Neighbor Discovery defines five new Internet Control Message Protocol (ICMP) messages. The messages serve the following purposes:
Router solicitation – When an interface becomes enabled, hosts can send router solicitation messages. The solicitations request routers to generate router advertisements immediately, rather than at their next scheduled time.
Router advertisement – Routers advertise their presence, various link parameters, and various Internet parameters. Routers advertise either periodically, or in response to a router solicitation message. Router advertisements contain prefixes that are used for on-link determination or address configuration, a suggested hop-limit value, and so on.
Neighbor solicitation – Nodes send neighbor solicitation messages to determine the link-layer address of a neighbor. Neighbor solicitation messages are also sent to verify that a neighbor is still reachable by a cached link-layer address. Neighbor solicitations are also used for duplicate address detection.
Neighbor advertisement – A node sends neighbor advertisement messages in response to a neighbor solicitation message. The node can also send unsolicited neighbor advertisements to announce a link-layer address change.