IPv6 NDP (Neighbour Discovery Protocol)

What is IPv6 NDP?

IPv6 NDP (Neighbor Discovery Protocol) is the Protocol that provide Network Nodes’ Discovery as its name implies. In the IPv4 World, we were using ARP, but in IPv6 world there is no ARP. Instead, ICMP based NDP is used for Neighbor Discovery. With NDP Mechanims, Local Network devices are learned.

NDP, uses five types of ICMPv6 Messages and a specific Multicast Address type, Solicited-node Multicast Address for its operations.

What are these five ICMP Messages. These are:

• Neighbor Solicitation Message
• Neighbour Advertisement Message
• Router Advertisement Message
• Router Solicitation Message
• Redirect Message

IPv6 NDP has some important roles. Let’s check these roles:

• Prefix / Parameter Discovery
• Neighbor and router reachability verification
• Router Discovery
• Neighbor Discovery
• Next-Hop Redirection
• SLAAC (Stateless Address Configuration)
• DAD (Duplicate Adderess Detection)
Etc.

Let’s explain these roles one by one and see NDP (Neighbor Discovery Protocol) Operations.

In IPv6, Prefix and Parameter Information is discovered with ICMPv6 Messages. These information is carried by Router Advertisement Messages. With this information, Hosts become familier that which IPv6 Prefixes are in that Link and what are the parameters.

In IPv6 NDP Concept, the next-hop is determined like IPv4, towards the longest match. If there is no match, then to the Default Route.

IPv6 NDP Concept also provides Neighbor unreachability detection. This is also done with ICMPv6.

Router Discovery

In IPv4 world, we can manually configure or rely on DHCP to give a Gateway Address to an Host. But this is a little different in IPv6. In IPv6, with Router Discovery Mechanism, Routers on the link can be detected by the Hosts. This is done with ICMPv6 Router Solicitation Messages (Type 133) and Router Advertisement Messages (Type 134).

A Host on the link sends a Multicast Router Solicitation Message that implies “Are there any Router on this Link?”. Routers on the link send Multicast Router Advertisement Messages as a response. With this message, they say that “I am a Router in this link”.

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