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RFC 2402 - IP Authentication Header

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Network Working Group                                            S. Kent
Request for Comments: 2402                                      BBN Corp
Obsoletes: 1826                                              R. Atkinson
Category: Standards Track                                  @Home Network
                                                           November 1998

                        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.

Copyright Notice

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

Table of Contents

  1. Introduction......................................................2
  2. Authentication Header Format......................................3
     2.1 Next Header...................................................4
     2.2 Payload Length................................................4
     2.3 Reserved......................................................4
     2.4 Security Parameters Index (SPI)...............................4
     2.5 Sequence Number...............................................5
     2.6 Authentication Data ..........................................5
  3. Authentication Header Processing..................................5
     3.1  Authentication Header Location...............................5
     3.2  Authentication Algorithms....................................7
     3.3  Outbound Packet Processing...................................8
        3.3.1  Security Association Lookup.............................8
        3.3.2  Sequence Number Generation..............................8
        3.3.3  Integrity Check Value Calculation.......................9
   Handling Mutable Fields............................9
      ICV Computation for IPv4.....................10
        Base Header Fields.......................10
      ICV Computation for IPv6.....................11
        Base Header Fields.......................11
        Extension Headers Containing Options.....11
        Extension Headers Not Containing Options.11
      Authentication Data Padding..................12

      Implicit Packet Padding......................12
        3.3.4  Fragmentation..........................................12
     3.4  Inbound Packet Processing...................................13
        3.4.1  Reassembly.............................................13
        3.4.2  Security Association Lookup............................13
        3.4.3  Sequence Number Verification...........................13
        3.4.4  Integrity Check Value Verification.....................15
  4. Auditing.........................................................15
  5. Conformance Requirements.........................................16
  6. Security Considerations..........................................16
  7. Differences from RFC 1826........................................16
  Appendix A -- Mutability of IP Options/Extension Headers............18
     A1. IPv4 Options.................................................18
     A2. IPv6 Extension Headers.......................................19
  Author Information..................................................22
  Full Copyright Statement............................................22

1.  Introduction

   The IP Authentication Header (AH) is used to provide connectionless
   integrity and data origin authentication for IP datagrams (hereafter
   referred to as just "authentication"), and to provide protection
   against replays.  This latter, optional service may be selected, by
   the receiver, when a Security Association is established. (Although
   the default calls for the sender to increment the Sequence Number
   used for anti-replay, the service is effective only if the receiver
   checks the Sequence Number.)  AH provides authentication for as much
   of the IP header as possible, as well as for upper level protocol
   data.  However, some IP header fields may change in transit and the
   value of these fields, when the packet arrives at the receiver, may
   not be predictable by the sender.  The values of such fields cannot
   be protected by AH.  Thus the protection provided to the IP header by
   AH is somewhat piecemeal.

   AH may be applied alone, in combination with the IP Encapsulating
   Security Payload (ESP) [KA97b], or in a nested fashion through the
   use of tunnel mode (see "Security Architecture for the Internet
   Protocol" [KA97a], hereafter referred to as the Security Architecture
   document).  Security services can be provided between a pair of
   communicating hosts, between a pair of communicating security
   gateways, or between a security gateway and a host.  ESP may be used
   to provide the same security services, and it also provides a
   confidentiality (encryption) service.  The primary difference between
   the authentication provided by ESP and AH is the extent of the
   coverage.  Specifically, ESP does not protect any IP header fields

   unless those fields are encapsulated by ESP (tunnel mode).  For more
   details on how to use AH and ESP in various network environments, see
   the Security Architecture document [KA97a].

   It is assumed that the reader is familiar with the terms and concepts
   described in the Security Architecture document.  In particular, the
   reader should be familiar with the definitions of security services
   offered by AH and ESP, the concept of Security Associations, the ways
   in which AH can be used in conjunction with ESP, and the different
   key management options available for AH and ESP.  (With regard to the
   last topic, the current key management options required for both AH
   and ESP are manual keying and automated keying via IKE [HC98].)

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in RFC 2119 [Bra97].

2.  Authentication Header Format

   The protocol header (IPv4, IPv6, or Extension) immediately preceding
   the AH header will contain the value 51 in its Protocol (IPv4) or
   Next Header (IPv6, Extension) field [STD-2].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   | Next Header   |  Payload Len  |          RESERVED             |
   |                 Security Parameters Index (SPI)               |
   |                    Sequence Number Field                      |
   |                                                               |
   +                Authentication Data (variable)                 |
   |                                                               |

   The following subsections define the fields that comprise the AH
   format.  All the fields described here are mandatory, i.e., they are
   always present in the AH format and are included in the Integrity
   Check Value (ICV) computation (see Sections 2.6 and 3.3.3).

