Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 162
IPv6 Tunneling Introduction

Tunneling is very simply the method of transporting IPv6 packets over IPv4 routing topologies. It is being used today with the 6Bone (www/6Bone.com). Two scenarios occur here. The first one is the following two tunneling methods:

Host to Router: Dual-stack IP hosts can tunnel IPv6 packets to an intermediary dual-stack IP router that is reachable via an IPv4 infrastructure.

Router to Router: A router that is running the dual-stack IP interconnected by an IPv4 infrastructure can tunnel IPv6 datagrams between routers.

With these types of tunnels, the tunnel endpoint is an intermediary router that must decapsulate the IPv6 packet and forward it to its final destination. The endpoint of the tunnel is different from the destination of the packet being tunneled. Therefore, the address in the IPv6 packet being tunneled does not provide the IPv4 address of the tunnel endpoint. The tunnel endpoint address must be determined from information that is configured on the node performing the tunneling. This is the configured tunnel approach. The endpoint is explicitly configured.

Tunnels are characterized by two endpoint IPv4 addresses. The IPv4 protocol identifier is 41, the assigned payload type number for IPv6.

Router to Host: Dual-stack IP routers can tunnel IPv6 packets to their final destination dual-stack IP host.

Host to Host: Dual-stack IP hosts can tunnel IPv6 packets between themselves over an IPv6 infrastructure (without the use of a router).

These two types provide for tunneling all the way to a final destination. In these cases, the tunnel endpoint is the node to which the IPv6 packet is addressed. This is automatic tunneling and it simply allows for IPv6 packets that are to be sent to IPv6 destinations using the IPv4-compatible address and located remotely (off-link) to be encapsulated in IPv4 headers and sent through the IPv4 infrastructure.