The ICMP Specification
ICMP is documented in RFC 792, which is included in STD 5 (the IP standard). As such, ICMP is a part of STD 5 and is therefore considered to be an Internet Standard protocol. However, RFC 792 contained some vagaries that were clarified in RFC 1122 (Host Network Requirements) and RFC 1812 (Router Requirements). In addition, much of ICMP’s core functionality has been redefined and clarified in STD 2. As such, ICMP implementations need to incorporate RFC 792, RFC 1122, RFC 1812, and STD 2 in order to work reliably and consistently with other implementations.
RFC 792 states that the Protocol ID for ICMP is 1. When a system receives an IP datagram that is marked as containing Protocol 1, it should pass the contents of the datagram to ICMP for further processing. However, ICMP is not a transport protocol and is not used to deliver application data. Rather, ICMP is a control protocol like IGMP, useful for informing devices of network events and changes.
RFC 792 states that ICMP must be used whenever IP itself needs to report a problem. Thus, although ICMP works at a layer above IP, IP also depends on ICMP in order to function properly. ICMP and IP are tightly interwoven, and for all practical purposes are inseparable. For this reason, every IP implementation is also required to include ICMP.