Internet Control Message Protocol (ICMP) ICMP is a maintenance protocol specified in RFC 792 and is normally considered part of the IP layer. ICMP messages are encapsulated within IP datagrams, so that they can be routed throughout an internetwork. Windows NT and Windows 2000 use ICMP to: Build and maintain route tables. Perform router discovery. Assist in Path Maximum Transmission Unit (PMTU) discovery. Diagnose problems (ping, tracert, pathping). Adjust flow control to prevent link or router saturation.