2.1  Next Header

   The Next Header is an 8-bit field that identifies the type of the
   next payload after the Authentication Header.  The value of this
   field is chosen from the set of IP Protocol Numbers defined in the
   most recent "Assigned Numbers" [STD-2] RFC from the Internet Assigned
   Numbers Authority (IANA).

2.2  Payload Length

   This 8-bit field specifies the length of AH in 32-bit words (4-byte
   units), minus "2".  (All IPv6 extension headers, as per RFC 1883,
   encode the "Hdr Ext Len" field by first subtracting 1 (64-bit word)
   from the header length (measured in 64-bit words).  AH is an IPv6
   extension header.  However, since its length is measured in 32-bit
   words, the "Payload Length" is calculated by subtracting 2 (32 bit
   words).)  In the "standard" case of a 96-bit authentication value
   plus the 3 32-bit word fixed portion, this length field will be "4".
   A "null" authentication algorithm may be used only for debugging
   purposes.  Its use would result in a "1" value for this field for
   IPv4 or a "2" for IPv6, as there would be no corresponding
   Authentication Data field (see Section on "Authentication
   Data Padding").

2.3  Reserved

   This 16-bit field is reserved for future use.  It MUST be set to
   "zero." (Note that the value is included in the Authentication Data
   calculation, but is otherwise ignored by the recipient.)

2.4  Security Parameters Index (SPI)

   The SPI is an arbitrary 32-bit value that, in combination with the
   destination IP address and security protocol (AH), uniquely
   identifies the Security Association for this datagram.  The set of
   SPI values in the range 1 through 255 are reserved by the Internet
   Assigned Numbers Authority (IANA) for future use; a reserved SPI
   value will not normally be assigned by IANA unless the use of the
   assigned SPI value is specified in an RFC.  It is ordinarily selected
   by the destination system upon establishment of an SA (see the
   Security Architecture document for more details).

   The SPI value of zero (0) is reserved for local, implementation-
   specific use and MUST NOT be sent on the wire.  For example, a key
   management implementation MAY use the zero SPI value to mean "No
   Security Association Exists" during the period when the IPsec
   implementation has requested that its key management entity establish
   a new SA, but the SA has not yet been established.

2.5  Sequence Number

   This unsigned 32-bit field contains a monotonically increasing
   counter value (sequence number).  It is mandatory and is always
   present even if the receiver does not elect to enable the anti-replay
   service for a specific SA.  Processing of the Sequence Number field
   is at the discretion of the receiver, i.e., the sender MUST always
   transmit this field, but the receiver need not act upon it (see the
   discussion of Sequence Number Verification in the "Inbound Packet
   Processing" section below).

   The sender's counter and the receiver's counter are initialized to 0
   when an SA is established.  (The first packet sent using a given SA
   will have a Sequence Number of 1; see Section 3.3.2 for more details
   on how the Sequence Number is generated.)  If anti-replay is enabled
   (the default), the transmitted Sequence Number must never be allowed
   to cycle.  Thus, the sender's counter and the receiver's counter MUST
   be reset (by establishing a new SA and thus a new key) prior to the
   transmission of the 2^32nd packet on an SA.

2.6  Authentication Data

   This is a variable-length field that contains the Integrity Check
   Value (ICV) for this packet.  The field must be an integral multiple
   of 32 bits in length.  The details of the ICV computation are
   described in Section 3.3.2 below.  This field may include explicit
   padding.  This padding is included to ensure that the length of the
   AH header is an integral multiple of 32 bits (IPv4) or 64 bits
   (IPv6).  All implementations MUST support such padding.  Details of
   how to compute the required padding length are provided below.  The
   authentication algorithm specification MUST specify the length of the
   ICV and the comparison rules and processing steps for validation.

3.  Authentication Header Processing

3.1  Authentication Header Location

   Like ESP, AH may be employed in two ways: transport mode or tunnel
   mode.  The former mode is applicable only to host implementations and
   provides protection for upper layer protocols, in addition to
   selected IP header fields.  (In this mode, note that for "bump-in-
   the-stack" or "bump-in-the-wire" implementations, as defined in the
   Security Architecture document, inbound and outbound IP fragments may
   require an IPsec implementation to perform extra IP
   reassembly/fragmentation in order to both conform to this
   specification and provide transparent IPsec support.  Special care is
   required to perform such operations within these implementations when
   multiple interfaces are in use.)

