An extension to the original Internet Protocol (IP) that reports errors that may have occurred in the processing of datagrams. For example, a datagram may be undeliverable or an incorrect route may have been chosen. ICMP supports the testing of a path to a distant host computer through an echo function known as the ping utility. ICMP also supports the requesting of a subnet mask. ICMP is integral to IP and must be implemented in both hosts and routers. See also datagram, echo, host, IP, ping, and utility.