Internet Core Protocols: The Definitive Guide

The Need for ICMP

Remember that IP is only responsible for getting datagrams from one host to another, one network at a time. Each IP datagram gets sent as an individual entity capable of following whatever path is available to it. Datagrams are moved across whatever hosts, routers, and networks are capable of getting that specific chunk of data closer to its destination.

In this model, any IP datagram can fail to get delivered for any number of reasons. Some datagrams will get discarded simply because the next-hop router is unavailable, and the current router is unable to forward them. Sometimes a datagram will be destroyed due to the user on the sending system providing a non-existent destination IP address or port number to their local application. In all of these cases, the system that detects an error will simply destroy the IP datagram that’s failing, and move on to the next datagram waiting to be processed.

Depending upon the exact cause of the failure, the system that destroyed the datagram may or may not choose to return an ICMP error message back to the original sender, notifying it of the failure and its cause. Typically this decision is made based on whether the failure is transient or semi-permanent.

Transient failures such as invalid checksums are generally ignored, since it is assumed that the sender will eventually notice the failure and retransmit any important data (which may be handled by TCP or by an application-specific reliability mechanism). The assumption ...