Internet Control Message Protocol (ICMP) An error reporting protocol. IP relies on ICMP to report errors. For example, if a router is congested and discards a packet, then ICMP returns an error message to ICMP on the source machine. ICMP messages are carried in the payload of IP packets. A ‘protocol’ value of 1 in the IP header means that the payload carries an ICMP Message. There is no flow control in IP. Routers can become congested.