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rfc:rfc4302

Network Working Group S. Kent Request for Comments: 4302 BBN Technologies Obsoletes: 2402 December 2005 Category: Standards Track

                      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 (2005).

Abstract

 This document describes an updated version of the IP Authentication
 Header (AH), which is designed to provide authentication services in
 IPv4 and IPv6.  This document obsoletes RFC 2402 (November 1998).

Table of Contents

 1. Introduction ....................................................3
 2. Authentication Header Format ....................................4
    2.1. Next Header ................................................5
    2.2. Payload Length .............................................5
    2.3. Reserved ...................................................6
    2.4. Security Parameters Index (SPI) ............................6
    2.5. Sequence Number ............................................8
         2.5.1. Extended (64-bit) Sequence Number ...................8
    2.6. Integrity Check Value (ICV) ................................9
 3. Authentication Header Processing ................................9
    3.1. Authentication Header Location .............................9
         3.1.1. Transport Mode ......................................9
         3.1.2. Tunnel Mode ........................................11
    3.2. Integrity Algorithms ......................................11
    3.3. Outbound Packet Processing ................................11
         3.3.1. Security Association Lookup ........................12
         3.3.2. Sequence Number Generation .........................12
         3.3.3. Integrity Check Value Calculation ..................13
                3.3.3.1. Handling Mutable Fields ...................13
                3.3.3.2. Padding and Extended Sequence Numbers .....16

Kent Standards Track [Page 1] RFC 4302 IP Authentication Header December 2005

         3.3.4. Fragmentation ......................................17
    3.4. Inbound Packet Processing .................................18
         3.4.1. Reassembly .........................................18
         3.4.2. Security Association Lookup ........................18
         3.4.3. Sequence Number Verification .......................19
         3.4.4. Integrity Check Value Verification .................20
 4. Auditing .......................................................21
 5. Conformance Requirements .......................................21
 6. Security Considerations ........................................22
 7. Differences from RFC 2402 ......................................22
 8. Acknowledgements ...............................................22
 9. References .....................................................22
    9.1. Normative References ......................................22
    9.2. Informative References ....................................23
 Appendix A: Mutability of IP Options/Extension Headers ............25
    A1. IPv4 Options ...............................................25
    A2. IPv6 Extension Headers .....................................26
 Appendix B: Extended (64-bit) Sequence Numbers ....................28
    B1. Overview ...................................................28
    B2. Anti-Replay Window .........................................28
        B2.1. Managing and Using the Anti-Replay Window ............29
        B2.2. Determining the Higher-Order Bits (Seqh) of the
              Sequence Number ......................................30
        B2.3. Pseudo-Code Example ..................................31
    B3. Handling Loss of Synchronization due to Significant
        Packet Loss ................................................32
        B3.1. Triggering Re-synchronization ........................33
        B3.2. Re-synchronization Process ...........................33

Kent Standards Track [Page 2] RFC 4302 IP Authentication Header December 2005

1. Introduction

 This document assumes that the reader is familiar with the terms and
 concepts described in the "Security Architecture for the Internet
 Protocol" [Ken-Arch], hereafter referred to as the Security
 Architecture document.  In particular, the reader should be familiar
 with the definitions of security services offered by the
 Encapsulating Security Payload (ESP) [Ken-ESP] and the IP
 Authentication Header (AH), the concept of Security Associations, the
 ways in which ESP can be used in conjunction with the Authentication
 Header (AH), and the different key management options available for
 ESP and AH.
 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].
 The IP Authentication Header (AH) is used to provide connectionless
 integrity and data origin authentication for IP datagrams (hereafter
 referred to as just "integrity") and to provide protection against
 replays.  This latter, optional service may be selected, by the
 receiver, when a Security Association (SA) is established.  (The
 protocol default requires the sender to increment the sequence number
 used for anti-replay, but the service is effective only if the
 receiver checks the sequence number.)  However, to make use of the
 Extended Sequence Number feature in an interoperable fashion, AH does
 impose a requirement on SA management protocols to be able to
 negotiate this new feature (see Section 2.5.1 below).
 AH provides authentication for as much of the IP header as possible,
 as well as for next 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 piecemeal.  (See
 Appendix A.)
 AH may be applied alone, in combination with the IP Encapsulating
 Security Payload (ESP) [Ken-ESP], or in a nested fashion (see
 Security Architecture document [Ken-Arch]).  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 anti-replay and similar
 integrity services, and it also provides a confidentiality
 (encryption) service.  The primary difference between the integrity
 provided by ESP and AH is the extent of the coverage.  Specifically,
 ESP does not protect any IP header fields unless those fields are

Kent Standards Track [Page 3] RFC 4302 IP Authentication Header December 2005

 encapsulated by ESP (e.g., via use of tunnel mode).  For more details
 on how to use AH and ESP in various network environments, see the
 Security Architecture document [Ken-Arch].
 Section 7 provides a brief review of the differences between this
 document and RFC 2402 [RFC2402].

