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IPv6 ND General Operational Overview: ND Functions, Functional Groups and Message Types
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As I mentioned in the
preceding overview, the name of the Neighbor
Discovery (ND) protocol really does not do it justice. The protocol
facilitates not merely the discovery of neighboring devices, but a substantial
number of functions related to local network connectivity, datagram
routing and configuration. Both regular hosts and routers in an IPv6
environment count on the ND protocol to facilitate important exchanges
of information that are necessary for proper internetwork operation.
The Neighbor Discovery protocol has
a number of similarities to the Internet
Control Message Protocol. An important
one is that like ICMP, ND is a messaging protocol. It
doesn't implement a single specific function but rather a group of activities
that are performed through the exchange of messages. This means we can't
explain the operation of ND through a specific description of “what
ND does”, but rather must define its operation by means of a list
of messages ND provides and specific ways that they are used.
Any local network on an internet
will have both regular hosts and routers, and the term neighbor
can refer to either. Of course, hosts and routers play different roles
on a network, and as a result neighbor discovery is very different
for each. The ND standard describes nine specific functions performed
by the protocol. To better understand these functions and how they are
related, we can divide them into three functional groups based on communication
type and the kinds of devices involved (see Figure 169).
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