IPv6 Neighbor Discovery Protocol

The IPv6 NDP (Neighbor Discovery Protocol) is defined under RFC 4861 and is a protocol that is useful for determining neighbor nodes. NDP is an IPv6 alternative for the below:

– ARP : Address Resolution Protocol

– ICMP Router Discovery

– ICMP Redirect Message

Nodes can perform the below with help of Neighbor Discovery Protocol:

– Resolve neighboring node link-layer address where IPv6 packet is sent

– Monitor and note the change of a link-layer address for a neighboring node

– To check if the neighboring node is reachable

Hosts use Neighbor discovery Protocol for:

– Discovering Neighbor routers

– To have address prefixes, routes , addresses and other configurations be auto configured

Routers use Neighbor Discovery for:

– Host configurations, routes are advertised on link prefixes.

– Share information about a better next hop address to the hosts

=> Different types of Neighbor Discovery Messages

ICMPv6 Type 133 -> Router Solicitation
(ICMPv6 Type 134 -> Router Advertisement
ICMPv6 Type 135 -> Neighbor Solicitation
ICMPv6 Type 136 -> Neighbor Advertisement
ICMPv6 Type 137 -> Redirect

1. Router Solicitation: IPv6 hosts send this message to find IPv6 routers on the link. The Router solicitation is sent by the host which makes the IPv6 enables routers respond to the message.

In Ethernet Header of Router Solicitation Message

Source Address -> Sending Network Adapter MAC address

Destination Address -> 33-33-00-00-00-02

In IPv6 Header of Router Solicitation Message

Source Address -> Link-Local IPv6 address or IPv6 unspecified address (::)

Destination Address -> Link Local scope Multicast for all routers (FF02::2)

2. Router Advertisement: This message contains the information that hosts require to determine link MTU, Link prefixes, specific routes, suggestion if auto configuration be used and validity for the address that will be created by auto configuration.

In Ethernet Header of Router Advertisement Message

Source Address -> Sending Network Adapter MAC address

Destination Address -> 33-33-00-00-00-01 or unicast MAC address of host from where the Router Solicitation arrived.

In IPv6 Header of Router Advertisement Message

Source Address -> Link-Local IPv6 address of sending interface

Destination Address -> Link Local scope Multicast for all nodes (FF02::1) or unicast IPv6 address of host from where the Router Solicitation arrived.

3. Neighbor Solicitation: This message is used to find link-layer address of IPv6 node. Usually it contains the Sender’s Link-Layer address. For Address Resolution these messages are multicast and after verifying the reach ability they are unicast.

In Ethernet Header of Neighbor Solicitation Message

Source Address -> Sending Network Adapter MAC address

Destination Address -> Ethernet MAC address that is corresponding to the target address of solicited node.

In IPv6 Header of Neighbor Solicitation Message

Source Address -> The Unicast IPv6 address of sending interface or IPv6 unspecified address (::)

Destination Address -> Target’s solicited Node address for Multicast Neighbor solicitation. Unicast IPv6 address of target for Unicast Neighbor Solicitation.

4. Neighbor Advertisement: Sent in response to the Neighbor solicitation message. The IPv6 Nodes also send this message informing about changes in link-layer addresses to the neighboring nodes.

In Ethernet Header of Neighbor Advertisement Message

Source Address -> Sending Network Adapter MAC address

Destination Address -> Initial Neighbors Solicitation sender’s Unicast MAC address for Solicited Neighbor Advertisement. Destination field set to 33-33-00-00-00-01 for Unsolicited Neighbor Advertisement.

In IPv6 Header of Neighbor Advertisement Message

Source Address -> The Unicast IPv6 address of sending interface

Destination Address -> Initial Neighbors Solicitation sender’s Unicast IP address for Solicited Neighbor Advertisement. Destination field set to link-local scope multicast address for all nodes for Unsolicited Neighbor Advertisement.

5. Redirect: This is sent to inform about a better first hop about a specific destination to an IPv6 host by the IPv6 router. These are sent by routers for unicast traffic are unicast to originating hosts and can be processed by hosts only.

In Ethernet Header of Redirect Message

Source Address -> Sending Network Adapter MAC address

Destination Address -> Unicast MAC address of originating router

In IPv6 Header of Redirect Message

Source Address -> The Unicast IPv6 address of sending interface

Destination Address -> Original Hosts Unicast IP address.

Read more