2. Authentication Header Format

 The protocol header (IPv4, IPv6, or IPv6 Extension) immediately
 preceding the AH header SHALL contain the value 51 in its Protocol
 (IPv4) or Next Header (IPv6, Extension) fields [DH98].  Figure 1
 illustrates the format for AH.
   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                      |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                                                               |
 +                Integrity Check Value-ICV (variable)           |
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Figure 1.  AH Format
 The following table refers to the fields that comprise AH,
 (illustrated in Figure 1), plus other fields included in the
 integrity computation, and illustrates which fields are covered by
 the ICV and what is transmitted.
                                                    What    What
                                   # of     Requ'd  Integ    is
                                   bytes     [1]    Covers  Xmtd
                                   ------   ------  ------  ------
        IP Header                  variable    M     [2]    plain
        Next Header                   1        M      Y     plain
        Payload Len                   1        M      Y     plain
        RESERVED                      2        M      Y     plain
        SPI                           4        M      Y     plain
        Seq# (low-order 32 bits)      4        M      Y     plain
        ICV                        variable    M      Y[3]  plain
        IP datagram [4]            variable    M      Y     plain
        Seq# (high-order 32 bits)     4      if ESN   Y     not xmtd
        ICV Padding                variable  if need  Y     not xmtd

Kent Standards Track [Page 4] RFC 4302 IP Authentication Header December 2005

     [1] - M = mandatory
     [2] - See Section 3.3.3, "Integrity Check Value Calculation", for
           details of which IP header fields are covered.
     [3] - Zeroed before ICV calculation (resulting ICV placed here
           after calculation)
     [4] - If tunnel mode -> IP datagram
           If transport mode -> next header and data
 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).
 Note: All of the cryptographic algorithms used in IPsec expect their
 input in canonical network byte order (see Appendix of RFC 791
 [RFC791]) and generate their output in canonical network byte order.
 IP packets are also transmitted in network byte order.
 AH does not contain a version number, therefore if there are concerns
 about backward compatibility, they MUST be addressed by using a
 signaling mechanism between the two IPsec peers to ensure compatible
 versions of AH, e.g., IKE [IKEv2] or an out-of-band configuration
 mechanism.

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 on the
 web page of Internet Assigned Numbers Authority (IANA).  For example,
 a value of 4 indicates IPv4, a value of 41 indicates IPv6, and a
 value of 6 indicates TCP.

2.2. Payload Length

 This 8-bit field specifies the length of AH in 32-bit words (4-byte
 units), minus "2".  Thus, for example, if an integrity algorithm
 yields a 96-bit authentication value, this length field will be "4"
 (3 32-bit word fixed fields plus 3 32-bit words for the ICV, minus
 2).  For IPv6, the total length of the header must be a multiple of
 8-octet units.  (Note that although IPv6 [DH98] characterizes AH as
 an extension header, its length is measured in 32-bit words, not the
 64-bit words used by other IPv6 extension headers.)  See Section 2.6,
 "Integrity Check Value (ICV)", for comments on padding of this field,
 and Section 3.3.3.2.1, "ICV Padding".

Kent Standards Track [Page 5] RFC 4302 IP Authentication Header December 2005

2.3. Reserved

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

2.4. Security Parameters Index (SPI)

 The SPI is an arbitrary 32-bit value that is used by a receiver to
 identify the SA to which an incoming packet is bound.  For a unicast
 SA, the SPI can be used by itself to specify an SA, or it may be used
 in conjunction with the IPsec protocol type (in this case AH).
 Because for unicast SAs the SPI value is generated by the receiver,
 whether the value is sufficient to identify an SA by itself or
 whether it must be used in conjunction with the IPsec protocol value
 is a local matter.  The SPI field is mandatory, and this mechanism
 for mapping inbound traffic to unicast SAs described above MUST be
 supported by all AH implementations.
 If an IPsec implementation supports multicast, then it MUST support
 multicast SAs using the algorithm below for mapping inbound IPsec
 datagrams to SAs.  Implementations that support only unicast traffic
 need not implement this de-multiplexing algorithm.
 In many secure multicast architectures, e.g., [RFC3740], a central
 Group Controller/Key Server unilaterally assigns the group security
 association's SPI.  This SPI assignment is not negotiated or
 coordinated with the key management (e.g., IKE) subsystems that
 reside in the individual end systems that comprise the group.
 Consequently, it is possible that a group security association and a
 unicast security association can simultaneously use the same SPI.  A
 multicast-capable IPsec implementation MUST correctly de-multiplex
 inbound traffic even in the context of SPI collisions.
 Each entry in the Security Association Database (SAD) [Ken-Arch] must
 indicate whether the SA lookup makes use of the destination, or
 destination and source, IP addresses, in addition to the SPI.  For
 multicast SAs, the protocol field is not employed for SA lookups.
 For each inbound, IPsec-protected packet, an implementation must
 conduct its search of the SAD such that it finds the entry that
 matches the "longest" SA identifier.  In this context, if two or more
 SAD entries match based on the SPI value, then the entry that also
 matches based on destination, or destination and source, address
 comparison (as indicated in the SAD entry) is the "longest" match.
 This implies a logical ordering of the SAD search as follows:

Kent Standards Track [Page 6] RFC 4302 IP Authentication Header December 2005

         1. Search the SAD for a match on {SPI, destination
            address, source address}.  If an SAD entry
            matches, then process the inbound AH packet with that
            matching SAD entry.  Otherwise, proceed to step 2.
         2. Search the SAD for a match on {SPI, destination
            address}.  If an SAD entry matches, then process
            the inbound AH packet with that matching SAD
            entry.  Otherwise, proceed to step 3.
         3. Search the SAD for a match on only {SPI} if the receiver
            has chosen to maintain a single SPI space for AH and ESP,
            or on {SPI, protocol} otherwise.  If an SAD
            entry matches, then process the inbound AH packet with
            that matching SAD entry.  Otherwise, discard the packet
            and log an auditable event.
 In practice, an implementation MAY choose any method to accelerate
 this search, although its externally visible behavior MUST be
 functionally equivalent to having searched the SAD in the above
 order.  For example, a software-based implementation could index into
 a hash table by the SPI.  The SAD entries in each hash table bucket's
 linked list are kept sorted to have those SAD entries with the
 longest SA identifiers first in that linked list.  Those SAD entries
 having the shortest SA identifiers are sorted so that they are the
 last entries in the linked list.  A hardware-based implementation may
 be able to effect the longest match search intrinsically, using
 commonly available Ternary Content-Addressable Memory (TCAM)
 features.
 The indication of whether source and destination address matching is
 required to map inbound IPsec traffic to SAs MUST be set either as a
 side effect of manual SA configuration or via negotiation using an SA
 management protocol, e.g., IKE or Group Domain of Interpretation
 (GDOI) [RFC3547].  Typically, Source-Specific Multicast (SSM) [HC03]
 groups use a 3-tuple SA identifier composed of an SPI, a destination
 multicast address, and source address.  An Any-Source Multicast group
 SA requires only an SPI and a destination multicast address as an
 identifier.
 The set of SPI values in the range 1 through 255 is 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.  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
 might use the zero SPI value to mean "No Security Association Exists"

