System Administration Guide: IP Services

Neighbor Solicitation and Unreachability

Neighbor solicitation messages can also be used to determine if more than one node has been assigned the same unicast address.

Neighbor unreachability detection detects the failure of a neighbor or the failure of the forward path to the neighbor. This detection requires positive confirmation that packets that are sent to a neighbor are actually reaching that neighbor and being processed properly by its IP layer. Neighbor unreachability detection uses confirmation from two sources. When possible, upper-layer protocols provide a positive confirmation that a connection is making forward progress. That is, data that was sent previously is known to have been delivered correctly. For example, new TCP acknowledgments were received recently. When positive confirmation is not forthcoming through such hints, a node sends unicast neighbor solicitation messages. These messages solicit neighbor advertisements as reachability confirmation from the next hop. To reduce unnecessary network traffic, probe messages are sent only to neighbors to which the node is actively sending packets.

In addition to addressing the previous general problems, neighbor discovery also handles the following situations.