IPv6 tunneling

Tunneling provides a way to use an existing IPv4 routing infrastructure to carry IPv6 traffic.

The key to a successful IPv6 transition is compatibility with the existing installed base of IPv4 hosts and routers. Maintaining compatibility with IPv4 while deploying IPv6 streamlines the task of transitioning the Internet to IPv6. While the IPv6 infrastructure is being deployed, the existing IPv4 routing infrastructure can remain functional, and can be used to carry IPv6 traffic.

IPv6 or IPv4 hosts and routers can tunnel IPv6 datagrams over regions of IPv4 routing topology by encapsulating them within IPv4 packets. Tunneling can be used in a variety of ways:

Item Description
Router-to-Router IPv6 or IPv4 routers interconnected by an IPv4 infrastructure can tunnel IPv6 packets between themselves. In this case, the tunnel spans one segment of the end-to-end path that the IPv6 packet takes.
Host-to-Router IPv6 or IPv4 hosts can tunnel IPv6 packets to an intermediary IPv6 or IPv4 router that is reachable through an IPv4 infrastructure. This type of tunnel spans the first segment of the packet's end-to-end path.
Host-to-Host IPv6 or IPv4 hosts that are interconnected by an IPv4 infrastructure can tunnel IPv6 packets between themselves. In this case, the tunnel spans the entire end-to-end path that the packet takes.
Router-to-Host IPv6/IPv4 routers can tunnel IPv6 packets to their final destination IPv6 or IPv4 host. This tunnel spans only the last segment of the end-to-end path.

Tunneling techniques are usually classified according to the mechanism by which the encapsulating node determines the address of the node at the end of the tunnel. In router-to-router or host-to-router methods, the IPv6 packet is being tunneled to a router. In host-to-host or router-to-host methods, the IPv6 packet is tunneled all the way to its final destination.

The entry node of the tunnel (the encapsulating node) creates an encapsulating IPv4 header and transmits the encapsulated packet. The exit node of the tunnel (the decapsulating node) receives the encapsulated packet, removes the IPv4 header, updates the IPv6 header, and processes the received IPv6 packet. However, the encapsulating node needs to maintain soft state information for each tunnel, such as the maximum transmission unit (MTU) of the tunnel, to process IPv6 packets forwarded into the tunnel.

There are two types of tunnels in IPv6:
automatic tunnels
Automatic tunnels are configured by using IPv4 address information embedded in an IPv6 address – the IPv6 address of the destination host includes information about which IPv4 address the packet should be tunneled to.
configured tunnels
Configured tunnels must be configured manually. These tunnels are used when using IPv6 addresses that do not have any embedded IPv4 information. The IPv6 and IPv4 addresses of the endpoints of the tunnel must be specified.

For information on configuring automatic and configured tunnels, see Setting up tunneling in IPv6.