Kent Standards Track [Page 7] RFC 4302 IP Authentication Header December 2005

 during the period when the IPsec implementation has requested that
 its key management entity establish a new SA, but the SA has not yet
 been established.)

2.5. Sequence Number

 This unsigned 32-bit field contains a counter value that increases by
 one for each packet sent, i.e., a per-SA packet sequence number.  For
 a unicast SA or a single-sender multicast SA, the sender MUST
 increment this field for every transmitted packet.  Sharing an SA
 among multiple senders is permitted, though generally not
 recommended.  AH provides no means of synchronizing packet counters
 among multiple senders or meaningfully managing a receiver packet
 counter and window in the context of multiple senders.  Thus, for a
 multi-sender SA, the anti-reply features of AH are not available (see
 Sections 3.3.2 and 3.4.3).
 The field is mandatory and MUST always be 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, but all AH implementations MUST be
 capable of performing the processing described in Section 3.3.2,
 "Sequence Number Generation", and Section 3.4.3, "Sequence Number
 Verification".  Thus, the sender MUST always transmit this field, but
 the receiver need not act upon it.
 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.

2.5.1. Extended (64-bit) Sequence Number

 To support high-speed IPsec implementations, a new option for
 sequence numbers SHOULD be offered, as an extension to the current,
 32-bit sequence number field.  Use of an Extended Sequence Number
 (ESN) MUST be negotiated by an SA management protocol.  Note that in
 IKEv2, this negotiation is implicit; the default is ESN unless 32-bit
 sequence numbers are explicitly negotiated.  (The ESN feature is
 applicable to multicast as well as unicast SAs.)
 The ESN facility allows use of a 64-bit sequence number for an SA.
 (See Appendix B, "Extended (64-bit) Sequence Numbers", for details.)
 Only the low-order 32 bits of the sequence number are transmitted in

Kent Standards Track [Page 8] RFC 4302 IP Authentication Header December 2005

 the AH header of each packet, thus minimizing packet overhead.  The
 high-order 32 bits are maintained as part of the sequence number
 counter by both transmitter and receiver and are included in the
 computation of the ICV, but are not transmitted.

2.6. Integrity Check Value (ICV)

 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 (IPv4 or IPv6) in length.  The details of ICV processing
 are described in Section 3.3.3, "Integrity Check Value Calculation",
 and Section 3.4.4, "Integrity Check Value Verification".  This field
 may include explicit padding, if required 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 and MUST
 insert only enough padding to satisfy the IPv4/IPv6 alignment
 requirements.  Details of how to compute the required padding length
 are provided below in Section 3.3.3.2, "Padding".  The integrity
 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

 AH may be employed in two ways: transport mode or tunnel mode.  (See
 the Security Architecture document for a description of when each
 should be used.)

3.1.1. Transport Mode

 In transport mode, AH is inserted after the IP header and before a
 next 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 next layer protocol.  (Note that
 the term "transport" mode should not be misconstrued as restricting
 its use to TCP and UDP.)  The following diagram illustrates AH
 transport mode positioning for a typical IPv4 packet, on a "before
 and after" basis.

Kent Standards Track [Page 9] RFC 4302 IP Authentication Header December 2005

                 BEFORE APPLYING AH
           ----------------------------
     IPv4  |orig IP hdr  |     |      |
           |(any options)| TCP | Data |
           ----------------------------
                 AFTER APPLYING AH
           -------------------------------------------------------
     IPv4  |original IP hdr (any options) | AH | TCP |    Data   |
           -------------------------------------------------------
           |<- mutable field processing ->|<- immutable fields ->|
           |<----- 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
 before or after or both before and 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 |
          ------------------------------------------------------------
          |<--- mutable field processing -->|<-- immutable fields -->|
          |<---- authenticated except for mutable fields ----------->|
  • = if present, could be before AH, after AH, or both
 ESP and AH headers can be combined in a variety of modes.  The IPsec
 Architecture document describes the combinations of security
 associations that must be supported.
 Note that in transport mode, 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 Standards Track [Page 10] RFC 4302 IP Authentication Header December 2005

3.1.2. Tunnel Mode

 In tunnel mode, the "inner" IP header carries the ultimate (IP)
 source and destination addresses, while an "outer" IP header contains
 the addresses of the IPsec "peers," e.g., addresses of security
 gateways.  Mixed inner and outer IP versions are allowed, i.e., IPv6
 over IPv4 and IPv4 over IPv6.  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 | | | orig IP hdr* | | |

      |new IP header * (any options) | AH | (any options) |TCP| Data |
      ----------------------------------------------------------------
      |<- mutable field processing ->|<------ immutable fields ----->|
      |<- 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|
      --------------------------------------------------------------
      |<--- mutable field -->|<--------- immutable fields -------->|
      |       processing     |
      |<-- authenticated except for mutable fields in new IP hdr ->|
  • = if present, construction of outer IP hdr/extensions and

modification of inner IP hdr/extensions is discussed in

            the Security Architecture document.

