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

Chapter 182
IPv6 Algorithm

If you are looking for more information on how IPv6 routes datagrams, you must first read RFC 1970. This is a very important RFC for your understanding of the IPv6 routing algorithm. It contains the Neighbor Discovery mechanism and includes everything on IPv6 subnets, such as hosts and routers. IPv6 does not use ARP; it uses Neighbor Discovery.

IPv6 needs the previously listed cache entries to assist in routing a datagram. If an IPv6 node needs to transmit a datagram, it must first find out the next hop towards the destination (known as next-hop determination). In other words, it must determine if the destination station is local or remote, and therefore sends the packet directly to the destination or utilizes a router. This process uses the Prefix List cache and the Destination cache. Once the next hop is known, it must determine the next hop’s link-layer address.

To route a datagram in IPv6, we consult the Destination cache, the Prefix List, and the Default Router List to determine the IP address of the appropriate next hop. The next-hop determination is invoked to create a Destination Cache entry. The results of next-hop determination computations are saved in the Destination cache (which also contains updates learned from Redirect messages).

Therefore, a sending node first looks in the Destination cache for a matching entry to the destination IP address. If one is not found, the Prefix List cache is consulted. The sending node compares the destination prefix mask with the entries in the Prefix List cache. If a match is found, it is then determined whether the destination is local or remote. If it is local, then the next-hop address is simply the destination address of the datagram; otherwise, the destination is remote and the node must select a router from the default router list. If there are no entries in the default router list, then the destination is assumed to be local. The results of this next-hop determination lookup are stored in the Destination cache (along with received ICMP redirects).

Once the next hop has been determined, the corresponding entry is added to the Destination cache and the Neighbor cache is used to determine the media address of that next-hop neighbor.

Once the IP address of the next-hop node is known, the sender examines the Neighbor cache for link-layer information about that neighbor. If no entry exists, the sender creates one, and then starts the address resolution procedure to complete the entry. The datagram to be transmitted must wait for this to complete. Once the neighbor entry is complete, this entry will be used for subsequent transfers to that destination station. No other procedures are needed.