2.17 ICMP Message

As illustrated in Animation 2.17-1 ICMP messages have a simple structure: The first field after the IP header is the type field with a length of one byte. This field indicates the function the message fulfills. The following field, the code field, has a length of one byte. It includes further information about the content of the message, e. g. a more specific description of an error. The checksum is a 2-byte number. It is applied to the ICMP message starting from its type field.