6. ICMP: control and error messages

In the unconnected mode of the Internet, each gateway and each machine operates autonomously. The routing and sending of datagrams takes place without coordination with the sender. This system works well, as long as all the machines have no problems and routing is correct, but this is not always the case.

Apart from network and terminal equipment failures, problems occur when a machine is temporarily or permanently disconnected from the network, or when the datagram lifetime expires, or when gateway congestion is too great.

To enable machines to report malfunctions, a protocol for sending control messages called ICMP (Internet Control Message Protocol) has been added to the Internet.

The recipient of an ICMP message is not an application process, but the Internet software of the machine concerned. When a message...