      X*   = X supports IPsec

B.3.1 Identifying the Originating Host(s)

The amount of information returned with the ICMP message is limited and this affects what selectors are available to identify security associations, originating hosts, etc. for use in further propagating the PMTU information.

In brief… An ICMP message must contain the following information from the "offending" packet:

  1. IPv4 (RFC 792) – IP header plus a minimum of 64 bits

Accordingly, in the IPv4 context, an ICMP PMTU may identify only the first (outermost) security association. This is because the ICMP PMTU may contain only 64 bits of the "offending" packet beyond the IP header, which would capture only the first SPI from AH or ESP. In the IPv6 context, an ICMP PMTU will probably provide all the SPIs and the selectors in the IP header, but maybe not the SRC/DST ports (in the transport header) or the encapsulated (TCP, UDP, etc.) protocol. Moreover, if ESP is used, the transport ports and protocol selectors may be encrypted.

Looking at the diagram below of a security gateway tunnel (as mentioned elsewhere, security gateways do not use transport mode)…

Kent & Atkinson Standards Track [Page 53] RFC 2401 Security Architecture for IP November 1998

   H1   ===================           H3
     \  |                 |          /
 H0 -- SG1* ---- R1 ---- SG2* ---- R2 -- H5
     /  ^        |                   \
   H2   |........|                    H4
 Suppose that the security policy for SG1 is to use a single SA to SG2
 for all the traffic between hosts H0, H1, and H2 and hosts H3, H4,
 and H5.  And suppose H0 sends a data packet to H5 which causes R1 to
 send an ICMP PMTU message to SG1.  If the PMTU message has only the
 SPI, SG1 will be able to look up the SA and find the list of possible
 hosts (H0, H1, H2, wildcard); but SG1 will have no way to figure out
 that H0 sent the traffic that triggered the ICMP PMTU message.
    original        after IPsec     ICMP
    packet          processing      packet
    --------        -----------     ------
                                    IP-3 header (S = R1, D = SG1)
                                    ICMP header (includes PMTU)
                    IP-2 header     IP-2 header (S = SG1, D = SG2)
                    ESP header      minimum of 64 bits of ESP hdr (*)
    IP-1 header     IP-1 header
    TCP header      TCP header
    TCP data        TCP data
                    ESP trailer
    (*) The 64 bits will include enough of the ESP (or AH) header to
        include the SPI.
            - ESP -- SPI (32 bits), Seq number (32 bits)
            - AH -- Next header (8 bits), Payload Len (8 bits),
              Reserved (16 bits), SPI (32 bits)
 This limitation on the amount of information returned with an ICMP
 message creates a problem in identifying the originating hosts for
 the packet (so as to know where to further propagate the ICMP PMTU
 information).  If the ICMP message contains only 64 bits of the IPsec
 header (minimum for IPv4), then the IPsec selectors (e.g., Source and
 Destination addresses, Next Protocol, Source and Destination ports,
 etc.) will have been lost.  But the ICMP error message will still
 provide SG1 with the SPI, the PMTU information and the source and
 destination gateways for the relevant security association.
 The destination security gateway and SPI uniquely define a security
 association which in turn defines a set of possible originating
 hosts.  At this point, SG1 could:

Kent & Atkinson Standards Track [Page 54] RFC 2401 Security Architecture for IP November 1998

 a. send the PMTU information to all the possible originating hosts.
    This would not work well if the host list is a wild card or if
    many/most of the hosts weren't sending to SG1; but it might work
    if the SPI/destination/etc mapped to just one or a small number of
    hosts.
 b. store the PMTU with the SPI/etc and wait until the next packet(s)
    arrive from the originating host(s) for the relevant security
    association.  If it/they are bigger than the PMTU, drop the
    packet(s), and compose ICMP PMTU message(s) with the new packet(s)
    and the updated PMTU, and send the originating host(s) the ICMP
    message(s) about the problem.  This involves a delay in notifying
    the originating host(s), but avoids the problems of (a).
 Since only the latter approach is feasible in all instances, a
 security gateway MUST provide such support, as an option.  However,
 if the ICMP message contains more information from the original
 packet, then there may be enough information to immediately determine
 to which host to propagate the ICMP/PMTU message and to provide that
 system with the 5 fields (source address, destination address, source
 port, destination port, and transport protocol) needed to determine
 where to store/update the PMTU.  Under such circumstances, a security
 gateway MUST generate an ICMP PMTU message immediately upon receipt
 of an ICMP PMTU from further down the path.  NOTE: The Next Protocol
 field may not be contained in the ICMP message and the use of ESP
 encryption may hide the selector fields that have been encrypted.

B.3.2 Calculation of PMTU

 The calculation of PMTU from an ICMP PMTU has to take into account
 the addition of any IPsec header by H1 -- AH and/or ESP transport, or
 ESP or AH tunnel.  Within a single host, multiple applications may
 share an SPI and nesting of security associations may occur.  (See
 Section 4.5 Basic Combinations of Security Associations for
 description of the combinations that MUST be supported).  The diagram
 below illustrates an example of security associations between a pair
 of hosts (as viewed from the perspective of one of the hosts.)  (ESPx
 or AHx = transport mode)
         Socket 1 -------------------------|
                                           |
         Socket 2 (ESPx/SPI-A) ---------- AHx (SPI-B) -- Internet
 In order to figure out the PMTU for each socket that maps to SPI-B,
 it will be necessary to have backpointers from SPI-B to each of the 2
 paths that lead to it -- Socket 1 and Socket 2/SPI-A.

Kent & Atkinson Standards Track [Page 55] RFC 2401 Security Architecture for IP November 1998

B.3.3 Granularity of Maintaining PMTU Data

 In hosts, the granularity with which PMTU ICMP processing can be done
 differs depending on the implementation situation.  Looking at a
 host, there are three situations that are of interest with respect to
 PMTU issues:
 a. Integration of IPsec into the native IP implementation
 b. Bump-in-the-stack implementations, where IPsec is implemented
    "underneath" an existing implementation of a TCP/IP protocol
    stack, between the native IP and the local network drivers
 c. No IPsec implementation -- This case is included because it is
    relevant in cases where a security gateway is sending PMTU
    information back to a host.
 Only in case (a) can the PMTU data be maintained at the same
 granularity as communication associations.  In the other cases, the
 IP layer will maintain PMTU data at the granularity of Source and
 Destination IP addresses (and optionally TOS/Class), as described in
 RFC 1191.  This is an important difference, because more than one
 communication association may map to the same source and destination
 IP addresses, and each communication association may have a different
 amount of IPsec header overhead (e.g., due to use of different
 transforms or different algorithms).  The examples below illustrate
 this.
 In cases (a) and (b)...  Suppose you have the following situation.
 H1 is sending to H2 and the packet to be sent from R1 to R2 exceeds
 the PMTU of the network hop between them.
               ==================================
               |                                |
              H1* --- R1 ----- R2 ---- R3 ---- H2*
               ^       |
               |.......|
 If R1 is configured to not fragment subscriber traffic, then R1 sends
 an ICMP PMTU message with the appropriate PMTU to H1.  H1's
 processing would vary with the nature of the implementation.  In case
 (a) (native IP), the security services are bound to sockets or the
 equivalent.  Here the IP/IPsec implementation in H1 can store/update
 the PMTU for the associated socket.  In case (b), the IP layer in H1
 can store/update the PMTU but only at the granularity of Source and
 Destination addresses and possibly TOS/Class, as noted above.  So the
 result may be sub-optimal, since the PMTU for a given
 SRC/DST/TOS/Class will be the subtraction of the largest amount of
 IPsec header used for any communication association between a given
 source and destination.

Kent & Atkinson Standards Track [Page 56] RFC 2401 Security Architecture for IP November 1998

 In case (c), there has to be a security gateway to have any IPsec
 processing.  So suppose you have the following situation.  H1 is
 sending to H2 and the packet to be sent from SG1 to R exceeds the
 PMTU of the network hop between them.
                       ================
                       |              |
              H1 ---- SG1* --- R --- SG2* ---- H2
               ^       |
               |.......|
 As described above for case (b), the IP layer in H1 can store/update
 the PMTU but only at the granularity of Source and Destination
 addresses, and possibly TOS/Class.  So the result may be sub-optimal,
 since the PMTU for a given SRC/DST/TOS/Class will be the subtraction
 of the largest amount of IPsec header used for any communication
 association between a given source and destination.

