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Overview
Though not a true protocol, routers and hosts (nodes)
use Neighbor Discovery (ND) messages to determine the link-layer addresses
of neighbors that reside on attached links and to overwrite invalid
cache entries. Hosts also use ND to find neighboring routers that
can forward packets on their behalf.
In addition, nodes use ND to actively track the
ability to reach neighbors. When a router (or the path to a router)
fails, nodes actively search for alternatives to reach the destination.
IPv6 Neighbor Discovery corresponds to a number
of the IPv4 protocols — ARP, ICMP Router Discovery, and ICMP
Redirect. However, Neighbor Discovery provides many improvements over
the IPv4 set of protocols. These improvements address the following:
- Router discovery—How a host locates routers residing
on an attached link.
- Prefix discovery—How a host discovers address prefixes
for destinations residing on an attached link. Nodes use prefixes
to distinguish between destinations that reside on an attached link
and those destinations that it can reach only through a router.
- Parameter discovery—How a node learns various parameters
(link parameters or Internet parameters) that it places in outgoing
packets.
- Address resolution—How a node uses only a destination
IPv6 address to determine a link-layer address for destinations on
an attached link.
- Next-hop determination—The algorithm that a node
uses for mapping an IPv6 destination address into a neighbor IPv6
address (either the next router hop or the destination itself) to
which it plans to send traffic for the destination.
- Neighbor unreachability detection—How a node determines
that it can no longer reach a neighbor.
- Duplicate address detection—How a node determines
whether an address is already in use by another node.
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