RFC 2473 (rfc2473) - Page 6 of 36
Generic Packet Tunneling in IPv6 Specification
Alternative Format: Original Text Document
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
Conta & Deering Standards Track