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rfc:rfc2402

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
         3.3.3.1  Handling Mutable Fields............................9
            3.3.3.1.1  ICV Computation for IPv4.....................10
               3.3.3.1.1.1 Base Header Fields.......................10
               3.3.3.1.1.2 Options..................................11
            3.3.3.1.2  ICV Computation for IPv6.....................11
               3.3.3.1.2.1 Base Header Fields.......................11
               3.3.3.1.2.2 Extension Headers Containing Options.....11
               3.3.3.1.2.3 Extension Headers Not Containing Options.11
         3.3.3.2  Padding...........................................12
            3.3.3.2.1  Authentication Data Padding..................12

Kent & Atkinson Standards Track [Page 1] RFC 2402 IP Authentication Header November 1998

            3.3.3.2.2  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
Acknowledgements....................................................17
Appendix A -- Mutability of IP Options/Extension Headers............18
   A1. IPv4 Options.................................................18
   A2. IPv6 Extension Headers.......................................19
References..........................................................20
Disclaimer..........................................................21
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

Kent & Atkinson Standards Track [Page 2] RFC 2402 IP Authentication Header November 1998

 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].)
 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].

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).

Kent & Atkinson Standards Track [Page 3] RFC 2402 IP Authentication Header November 1998

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 3.3.3.2.1 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.

Kent & Atkinson Standards Track [Page 4] RFC 2402 IP Authentication Header November 1998

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.)

Kent & Atkinson Standards Track [Page 5] RFC 2402 IP Authentication Header November 1998

 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

Kent & Atkinson Standards Track [Page 6] RFC 2402 IP Authentication Header November 1998

 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
 packets.
  1. ———————————————–

IPv4 | new IP hdr* | | orig IP hdr* | | |

        |(any options)| AH | (any options) |TCP | Data |
        ------------------------------------------------
        |<- authenticated except for mutable fields -->|
        |           in the new IP hdr                  |
  1. ————————————————————-

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
 supported.

Kent & Atkinson Standards Track [Page 7] RFC 2402 IP Authentication Header November 1998

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).

Kent & Atkinson Standards Track [Page 8] RFC 2402 IP Authentication Header November 1998

 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

3.3.3.1 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.

Kent & Atkinson Standards Track [Page 9] RFC 2402 IP Authentication Header November 1998

3.3.3.1.1 ICV Computation for IPv4

3.3.3.1.1.1 Base Header Fields

 The IPv4 base header fields are classified as follows:
 Immutable
           Version
           Internet Header Length
           Total Length
           Identification
           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)
           Flags
           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
           field.
    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.

Kent & Atkinson Standards Track [Page 10] RFC 2402 IP Authentication Header November 1998

3.3.3.1.1.2 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.

3.3.3.1.2 ICV Computation for IPv6

3.3.3.1.2.1 Base Header Fields

 The IPv6 base header fields are classified as follows:
 Immutable
           Version
           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)
           Class
           Flow Label
           Hop Limit

3.3.3.1.2.2 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.

3.3.3.1.2.3 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.

Kent & Atkinson Standards Track [Page 11] RFC 2402 IP Authentication Header November 1998

3.3.3.2 Padding

3.3.3.2.1 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:
  1. the length of the ICV
  2. 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.

3.3.3.2.2 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.

Kent & Atkinson Standards Track [Page 12] RFC 2402 IP Authentication Header November 1998

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.)

Kent & Atkinson Standards Track [Page 13] RFC 2402 IP Authentication Header November 1998

 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
 size.)
 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.

Kent & Atkinson Standards Track [Page 14] RFC 2402 IP Authentication Header November 1998

 DISCUSSION:
    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
    data.

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
 ID.
 DISCUSSION:
    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 3.3.3.1
    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

Kent & Atkinson Standards Track [Page 15] RFC 2402 IP Authentication Header November 1998

 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:
  1. HMAC with MD5 [MG97a]
  2. 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

Kent & Atkinson Standards Track [Page 16] RFC 2402 IP Authentication Header November 1998

 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.

Acknowledgements

 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.

Kent & Atkinson Standards Track [Page 17] RFC 2402 IP Authentication Header November 1998

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).
         Opt.

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]

EXPERIMENTAL, SUPERCEDED – zeroed

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.

Kent & Atkinson Standards Track [Page 18] RFC 2402 IP Authentication Header November 1998

 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
 IPsec.
 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]

BIT INDICATES IF OPTION IS MUTABLE (CHANGES UNPREDICTABLY DURING TRANSIT)

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

NOT APPLICABLE

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

Kent & Atkinson Standards Track [Page 19] RFC 2402 IP Authentication Header November 1998

           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
           computation.
    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.

References

 [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.

Kent & Atkinson Standards Track [Page 20] RFC 2402 IP Authentication Header 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:
           http://www.iana.org/numbers.html

Disclaimer

 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
 specification.

Author Information

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

Kent & Atkinson Standards Track [Page 21] RFC 2402 IP Authentication Header November 1998

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

 This document and translations of it may be copied and furnished to
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Kent & Atkinson Standards Track [Page 22]

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