B.3.4 Per Socket Maintenance of PMTU Data

 Implementation of the calculation of PMTU (Section B.3.2) and support
 for PMTUs at the granularity of individual "communication
 associations" (Section B.3.3) is a local matter.  However, a socket-
 based implementation of IPsec in a host SHOULD maintain the
 information on a per socket basis.  Bump in the stack systems MUST
 pass an ICMP PMTU to the host IP implementation, after adjusting it
 for any IPsec header overhead added by these systems.  The
 determination of the overhead SHOULD be determined by analysis of the
 SPI and any other selector information present in a returned ICMP
 PMTU message.

B.3.5 Delivery of PMTU Data to the Transport Layer

 The host mechanism for getting the updated PMTU to the transport
 layer is unchanged, as specified in RFC 1191 (Path MTU Discovery).

B.3.6 Aging of PMTU Data

 This topic is covered in Section 6.1.2.4.

Kent & Atkinson Standards Track [Page 57] RFC 2401 Security Architecture for IP November 1998

Appendix C – Sequence Space Window Code Example

 This appendix contains a routine that implements a bitmask check for
 a 32 packet window.  It was provided by James Hughes
 (jim_hughes@stortek.com) and Harry Varnis (hgv@anubis.network.com)
 and is intended as an implementation example.  Note that this code
 both checks for a replay and updates the window.  Thus the algorithm,
 as shown, should only be called AFTER the packet has been
 authenticated.  Implementers might wish to consider splitting the
 code to do the check for replays before computing the ICV.  If the
 packet is not a replay, the code would then compute the ICV, (discard
 any bad packets), and if the packet is OK, update the window.

#include <stdio.h> #include <stdlib.h> typedef unsigned long u_long;

enum {

  ReplayWindowSize = 32

};

u_long bitmap = 0; /* session state - must be 32 bits */ u_long lastSeq = 0; /* session state */

/* Returns 0 if packet disallowed, 1 if packet permitted */ int ChkReplayWindow(u_long seq);

int ChkReplayWindow(u_long seq) {

  u_long diff;
  if (seq == 0) return 0;             /* first == 0 or wrapped */
  if (seq > lastSeq) {                /* new larger sequence number */
      diff = seq - lastSeq;
      if (diff < ReplayWindowSize) {  /* In window */
          bitmap <<= diff;
          bitmap |= 1;                /* set bit for this packet */
      } else bitmap = 1;          /* This packet has a "way larger" */
      lastSeq = seq;
      return 1;                       /* larger is good */
  }
  diff = lastSeq - seq;
  if (diff >= ReplayWindowSize) return 0; /* too old or wrapped */
  if (bitmap & ((u_long)1 << diff)) return 0; /* already seen */
  bitmap |= ((u_long)1 << diff);              /* mark as seen */
  return 1;                           /* out of order but good */

}

char string_buffer[512];

Kent & Atkinson Standards Track [Page 58] RFC 2401 Security Architecture for IP November 1998

#define STRING_BUFFER_SIZE sizeof(string_buffer)

int main() {

  int result;
  u_long last, current, bits;
  printf("Input initial state (bits in hex, last msgnum):\n");
  if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) exit(0);
  sscanf(string_buffer, "%lx %lu", &bits, &last);
  if (last != 0)
  bits |= 1;
  bitmap = bits;
  lastSeq = last;
  printf("bits:%08lx last:%lu\n", bitmap, lastSeq);
  printf("Input value to test (current):\n");
  while (1) {
      if (!fgets(string_buffer, STRING_BUFFER_SIZE, stdin)) break;
      sscanf(string_buffer, "%lu", &current);
      result = ChkReplayWindow(current);
      printf("%-3s", result ? "OK" : "BAD");
      printf(" bits:%08lx last:%lu\n", bitmap, lastSeq);
  }
  return 0;

}

Kent & Atkinson Standards Track [Page 59] RFC 2401 Security Architecture for IP November 1998

Appendix D – Categorization of ICMP messages

The tables below characterize ICMP messages as being either host generated, router generated, both, unassigned/unknown. The first set are IPv4. The second set are IPv6.

