RFC 2473 (rfc2473) - Page 7 of 36

Generic Packet Tunneling in IPv6 Specification

RFC 2473            Generic Packet Tunneling in IPv6       December 1998


   Tunnel extension headers should appear in the order recommended by
   the specifications that define the extension headers, such as [IPv6-
   Spec].

   A source of original packets and a tunnel entry-point that
   encapsulates those packets can be the same node.

3.2 Packet Processing in Tunnels

   The intermediate nodes in the tunnel process the IPv6 tunnel packets
   according to the IPv6 protocol.  For example, a tunnel Hop by Hop
   Options extension header is processed by each receiving node in the
   tunnel; a tunnel Routing extension header identifies the intermediate
   processing nodes, and controls at a finer granularity the forwarding
   path of the tunnel packet through the tunnel; a tunnel Destination
   Options extension header is processed at the tunnel exit-point node.

3.3 IPv6 Decapsulation

   Decapsulation is graphically shown in Fig.4:

     +---------+- - - - - -+----------------------------------//-----+
     | IPv6    | IPv6      |                                         |
     |         | Extension |        Original Packet                  |
     | Header  | Headers   |                                         |
     +---------+- - - - - -+----------------------------------//-----+
      <                      Tunnel IPv6 Packet                     >
                                      |
                                      v
                           +----------------------------------//-----+
                           | Original |                              |
                           |          |   Original Packet Payload    |
                           | Headers  |                              |
                           +----------------------------------//-----+
                            <            Original Packet            >


                     Fig.4 Decapsulating a Packet

   Upon receiving an IPv6 packet destined to an IPv6 address of a tunnel
   exit-point node, its IPv6 protocol layer processes the tunnel
   headers. The strict left-to-right processing rules for extension
   headers is applied. When processing is complete, control is handed to
   the next protocol engine, which is identified by the Next Header
   field value in the last header processed. If this is set to a tunnel
   protocol value, the tunnel protocol engine discards the tunnel
   headers and passes the resulting original packet to the Internet or
   lower layer protocol identified by that value for further processing.



Conta & Deering             Standards Track