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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 Concepts and General Operation

ICMP Concepts and General Operation 1 2 3 ICMP General Operation

ICMP Standards for IPv4 and IPv6

The defining standard I just quoted, by the way, is RFC 792. This was the initial defining standard for ICMP, titled simply Internet Control Message Protocol. It was published at the same time as the standard for IP, which was RFC 791. This is further indication that IP and ICMP really are a “team” of sorts.

Due to the close relationship between the two, when the new version 6 of the Internet Protocol (IPv6) was developed in the mid-1990s, it was necessary to define a new version of ICMP as well. This was of course called the Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification, first published as RFC 1885 in 1995, and revised in RFC 2463 in 1998. Just as the original IP is now often called IPv4 to differentiate it from IPv6, the original ICMP is now also called ICMPv4.

These two RFCs, 792 and 2463, define the basic operation of ICMPv4 and ICMPv6 respectively, and also describe some of the ICMP message types supported by each version of the protocol. ICMPv4 and ICMPv6 are very similar in most general respects, though they have some differences, most of which are a direct result of the changes made to IP itself. Another document, RFC 1122 (Requirements for Internet Hosts - Communication Layers) contains rules for how ICMPv4 is used, as we will see in the topic on ICMP message creation and processing conventions. RFC 1812 (Requirements for IP Version 4 Routers) is also relevant.

ICMP Concepts and General Operation 1 2 3 ICMP General Operation