   In transport mode, AH is inserted after the IP header and before an
   upper layer protocol, e.g., TCP, UDP, ICMP, etc. or before any other
   IPsec headers that have already been inserted.  In the context of
   IPv4, this calls for placing AH after the IP header (and any options
   that it contains), but before the upper layer protocol.  (Note that
   the term "transport" mode should not be misconstrued as restricting
   its use to TCP and UDP.  For example, an ICMP message MAY be sent
   using either "transport" mode or "tunnel" mode.)  The following
   diagram illustrates AH transport mode positioning for a typical IPv4
   packet, on a "before and after" basis.

                  BEFORE APPLYING AH
      IPv4  |orig IP hdr  |     |      |
            |(any options)| TCP | Data |

                  AFTER APPLYING AH
      IPv4  |orig IP hdr  |    |     |      |
            |(any options)| AH | TCP | Data |
            |<------- authenticated ------->|
                 except for mutable fields

   In the IPv6 context, AH is viewed as an end-to-end payload, and thus
   should appear after hop-by-hop, routing, and fragmentation extension
   headers.  The destination options extension header(s) could appear
   either before or after the AH header depending on the semantics
   desired.  The following diagram illustrates AH transport mode
   positioning for a typical IPv6 packet.

                       BEFORE APPLYING AH
      IPv6  |             | ext hdrs |     |      |
            | orig IP hdr |if present| TCP | Data |

                      AFTER APPLYING AH
      IPv6  |             |hop-by-hop, dest*, |    | dest |     |      |
            |orig IP hdr  |routing, fragment. | AH | opt* | TCP | Data |
            |<---- authenticated except for mutable fields ----------->|

                 * = if present, could be before AH, after AH, or both

   ESP and AH headers can be combined in a variety of modes.  The IPsec
   Architecture document describes the combinations of security
   associations that must be supported.

   Tunnel mode AH may be employed in either hosts or security gateways
   (or in so-called "bump-in-the-stack" or "bump-in-the-wire"
   implementations, as defined in the Security Architecture document).
   When AH is implemented in a security gateway (to protect transit
   traffic), tunnel mode must be used.  In tunnel mode, the "inner" IP
   header carries the ultimate source and destination addresses, while
   an "outer" IP header may contain distinct IP addresses, e.g.,
   addresses of security gateways.  In tunnel mode, AH protects the
   entire inner IP packet, including the entire inner IP header. The
   position of AH in tunnel mode, relative to the outer IP header, is
   the same as for AH in transport mode.  The following diagram
   illustrates AH tunnel mode positioning for typical IPv4 and IPv6

    IPv4  | new IP hdr* |    | orig IP hdr*  |    |      |
          |(any options)| AH | (any options) |TCP | Data |
          |<- authenticated except for mutable fields -->|
          |           in the new IP hdr                  |

    IPv6  |           | ext hdrs*|    |            | ext hdrs*|   |    |
          |new IP hdr*|if present| AH |orig IP hdr*|if present|TCP|Data|
          |<-- authenticated except for mutable fields in new IP hdr ->|

           * = construction of outer IP hdr/extensions and modification
               of inner IP hdr/extensions is discussed below.

3.2  Authentication Algorithms

   The authentication algorithm employed for the ICV computation is
   specified by the SA.  For point-to-point communication, suitable
   authentication algorithms include keyed Message Authentication Codes
   (MACs) based on symmetric encryption algorithms (e.g., DES) or on
   one-way hash functions (e.g., MD5 or SHA-1).  For multicast
   communication, one-way hash algorithms combined with asymmetric
   signature algorithms are appropriate, though performance and space
   considerations currently preclude use of such algorithms.  The
   mandatory-to-implement authentication algorithms are described in
   Section 5 "Conformance Requirements".  Other algorithms MAY be

3.3  Outbound Packet Processing

   In transport mode, the sender inserts the AH header after the IP
   header and before an upper layer protocol header, as described above.
   In tunnel mode, the outer and inner IP header/extensions can be
   inter-related in a variety of ways.  The construction of the outer IP
   header/extensions during the encapsulation process is described in
   the Security Architecture document.

   If there is more than one IPsec header/extension required, the order
   of the application of the security headers MUST be defined by
   security policy.  For simplicity of processing, each IPsec header
   SHOULD ignore the existence (i.e., not zero the contents or try to
   predict the contents) of IPsec headers to be applied later.  (While a
   native IP or bump-in-the-stack implementation could predict the
   contents of later IPsec headers that it applies itself, it won't be
   possible for it to predict any IPsec headers added by a bump-in-the-
   wire implementation between the host and the network.)

