(第2空)ICMP is short for Internet ( ) Message Protocol, and is an integral part of the Internet ( ) suite (commonly referred to as TCP/IP). It is a ( ) layer protocol used by network devices to diagnose network communication issues. IP is not designed to be absolutely reliable. ICMP is to provide feedback about problems in the communication environment, not to make IP ( ).One of the most well-known and useful messages in an ICMP datagram is the Destination ( ) message which is generated for several reasons, including unable to reach a network, a host, a port, or a protocol.