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

Chapter 179
ICMP and Neighbor Discovery

Address resolution is accomplished via Neighbor Discovery messages, which are generated and processed by ICMP. The following are those messages:

Router Solicitation. Hosts send Router Solicitations in order to prompt routers to generate Router Advertisements quickly.

Router Advertisement. Routers send out Router Advertisement messages periodically, or in response to Router Solicitations. These messages contain information related to the local prefixes and if the router can act as a default router.

Neighbor Solicitation. Nodes send Neighbor Solicitations to request the link-layer address of a target node while also providing their own link-layer address to the target. Neighbor Solicitations are multicast when the node needs to resolve an address and unicast when the node seeks to verify the reachability of a neighbor.

Neighbor Advertisement. A node sends Neighbor Advertisements in response to Neighbor Solicitations, and sends unsolicited Neighbor Advertisements in order to (unreliably) propagate new information quickly. For example, if a node has determined some changes, such a link-level address change, it can quickly relay this information to its neighbors.

Redirect. Routers send Redirect packets to inform a host of a better first-hop node on the path to a destination. Hosts can be redirected to a better first-hop router, but can also be informed by a redirect that the destination is in fact a neighbor. The latter is accomplished by setting the ICMP Target Address equal to the ICMP Destination Address.