Internet Control Message Protocol ICMP

IP is a connection less, unreliable delivery service. There is no provision in IP to report errors that occur in the intervening communication channel, routing between hosts. The Internet Control Message Protocol (ICMP) provides this reporting. As the name suggests ICMP is a separate protocol from IP, but is an integral part of IP's operation.

Description from RFC792:-

The Internet Protocol IP is not designed to be absolutely reliable. The purpose of these control messages (ICMP) is to provide feedback about problems in the communication environment, not to make IP reliable. There are still no guarantees that a datagram will be delivered or a control message will be returned. Some datagrams may still be undelivered without any report of their loss. The higher level protocols that use IP TCP must implement their own reliability procedures if reliable communication is required.

The ICMP messages typically report errors in the processing of datagrams. To avoid the infinite regress of messages about messages etc., no ICMP messages are sent about ICMP messages. Also ICMP messages are only sent about errors in handling fragment zero of fragmented datagrams. (Fragment zero has the fragment offset equal zero).

Summary of ICMP message types:-

The most common use for ICMP is by PING and TRACERT. ICMP is IP protocol number 1, there is no concept of source and destination port numbers only message types, so for example a PING will be message type 8 at the source, and message type 8 at the destination.

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Last modified: 25 July 2011

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