3.4.2  Security Association Lookup

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

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

3.4.3  Sequence Number Verification

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





 
RFC 2402                IP Authentication Header           November 1998


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

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

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

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

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









 
RFC 2402                IP Authentication Header           November 1998


   DISCUSSION:

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

3.4.4  Integrity Check Value Verification

   The receiver computes the ICV over the appropriate fields of the