ICMP ICMP is the Internet Control Message Protocol. It is a network layer protocol of the OSI Model which can report errors and status information. The ping command uses the ICMP echo request and echo reply packets to detect network or host communication failures and troubleshoot common TCP/IP connectivity problems. ICMP Message types Echo Reply, Destination unreachable, Redirect, Echo, Router Advertisement, Traceroute. The two most important ICMP messages are Echo Request (8) and Echo Reply (0).