ne of the main responsibilities of ICMP is error reporting, that is only detecting the error. It does not correct the detected errors.

Following are the error reporting message:

Destination Unreachable:

This is generated by a router or a host when it does not find a route for sending the datagram. The datagram is discarded and destination unreachable message is send to the sender.

Source Quench:

IP protocol is a connectionless protocol and it does not support flow control mechanism. This message is generated again by a router or a host when it does want to sender to slow down(quench) its rate of transmission to avoid any congestion.

Time Exceeded: This message is generated in two cases:

• It is generated by a router when it receives a packet with TTL field set to 0.
• Second in the case of fragmentation, that is when the first fragment is received, it starts a timer and if all the fragments is not received within the specified time, timeout message is generated.

At each hop, the TTL value is decremented by 1, it avoid loop in the network as packet cannot travel in the network forever.

Parameter Problem:

This is generated by a router or a host, if they receive a header with some missing values or the integrity of message is lots(detected by checksum). The datagram is discarded.

Source Redirect:

• Suppose the source does not take part in routing update because of efficiency and it has a route as next hop “R1” to reach destination.
• Source sends the packet to “R1”, and “R1” finds in its routing table that there is best path to reach destination that is via “R4”.
• So it forwards the packet to “R2”, but at the same time also sends a message to source that there exist a better path via “R4” to reach destination.
• This complete process is known as source redirect.

Categories: Networking