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.
Link-layer address change – A node that knows its link-layer address has been changed can multicast a few (unsolicited) neighbor advertisement packets. The node can multicast to all nodes to update cached link-layer addresses that have become invalid. The sending of unsolicited advertisements is a performance enhancement only. The detection algorithm for neighbor unreachability ensures that all nodes reliably discover the new address, though the delay might be somewhat longer.
Inbound load balancing – Nodes with replicated interfaces might want to load-balance the reception of incoming packets across multiple network interfaces on the same link. Such nodes have multiple link-layer addresses assigned to the same interface. For example, a single network driver can represent multiple network interface cards as a single logical interface that has multiple link-layer addresses.
Load balancing is handled by allowing routers to omit the source link-layer address from router advertisement packets. Consequently, neighbors must use neighbor solicitation messages to learn link-layer addresses of routers. Returned neighbor advertisement messages can then contain link-layer addresses that differ, depending on who issued the solicitation.
Anycast addresses – Anycast addresses identify one of a set of nodes that provide an equivalent service. Multiple nodes on the same link can be configured to recognize the same anycast address. Neighbor discovery handles anycasts by setting nodes to expect to receive multiple neighbor advertisements for the same target. All advertisements for anycast addresses are tagged as being non-override advertisements. Non-override advertisements invoke specific rules to determine which of potentially multiple advertisements should be used.
Proxy advertisements – A router that accept packets on behalf of a target address that is unable to respond to neighbor solicitations can issue non-override neighbor advertisements. Currently, the use of proxy is not specified. However, proxy advertising can potentially be used to handle cases like mobile nodes that have moved off-link. However, the use of proxy is not intended as a general mechanism to handle nodes that do not implement this protocol.