Internet Control Message Protocol (ICMP) :
- The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, like routers, to send error messages and operational information indicating, for example, that a requested service is not available or that a host or router could not be reached.
- It differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications
- It is an error-reporting protocol network devices like routers use to generate error messages to the source IP address when network problems prevent delivery of IP packets. It creates and sends messages to the source IP address indicating that a gateway to the Internet that a router, service or host cannot be reached for packet delivery.
- Its messages are transmitted as datagrams and consist of an IP header that encapsulates the data. ICMP packets are IP packets with ICMP in the IP data portion. The messages also contain the entire IP header from the original message, so the end system knows which packet failed