Transmission of ICMP Messages
ICMP messages cannot be directly sent to the Network Interface layer for transmission. They are encapsulated in an IP datagram and sent to the next layer. The data part of an IP datagram consists of the ICMP message. The Protocol field in the IP header is set to 1 when an ICMP message is transmitted through an IP datagram.
The following are the key facts to remember about ICMP:
ICMP reports errors that occur during data transmission, but is not responsible for correcting the problems due to the errors. It is the responsibility of the Transport layer protocols to rectify the problems. For example, if a router discards a data packet, the function of ICMP is to report the error to the Transport layer protocols. ...