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ICMP Concepts and General Operation
The Internet Control Message Protocol
(ICMP) is one of the under-appreciated worker bees of the
networking world. Everyone knows how important key protocols such as
the Internet Protocol are to TCP/IP, but few realize that the suite
as a whole relies on many functions that ICMP provides. Originally created
to allow the reporting of a small set of error conditions, ICMP messages
are now used to implement a wide range of error-reporting, feedback
and testing capabilities. While each message type is unique, they are
all implemented using a common message format, sent and received based
on relatively simple protocol rules. This makes ICMP one of the easiest
TCP/IP protocols to understand. (Yes, I actually said something in this
Guide was easy!)
In this section I provide a general
description of ICMP. I begin with an overview of ICMP, discussing its
purpose, history, and the versions and standards that define it. I describe
the general method by which ICMP operates, and also discuss the rules
that govern how and when ICMP messages are created and processed. I
then outline the common format used for ICMP messages in ICMPv4 and
ICMPv6, and how data is encapsulated in them in general terms. I conclude
with a discussion of ICMP message classifications, and a summary of
different message types and codes for both version 4 and version 6.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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