ICMP General Operation
ICMP is one of the simplest protocols in the TCP/IP protocol suite. Most protocols implement a particular type of functionality to either facilitate basic operation of a part of the network stack or an application. To this end, they include many specific algorithms and tasks that define the protocol, which is where most of the complexity lies. ICMP, in contrast, is exactly what its name suggests: a protocol that defines control messages. As such, pretty much all of what ICMP is about is providing a mechanism for any IP device to send control messages to another device.
The ICMP Message-Passing Service
Various message types are defined in ICMP that allow different types of information to be exchanged. These are usually either ...