Internet Core Protocols: The Definitive Guide

Reporting on Delivery Problems

As mentioned earlier, ICMP error messages are used when it is necessary to report a problem that is preventing delivery from occurring. Although IP is an unreliable protocol that may fail without warning, it is important for the network to know when problems occur that will prevent delivery from ever occurring.

This is a fundamentally different concept from packets simply becoming lost or corrupted. If a system is trying to send data to a destination network that is totally unavailable, then the sending system ought to know that no datagrams are ever going to make it through to the destination system. The problem should be reported to the sending application so it can stop sending data to that destination.

Although ICMP can be used to report on IP failures, it is important to note that ICMP does not make IP a reliable protocol. IP is still capable of losing packets, sending duplicate or out-of-sequence packets, or doing anything else that it wants to. Nor does the lack of ICMP messages mean that the network is functioning perfectly; a host may be ignoring messages for any number of reasons without an ICMP message ever getting returned to the sender.

By the same token, TCP is a reliable transport that uses negotiated connections for data exchange, and as such many of ICMP’s error messages are not needed or used by TCP. For example, if an application specified an invalid port number on a destination system, the remote system’s TCP software would simply ...