Neighbor Discovery Protocol – NDP Overview

NDP stands for Neighbor Discovery Protocol, an IPv6 protocol responsible for tasks such as stateless autoconfiguration, address resolution, Neighbor Unreachability Detection (NUD), and Duplicate Address Detection (DAD). It operates at Layer 2, the Data Link Layer, of the OSI model and was developed to improve data transmission efficiency and consistency across multiple networks and processes.

Unlike IPv4, we no longer use Address Resolution Protocol or ARP in IPv6. IPv6 Neighbor Discovery replaces this function.


Neighbor Discovery Protocol Features

Now, let’s discuss the different functions of NDP:

  1. Stateless Address Autoconfiguration (SLAAC) – enables each host on the network to auto-configure its unique IPv6 link-local address and global unicast address without the help of a DHCP server. Link-local addresses can be used to talk with other hosts on the same network, while global unicast addresses are routable on the Internet.
  2. Address Resolution – The basic concepts of address resolution in IPv6 are not that different from those in IPv4 ARP. Resolution is still dynamic and based on using a cache table that maintains pairings of IPv6 addresses and MAC addresses.
  3. Neighbor Unreachability Detection (NUD) – detects when a host is no longer reachable.
  4. Duplicate Address Detection (DAD) – verifies that a unicast IPv6 address is unique before being assigned to a host interface.


NDP ICMPv6 Message Types

Neighbor Discovery Protocol uses ICMPv6 messages to perform all its functions. Let’s discuss the five different types of ICMPv6:

  1. Router Solicitation – Router Solicitation messages (RS) are sent by hosts when they boot up to find any routers in a local segment and to request that they advertise their presence on the network.
  2. Router Advertisement – Router Advertisement messages (RA) are used by an IPv6 router to advertise its presence on the network. These Router Advertisements contain information like the router’s IPv6 address, MAC address, MTU, etc.
  3. Neighbor Solicitation Message – Neighbor Solicitation messages (NS) are sent by a host to determine a remote host’s link-layer IPv6 address. The destination address will be the solicited-node multicast address of the remote host. It verifies that a neighbor or remote host is still reachable via a cached link-layer address.
  4. Neighbor Advertisement Message – A host use Neighbor Advertisement messages (NA) to respond to NS message. If a remote host receives an NS message, it returns a NA message to the sender host. A host also uses this message to announce a link-layer address change.
  5. Redirect – IPv6 routers use this message to notify an originating host of a better next-hop address for a specific destination. Only routers can send unicast traffic redirect messages. Only hosts process redirect messages.

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