Internet Core Protocols: The Definitive Guide

Housekeeping and Maintenance

Every system that receives a packet—whether the system is the final destination or a router along the delivery path—will inspect it. If the packet has become corrupt or has experienced some other form of temporary failure, then the packet will be destroyed right then and there. Whenever one of these transient errors occurs, the datagram is destroyed rather than being forwarded on.

However, if a problem occurs that is semi-permanent—for example, if the current device does not have a routing table entry for the destination network, or if the packet does not meet certain criteria for forwarding across the next-hop network—then IP may call upon the Internet Control Message Protocol (ICMP) to return an error message back to the original sender, informing them of the failure. Although the datagram will still be destroyed by the last-hop device, it will also inform the sender of the problem, thereby allowing it to correct whatever condition was causing the failure to occur.

This distinction between transient and semi-permanent failures is important. Transient errors are caused by no fault of the sender (such as can happen when the Time-to-Live timer expires, or a checksum is miscalculated), while semi-permanent failures are problems with the packet or network that will always prevent delivery from occurring over this path. In the latter case, it is best either to inform the sender of the problem so that it can take whatever corrective actions are required, ...