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| Communication protocol | |
 When defining its messages, NDP follows ICMPv6 message format. | |
| Purpose | Auxiliary protocol for IPv6 |
|---|---|
| Developer(s) | Internet Engineering Task Force |
| Introduction | March 1996 |
| OSI layer | Network layer |
| RFC(s) | 1970, 2461, 4861 |
The Neighbor Discovery Protocol (NDP), or simply Neighbor Discovery (ND), is a protocol of the Internet protocol suite used with Internet Protocol Version 6 (IPv6).[1]: §1 It operates at the internet layer of the Internet model,[2] and is responsible for gathering various information required for network communication, including the configuration of local connections and the domain name servers and gateways.
The protocol defines five 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. It provides many improvements over its IPv4 counterparts.[1]: §3.1 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.
The Inverse Neighbor Discovery (IND) protocol extension allows nodes to determine and advertise an IPv6 address corresponding to a given link-layer address, similar to Inverse ARP for IPv4.[3]
The Secure Neighbor Discovery Protocol (SEND), a security extension of NDP, uses Cryptographically Generated Addresses (CGA) and the Resource Public Key Infrastructure (RPKI) to provide an alternative mechanism for securing NDP with a cryptographic method that is independent of IPsec. Neighbor Discovery Proxy (ND Proxy) provides a service similar to IPv4 Proxy ARP and allows bridging multiple network segments within a single subnet prefix when bridging cannot be done at the link layer.[4]
| Internet protocol suite |
|---|
| Application layer |
| Transport layer |
| Internet layer |
| Link layer |
NDP defines five ICMPv6 packet types for the purpose of router solicitation, router advertisement, neighbor solicitation, neighbor advertisement, and network redirects.[1]
These messages are used to provide the following functionality:
IANA maintains a list of all current NDP options as they are published.[7]
Two computers, A and B are connected to the same local area network with no intervening gateway or router. A has a packet to send to IP address 2001:db8::55 which happens to be the address of B.
Before sending the packet to B, A creates a solicited-node multicast address by appending the least-significant 24 bits of B's address to the prefix ff02::1:ff00:0/104, which is ff02::1:ff00:55 and creates a solicited-node multicast MAC address by appending the least-significant 24 bits of B's solicited-node multicast address to the prefix 33:33:FF:xx:xx:xx,[8] which is 33:33:FF:00:00:55. A sends a neighbor solicitation message requesting an answer for 2001:db8::55 (destination ff02::1:ff00:55 IP address and destination 33:33:FF:00:00:55 MAC address), which is accepted by B which is listening on its own solicited-node multicast address on the local network. B responds with a neighbor advertisement message containing its MAC and IP addresses. A receives the response and sends the packet on the link with B's MAC address.
Typically, network nodes maintain a lookup cache that associates IP and MAC addressees. In this example, if A had the lookup cached, then it would not need to send the NDP request. Also, when B received the request, it could cache the lookup to A so that if B needs to send a packet to A later, it does not need to use NDP to lookup its MAC address. Finally, when A receives the NDP response, it can cache the lookup for future messages addressed to the same IP address.
