value 51 in its Next Header (or Protocol) field [STD-2].

   Example high-level diagrams of IP datagrams with the Authentication
   Header follow.

 +------------+-------------------+------------+-------+---------------+
 | IPv6 Header| Hop-by-Hop/Routing| Auth Header| Others| Upper Protocol|
 +------------+-------------------+------------+-------+---------------+

                Figure 1: IPv6 Example
















 
RFC 1826                IP Authentication Header             August 1995


   When used with IPv6, the Authentication Header normally appears after
   the IPv6 Hop-by-Hop Header and before the IPv6 Destination Options.

    +-------------+--------------+-------------------------------+
    | IPv4 Header |  Auth Header | Upper Protocol (e.g. TCP, UDP)|
    +-------------+--------------+-------------------------------+

                   Figure 2:  IPv4 Example


   When used with IPv4, the Authentication Header normally follows the
   main IPv4 header.

3.1 Authentication Header Syntax

   The authentication data is the output of the authentication algorithm
   calculated over the the entire IP datagram as described in more
   detail later in this document.  The authentication calculation must
   treat the Authentication Data field itself and all fields that are
   normally modified in transit (e.g., TTL or Hop Limit) as if those
   fields contained all zeros.  All other Authentication Header fields
   are included in the authentication calculation normally.

   The IP Authentication Header has the following syntax:

     +---------------+---------------+---------------+---------------+
     | Next Header   | Length        |           RESERVED            |
     +---------------+---------------+---------------+---------------+
     |                    Security Parameters Index                  |
     +---------------+---------------+---------------+---------------+
     |                                                               |
     +     Authentication Data (variable number of 32-bit words)     |
     |                                                               |
     +---------------+---------------+---------------+---------------+
      1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8


                   Figure 3:  Authentication Header syntax














 
RFC 1826                IP Authentication Header             August 1995


3.2 Fields of the Authentication Header

   NEXT HEADER
      8 bits wide.  Identifies the next payload after the Authentication
      Payload.  This values in this field are the set of IP Protocol
      Numbers as defined in the most recent RFC from the Internet
      Assigned Numbers Authority (IANA) describing "Assigned Numbers"
      [STD-2].

   PAYLOAD LENGTH
      8 bits wide.  The length of the Authentication Data field in 32-
      bit words.  Minimum value is 0 words, which is only used in the
      degenerate case of a "null" authentication algorithm.