3.2. Integrity Algorithms

 The integrity algorithm employed for the ICV computation is specified
 by the SA.  For point-to-point communication, suitable integrity
 algorithms include keyed Message Authentication Codes (MACs) based on
 symmetric encryption algorithms (e.g., AES [AES]) or on one-way hash
 functions (e.g., MD5, SHA-1, SHA-256, etc.).  For multicast
 communication, a variety of cryptographic strategies for providing
 integrity have been developed and research continues in this area.

3.3. Outbound Packet Processing

 In transport mode, the sender inserts the AH header after the IP
 header and before a next layer protocol header, as described above.
 In tunnel mode, the outer and inner IP header/extensions can be

Kent Standards Track [Page 11] RFC 4302 IP Authentication Header December 2005

 interrelated in a variety of ways.  The construction of the outer IP
 header/extensions during the encapsulation process is described in
 the Security Architecture document.

3.3.1. 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 (or ESN) counter for this
 SA and inserts the low-order 32 bits of the value into the Sequence
 Number field.  Thus, the first packet sent using a given SA will
 contain 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.  The audit log entry for this event
 SHOULD include the SPI value, current date/time, Source Address,
 Destination Address, and (in IPv6) the cleartext Flow ID.
 The sender assumes anti-replay is enabled as a default, unless
 otherwise notified by the receiver (see Section 3.4.3) or if the SA
 was configured using manual key management.  Thus, typical behavior
 of an AH implementation calls for the sender to establish a new SA
 when the Sequence Number (or ESN) cycles, or in anticipation of this
 value cycling.
 If anti-replay is disabled (as noted above), 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.  (This behavior is recommended for multi-sender,
 multicast SAs, unless anti-replay mechanisms outside the scope of
 this standard are negotiated between the sender and receiver.)
 If ESN (see Appendix B) is selected, only the low-order 32 bits of
 the sequence number are transmitted in the Sequence Number field,
 although both sender and receiver maintain full 64-bit ESN counters.
 However, the high-order 32 bits are included in the ICV calculation.

Kent Standards Track [Page 12] RFC 4302 IP Authentication Header December 2005

 Note: If a receiver chooses not to enable anti-replay for an SA, then
 the receiver SHOULD NOT negotiate ESN in an SA management protocol.
 Use of ESN creates a need for the receiver to manage the anti-replay
 window (in order to determine the correct value for the high-order
 bits of the ESN, which are employed in the ICV computation), which is
 generally contrary to the notion of disabling anti-replay for an SA.

3.3.3. Integrity Check Value Calculation

 The AH ICV is computed over:
      o IP or extension header fields before the AH header 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 (low-order 32 bits), and the ICV (which is set
        to zero for this computation), and explicit padding bytes (if
        any))
      o everything after AH is assumed to be immutable in transit
      o the high-order bits of the ESN (if employed), and any implicit
        padding required by the integrity algorithm

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 Integrity Check Value 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 the Hop-by-Hop Extension Header)
 contain a flag indicating mutability, which determines appropriate
 processing for such options.

Kent Standards Track [Page 13] RFC 4302 IP Authentication Header December 2005

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)
         Differentiated Services Code Point (DSCP)
            (6 bits, see RFC 2474 [NBBB98])
         Explicit Congestion Notification (ECN)
            (2 bits, see RFC 3168 [RFB01])
         Flags
         Fragment Offset
         Time to Live (TTL)
         Header Checksum
 DSCP - Routers may rewrite the DS field as needed to provide a
 desired local or end-to-end service, thus its value upon reception
 cannot be predicted by the sender.
 ECN - This will change if a router along the route experiences
 congestion, and thus its value upon reception cannot be predicted by
 the sender.
 Flags - This field is excluded because 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.

Kent Standards Track [Page 14] RFC 4302 IP Authentication Header December 2005

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

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
         Source Address
         Destination Address (without Routing Extension Header)
 Mutable but predictable
         Destination Address (with Routing Extension Header)
 Mutable (zeroed prior to ICV calculation)
         DSCP (6 bits, see RFC2474 [NBBB98])
         ECN (2 bits, see RFC3168 [RFB01])
         Flow Label (*)
         Hop Limit
      (*) The flow label described in AHv1 was mutable, and in
          RFC 2460 [DH98] was potentially mutable.  To retain
          compatibility with existing AH implementations, the
          flow label is not included in the ICV in AHv2.

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

Kent Standards Track [Page 15] RFC 4302 IP Authentication Header December 2005

 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 [DH98] 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.

3.3.3.2. Padding and Extended Sequence Numbers

3.3.3.2.1. ICV Padding

 As mentioned in Section 2.6, the ICV field may include explicit
 padding if required 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 IPv4 or 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 ICV calculation,
 counted as part of the Payload Length, and transmitted at the end of
 the ICV field to enable the receiver to perform the ICV calculation.
 Inclusion of padding in excess of the minimum amount required to
 satisfy IPv4/IPv6 alignment requirements is prohibited.

3.3.3.2.2. Implicit Packet Padding and ESN

 If the ESN option is elected for an SA, then the high-order 32 bits
 of the ESN must be included in the ICV computation.  For purposes of
 ICV computation, these bits are appended (implicitly) immediately
 after the end of the payload, and before any implicit packet padding.
 For some integrity 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 and the 32
 high-order bits of the ESN, if enabled) 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

Kent Standards Track [Page 16] RFC 4302 IP Authentication Header December 2005

 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.  The document that
 defines an integrity algorithm MUST be consulted to determine if
 implicit padding is required as described above.  If the document
 does not specify an answer to this, then the default is to assume
 that implicit padding is required (as needed to match the packet
 length to the algorithm's blocksize.)  If padding bytes are needed
 but the algorithm does not specify the padding contents, then the
 padding octets MUST have a value of zero.

