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Versions: 00 01 02 03 04 05 06 RFC 2402

Network Working Group                             Stephen Kent, BBN Corp
Internet Draft                           Randall Atkinson, @Home Network
draft-ietf-ipsec-auth-header-01.txt                         July 21 1997





                        IP Authentication Header




Status of This Memo

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of 6 months.
   Internet Drafts may be updated, replaced, or obsoleted by other
   documents at any time. It is not appropriate to use Internet Drafts
   as reference material or to cite them other than as a "working draft"
   or "work in progress". Please check the I-D abstract listing
   contained in each Internet Draft directory to learn the current
   status of this or any other Internet Draft.

   This particular Internet Draft is a product of the IETF's IPsec
   Working Group. It is intended that a future version of this draft
   will be submitted for consideration as a standards-track document.
   Distribution of this document is unlimited.























Kent, Atkinson                                                  [Page 1]


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Table of Contents

   1. Introduction......................................................3
   2. Authentication Header Format......................................4
      2.1 Next Header...................................................4
      2.2 Payload Length................................................4
      2.3 Reserved......................................................5
      2.4 Security Parameters Index (SPI)...............................5
      2.5 Sequence Number...............................................5
      2.6 Authentication Data ..........................................5
   3. Authentication Header Processing..................................6
      3.1  Authentication Header Location...............................6
      3.2  Outbound Packet Processing...................................8
         3.2.1  Security Association Lookup.............................8
         3.2.2  Sequence Number Generation..............................8
         3.2.3  Integrity Check Value Calculation.......................9
            3.2.3.1  Handling Mutable Fields............................9
               3.2.3.1.1  ICV Computation for IPv4......................9
                  3.2.3.1.1.1 Base Header Fields........................9
                  3.2.3.1.1.2 Options..................................10
               3.2.3.1.2  ICV Computation for IPv6.....................10
                  3.2.3.1.2.1 Base Header Fields.......................10
                  3.2.3.1.2.2 Extension Headers -- Options.............11
                  3.2.3.1.2.3 Extension Headers -- non-Options.........11
            3.2.3.2  Padding...........................................11
               3.2.3.2.1  Authentication Data Padding..................11
               3.2.3.2.2  Implicit Packet Padding......................12
            3.2.3.3  Authentication Algorithms.........................12
         3.2.4  Fragmentation..........................................12
      3.3  Inbound Packet Processing...................................13
         3.3.1  Reassembly.............................................13
         3.3.2  Security Association Lookup............................13
         3.3.3  Sequence Number Verification...........................13
         3.3.4  Integrity Check Value Verification.....................14
   4. Auditing.........................................................15
   5. Conformance Requirements.........................................15
   6. Security Considerations..........................................16
   7. Differences from RFC 1826........................................16
   Acknowledgements....................................................17
   Appendix A -- Mutability of IP Options/Extension Headers............18
      1. IPv4 Options..................................................18
      2. IPv6 Extension Headers........................................19
   References..........................................................21
   Disclaimer..........................................................22
   Author Information..................................................22





Kent, Atkinson                                                  [Page 2]


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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.  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
   transmitter.  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 Oakley/ISAKMP.)












Kent, Atkinson                                                  [Page 3]


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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 ICV
   computation.

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," i.e., the fixed portion (as defined in the
   original AH spec) of AH is not counted.  (Since the Sequence Number
   field is always present, the fixed portion of AH is now three 32-bit
   words.  However, the "minus 2" length adjustment has been retained
   for backwards compatibility.)  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, as there would be no corresponding
   Authentication Data field.



Kent, Atkinson                                                  [Page 4]


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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 uniquely identifies the
   Security Association for this datagram, relative to the destination
   IP address contained in the IP header with which this security header
   is associated, and relative to the security protocol employed.  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).  (A zero value may
   be used for local debugging purposes, but no AH packets should be
   transmitted with a zero SPI value.)

2.5 Sequence Number

   This unsigned 32-bit field contains a monotonically increasing
   counter value (sequence number).  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 3.2.2 for more details on how the Sequence Number
   is generated.) 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 2^32nd packet on an SA.

   This field is always present, even if the receiver does not elect to
   enable the anti-replay service for a specific SA, in order to ensure
   8-byte alignment for the IPv6 environment, when the default integrity
   algorithms are employed.

   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 Processing" section below).

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


Kent, Atkinson                                                  [Page 5]


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   described in Section 3.2.3 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.