3.3.1  Security Association Lookup

   AH is applied to an outbound packet only after an IPsec
   implementation determines that the packet is associated with an SA
   that calls for AH processing.  The process of determining what, if
   any, IPsec processing is applied to outbound traffic is described in
   the Security Architecture document.

3.3.2  Sequence Number Generation

   The sender's counter is initialized to 0 when an SA is established.
   The sender increments the Sequence Number for this SA and inserts the
   new value into the Sequence Number Field.  Thus the first packet sent
   using a given SA will have a Sequence Number of 1.

   If anti-replay is enabled (the default), the sender checks to ensure
   that the counter has not cycled before inserting the new value in the
   Sequence Number field.  In other words, the sender MUST NOT send a
   packet on an SA if doing so would cause the Sequence Number to cycle.
   An attempt to transmit a packet that would result in Sequence Number
   overflow is an auditable event.  (Note that this approach to Sequence
   Number management does not require use of modular arithmetic.)

   The sender assumes anti-replay is enabled as a default, unless
   otherwise notified by the receiver (see 3.4.3).  Thus, if the counter
   has cycled, the sender will set up a new SA and key (unless the SA
   was configured with manual key management).

   If anti-replay is disabled, the sender does not need to monitor or
   reset the counter, e.g., in the case of manual key management (see
   Section 5.) However, the sender still increments the counter and when
   it reaches the maximum value, the counter rolls over back to zero.

3.3.3  Integrity Check Value Calculation

   The AH ICV is computed over:
           o IP header fields that are either immutable in transit or
             that are predictable in value upon arrival at the endpoint
             for the AH SA
           o the AH header (Next Header, Payload Len, Reserved, SPI,
             Sequence Number, and the Authentication Data (which is set
             to zero for this computation), and explicit padding bytes
             (if any))
           o the upper level protocol data, which is assumed to be
             immutable in transit  Handling Mutable Fields

   If a field may be modified during transit, the value of the field is
   set to zero for purposes of the ICV computation.  If a field is
   mutable, but its value at the (IPsec) receiver is predictable, then
   that value is inserted into the field for purposes of the ICV
   calculation.  The Authentication Data field is also set to zero in
   preparation for this computation.  Note that by replacing each
   field's value with zero, rather than omitting the field, alignment is
   preserved for the ICV calculation.  Also, the zero-fill approach
   ensures that the length of the fields that are so handled cannot be
   changed during transit, even though their contents are not explicitly
   covered by the ICV.

   As a new extension header or IPv4 option is created, it will be
   defined in its own RFC and SHOULD include (in the Security
   Considerations section) directions for how it should be handled when
   calculating the AH ICV.  If the IP (v4 or v6) implementation
   encounters an extension header that it does not recognize, it will
   discard the packet and send an ICMP message.  IPsec will never see
   the packet.  If the IPsec implementation encounters an IPv4 option
   that it does not recognize, it should zero the whole option, using
   the second byte of the option as the length.  IPv6 options (in
   Destination extension headers or Hop by Hop extension header) contain
   a flag indicating mutability, which determines appropriate processing
   for such options.  ICV Computation for IPv4  Base Header Fields

   The IPv4 base header fields are classified as follows:

             Internet Header Length
             Total Length
             Protocol (This should be the value for AH.)
             Source Address
             Destination Address (without loose or strict source routing)

   Mutable but predictable
             Destination Address (with loose or strict source routing)

   Mutable (zeroed prior to ICV calculation)
             Type of Service (TOS)
             Fragment Offset
             Time to Live (TTL)
             Header Checksum

      TOS -- This field is excluded because some routers are known to
             change the value of this field, even though the IP
             specification does not consider TOS to be a mutable header

      Flags -- This field is excluded since an intermediate router might
             set the DF bit, even if the source did not select it.

      Fragment Offset -- Since AH is applied only to non-fragmented IP
             packets, the Offset Field must always be zero, and thus it
             is excluded (even though it is predictable).

      TTL -- This is changed en-route as a normal course of processing
             by routers, and thus its value at the receiver is not
             predictable by the sender.