                              IPv4

Type Name/Codes Reference

HOST GENERATED:

3     Destination Unreachable
       2  Protocol Unreachable                               [RFC792]
       3  Port Unreachable                                   [RFC792]
       8  Source Host Isolated                               [RFC792]
      14  Host Precedence Violation                          [RFC1812]

10 Router Selection [RFC1256]

Type Name/Codes Reference

ROUTER GENERATED:

3     Destination Unreachable
       0  Net Unreachable                                    [RFC792]
       4  Fragmentation Needed, Don't Fragment was Set       [RFC792]
       5  Source Route Failed                                [RFC792]
       6  Destination Network Unknown                        [RFC792]
       7  Destination Host Unknown                           [RFC792]
       9  Comm. w/Dest. Net. is Administratively Prohibited  [RFC792]
      11  Destination Network Unreachable for Type of Service[RFC792]
5     Redirect
       0  Redirect Datagram for the Network (or subnet)      [RFC792]
       2  Redirect Datagram for the Type of Service & Network[RFC792]
9     Router Advertisement                                   [RFC1256]

18 Address Mask Reply [RFC950]

Kent & Atkinson Standards Track [Page 60] RFC 2401 Security Architecture for IP November 1998

                              IPv4

Type Name/Codes Reference

BOTH ROUTER AND HOST GENERATED:

0     Echo Reply                                             [RFC792]
3     Destination Unreachable
       1  Host Unreachable                                   [RFC792]
      10  Comm. w/Dest. Host is Administratively Prohibited  [RFC792]
      12  Destination Host Unreachable for Type of Service   [RFC792]
      13  Communication Administratively Prohibited          [RFC1812]
      15  Precedence cutoff in effect                        [RFC1812]
4     Source Quench                                          [RFC792]
5     Redirect
       1  Redirect Datagram for the Host                     [RFC792]
       3  Redirect Datagram for the Type of Service and Host [RFC792]
6     Alternate Host Address                                 [JBP]
8     Echo                                                   [RFC792]

11 Time Exceeded [RFC792] 12 Parameter Problem [RFC792,RFC1108] 13 Timestamp [RFC792] 14 Timestamp Reply [RFC792] 15 Information Request [RFC792] 16 Information Reply [RFC792] 17 Address Mask Request [RFC950] 30 Traceroute [RFC1393] 31 Datagram Conversion Error [RFC1475] 32 Mobile Host Redirect [Johnson] 39 SKIP [Markson] 40 Photuris [Simpson]

Type Name/Codes Reference

UNASSIGNED TYPE OR UNKNOWN GENERATOR:

1     Unassigned                                             [JBP]
2     Unassigned                                             [JBP]
7     Unassigned                                             [JBP]

19 Reserved (for Security) [Solo] 20-29 Reserved (for Robustness Experiment) [ZSu] 33 IPv6 Where-Are-You [Simpson] 34 IPv6 I-Am-Here [Simpson] 35 Mobile Registration Request [Simpson] 36 Mobile Registration Reply [Simpson] 37 Domain Name Request [Simpson] 38 Domain Name Reply [Simpson] 41-255 Reserved [JBP]

Kent & Atkinson Standards Track [Page 61] RFC 2401 Security Architecture for IP November 1998

                              IPv6

Type Name/Codes Reference

HOST GENERATED:

1     Destination Unreachable                                [RFC 1885]
       4  Port Unreachable

Type Name/Codes Reference

ROUTER GENERATED:

1     Destination Unreachable                                [RFC1885]
       0  No Route to Destination
       1  Comm. w/Destination is Administratively Prohibited
       2  Not a Neighbor
       3  Address Unreachable
2     Packet Too Big                                         [RFC1885]
       0
3     Time Exceeded                                          [RFC1885]
       0  Hop Limit Exceeded in Transit
       1  Fragment reassembly time exceeded

Type Name/Codes Reference

BOTH ROUTER AND HOST GENERATED:

4     Parameter Problem                                      [RFC1885]
       0  Erroneous Header Field Encountered
       1  Unrecognized Next Header Type Encountered
       2  Unrecognized IPv6 Option Encountered

Kent & Atkinson Standards Track [Page 62] RFC 2401 Security Architecture for IP November 1998

