Saturday, December 22, 2012

Internet Control Message Protocol (ICMP)

IP is always configured with a companion protocol, known as the Internet Control Message Protocol  (ICMP), that defines a collection of error messages that are sent back to the source host whenever a router or host is unable to process an IP datagram successfully. For example, ICMP defines error messages indicating that the destination host is unreachable (perhaps due to a link failure), that the reassembly process failed, that the TTL had reached 0, that the IP header checksum failed, and so on.

ICMP also defines a handful of control messages that a router can send back to a source host. One of the most useful control messages, called an ICMP-Redirect, tells the source host that there is a better route to the destination.

ICMP is the basis for widely used debugging tools, Ping. Ping uses ICMP echo messages to determine if a node is reachable and alive.

Computer Networks: Peterson & Davie


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