      Header Checksum -- This will change if any of these other fields
             changes, and thus its value upon reception cannot be
             predicted by the sender.  Options

   For IPv4 (unlike IPv6), there is no mechanism for tagging options as
   mutable in transit.  Hence the IPv4 options are explicitly listed in
   Appendix A and classified as immutable, mutable but predictable, or
   mutable.  For IPv4, the entire option is viewed as a unit; so even
   though the type and length fields within most options are immutable
   in transit, if an option is classified as mutable, the entire option
   is zeroed for ICV computation purposes.  ICV Computation for IPv6  Base Header Fields

   The IPv6 base header fields are classified as follows:

             Payload Length
             Next Header (This should be the value for AH.)
             Source Address
             Destination Address (without Routing Extension Header)

   Mutable but predictable
             Destination Address (with Routing Extension Header)

   Mutable (zeroed prior to ICV calculation)
             Flow Label
             Hop Limit  Extension Headers Containing Options

   IPv6 options in the Hop-by-Hop and Destination Extension Headers
   contain a bit that indicates whether the option might change
   (unpredictably) during transit.  For any option for which contents
   may change en-route, the entire "Option Data" field must be treated
   as zero-valued octets when computing or verifying the ICV.  The
   Option Type and Opt Data Len are included in the ICV calculation.
   All options for which the bit indicates immutability are included in
   the ICV calculation.  See the IPv6 specification [DH95] for more
   information.  Extension Headers Not Containing Options

   The IPv6 extension headers that do not contain options are explicitly
   listed in Appendix A and classified as immutable, mutable but
   predictable, or mutable.  Padding  Authentication Data Padding

   As mentioned in section 2.6, the Authentication Data field explicitly
   includes padding to ensure that the AH header is a multiple of 32
   bits (IPv4) or 64 bits (IPv6).  If padding is required, its length is
   determined by two factors:

             - the length of the ICV
             - the IP protocol version (v4 or v6)

   For example, if the output of the selected algorithm is 96-bits, no
   padding is required for either IPv4 or for IPv6.  However, if a
   different length ICV is generated, due to use of a different
   algorithm, then padding may be required depending on the length and
   IP protocol version.  The content of the padding field is arbitrarily
   selected by the sender.  (The padding is arbitrary, but need not be
   random to achieve security.)  These padding bytes are included in the
   Authentication Data calculation, counted as part of the Payload
   Length, and transmitted at the end of the Authentication Data field
   to enable the receiver to perform the ICV calculation.  Implicit Packet Padding

   For some authentication algorithms, the byte string over which the
   ICV computation is performed must be a multiple of a blocksize
   specified by the algorithm.  If the IP packet length (including AH)
   does not match the blocksize requirements for the algorithm, implicit
   padding MUST be appended to the end of the packet, prior to ICV
   computation.  The padding octets MUST have a value of zero.  The
   blocksize (and hence the length of the padding) is specified by the
   algorithm specification.  This padding is not transmitted with the
   packet.  Note that MD5 and SHA-1 are viewed as having a 1-byte
   blocksize because of their internal padding conventions.

3.3.4  Fragmentation

   If required, IP fragmentation occurs after AH processing within an
   IPsec implementation.  Thus, transport mode AH is applied only to
   whole IP datagrams (not to IP fragments).  An IP packet to which AH
   has been applied may itself be fragmented by routers en route, and
   such fragments must be reassembled prior to AH processing at a
   receiver.  In tunnel mode, AH is applied to an IP packet, the payload
   of which may be a fragmented IP packet.  For example, a security
   gateway or a "bump-in-the-stack" or "bump-in-the-wire" IPsec
   implementation (see the Security Architecture document for details)
   may apply tunnel mode AH to such fragments.

3.4  Inbound Packet Processing

   If there is more than one IPsec header/extension present, the
   processing for each one ignores (does not zero, does not use) any
   IPsec headers applied subsequent to the header being processed.

3.4.1  Reassembly

   If required, reassembly is performed prior to AH processing.  If a
   packet offered to AH for processing appears to be an IP fragment,
   i.e., the OFFSET field is non-zero or the MORE FRAGMENTS flag is set,
   the receiver MUST discard the packet; this is an auditable event. The
   audit log entry for this event SHOULD include the SPI value,
   date/time, Source Address, Destination Address, and (in IPv6) the
   Flow ID.

   NOTE: For packet reassembly, the current IPv4 spec does NOT require
   either the zero'ing of the OFFSET field or the clearing of the MORE
   FRAGMENTS flag.  In order for a reassembled packet to be processed by
   IPsec (as opposed to discarded as an apparent fragment), the IP code
   must do these two things after it reassembles a packet.

3.4.2  Security Association Lookup

   Upon receipt of a packet containing an IP Authentication Header, the
   receiver determines the appropriate (unidirectional) SA, based on the
   destination IP address, security protocol (AH), and the SPI.  (This
   process is described in more detail in the Security Architecture
   document.)  The SA indicates whether the Sequence Number field will
   be checked, specifies the algorithm(s) employed for ICV computation,
   and indicates the key(s) required to validate the ICV.

