Internet Control Message Protocol (ICMP)
If a router can’t forward a packet because of some reason. It will send an error message back to the sender to report the problem. The protocol that handles error and control messages is known as ICMP. Although ICMP messages are encapsulated by IP packets, it is considered to be in the same layer as IP. Each ICMP message begins with a type field that identifies the messages.
|Type||It identifies a particular ICMP message|
|Code||It is used to specify different conditions for the same type of field value|
|Checksum||It covers entire ICMP message|