ICMP Common Message Format and Data Encapsulation
As you have seen so far in this chapter, ICMP is not so much a protocol that performs a specific function as a framework for the exchange of error reports and information. Since each of the message types is used for a different purpose, they differ in the types of information they contain. This means each ICMP message has a slightly different format. At the same time, however, ICMP message types also have a degree of commonality—a portion of each message is common between message types.
ICMP Common Message Format
You can think of the structure of an ICMP message as having a common part and a unique part. The common part consists of three fields that have the same size and same meaning in all ICMP ...