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 IPv4 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.  (This does not apply to IPv6, where there is no router-
 initiated fragmentation.)  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.
 NOTE: For transport mode -- As mentioned at the end of Section 3.1.1,
 bump-in-the-stack and bump-in-the-wire implementations may have to
 first reassemble a packet fragmented by the local IP layer, then
 apply IPsec, and then fragment the resulting packet.
 NOTE: For IPv6 -- For bump-in-the-stack and bump-in-the-wire
 implementations, it will be necessary to examine all the extension
 headers to determine if there is a fragmentation header and hence
 that the packet needs reassembling prior to IPsec processing.
 Fragmentation, whether performed by an IPsec implementation or by
 routers along the path between IPsec peers, significantly reduces
 performance.  Moreover, the requirement for an AH receiver to accept
 fragments for reassembly creates denial of service vulnerabilities.
 Thus, an AH implementation MAY choose to not support fragmentation
 and may mark transmitted packets with the DF bit, to facilitate Path
 MTU (PMTU) discovery.  In any case, an AH implementation MUST support
 generation of ICMP PMTU messages (or equivalent internal signaling
 for native host implementations) to minimize the likelihood of
 fragmentation.  Details of the support required for MTU management
 are contained in the Security Architecture document.

Kent Standards Track [Page 17] RFC 4302 IP Authentication Header December 2005

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 nonzero 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 zeroing 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 via lookup in
 the SAD.  For a unicast SA, this determination is based on the SPI or
 the SPI plus protocol field, as described in Section 2.4.  If an
 implementation supports multicast traffic, the destination address is
 also employed in the lookup (in addition to the SPI), and the sender
 address also may be employed, as described in Section 2.4.  (This
 process is described in more detail in the Security Architecture
 document.)  The SAD entry for the SA also indicates whether the
 Sequence Number field will be checked and whether 32- or 64-bit
 sequence numbers are employed for the SA.  The SAD entry for the SA
 also specifies the algorithm(s) employed for ICV computation, and
 indicates the key required to validate the ICV.
 If no valid Security Association exists for this packet 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 that SA management traffic, such as IKE packets, does not need
 to be processed based on SPI, i.e., one can de-multiplex this traffic
 separately based on Next Protocol and Port fields, for example.)

Kent Standards Track [Page 18] RFC 4302 IP Authentication Header December 2005

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.
 Anti-replay is applicable to unicast as well as multicast SAs.
 However, this standard specifies no mechanisms for providing anti-
 replay for a multi-sender SA (unicast or multicast).  In the absence
 of negotiation (or manual configuration) of an anti-replay mechanism
 for such an SA, it is recommended that sender and receiver checking
 of the Sequence Number for the SA be disabled (via negotiation or
 manual configuration), as noted below.
 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, "Sequence Number Generation"), 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
 receive 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.
 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.
 If the ESN option is selected for an SA, only the low-order 32 bits
 of the sequence number are explicitly transmitted, but the receiver
 employs the full sequence number computed using the high-order 32
 bits for the indicated SA (from his local counter) when checking the
 received Sequence Number against the receive window.  In constructing
 the full sequence number, if the low-order 32 bits carried in the

Kent Standards Track [Page 19] RFC 4302 IP Authentication Header December 2005

 packet are lower in value than the low-order 32 bits of the
 receiver's sequence number counter, the receiver assumes that the
 high-order 32 bits have been incremented, moving to a new sequence
 number subspace.  (This algorithm accommodates gaps in reception for
 a single SA as large as 2**32-1 packets.  If a larger gap occurs,
 additional, heuristic checks for re-synchronization of the receiver's
 sequence number counter MAY be employed, as described in Appendix B.)
 If the received packet falls within the window and is not a
 duplicate, 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.
 A MINIMUM window size of 32 packets 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 receive window size should be increased for higher-speed
 environments, irrespective of assurance issues.  Values for minimum
 and recommended receive window sizes for very high-speed (e.g.,
 multi-gigabit/second) devices are not specified by this standard.

3.4.4. Integrity Check Value Verification

 The receiver computes the ICV over the appropriate fields of the
 packet, using the specified integrity algorithm, and verifies that it
 is the same as the ICV included in the ICV field of the packet.
 Details of the computation are provided below.
 If the computed and received ICVs 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.
 Implementation Note:
    Implementations can use any set of steps that results in the same
    result as the following set of steps.  Begin by saving the ICV
    value and replacing it (but not any ICV field padding) with zero.
    Zero all other fields that may have been modified during transit.
    (See Section 3.3.3.1, "Handling Mutable Fields", for a discussion
    of which fields are zeroed before performing the ICV calculation.)

Kent Standards Track [Page 20] RFC 4302 IP Authentication Header December 2005

    If the ESN option is elected for this SA, append the high-order 32
    bits of the ESN after the end of the packet.  Check the overall
    length of the packet (as described above), and if it requires
    implicit padding based on the requirements of the integrity
    algorithm, append zero-filled bytes to the end of the packet
    (after the ESN if present) as required.  Perform the ICV
    computation and compare the result with the saved value, using the
    comparison rules defined by the algorithm specification.  (For
    example, if a digital signature and one-way hash are used for the
    ICV computation, the matching process is more complex.)

4. Auditing

 Not all systems that implement AH will implement auditing.  However,
 if AH is incorporated into a system that supports auditing, then the
 AH implementation MUST also support auditing and MUST allow a system
 administrator to enable or disable auditing for AH.  For the most
 part, the granularity of auditing is a local matter.  However,
 several auditable events are identified in this specification, and
 for each of these events a minimum set of information that SHOULD be
 included in an audit log is defined.  Additional information also MAY
 be included in the audit log for each of these events, and additional
 events, not explicitly called out in this specification, also MAY
 result in audit log entries.  There is no requirement for the
 receiver to transmit any message to the purported sender in response
 to the detection of an auditable event, because of the potential to
 induce denial of service via such action.

