ICMP Message Creation and Processing Conventions and Rules
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In the overview of ICMP I compared the relationship between IP and ICMP to that between an executive and an administrative assistant. One of the characteristics that many executives value in a good assistant is that the assistant does his or her work independently, and without causing unnecessary disruption. A good assistant should save the executive time, not cost him or her time.
As the assistant to IP, ICMP must similarly help IP function without taking up too much of its time. Here, the resource being conserved is not so much time as bandwidth. ICMP messages are important, but must be considered part of the overhead of running a network. They carry no user data, so each one represents a small loss of overall end-user bandwidth on the network. For this reason, we want to send them only when necessary, and to carefully control the circumstances under which they are generated.
Now, administrative assistants have some serious advantages over networking protocols: common sense and experience. They usually know where the line is drawn between help and hindrance; computers don't. To partially compensate, ICMP's operation is guided by a set of conventions or rules for how messages are created and processed. For ICMPv4, these conventions are described in part in the defining RFC 792, but much more in RFC 1122 (Requirements for Internet Hosts Communication Layers), which provides specific information on implementing TCP/IP in host devices. In ICMPv6, the information related to ICMP implementation that appears in RFC 1122 has been largely incorporated into the main document that defines ICMPv6, RFC 2463.
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