rfc1826.txt

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Network Working Group                                        R. Atkinson
Request for Comments: 1826                     Naval Research Laboratory
Category: Standards Track                                    August 1995


                        IP Authentication Header

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.

ABSTRACT

   This document describes a mechanism for providing cryptographic
   authentication for IPv4 and IPv6 datagrams.  An Authentication Header
   (AH) is normally inserted after an IP header and before the other
   information being authenticated.

1. INTRODUCTION

   The Authentication Header is a mechanism for providing strong
   integrity and authentication for IP datagrams.  It might also provide
   non-repudiation, depending on which cryptographic algorithm is used
   and how keying is performed.  For example, use of an asymmetric
   digital signature algorithm, such as RSA, could provide non-
   repudiation.

   Confidentiality, and protection from traffic analysis are not
   provided by the Authentication Header.  Users desiring
   confidentiality should consider using the IP Encapsulating Security
   Protocol (ESP) either in lieu of or in conjunction with the
   Authentication Header [Atk95b].  This document assumes the reader has
   previously read the related IP Security Architecture document which
   defines the overall security architecture for IP and provides
   important background information for this specification [Atk95a].

1.1 Overview

   The IP Authentication Header seeks to provide security by adding
   authentication information to an IP datagram. This authentication
   information is calculated using all of the fields in the IP datagram
   (including not only the IP Header but also other headers and the user
   data) which do not change in transit.  Fields or options which need
   to change in transit (e.g., "hop count", "time to live", "ident",



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RFC 1826                IP Authentication Header             August 1995


   "fragment offset", or "routing pointer") are considered to be zero
   for the calculation of the authentication data.  This provides
   significantly more security than is currently present in IPv4 and
   might be sufficient for the needs of many users.

   Use of this specification will increase the IP protocol processing
   costs in participating end systems and will also increase the
   communications latency.  The increased latency is primarily due to
   the calculation of the authentication data by the sender and the
   calculation and comparison of the authentication data by the receiver
   for each IP datagram containing an Authentication Header.  The impact
   will vary with authentication algorithm used and other factors.

   In order for the Authentication Header to work properly without
   changing the entire Internet infrastructure, the authentication data
   is carried in its own payload.  Systems that aren't participating in
   the authentication MAY ignore the Authentication Data.  When used
   with IPv6, the Authentication Header is normally placed after the
   Fragmentation and End-to-End headers and before the ESP and
   transport-layer headers.  The information in the other IP headers is
   used to route the datagram from origin to destination.  When used
   with IPv4, the Authentication Header immediately follows an IPv4
   header.

   If a symmetric authentication algorithm is used and intermediate
   authentication is desired, then the nodes performing such
   intermediate authentication would need to be provided with the
   appropriate keys.  Possession of those keys would permit any one of
   those systems to forge traffic claiming to be from the legitimate
   sender to the legitimate receiver or to modify the contents of
   otherwise legitimate traffic.  In some environments such intermediate
   authentication might be desirable [BCCH94].  If an asymmetric
   authentication algorithm is used and the routers are aware of the
   appropriate public keys and authentication algorithm, then the
   routers possessing the authentication public key could authenticate
   the traffic being handled without being able to forge or modify
   otherwise legitimate traffic.  Also, Path MTU Discovery MUST be used
   when intermediate authentication of the Authentication Header is
   desired and IPv4 is in use because with this method it is not
   possible to authenticate a fragment of a packet [MD90] [Kno93].











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RFC 1826                IP Authentication Header             August 1995


1.2 Requirements Terminology

   In this document, the words that are used to define the significance
   of each particular requirement are usually capitalised.  These words
   are:

   - MUST

      This word or the adjective "REQUIRED" means that the item is an
      absolute requirement of the specification.

   - SHOULD

      This word or the adjective "RECOMMENDED" means that there might
      exist valid reasons in particular circumstances to ignore this
      item, but the full implications should be understood and the case
      carefully weighed before taking a different course.

   - MAY

      This word or the adjective "OPTIONAL" means that this item is
      truly optional.  One vendor might choose to include the item
      because a particular marketplace requires it or because it
      enhances the product, for example; another vendor may omit the
      same item.

2. KEY MANAGEMENT

   Key management is an important part of the IP security architecture.
   However, it is not integrated with this specification because of a
   long history in the public literature of subtle flaws in key
   management algorithms and protocols.  The IP Authentication Header
   tries to decouple the key management mechanisms from the security
   protocol mechanisms.  The only coupling between the key management
   protocol and the security protocol is with the Security Parameters
   Index (SPI), which is described in more detail below.  This
   decoupling permits several different key management mechanisms to be
   used.  More importantly, it permits the key management protocol to be
   changed or corrected without unduly impacting the security protocol
   implementations.

   The key management mechanism is used to negotiate a number of
   parameters for each "Security Association", including not only the
   keys but also other information (e.g., the authentication algorithm
   and mode) used by the communicating parties.  The key management
   mechanism creates and maintains a logical table containing the
   several parameters for each current security association.  An
   implementation of the IP Authentication Header will need to read that



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RFC 1826                IP Authentication Header             August 1995


   logical table of security parameters to determine how to process each
   datagram containing an Authentication Header (e.g., to determine
   which algorithm/mode and key to use in authentication).

   Security Associations are unidirectional.  A bidirectional
   communications session will normally have one Security Association in
   each direction.  For example, when a TCP session exists between two
   systems A and B, there will normally be one Security Association from
   A to B and a separate second Security Assocation from B to A.  The
   receiver assigns the SPI value to the the Security Association with
   that sender.  The other parameters of the Security Association are
   determined in a manner specified by the key management mechanism.
   Section 4 of this document describes in detail the process of
   selecting a Security Association for an outgoing packet and
   identifying the Security Assocation for an incoming packet.

   The IP Security Architecture document describes key management in
   detail.  It includes specification of the key management requirements
   for this protocol, and is incorporated here by reference [Atk95a].

3. AUTHENTICATION HEADER SYNTAX

   The Authentication Header (AH) may appear after any other headers
   which are examined at each hop, and before any other headers which
   are not examined at an intermediate hop.  The IPv4 or IPv6 header
   immediately preceding the Authentication Header will contain the
   value 51 in its Next Header (or Protocol) field [STD-2].

   Example high-level diagrams of IP datagrams with the Authentication
   Header follow.

 +------------+-------------------+------------+-------+---------------+
 | IPv6 Header| Hop-by-Hop/Routing| Auth Header| Others| Upper Protocol|
 +------------+-------------------+------------+-------+---------------+

                Figure 1: IPv6 Example















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RFC 1826                IP Authentication Header             August 1995


   When used with IPv6, the Authentication Header normally appears after
   the IPv6 Hop-by-Hop Header and before the IPv6 Destination Options.

    +-------------+--------------+-------------------------------+
    | IPv4 Header |  Auth Header | Upper Protocol (e.g. TCP, UDP)|
    +-------------+--------------+-------------------------------+

                   Figure 2:  IPv4 Example


   When used with IPv4, the Authentication Header normally follows the
   main IPv4 header.

3.1 Authentication Header Syntax