1.11 ICMP 3/3
nICMP messages are carried (encapsulated) within IP datagrams.
nThe ICMP message has a common 4 byte header:
–1 byte indicates message type
–1 byte indicates the code
–2 bytes are used for checksum
nWhen reporting error messages, its data field contains the IP header (incl. Options) and the 8 bytes that follow the IP header (indicating TCP, or UDP, port numbers, etc.) of the message in error.