Complete Protocol dictionary, glossary and reference
ICMP: Internet Control Message Protocol
Interne Control Message Protocol (ICMP) is an integrated part of the IP suite. ICMP messages, delivered in IP packets, are used for out-of-band messages related to network operation or mis-operation. ICMP packet delivery is unreliable, so hosts can't count on receiving ICMP packets for any network problems.
RFC792 and 950