5. Conformance Requirements

 Implementations that claim conformance or compliance with this
 specification MUST fully implement the AH syntax and processing
 described here for unicast traffic, and MUST comply with all
 requirements of the Security Architecture document [Ken-Arch].
 Additionally, if an implementation claims to support multicast
 traffic, it MUST comply with the additional requirements specified
 for support of such traffic.  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.
 The mandatory-to-implement algorithms for use with AH are described
 in a separate RFC [Eas04], to facilitate updating the algorithm
 requirements independently from the protocol per se.  Additional
 algorithms, beyond those mandated for AH, MAY be supported.

Kent Standards Track [Page 21] RFC 4302 IP Authentication Header December 2005

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 2402

 This document differs from RFC 2402 [RFC2402] in the following ways.
      o SPI -- modified to specify a uniform algorithm for SAD lookup
        for unicast and multicast SAs, covering a wider range of
        multicast technologies.  For unicast, the SPI may be used
        alone to select an SA, or may be combined with the protocol,
        at the option of the receiver.  For multicast SAs, the SPI is
        combined with the destination address, and optionally the
        source address, to select an SA.
      o Extended Sequence Number -- added a new option for a 64-bit
        sequence number for very high-speed communications.  Clarified
        sender and receiver processing requirements for multicast SAs
        and multi-sender SAs.
      o Moved references to mandatory algorithms to a separate
        document [Eas04].

8. Acknowledgements

 The author would like to acknowledge the contributions of Ran
 Atkinson, who played a critical role in initial IPsec activities, and
 who authored the first series of IPsec standards: RFCs 1825-1827.
 Karen Seo deserves special thanks for providing help in the editing
 of this and the previous version of this specification.  The author
 also would like to thank the members of the IPsec and MSEC working
 groups who have contributed to the development of this protocol
 specification.

9. References

9.1. Normative References

 [Bra97]    Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Level", BCP 14, RFC 2119, March 1997.
 [DH98]     Deering, S. and R.  Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.

Kent Standards Track [Page 22] RFC 4302 IP Authentication Header December 2005

 [Eas04]    3rd Eastlake, D., "Cryptographic Algorithm Implementation
            Requirements for Encapsulating Security Payload (ESP) and
            Authentication Header (AH)", RFC 4305, December 2005.
 [Ken-Arch] Kent, S. and K. Seo, "Security Architecture for the
            Internet Protocol", RFC 4301, December 2005.
 [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791, September
            1981.
 [RFC1108]  Kent, S., "U.S. Department of Defense Security Options for
            the Internet Protocol", RFC 1108, November 1991.

9.2. Informative References

 [AES]      Advanced Encryption Standard (AES), Federal Information
            Processing Standard 197, National Institutes of Standards
            and Technology, November 26, 2001.
 [HC03]     Holbrook, H. and B. Cain, "Source Specific Multicast for
            IP", Work in Progress, November 3, 2002.
 [IKEv2]    Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
            Protocol", RFC 4306, December 2005.
 [Ken-ESP]  Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
            4303, December 2005.
 [NBBB98]   Nichols, K., Blake, S., Baker, F., and D. Black,
            "Definition of the Differentiated Services Field (DS
            Field) in the IPv4 and IPv6 Headers", RFC 2474, December
            1998.
 [RFB01]    Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
            of Explicit Congestion Notification (ECN) to IP", RFC
            3168, September 2001.
 [RFC1063]  Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
            MTU discovery options", RFC 1063, July 1988.
 [RFC1122]  Braden, R., "Requirements for Internet Hosts -
            Communication Layers", STD 3, RFC 1122, October 1989.
 [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
            November 1990.
 [RFC1385]  Wang, Z., "EIP: The Extended Internet Protocol", RFC 1385,
            November 1992.

Kent Standards Track [Page 23] RFC 4302 IP Authentication Header December 2005

 [RFC1393]  Malkin, G., "Traceroute Using an IP Option", RFC 1393,
            January 1993.
 [RFC1770]  Graff, C., "IPv4 Option for Sender Directed Multi-
            Destination Delivery", RFC 1770, March 1995.
 [RFC2113]  Katz, D., "IP Router Alert Option", RFC 2113, February
            1997.
 [RFC2402]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC
            2402, November 1998.
 [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
            Group Domain of Interpretation", RFC 3547, July 2003.
 [RFC3740]  Hardjono, T. and B. Weis, "The Multicast Group Security
            Architecture", RFC 3740, March 2004.

Kent Standards Track [Page 24] RFC 4302 IP Authentication Header December 2005

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 RFC 1700, "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
    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, DH98 (IPv6)]
    0   0     10  Experimental Measurement
    1   2     13  Experimental Flow Control
    1   0     14  Experimental Access Ctl
    0   0     15  ???
    1   0     16  IMI Traffic Descriptor
    1   0     19  Address Extension
 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

Kent Standards Track [Page 25] RFC 4302 IP Authentication Header December 2005

 being processed by the application to which the option contents are
 directed (e.g., Resource Reservation Protocol (RSVP)/Internet Group
 Management Protocol (IGMP)), the packet should encounter AH
 processing.  However, AH processing would require that each router
 along the path is a member of a multicast-SA defined by the SPI.
 This might pose problems for packets that are not strictly source
 routed, and it requires multicast support techniques not currently
 available.
 NOTE: Addition or removal of security labels (e.g., Basic Security
 Option (BSO), Extended Security Option (ESO), or Commercial Internet
 Protocol Security Option (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 that 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)                    [DH98]
     BIT INDICATES IF OPTION IS MUTABLE (CHANGES UNPREDICTABLY DURING
     TRANSIT)
       Hop-by-Hop options                  [DH98]
       Destination options                 [DH98]
     NOT APPLICABLE
       Fragmentation                       [DH98]
     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
 [DH98] for more information.

