Neighbor discovery

Neighbor discovery (ND) corresponds to a combination of the IPv4 protocols ARP, ICMP Router Discovery, and ICMP Redirect. Nodes (hosts and routers) use ND to determine the link-layer addresses for neighbors that are known to reside on attached links and to quickly purge cached values that become invalid. Hosts also use ND to find neighboring routers that are able to forward packets on their behalf. ND also defines a Neighbor Unreachability Detection algorithm. IPv4 does not contain a generally agreed upon protocol for performing Neighbor Unreachability Detection, although Dead Gateway Detection does address a subset of the problems that Neighbor Unreachability Detection solves.

Neighbor Discovery is used to do the following tasks:

Obtain configuration information that includes:
Router Discovery

Defines how hosts can automatically locate routers that reside on an attached link.

Prefix Discovery
Specifies how hosts discover the following sets of prefixes:
- Prefixes that are defined as being on-link (IPv6 address prefixes that reside on the shared link, such as an Ethernet link)
- Prefixes that are defined as being off-link (IPv6 address prefixes that can be reached by using an adjacent router)
- Prefixes that are to be used for Stateless Address Autoconfiguration
Parameter Discovery

Allows a host to learn link parameters, such as the link MTU, and IP parameters, such as the hop limit to place in outgoing packets.
Perform address resolution. Address resolution allows a node to determine the link-layer address of an on-link destination given the destination IP address.
Dynamically learn routes which can be used in next-hop determination. This specifies the algorithm for mapping the IP destination address into the IP address of the neighbor to which traffic is to be sent. The next-hop can be either a router or the destination itself. Next-hop determination uses the on-link prefixes learned as part of Prefix Discovery to determine when the next hop is the destination itself.
Determine when a neighbor is no longer reachable using Neighbor Unreachability Detection.
Process Redirect messages. Routers use Redirect messages to notify a node that a better next-hop node is to be used when forwarding packets to a particular destination. The new next-hop could be the actual destination, if the destination is on-link, or a different router, if the destination is off-link.