Internet Control Message Protocol (ICMP) is the utility protocol of TCP/IP, responsible for providing information regarding the availability of devices, services, or routes on a TCP/IP network. Most network troubleshooting techniques and tools center around common ICMP message types. ICMP is defined in RFC 792.
ICMP is part of IP, and it relies on IP to transmit its messages. ICMP contains a relatively small header that changes depending on its purpose. As shown in Figure 6-29, the ICMP header contains the following fields:
|Type The type or classification of the ICMP message, based on the RFC specification|
|Code The subclassification of the ICMP message, based on the RFC specification|
|Checksum Used to ...|