References

 [BL73]    Bell, D.E. & LaPadula, L.J., "Secure Computer Systems:
           Mathematical Foundations and Model", Technical Report M74-
           244, The MITRE Corporation, Bedford, MA, May 1973.
 [Bra97]   Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Level", BCP 14, RFC 2119, March 1997.
 [DoD85]   US National Computer Security Center, "Department of
           Defense Trusted Computer System Evaluation Criteria", DoD
           5200.28-STD, US Department of Defense, Ft. Meade, MD.,
           December 1985.
 [DoD87]   US National Computer Security Center, "Trusted Network
           Interpretation of the Trusted Computer System Evaluation
           Criteria", NCSC-TG-005, Version 1, US Department of
           Defense, Ft. Meade, MD., 31 July 1987.
 [HA94]    Haller, N., and R. Atkinson, "On Internet Authentication",
           RFC 1704, October 1994.
 [HC98]    Harkins, D., and D. Carrel, "The Internet Key Exchange
           (IKE)", RFC 2409, November 1998.
 [HM97]    Harney, H., and C.  Muckenhirn, "Group Key Management
           Protocol (GKMP) Architecture", RFC 2094, July 1997.
 [ISO]     ISO/IEC JTC1/SC6, Network Layer Security Protocol, ISO-IEC
           DIS 11577, International Standards Organisation, Geneva,
           Switzerland, 29 November 1992.
 [IB93]    John Ioannidis and Matt Blaze, "Architecture and
           Implementation of Network-layer Security Under Unix",
           Proceedings of USENIX Security Symposium, Santa Clara, CA,
           October 1993.
 [IBK93]   John Ioannidis, Matt Blaze, & Phil Karn, "swIPe: Network-
           Layer Security for IP", presentation at the Spring 1993
           IETF Meeting, Columbus, Ohio
 [KA98a]   Kent, S., and R. Atkinson, "IP Authentication Header", RFC
           2402, November 1998.
 [KA98b]   Kent, S., and R. Atkinson, "IP Encapsulating Security
           Payload (ESP)", RFC 2406, November 1998.

Kent & Atkinson Standards Track [Page 63] RFC 2401 Security Architecture for IP November 1998

 [Ken91]   Kent, S., "US DoD Security Options for the Internet
           Protocol", RFC 1108, November 1991.
 [MSST97]  Maughan, D., Schertler, M., Schneider, M., and J. Turner,
           "Internet Security Association and Key Management Protocol
           (ISAKMP)", RFC 2408, November 1998.
 [Orm97]   Orman, H., "The OAKLEY Key Determination Protocol", RFC
           2412, November 1998.
 [Pip98]   Piper, D., "The Internet IP Security Domain of
           Interpretation for ISAKMP", RFC 2407, November 1998.
 [Sch94]   Bruce Schneier, Applied Cryptography, Section 8.6, John
           Wiley & Sons, New York, NY, 1994.
 [SDNS]    SDNS Secure Data Network System, Security Protocol 3, SP3,
           Document SDN.301, Revision 1.5, 15 May 1989, published in
           NIST Publication NIST-IR-90-4250, February 1990.
 [SMPT98]  Shacham, A., Monsour, R., Pereira, R., and M. Thomas, "IP
           Payload Compression Protocol (IPComp)", RFC 2393, August
           1998.
 [TDG97]   Thayer, R., Doraswamy, N., and R. Glenn, "IP Security
           Document Roadmap", RFC 2411, November 1998.
 [VK83]    V.L. Voydock & S.T. Kent, "Security Mechanisms in High-
           level Networks", ACM Computing Surveys, Vol. 15, No. 2,
           June 1983.

Disclaimer

 The views and specification expressed in this document are those of
 the authors and are not necessarily those of their employers.  The
 authors and their employers specifically disclaim responsibility for
 any problems arising from correct or incorrect implementation or use
 of this design.

Kent & Atkinson Standards Track [Page 64] RFC 2401 Security Architecture for IP November 1998

Author Information

 Stephen Kent
 BBN Corporation
 70 Fawcett Street
 Cambridge, MA  02140
 USA
 Phone: +1 (617) 873-3988
 EMail: kent@bbn.com
 Randall Atkinson
 @Home Network
 425 Broadway
 Redwood City, CA 94063
 USA
 Phone: +1 (415) 569-5000
 EMail: rja@corp.home.net

Kent & Atkinson Standards Track [Page 65] RFC 2401 Security Architecture for IP November 1998

Copyright (C) The Internet Society (1998). All Rights Reserved.

 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Kent & Atkinson Standards Track [Page 66]

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