Overview

The Internet Protocol (IP) has no way in which to report errors or to send control messages to transmitting hosts. So what happens when a datagram cannot be delivered for some reason or when a problem is detected within a datagram? And how can hosts be informed of better routes when they become available? The Internet Control Message Protocol (ICMP) is the mechanism through which all of these functions are performed. ICMP is an integral part of IP, and must be implemented in all IP modules. However, since it utilizes IP for delivery, it is discussed here as a totally separate protocol.

Because ICMP relies upon IP, ICMP does not make IP more reliable. Where extra reliability is desired, an upper layer protocol must implement these mechanisms. ICMP messages are typically sent to report errors that have occurred in the processing of datagrams. They may also be sent where a datagram cannot be delivered to its destination, where the router has insufficient resource to buffer an incoming datagram, and where it would be desirable to send datagrams via a shorter route. Since ICMP is used to report errors, it is possible that a datagram carrying an ICMP error message may also be in error. The one exception to the operation described above, is that where an ICMP error message itself causes an error, no error message will be generated. This avoids the situation of recurrent error messages being generated. Equally, where an error is detected on a multi-fragment...