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Table Of Contents  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)

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Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
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ICMP Overview, History, Versions and Standards
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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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