Neighbor Discovery Protocol

The Neighbor Discovery Protocol (NDP) is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 (IPv6). It operates in the Link Layer of the Internet model and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer addresses of other nodes, duplicate address detection, finding available routers and Domain Name System (DNS) servers, address prefix discovery, and maintaining reachability information about the paths to other active neighbor nodes.

The protocol defines five different ICMPv6 packet types to perform functions for IPv6 similar to the Address Resolution Protocol (ARP) and Internet Control Message Protocol (ICMP) Router Discovery and Router Redirect protocols for IPv4. However, it provides many improvements over its IPv4 counterparts. For example, it includes Neighbor Unreachability Detection (NUD), thus improving robustness of packet delivery in the presence of failing routers or links, or mobile nodes.

Technical details

NDP defines the following five ICMPv6 packet types and their purposes:

1. Router Solicitation - used by hosts to locate routers on an attached link. Nodes which forward packets not addressed to them generate Router Advertisements immediately upon receipt of this message rather than at their next scheduled time.
2. Router Advertisement - used by routers to advertise their presence together with various link and Internet parameters either periodically, or in response to a Router Solicitation message
3. Neighbor Solicitation - used by nodes to determine the link-layer address of a neighbor, or to verify that a neighbor is still reachable via a cached link-layer address
4. Neighbor Advertisement - used by nodes to respond to a Neighbor Solicitation message
5. Redirect - used by routers to inform hosts of a better first hop for a destination

These messages are used to provide the following functionality:

• Router discovery: hosts can locate routers residing on attached links.
• Prefix discovery: hosts can discover address prefixes that are on-link for attached links.
• Parameter discovery: hosts can find link parameters (e.g., MTU).
• Address autoconfiguration: stateless configuration of addresses of network interfaces.
• Address resolution: mapping between IP addresses and link-layer addresses.
• Next-hop determination: hosts can find next-hop routers for a destination.
• Neighbor unreachability detection (NUD): determine that a neighbor is no longer reachable on the link.
• Duplicate address detection (DAD): nodes can check whether an address is already in use.
• Redirect: router can inform a node about better first-hop routers.
• Recursive DNS Server (RDNSS) and DNS Search List (DNSSL) assignment via a router advertisement (RA) options. This is a new feature and not widely supported by clients.