   If no valid Security Association exists for this session (e.g., the
   receiver has no key), the receiver MUST discard the packet; this is
   an auditable event.  The audit log entry for this event SHOULD
   include the SPI value, date/time, Source Address, Destination
   Address, and (in IPv6) the Flow ID.

3.4.3  Sequence Number Verification

   All AH implementations MUST support the anti-replay service, though
   its use may be enabled or disabled by the receiver on a per-SA basis.
   (Note that there are no provisions for managing transmitted Sequence
   Number values among multiple senders directing traffic to a single SA
   (irrespective of whether the destination address is unicast,
   broadcast, or multicast).  Thus the anti-replay service SHOULD NOT be
   used in a multi-sender environment that employs a single SA.)

   If the receiver does not enable anti-replay for an SA, no inbound
   checks are performed on the Sequence Number.  However, from the
   perspective of the sender, the default is to assume that anti-replay
   is enabled at the receiver.  To avoid having the sender do
   unnecessary sequence number monitoring and SA setup (see section
   3.3.2), if an SA establishment protocol such as IKE is employed, the
   receiver SHOULD notify the sender, during SA establishment, if the
   receiver will not provide anti-replay protection.

   If the receiver has enabled the anti-replay service for this SA, the
   receiver packet counter for the SA MUST be initialized to zero when
   the SA is established.  For each received packet, the receiver MUST
   verify that the packet contains a Sequence Number that does not
   duplicate the Sequence Number of any other packets received during
   the life of this SA.  This SHOULD be the first AH check applied to a
   packet after it has been matched to an SA, to speed rejection of
   duplicate packets.

   Duplicates are rejected through the use of a sliding receive window.
   (How the window is implemented is a local matter, but the following
   text describes the functionality that the implementation must
   exhibit.)  A MINIMUM window size of 32 MUST be supported; but a
   window size of 64 is preferred and SHOULD be employed as the default.
   Another window size (larger than the MINIMUM) MAY be chosen by the
   receiver.  (The receiver does NOT notify the sender of the window

   The "right" edge of the window represents the highest, validated
   Sequence Number value received on this SA.  Packets that contain
   Sequence Numbers lower than the "left" edge of the window are
   rejected.  Packets falling within the window are checked against a
   list of received packets within the window.  An efficient means for
   performing this check, based on the use of a bit mask, is described
   in the Security Architecture document.

   If the received packet falls within the window and is new, or if the
   packet is to the right of the window, then the receiver proceeds to
   ICV verification.  If the ICV validation fails, the receiver MUST
   discard the received IP datagram as invalid; this is an auditable
   event.  The audit log entry for this event SHOULD include the SPI
   value, date/time, Source Address, Destination Address, the Sequence
   Number, and (in IPv6) the Flow ID.  The receive window is updated
   only if the ICV verification succeeds.


      Note that if the packet is either inside the window and new, or is
      outside the window on the "right" side, the receiver MUST
      authenticate the packet before updating the Sequence Number window

3.4.4  Integrity Check Value Verification

   The receiver computes the ICV over the appropriate fields of the
   packet, using the specified authentication algorithm, and verifies
   that it is the same as the ICV included in the Authentication Data
   field of the packet.  Details of the computation are provided below.

   If the computed and received ICV's match, then the datagram is valid,
   and it is accepted.  If the test fails, then the receiver MUST
   discard the received IP datagram as invalid; this is an auditable
   event.  The audit log entry SHOULD include the SPI value, date/time
   received, Source Address, Destination Address, and (in IPv6) the Flow


      Begin by saving the ICV value and replacing it (but not any
      Authentication Data padding) with zero.  Zero all other fields
      that may have been modified during transit.  (See section
      for a discussion of which fields are zeroed before performing the
      ICV calculation.)  Check the overall length of the packet, and if
      it requires implicit padding based on the requirements of the
      authentication algorithm, append zero-filled bytes to the end of
      the packet as required.  Perform the ICV computation and compare
      the result with the saved value, using the comparison rules
      defined by the algorithm specification.  (For example, if a
      digital signature and one-way hash are used for the ICV
      computation, the matching process is more complex.)

4.  Auditing

   Not all systems that implement AH will implement auditing.  However,
   if AH is incorporated into a system that supports auditing, then the
   AH implementation MUST also support auditing and MUST allow a system
   administrator to enable or disable auditing for AH.  For the most
   part, the granularity of auditing is a local matter.  However,
   several auditable events are identified in this specification 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 sender in response
   to the detection of an auditable event, because of the potential to
   induce denial of service via such action.

