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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

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Mobile IP Security Considerations
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ICMP Concepts and General Operation
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Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)

The Internet Protocol (IP) is the key network layer protocol that implements the TCP/IP protocol suite. Since IP is the protocol that provides the mechanism for delivering datagrams between devices, it is designed to be relatively basic, and to function with few “bells and whistles”. One of the provisions it lacks is some way to allow errors to be reported back to a transmitting device, and for tests and special tasks to be accomplished. However, these capabilities are necessary for the operation of an internetwork, so TCP/IP defines an adjunct protocol for IP that provides them: the Internet Control Message Protocol (ICMP)

In this section I describe the operation of ICMP using two main subsections. The first subsection describes ICMP concepts and operation in general terms, including a look at the overall method by which ICMP messages are packaged and the circumstances under which they are sent. The second subsection provides detailed descriptions of each of the individual message types supported by ICMP.

ICMP was developed to be a companion to the original Internet Protocol, version 4. With the creation of IP version 6 (IPv6), a new version of ICMP called (ta-da) ICMP version 6 (ICMPv6) was created as well, and the original ICMP is now sometimes called ICMPv4 to differentiate it, just as the “old” IP is now often called “IPv4”. These two versions have some differences in their specifics, but really are very similar in overall operation. For this reason, I decided to integrate the general operation description of both versions of ICMP, and just point out where the differences are between them. The area where IPv4 and IPv6 most differ is in specific message types and formats, so these have been described separately in the second subsection.

Background Information: Due to the close relationship between ICMP and IP, this section assumes familiarity with basic IP concepts, including IP addressing, the general format of IP datagrams and how they are routed. To better understand ICMPv6 details, you may also want to reference the IPv6 addressing and datagram encapsulation topics, and especially the IPv6 Neighbor Discovery (ND) protocol.


Quick navigation to subsections and regular topics in this section



Previous Topic/Section
Mobile IP Security Considerations
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ICMP Concepts and General Operation
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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