Kent Standards Track [Page 26] RFC 4302 IP Authentication Header December 2005

     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 Integrity Check Value 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.

Kent Standards Track [Page 27] RFC 4302 IP Authentication Header December 2005

Appendix B: Extended (64-bit) Sequence Numbers

B1. Overview

 This appendix describes an Extended Sequence Number (ESN) scheme for
 use with IPsec (ESP and AH) that employs a 64-bit sequence number,
 but in which only the low-order 32 bits are transmitted as part of
 each packet.  It covers both the window scheme used to detect
 replayed packets and the determination of the high-order bits of the
 sequence number that are used both for replay rejection and for
 computation of the ICV.  It also discusses a mechanism for handling
 loss of synchronization relative to the (not transmitted) high-order
 bits.

B2. Anti-Replay Window

 The receiver will maintain an anti-replay window of size W.  This
 window will limit how far out of order a packet can be, relative to
 the packet with the highest sequence number that has been
 authenticated so far.  (No requirement is established for minimum or
 recommended sizes for this window, beyond the 32- and 64-packet
 values already established for 32-bit sequence number windows.
 However, it is suggested that an implementer scale these values
 consistent with the interface speed supported by an implementation
 that makes use of the ESN option.  Also, the algorithm described
 below assumes that the window is no greater than 2^31 packets in
 width.)  All 2^32 sequence numbers associated with any fixed value
 for the high-order 32 bits (Seqh) will hereafter be called a sequence
 number subspace.  The following table lists pertinent variables and
 their definitions.
      Var.   Size
      Name  (bits)             Meaning
      ----  ------   ---------------------------
      W       32     Size of window
      T       64     Highest sequence number authenticated so far,
                     upper bound of window
        Tl      32     Lower 32 bits of T
        Th      32     Upper 32 bits of T
      B       64     Lower bound of window
        Bl      32     Lower 32 bits of B
        Bh      32     Upper 32 bits of B
      Seq     64     Sequence Number of received packet
        Seql    32     Lower 32 bits of Seq
        Seqh    32     Upper 32 bits of Seq

Kent Standards Track [Page 28] RFC 4302 IP Authentication Header December 2005

 When performing the anti-replay check, or when determining which
 high-order bits to use to authenticate an incoming packet, there are
 two cases:
   + Case A: Tl >= (W - 1). In this case, the window is within one
                            sequence number subspace.  (See Figure 1)
   + Case B: Tl < (W - 1).  In this case, the window spans two
                            sequence number subspaces.  (See Figure 2)
 In the figures below, the bottom line ("----") shows two consecutive
 sequence number subspaces, with zeros indicating the beginning of
 each subspace.  The two shorter lines above it show the higher-order
 bits that apply.  The "====" represents the window.  The "****"
 represents future sequence numbers, i.e., those beyond the current
 highest sequence number authenticated (ThTl).
      Th+1                         *********
      Th               =======*****
  1. -0——–+—–+—–0——–+———–0–

Bl Tl Bl

                                      (Bl+2^32) mod 2^32
                          Figure 1 -- Case A
      Th                           ====**************
      Th-1                      ===
  1. -0—————–+–0–+————–+–0–

Bl Tl Bl

                                               (Bl+2^32) mod 2^32
                          Figure 2 -- Case B

B2.1. Managing and Using the Anti-Replay Window

 The anti-replay window can be thought of as a string of bits where
 `W' defines the length of the string.  W = T - B + 1 and cannot
 exceed 2^32 - 1 in value.  The bottom-most bit corresponds to B and
 the top-most bit corresponds to T, and each sequence number from Bl
 through Tl is represented by a corresponding bit.  The value of the
 bit indicates whether or not a packet with that sequence number has
 been received and authenticated, so that replays can be detected and
 rejected.

Kent Standards Track [Page 29] RFC 4302 IP Authentication Header December 2005

 When a packet with a 64-bit sequence number (Seq) greater than T is
 received and validated,
    + B is increased by (Seq - T)
    + (Seq - T) bits are dropped from the low end of the window
    + (Seq - T) bits are added to the high end of the window
    + The top bit is set to indicate that a packet with that sequence
      number has been received and authenticated
    + The new bits between T and the top bit are set to indicate that
      no packets with those sequence numbers have been received yet.
    + T is set to the new sequence number
 In checking for replayed packets,
    + Under Case A: If Seql >= Bl (where Bl = Tl - W + 1) AND
      Seql <= Tl, then check the corresponding bit in the window to
      see if this Seql has already been seen.  If yes, reject the
      packet.  If no, perform integrity check (see Appendix B2.2
      below for determination of SeqH).
    + Under Case B: If Seql >= Bl (where Bl = Tl - W + 1) OR
      Seql <= Tl, then check the corresponding bit in the window to
      see if this Seql has already been seen.  If yes, reject the
      packet.  If no, perform integrity check (see Appendix B2.2
      below for determination of Seqh).

