ICMP messages contain a Type, Code and Checksum. ICMP is best known as the facility that supports the PING utility. ICMP allows connectivity to be tested and has facilities to synchronize time. There are 18 message types, several of which are obsolete. Initially ICMP was used to negotiate IP addresses as well as to test connectivity, but that usage has been replaced by the DHCP and BOOTP protocols.
ICMP is defined in RFC950, RFC792, and RFC1122.
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