ICMP Version 6 (ICMPv6) Informational Message Types and Formats
In the previous section we explored a number of ICMPv6 error messages, which are sent back to the originator of an IPv6 datagram when an error is detected in it that makes it impossible to be delivered. Just as was the case with the original version of ICMP (ICMPv4), ICMPv6 also defines another class of message: informational messages. These ICMPv6 messages are used not to report errors, but to allow the sharing of information required to implement various test, diagnostic and support functions critical to the operation of IPv6.
In this section I describe eight different ICMPv6 informational messages in five topics (six of these messages are used in matching pairs, and the pairs are described together). I begin by describing ICMPv6 Echo Request and Echo Reply messages, used for network connectivity testing. I explain the format of Router Advertisement and Router Solicitation messages, used to let hosts discover local routers and learn necessary parameters from them. I then describe ICMPv6 Neighbor Advertisement and Neighbor Solicitation messages, used for various communications between hosts on a local network, including IPv6 address resolution. I discuss IPv6 Redirect messages, which let routers inform hosts of better first-hop routers, and IPv6 Router Renumbering messages.
Several of the ICMPv6 informational messages include additional information that is either optional, recommended or mandatory, depending on the circumstances under which the message is generated. Some of these are shared between message types, so they are described in a separate topic at the end of the section.
In IPv4, the use of many of the ICMP informational messages was described in a variety of different standards. In IPv6, many of the functions using informational messages have been gathered together and formalized in the IPv6 Neighbor Discovery (ND) Protocol. The solicitation and advertisement of local routers and neighboring hosts, as well as communication of redirection information are both examples of activities for which ND is responsible. In fact, five of the ICMP messages described in this section are actually defined in the Neighbor Discovery standard, RFC 2461.
ND and ICMPv6 are obviously closely related, given that ND describes the use of several of the ICMP messages: Router Advertisement, Router Solicitation, Neighbor Advertisement, Neighbor Solicitation and Redirect. Thus, just as ICMPv4 is an important assistant to IPv4, both ICMPv6 and ND are important helpers for IPv6. In this Guide, I provide most of the description of how these messages are used in the detailed section on ND. In this section, I provide only a brief summary of their use, while focusing primarily on message format and the meaning of each of the fields in that format.
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