ICMP (Internet Control Message Protocol) is a protocol used to report errors.
It is protocol within TCP/IP.
It is used by network devices such as routers to generate error messages and to communicate with the data packet source when the delivery of packets is prevented due to some network connectivity issue.
It sends messages as datagrams and comprises of IP header that hides ICMP data.
Messages like destination network unreachable, source route failed etc are sent by ICMP.
It contains data packet with header and data portion.
ICMP messages often include from the original message, the IP header, which helps in identifying which packet failed.
IP network devices are able to send, receive or process ICMP messages.
ICMP takes source IP from the discarded packet and indicates to source by sending source quench message after which source lessens transmission speed to remove congestion in the router.
When a packet is eventually sent in the wrong direction it is routed in the current direction, otherwise ICMP must send redirected message.
So ICMP protocol informs sender that something in the transmitting phase has gone wrong and that packets have not been sent.
