Internet Core Protocols: The Definitive Guide

Chapter 5. The Internet Control Message Protocol

Summary	The Internet Control Message Protocol provides a mechanism for IP devices to use when they need to exchange information about network problems that are preventing delivery. Although IP is an unreliable protocol that does not guarantee delivery, it is important to be able to inform a sender when delivery is not possible due to a semi-permanent, nontransient error.
Protocol ID	1
Relevant STDs	2 (http://www.iana.org/); 3 (includes RFCs 1122 and 1123); 4 (RFC 1812, republished); 5 (includes RFCs 791, 792, 919, 922, 950, and 1112)
Relevant RFCs	792 (Internet Control Message Protocol); 896 (Source Quench); 950 (Address Mask Extension); 1122 (Host Network Requirements); 1191 (Path MTU Discovery); 1256 (Router Discovery); 1812 (Router Requirements)
Related RFCs	1393 (Traceroute Extension)

IP is an unreliable protocol, and as such, delivery is not guaranteed. In this model, if important datagrams are lost, then a higher-layer protocol (such as a transport-layer protocol like TCP or an application-layer protocol like TFTP) will eventually recognize that a problem has occurred and will deal with it. As the theory goes, important data will eventually get through.

However, sometimes a problem crops up that prevents all datagrams from getting through to their destination. When these kinds of nontransient errors occur, IP fails for a specific and avoidable reason, and the sender should be notified about the problem so that it can either stop sending ...