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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats                      9  ICMP Version 6 (ICMPv6) Informational Message Types and Formats

ICMPv6 Echo Request and Echo Reply Messages 1 2 3 ICMPv6 Neighbor Advertisement and Neighbor Solicitation Messages

At the highest level, we can separate IP devices into two groups: hosts and routers. Both participate in the use of the internetwork, but they have different roles. An important IP principle related to this division is that routers take care of routing—moving data between networks—while hosts generally don't need to worry about this job. Hosts rely on the routers on their local networks to facilitate communication to all other hosts except those on the local network.

The implications of this are clear: a host cannot really use an internetwork until it knows the identity of at least one local router, and the method by which that router is to be used. In IPv4, a technique known as router discovery was invented, which provides a means by which a host can locate a router and learn important parameters related to the operation of the local network.

Router discovery in IPv6 works in a very similar manner, by having routers send Router Advertisement messages both on a regular basis and in response to hosts prompting for them using Router Solicitation messages. The router discovery function has been incorporated into the Neighbor Discovery (ND) protocol, where it is part of a larger class of tools that I call host-router discovery functions.

ICMPv6 Echo Request and Echo Reply Messages 1 2 3 ICMPv6 Neighbor Advertisement and Neighbor Solicitation Messages