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, e.g., ESP.  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


Kent, Atkinson                                                  [Page 6]


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   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  |             |hxh,rtg,frag| dest |    | dest |     |      |
            |orig IP hdr  |if present**| opt* | AH | opt* | TCP | Data |
            ------------------------------------------------------------
            |<---- authenticated except for mutable fields ----------->|

                * = if present, could be before AH, after AH, or both
               ** = hop by hop, routing, fragmentation headers

   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 subscriber
   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.















Kent, Atkinson                                                  [Page 7]


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

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

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

3.2  Outbound Packet Processing

   In transport mode, the transmitter 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.

3.2.1  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.

3.2.2  Sequence Number Generation

   The sender's counter is initialized to 0 when an SA is established.
   The transmitter increments the Sequence Number for this SA, checks to
   ensure that the counter has not cycled, 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.  A transmitter 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.)






Kent, Atkinson                                                  [Page 8]


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3.2.3  Integrity Check Value Calculation

3.2.3.1  Handling Mutable Fields

   The AH ICV is computed over IP header fields that are either
   immutable in transit or that are predictable in value upon arrival at
   the endpoint for the AH SA.  The ICV also encompasses the upper level
   protocol data, which is assumed to be immutable in transit.  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 also is 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 IPSEC implementation encounters an
   extension header that it does not recognize, it MUST zero the whole
   header except for the Next Header and Hdr Ext Len fields.  The length
   of the extension header MUST be computed by 8 * Hdr Ext Len value +
   8.  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 contain a flag
   indicating mutability, which determines appropriate processing for
   such options.)

3.2.3.1.1  ICV Computation for IPv4

3.2.3.1.1.1 Base Header Fields

   The IPv4 base header fields are classified as follows:

   Immutable
             Version
             Internet Header Length
             Total Length
             Identification
             Protocol
             Source Address
             Destination Address (without loose or strict source routing)




Kent, Atkinson                                                  [Page 9]


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   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.

3.2.3.1.1.2 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.

3.2.3.1.2  ICV Computation for IPv6

3.2.3.1.2.1 Base Header Fields

   The IPv6 base header fields are classified as follows:






Kent, Atkinson                                                 [Page 10]


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   Immutable
             Version
             Payload Length
             Next Header
             Source Address
             Destination Address (without Routing Extension Header)

   Mutable but predictable
             Destination Address (with Routing Extension Header)

   Mutable (zeroed prior to ICV calculation)
             Priority
             Flow Label
             Hop Limit

3.2.3.1.2.2 Extension Headers -- Options

   The IPv6 extension headers (that are options) are explicitly listed
   in Appendix A and classified as immutable, mutable but predictable,
   or mutable.

   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.

3.2.3.1.2.3 Extension Headers -- non-Options

   The IPv6 extension headers (that are not options) are explicitly
   listed in Appendix A and classified as immutable, mutable but
   predictable, or mutable.

3.2.3.2  Padding

3.2.3.2.1  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:

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


Kent, Atkinson                                                 [Page 11]


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   For example, if a default, 96-bit truncated (see Section 3.2.3.3)
   HMAC algorithm is selected no padding is required for either IPv4 nor
   for IPv6.  However, if a different length ICV is generated, due to
   use of a different algorithm, then padding may be required for the
   IPv6 environment.  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.

3.2.3.2.2 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.

3.2.3.3  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.  As of this
   writing, the mandatory-to-implement authentication algorithms are
   based on the former class, i.e., HMAC [KBC97] with SHA-1 [SHA] or
   HMAC with MD5 [Riv92].  The output of the HMAC computation is
   truncated to the leftmost 96 bits.  Other algorithms, possibly with
   different ICV lengths, MAY be supported.

3.2.4  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


Kent, Atkinson                                                 [Page 12]


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   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.

3.3  Inbound Packet Processing

3.3.1  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.

3.3.2  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 and the SPI.  (This process is described in
   more detail in the Security Architecture document.)  The SA dictates
   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.

3.3.3  Sequence Number Verification

   All AH implementations MUST support the anti-replay service, though
   its use may be enabled or disabled 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,
   multicast SA.  Thus the anti-replay service SHOULD NOT be used in a
   multi-sender multicast environment that employs a single, multicast
   SA.)  If an SA establishment protocol such as Oakley/ISAKMP is
   employed, then the receiver SHOULD notify the transmitter, during SA
   establishment, if the receiver will provide anti-replay protection
   and SHOULD inform the transmitter of the window size.

