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ICMP (Internet Control Message Protocol)

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ICMP is an error-reporting protocol that works in concert with IP. If an error on the network occurs, such as a failure in one of the paths, IP sends an ICMP error message within an IP datagram. ICMP, therefore, requires IP as its transport mechanism.

Routers send ICMP messages in response to datagrams that cannot be delivered or had other problems. The router puts an ICMP message in an IP datagram and sends it back to the source of the datagram that could not be delivered.

The ping command uses ICMP as a probe to test whether a station is reachable. Ping packages an ICMP echo request message in a datagram and sends it to a selected destination. The user chooses the destination by specifying its IP address or name on the command line in a form such as:


When the destination receives the echo request message, it responds by sending an ICMP echo reply message. If a reply is not returned within a set time, ping resends the echo request several more times. If no reply arrives, ping indicates that the destination is unreachable.

There are five ICMP error messages that may be returned to a sender in response to problems on the network or with datagrams. These messages can indicate information such as depleted buffers at the destination, lost packets, unreachable hosts, alternate network paths, and the need to fragment datagrams.

ICMP is discussed in the following RFCs:

  • RFC 792 (Internet Control Message Protocol, September 1981)

  • RFC 1256 (ICMP Router Discovery Messages, September 1991)

  • RFC 1788 (ICMP Domain Name Messages, April 1995)

  • RFC 2463 (Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6, December 1998)

  • RFC 2521 (ICMP Security Failures Messages, March 1999)

Another utility that uses ICMP is traceroute, which provides a list of all the routers along the path to a specified destination.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.