B2.2. Determining the Higher-Order Bits (Seqh) of the Sequence Number

 Because only `Seql' will be transmitted with the packet, the receiver
 must deduce and track the sequence number subspace into which each
 packet falls, i.e., determine the value of Seqh.  The following
 equations define how to select Seqh under "normal" conditions; see
 Appendix B3 for a discussion of how to recover from extreme packet
 loss.
    + Under Case A (Figure 1):
      If Seql >= Bl (where Bl = Tl - W + 1), then Seqh = Th
      If Seql <  Bl (where Bl = Tl - W + 1), then Seqh = Th + 1
    + Under Case B (Figure 2):
      If Seql >= Bl (where Bl = Tl - W + 1), then Seqh = Th - 1
      If Seql <  Bl (where Bl = Tl - W + 1), then Seqh = Th

Kent Standards Track [Page 30] RFC 4302 IP Authentication Header December 2005

B2.3. Pseudo-Code Example

 The following pseudo-code illustrates the above algorithms for anti-
 replay and integrity checks.  The values for `Seql', `Tl', `Th', and
 `W' are 32-bit unsigned integers.  Arithmetic is mod 2^32.
      If (Tl >= W - 1)                            Case A
          If (Seql >= Tl - W + 1)
              Seqh = Th
              If (Seql <= Tl)
                  If (pass replay check)
                      If (pass integrity check)
                          Set bit corresponding to Seql
                          Pass the packet on
                      Else reject packet
                  Else reject packet
              Else
                  If (pass integrity check)
                      Tl = Seql (shift bits)
                      Set bit corresponding to Seql
                      Pass the packet on
                  Else reject packet
          Else
              Seqh = Th + 1
              If (pass integrity check)
                  Tl = Seql (shift bits)
                  Th = Th + 1
                  Set bit corresponding to Seql
                  Pass the packet on
              Else reject packet
      Else                                    Case B
          If (Seql >= Tl - W + 1)
              Seqh = Th - 1
              If (pass replay check)
                  If (pass integrity check)
                      Set the bit corresponding to Seql
                      Pass packet on
                  Else reject packet
              Else reject packet
          Else
              Seqh = Th
              If (Seql <= Tl)
                  If (pass replay check)
                      If (pass integrity check)
                          Set the bit corresponding to Seql
                          Pass packet on
                      Else reject packet
                  Else reject packet

Kent Standards Track [Page 31] RFC 4302 IP Authentication Header December 2005

              Else
                  If (pass integrity check)
                      Tl = Seql (shift bits)
                      Set the bit corresponding to Seql
                      Pass packet on
                  Else reject packet

B3. Handling Loss of Synchronization due to Significant Packet Loss

 If there is an undetected packet loss of 2^32 or more consecutive
 packets on a single SA, then the transmitter and receiver will lose
 synchronization of the high-order bits, i.e., the equations in
 Appendix B2.2. will fail to yield the correct value.  Unless this
 problem is detected and addressed, subsequent packets on this SA will
 fail authentication checks and be discarded.  The following procedure
 SHOULD be implemented by any IPsec (ESP or AH) implementation that
 supports the ESN option.
 Note that this sort of extended traffic loss seems unlikely to occur
 if any significant fraction of the traffic on the SA in question is
 TCP, because the source would fail to receive ACKs and would stop
 sending long before 2^32 packets had been lost.  Also, for any bi-
 directional application, even ones operating above UDP, such an
 extended outage would likely result in triggering some form of
 timeout.  However, a unidirectional application, operating over UDP,
 might lack feedback that would cause automatic detection of a loss of
 this magnitude, hence the motivation to develop a recovery method for
 this case.
 The solution we've chosen was selected to:
   + minimize the impact on normal traffic processing.
   + avoid creating an opportunity for a new denial of service attack
     such as might occur by allowing an attacker to force diversion of
     resources to a re-synchronization process.
   + limit the recovery mechanism to the receiver because anti-replay
     is a service only for the receiver, and the transmitter generally
     is not aware of whether the receiver is using sequence numbers in
     support of this optional service.  It is preferable for recovery
     mechanisms to be local to the receiver.  This also allows for
     backward compatibility.

Kent Standards Track [Page 32] RFC 4302 IP Authentication Header December 2005

B3.1. Triggering Re-synchronization

 For each SA, the receiver records the number of consecutive packets
 that fail authentication.  This count is used to trigger the re-
 synchronization process, which should be performed in the background
 or using a separate processor.  Receipt of a valid packet on the SA
 resets the counter to zero.  The value used to trigger the re-
 synchronization process is a local parameter.  There is no
 requirement to support distinct trigger values for different SAs,
 although an implementer may choose to do so.

B3.2. Re-synchronization Process

 When the above trigger point is reached, a "bad" packet is selected
 for which authentication is retried using successively larger values
 for the upper half of the sequence number (Seqh).  These values are
 generated by incrementing by one for each retry.  The number of
 retries should be limited, in case this is a packet from the "past"
 or a bogus packet.  The limit value is a local parameter.  (Because
 the Seqh value is implicitly placed after the AH (or ESP) payload, it
 may be possible to optimize this procedure by executing the integrity
 algorithm over the packet up to the endpoint of the payload, then
 compute different candidate ICVs by varying the value of Seqh.)
 Successful authentication of a packet via this procedure resets the
 consecutive failure count and sets the value of T to that of the
 received packet.
 This solution requires support only on the part of the receiver,
 thereby allowing for backward compatibility.  Also, because re-
 synchronization efforts would either occur in the background or
 utilize an additional processor, this solution does not impact
 traffic processing and a denial of service attack cannot divert
 resources away from traffic processing.

Author's Address

 Stephen Kent
 BBN Technologies
 10 Moulton Street
 Cambridge, MA  02138
 USA
 Phone: +1 (617) 873-3988
 EMail: kent@bbn.com

Kent Standards Track [Page 33] RFC 4302 IP Authentication Header December 2005

Full Copyright Statement

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 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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Acknowledgement

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 Internet Society.

Kent Standards Track [Page 34]

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