Internet Control Message Protocol (ICMP) ExplainedJan 20, 2010, 04:33 (0 Talkback[s])
(Other stories by Swayam Prakasha)
[ 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.
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