Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 174
ICMP

Like IPv4, IPv6 does not provide error detection. This is a function of ICMP. ICMP for IPv6 has been addressed as well.

Internet Control Message Protocol (ICMP) for IPv6 is found in RFCs 1885 and 1970. ICMP for IPv6 is explained in RFC 1885, but the individual functions of ICMP for IPv6 (for example, path MTU discovery) are detailed in existing RFCs such as RFC 1191. Hopefully, you are not too confused? If you are, welcome to the world of RFCs. IPv6 and its extension protocols used previous RFCs if they were found relevant to the protocol.

ICMPv6 is an IPv4 extension. It is originally documented in RFC 792 and is an integral part of the IP. Along the years, other functions that utilize ICMP were added, such as router discovery (RFC 1256). ICMPv6 is a version of ICMP for IPv6 (seeing a “v6” after a protocol name is a very common way of specifying which version of IP you are representing when explaining the protocol). There are currently two RFCs that define all the ICMP functions for IPv6: RFC 1885 and RFC 1970. RFC 1885 provides for information on new functions and names the older functions that made it through the review process. RFC 1970 includes the discovery protocols of RFC 1256 and a few other discovery protocols. It also includes the redirect message.

ICMPv6, as defined in RFCs 1885 and 1970, is currently using control and information messages previously defined in RFCs 791, 1112, and 1191. Therefore, the procedures for certain ICMP functions continue to be defined in their respective RFCs. You must read the original RFCs to fully explain the procedures used.

As indicated in the previous text on IP, the Internet Protocol is an “unreliable” protocol. ICMP is an add-on protocol that does not make IP reliable, but is a control message protocol, the purpose of which is to provide feedback about problems in the communication environment. There are still no guarantees that a datagram will be delivered or a control message will be returned. Some datagrams may still be undelivered without any report of their loss. The higher-level protocols that use IPv6 must still implement their own reliability procedures if reliable communication is required.

The ICMP messages typically report errors in the processing of datagrams. To avoid the problem of ICMP messages about ICMP messages, none are sent.

ICMPv6 is used by IPv6 nodes to report errors encountered in processing packets, and to perform other internet-layer functions, such as diagnostics (ICMPv6 “ping”) and multicast membership reporting. ICMPv6 is an integral part of IPv6 and must be fully implemented by every IPv6 node.