Internet Control Message Protocol (ICMP)

ICMP allows gateways and hosts to exchange bootstrap and error information. Gateways send ICMP datagrams when they cannot deliver a datagram, or to direct hosts to use another gateway. Hosts send ICMP datagrams to test the 'liveness' of their network.

As an example, the Unix program ping sends ICMP echo messages to a specified machine. Upon receipt of the echo request, the destination returns an ICMP echo reply. ping hence both checks that a host is up and that the path to a host is viable.

    prompt> /bin/ping elvis
    elvis is alive

    prompt> time /bin/ping sophia.inria.fr
    sophia.inria.fr is alive
    3.006s real 0.040s usr 0.060s sys

    prompt> time /bin/ping sophia.inria.fr
    sophia.inria.fr is alive
    0.591s real 0.020s usr 0.090s sys

If a gateway must discard a datagram due to lack of resources it sends a source quench to the datagram's sender. If a datagram cannot be delivered because a host is down or no route exists, a ICMP destination unreachable datagram is generated.

The TCP/IP Protocol suite defines over 25 (in-use) ICMP error message types, including: