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Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
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
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.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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