RFC 2473 (rfc2473) - Page 6 of 36

Generic Packet Tunneling in IPv6 Specification

RFC 2473            Generic Packet Tunneling in IPv6       December 1998


   Bi-directional tunneling is achieved by merging two unidirectional
   mechanisms, that is, configuring two tunnels, each in opposite
   direction to the other - the entry-point node of one tunnel is the
   exit-point node of the other tunnel (see Fig.2).

3.1 IPv6 Encapsulation

   IPv6 encapsulation consists of prepending to the original packet an
   IPv6 header and, optionally, a set of IPv6 extension headers (see
   Fig.3), which are collectively called tunnel IPv6 headers.  The
   encapsulation takes place in an IPv6 tunnel entry-point node, as the
   result of an original packet being forwarded onto the virtual link
   represented by the tunnel. The original packet is processed during
   forwarding according to the forwarding rules of the protocol of that
   packet. For instance if the original packet is an:

    (a)  IPv6 packet, the IPv6 original header hop limit is  decremented
         by one.

    (b)  IPv4 packet, the IPv4 original header time to live field (TTL)
         is decremented by one.

   At encapsulation, the source field of the tunnel IPv6 header is
   filled with an IPv6 address of the tunnel entry-point node, and the
   destination field with an IPv6 address of the tunnel exit-point.
   Subsequently, the tunnel packet resulting from encapsulation is sent
   towards the tunnel exit-point node.

                            +----------------------------------//-----+
                            | Original |                              |
                            |          |   Original Packet Payload    |
                            | Header   |                              |
                            +----------------------------------//-----+
                             <            Original Packet            >
                                              |
                                              v
        <       Original Packet                 >

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

                       Fig.3 Encapsulating a Packet





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