RFC 2473 Generic Packet Tunneling in IPv6 December 1998 Note: This document refers in particular to tunnels between two nodes identified by unicast addresses - such tunnels look like "virtual point to point links". The mechanisms described herein apply also to tunnels in which the exit-point nodes are identified by other types of addresses, such as anycast or multicast. These tunnels may look like "virtual point to multipoint links". At the time of writing this document, IPv6 anycast addresses are a subject of ongoing specification and experimental work. Tunnel from node B to node C <----------------------> Tunnel Tunnel Entry-Point Exit-Point Node Node +-+ +-+ +-+ +-+ |A|-->--//-->--|B|=====>=====//=====>=====|C|-->--//-->--|D| +-+ +-+ +-+ +-+ Original Original Packet Packet Source Destination Node Node Fig.1 Tunnel An IPv6 tunnel is a unidirectional mechanism - tunnel packet flow takes place in one direction between the IPv6 tunnel entry-point and exit-point nodes (see Fig.1). Tunnel from Node B to Node C <------------------------> Tunnel Tunnel Original Entry-Point Exit-Point Original Packet Node Node Packet Source Destination Node Node +-+ +-+ +-+ +-+ | |-->--//-->--| |=====>=====//=====>======| |-->--//-->--| | |A| |B| |C| |D| | |--<--//--<--| |=====<=====//=====<======| |--<--//--<--| | +-+ +-+ +-+ +-+ Original Original Packet Packet Destination Tunnel Tunnel Source Node Exit-Point Entry-Point Node Node Node <-------------------------> Tunnel from Node C to Node B Fig.2 Bi-directional Tunneling Mechanism Conta & Deering Standards Track