5.  Conformance Requirements

   Implementations that claim conformance or compliance with this
   specification MUST fully implement the AH syntax and processing
   described here and MUST comply with all requirements of the Security
   Architecture document.  If the key used to compute an ICV is manually
   distributed, correct provision of the anti-replay service would
   require correct maintenance of the counter state at the sender, until
   the key is replaced, and there likely would be no automated recovery
   provision if counter overflow were imminent.  Thus a compliant
   implementation SHOULD NOT provide this service in conjunction with
   SAs that are manually keyed.  A compliant AH implementation MUST
   support the following mandatory-to-implement algorithms:

             - HMAC with MD5 [MG97a]
             - HMAC with SHA-1 [MG97b]

6.  Security Considerations

   Security is central to the design of this protocol, and these
   security considerations permeate the specification.  Additional
   security-relevant aspects of using the IPsec protocol are discussed
   in the Security Architecture document.

7.  Differences from RFC 1826

   This specification of AH differs from RFC 1826 [ATK95] in several
   important respects, but the fundamental features of AH remain intact.
   One goal of the revision of RFC 1826 was to provide a complete
   framework for AH, with ancillary RFCs required only for algorithm
   specification.  For example, the anti-replay service is now an
   integral, mandatory part of AH, not a feature of a transform defined
   in another RFC.  Carriage of a sequence number to support this
   service is now required at all times.  The default algorithms
   required for interoperability have been changed to HMAC with MD5 or
   SHA-1 (vs. keyed MD5), for security reasons.  The list of IPv4 header
   fields excluded from the ICV computation has been expanded to include
   the OFFSET and FLAGS fields.

   Another motivation for revision was to provide additional detail and
   clarification of subtle points.  This specification provides
   rationale for exclusion of selected IPv4 header fields from AH
   coverage and provides examples on positioning of AH in both the IPv4

   and v6 contexts.  Auditing requirements have been clarified in this
   version of the specification.  Tunnel mode AH was mentioned only in
   passing in RFC 1826, but now is a mandatory feature of AH.
   Discussion of interactions with key management and with security
   labels have been moved to the Security Architecture document.


   For over 3 years, 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, Francis Dupont, Phil Karn, Frank
   Kastenholz, Perry Metzger, David Mihelcic, Hilarie Orman, Norman
   Shulman, William Simpson, and Nina Yuan.

Appendix A -- Mutability of IP Options/Extension Headers

A1.  IPv4 Options

   This table shows how the IPv4 options are classified with regard to
   "mutability".  Where two references are provided, the second one
   supercedes the first.  This table is based in part on information
   provided in RFC1700, "ASSIGNED NUMBERS", (October 1994).

Copy Class  #   Name                      Reference
---- ----- ---  ------------------------  ---------
IMMUTABLE -- included in ICV calculation
  0   0     0   End of Options List       [RFC791]
  0   0     1   No Operation              [RFC791]
  1   0     2   Security                  [RFC1108(historic but in use)]
  1   0     5   Extended Security         [RFC1108(historic but in use)]
  1   0     6   Commercial Security       [expired I-D, now US MIL STD]
  1   0    20   Router Alert              [RFC2113]
  1   0    21   Sender Directed Multi-    [RFC1770]
                Destination Delivery
MUTABLE -- zeroed
  1   0      3  Loose Source Route        [RFC791]
  0   2      4  Time Stamp                [RFC791]
  0   0      7  Record Route              [RFC791]
  1   0      9  Strict Source Route       [RFC791]
  0   2     18  Traceroute                [RFC1393]

  1   0      8  Stream ID                 [RFC791, RFC1122 (Host Req)]
  0   0     11  MTU Probe                 [RFC1063, RFC1191 (PMTU)]
  0   0     12  MTU Reply                 [RFC1063, RFC1191 (PMTU)]
  1   0     17  Extended Internet Proto   [RFC1385, RFC1883 (IPv6)]
  0   0     10  Experimental Measurement  [ZSu]
  1   2     13  Experimental Flow Control [Finn]
  1   0     14  Experimental Access Ctl   [Estrin]
  0   0     15  ???                       [VerSteeg]
  1   0     16  IMI Traffic Descriptor    [Lee]
  1   0     19  Address Extension         [Ullmann IPv7]

   NOTE: Use of the Router Alert option is potentially incompatible with
   use of IPsec.  Although the option is immutable, its use implies that
   each router along a packet's path will "process" the packet and
   consequently might change the packet.  This would happen on a hop by
   hop basis as the packet goes from router to router.  Prior to being
   processed by the application to which the option contents are
   directed, e.g., RSVP/IGMP, the packet should encounter AH processing.

