The informational and error messages found in ICMPv6 are very similar to the messages implemented by ICMPv4. But, ICMPv6 has new and improved features and functionality which not found in ICMPv4. ICMPv6 messages are encapsulated in IPv6. ICMPv6 has four new protocols as part of the Neighbor Discovery Protocol. Messaging between an IPv6 enabled router and an IPv6 enabled devices are following:
- Router Solicitation (RS) message
- Router Advertisement (RA) message
- Neighbor Solicitation(NS) message
- Neighbor Advertisement (NA) message
Router Solicitation (RS) message
When a host is configured to get its addressing information automatically using SLAAC, the host will send an RS message to the router at a system start-up time. The RS message is sent as an IPv6 all-routers multicast message to address FF02::2.
Router Advertisement (RA) message
RA messages are sent periodically by a router to provide the following information to hosts:
- Addressing information using SLAAC such as the prefix, prefix length, DNS address and domain name.
- IPv6 prefixes for automatic address configuration
- Information such as MTU, hop limit reachable time, retransmission time for neighbor solicitations
Neighbor Solicitation(NS) message and Neighbor Advertisement (NA) message are used for Address resolution and Duplicate Address Detection (DAD).
Address Resolution
Address Resolution is used when a device knows the IPv6 unicast address of the destination device on the same link but does not know its MAC address. This is equivalent to an ARP Request for IPv4. To determine the MAC address of the destination, the device will send an NS message to the solicited-node address. The message will contain the known destination IPv6 address. The destination device that will respond with an NA message containing its Ethernet MAC address.
Duplicate Address Detection
IPv4 nodes use ARP Request messages and a method called gratuitous ARP to detect a duplicate unicast IPv4 address on the local link. in the same way, IPv6 nodes use Neighbor Solicitation (NS) messages to detect the uniqueness of an address on the local link in a process known as duplicate address detection (DAD). The device will send an NS message with its own IPv6 address as the targeted IPv6 address. If another device on the network has this address, it will respond with an NA message. This NA message will notify the sending device that the address is in use. If a corresponding NA message is not returned within a certain period of time, the unicast address is unique and acceptable for use.
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