   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


Kent, Atkinson                                                 [Page 13]


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   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.
   A window size of 64 or larger MAY be chosen by the receiver.  If a
   larger window size is chosen, it MUST be a multiple of 32.  If any
   window size other than the default of 64 is employed by the receiver,
   it MUST be reported to the transmitter during SA negotiation.

   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.


   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.

3.3.4  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.


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   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,
   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 3.2.3.1
      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.  Now perform the ICV computation and
      compare the result with the saved value.  (For the mandatory-to-
      implement authentication algorithms, HMAC [KBC97] with SHA-1 [SHA]
      or HMAC with MD5 [Riv92], the output of the HMAC computation is
      truncated to the leftmost 96 bits.  Other algorithms may have
      different ICV lengths.) (If a digital signature and one-way hash
      are used for the ICV computation, the matching process is more
      complex and will be described in the algorithm specification.)


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 transmitter 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


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   distributed, correct provision of the anti-replay service would
   require correct maintenance of the counter state at the transmitter,
   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 (specified in [KBC97]):

             - HMAC with MD5
             - HMAC with SHA-1

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, to meet IPv6 alignment
   requirements (even when anti-replay is not enabled for an SA).  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.





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Acknowledgements

   For over 2 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, William
   Simpson, and Nina Yuan.





































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Appendix A -- Mutability of IP Options/Extension Headers

   1. 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 Protocol [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


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   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.

   2. 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 IPv6 Routing Header "Type 0" is
             included in the Authentication Data calculation as mutable but
             predictable.  The transmitter 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 3.2.4) and reassembly occurs before inbound IPSEC
             processing (section 3.3.1).  So the Fragmentation Extension


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             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.

































Kent, Atkinson                                                 [Page 20]


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References

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

   [BCCH94]  R. Braden, D. Clark, S. Crocker, & C.Huitema, "Report of
             IAB Workshop on Security in the Internet Architecture",
             RFC-1636, 9 June 1994, pp. 21-34.

   [Bel89]   Steven M. Bellovin, "Security Problems in the TCP/IP
             Protocol Suite", ACM Computer Communications Review, Vol.
             19, No. 2, March 1989.

   [CER95]   Computer Emergency Response Team (CERT), "IP Spoofing
             Attacks and Hijacked Terminal Connections", CA-95:01,
             January 1995. Available via anonymous ftp from
             info.cert.org in /pub/cert_advisories.

   [DH95]    Steve Deering & Bob Hinden, "Internet Protocol version 6
             (IPv6) Specification", RFC-1883, December 1995.

   [GM93]    James Galvin & Keith McCloghrie, Security Protocols for
             version 2 of the Simple Network Management Protocol
             (SNMPv2), RFC-1446, April 1993.

   [KA97a]   Steve Kent, Randall Atkinson, "Security Architecture for
             the Internet Protocol", Internet Draft, ?? 1997.

   [KA97b]   Steve Kent, Randall Atkinson, "IP Encapsulating Security
             Payload (ESP)", Internet Draft, ?? 1997.

   [KA97c]   Steve Kent, Randall Atkinson, "IP Authentication Header",
             Internet Draft, ?? 1997.

   [KBC97]   Hugo Krawczyk, Mihir Bellare, and Ran Canetti, "HMAC:
             Keyed-Hashing for Message Authentication", RFC-2104,
             February 1997.

   [Ken91]   Steve Kent, "US DoD Security Options for the Internet
             Protocol", RFC-1108, November 1991.

   [KA97a]   Steve Kent, Randall Atkinson, "Security Architecture for
             the Internet Protocol", Internet Draft, ?? 1997.

   [Riv92]   Ronald Rivest, "The MD5 Message Digest Algorithm," RFC-
             1321, April 1992.

   [SHA]     NIST, FIPS PUB 180-1: Secure Hash Standard, April 1995


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   [STD-1]   J. Postel, "Internet Official Protocol Standards", STD-1,
             March 1996.

   [STD-2]   J. Reynolds & J. Postel, "Assigned Numbers", STD-2, 20
             October 1994.


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
   E-mail: kent@bbn.com
   Telephone: +1 (617) 873-3988

   Randall Atkinson
   @Home Network
   385 Ravendale Drive
   Mountain View, CA 94043
   USA
   E-mail: rja@inet.org

















Kent, Atkinson                                                 [Page 22]


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