IP provides unreliable and connectionless datagram delivery. It was designed this way to make efficient use of network resources. IP is a best effort delivery service that delivers a datagram from its original source to its final destination. However it has two deficiencies, lack of error control and lack of assistance mechanisms.
IP has no error reporting or error correcting mechanism.There is no mechanism to overcome if anything goes wrong and if a router has to discard a datagram because it could not find a router to the final destination and if the final destination host has to discard all fragments of a datagram because it has not received all fragments within a predetermined time limit. IP also lacks a mechanism for host and management queries. A host sometimes needs to determine if a router or another host is alive. And sometimes a network manager needs information from another host or router.
The Internet Control Message Protocol(ICMP) has been designed to compensate for the above two deficiencies. It is a companion to the IP. ICMP itself is a network layer protocol. However, its messages are not passed directly to the data link layers as would be expected. Instead, the messages are first encapsulated inside IP datagrams before going to the lower layer.
IP has no error reporting or error correcting mechanism.There is no mechanism to overcome if anything goes wrong and if a router has to discard a datagram because it could not find a router to the final destination and if the final destination host has to discard all fragments of a datagram because it has not received all fragments within a predetermined time limit. IP also lacks a mechanism for host and management queries. A host sometimes needs to determine if a router or another host is alive. And sometimes a network manager needs information from another host or router.
The Internet Control Message Protocol(ICMP) has been designed to compensate for the above two deficiencies. It is a companion to the IP. ICMP itself is a network layer protocol. However, its messages are not passed directly to the data link layers as would be expected. Instead, the messages are first encapsulated inside IP datagrams before going to the lower layer.