   However, AH processing would require that each router along the path
   is a member of a multicast-SA defined by the SPI.  This might pose
   problems for packets that are not strictly source routed, and it
   requires multicast support techniques not currently available.

   NOTE: Addition or removal of any security labels (BSO, ESO, CIPSO) by
   systems along a packet's path conflicts with the classification of
   these IP Options as immutable and is incompatible with the use of

   NOTE: End of Options List options SHOULD be repeated as necessary to
   ensure that the IP header ends on a 4 byte boundary in order to
   ensure that there are no unspecified bytes which could be used for a
   covert channel.

A2.  IPv6 Extension Headers

   This table shows how the IPv6 Extension Headers are classified with
   regard to "mutability".

Option/Extension Name                  Reference
-----------------------------------    ---------
MUTABLE BUT PREDICTABLE -- included in ICV calculation
  Routing (Type 0)                    [RFC1883]

  Hop by Hop options                  [RFC1883]
  Destination options                 [RFC1883]

  Fragmentation                       [RFC1883]

      Options -- IPv6 options in the Hop-by-Hop and Destination
             Extension Headers contain a bit that indicates whether the
             option might change (unpredictably) during transit.  For
             any option for which contents may change en-route, the
             entire "Option Data" field must be treated as zero-valued
             octets when computing or verifying the ICV.  The Option
             Type and Opt Data Len are included in the ICV calculation.
             All options for which the bit indicates immutability are
             included in the ICV calculation.  See the IPv6
             specification [DH95] for more information.

      Routing (Type 0) -- The IPv6 Routing Header "Type 0" will
             rearrange the address fields within the packet during
             transit from source to destination.  However, the contents
             of the packet as it will appear at the receiver are known
             to the sender and to all intermediate hops.  Hence, the

             IPv6 Routing Header "Type 0" is included in the
             Authentication Data calculation as mutable but predictable.
             The sender must order the field so that it appears as it
             will at the receiver, prior to performing the ICV

      Fragmentation -- Fragmentation occurs after outbound IPsec
             processing (section 3.3) and reassembly occurs before
             inbound IPsec processing (section 3.4).  So the
             Fragmentation Extension Header, if it exists, is not seen
             by IPsec.

             Note that on the receive side, the IP implementation could
             leave a Fragmentation Extension Header in place when it
             does re-assembly.  If this happens, then when AH receives
             the packet, before doing ICV processing, AH MUST "remove"
             (or skip over) this header and change the previous header's
             "Next Header" field to be the "Next Header" field in the
             Fragmentation Extension Header.

             Note that on the send side, the IP implementation could
             give the IPsec code a packet with a Fragmentation Extension
             Header with Offset of 0 (first fragment) and a More
             Fragments Flag of 0 (last fragment).  If this happens, then
             before doing ICV processing, AH MUST first "remove" (or
             skip over) this header and change the previous header's
             "Next Header" field to be the "Next Header" field in the
             Fragmentation Extension Header.


   [ATK95]   Atkinson, R., "The IP Authentication Header", RFC 1826,
             August 1995.

   [Bra97]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Level", BCP 14, RFC 2119, March 1997.

   [DH95]    Deering, S., and B. Hinden, "Internet Protocol version 6
             (IPv6) Specification", RFC 1883, December 1995.

   [HC98]    Harkins, D., and D. Carrel, "The Internet Key Exchange
             (IKE)", RFC 2409, November 1998.

   [KA97a]   Kent, S., and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.

   [KA97b]   Kent, S., and R. Atkinson, "IP Encapsulating Security
             Payload (ESP)", RFC 2406, November 1998.

   [MG97a]   Madson, C., and R. Glenn, "The Use of HMAC-MD5-96 within
             ESP and AH", RFC 2403, November 1998.

   [MG97b]   Madson, C., and R. Glenn, "The Use of HMAC-SHA-1-96 within
             ESP and AH", RFC 2404, November 1998.

   [STD-2]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
             1700, October 1994.  See also:


   The views and specification here 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

Author Information

   Stephen Kent
   BBN Corporation
   70 Fawcett Street
   Cambridge, MA  02140

   Phone: +1 (617) 873-3988
   EMail: kent@bbn.com

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

   Phone: +1 (415) 569-5000
   EMail: rja@corp.home.net

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