Internet Control Message Protocol

protocol used to send control messages and error messages during the transmission of IP-type data packets, ICMP

The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. It is chiefly used by networked computers' operating systems to send error messages—indicating, for instance, that a requested service is not available or that a host or router could not be reached.
See more at Wikipedia.org...

<protocol> (ICMP) An extension to the Internet Protocol (IP) that allows for the generation of error messages, test packets, and informational messages related to IP. It is defined in STD 5, RFC 792.
(1999-09-18)