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



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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




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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







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  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



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  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



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  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





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         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.



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       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



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  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.





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  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



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  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



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            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.




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  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.





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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



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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



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             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



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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



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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.

     - 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]

     - 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]

     - 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.,
                      [email protected]
                   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".





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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]

     - Data sensitivity level: (IPSO/CIPSO labels)
       [REQUIRED for all systems providing information flow security as
       per Section 8, OPTIONAL for all other systems.]

     - 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.

     - 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.





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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



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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



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  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



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          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.





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        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]






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       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



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       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.




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  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





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  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:




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       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].



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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.



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  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



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      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).



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                       <-- 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



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             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.





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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.






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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



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          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




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       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.








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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.





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  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



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  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:

          - 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





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  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,



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  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.








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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.
































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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.)





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    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




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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.























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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.








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  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.




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               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



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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).




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         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)



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RFC 2401              Security Architecture for IP         November 1998


       ---- = 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:
       - 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)...








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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
  ([email protected]) and Harry Varnis ([email protected])
  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: [email protected]


  Randall Atkinson
  @Home Network
  425 Broadway
  Redwood City, CA 94063
  USA

  Phone: +1 (415) 569-5000
  EMail: [email protected]































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
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