Internet Control Message Protocol (ICMP) Explained

[ Thanks to Linux User & Developer magazine for this link. ]

"Let us see an example of how ICMP facilitates this. When a router is not able to forward a datagram, it informs the original source about this error, and does not specify what action needs to be taken to overcome the error. It is up to the source to take the necessary action in order to correct the problem. It is important to notice here that ICMP cannot be used to inform the intermediate routers when a problem occurs. This is basically because a datagram contains fields that specify the original source and the ultimate destination. So when a router gets a datagram, it does not know the path taken by the datagram to arrive there. Instead of discarding a datagram when an error occurs, it informs the source from which the datagram originated. Hence the connectivity and configuration of a network is easily tested.

"ICMP is a classic example of a client-server application. The fundamental purpose of this protocol is to report problems with the delivery of IP datagrams. The protocol is also frequently used by internet managers to verify correct operations of End Systems (ES) and to check that routers are correctly routing packets to the specified destination address. It is the responsibility of the network-layer protocol to ensure that the ICMP message is sent to the correct destination. This is achieved by setting the destination address of the IP packet carrying the ICMP message."

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