Appendix H. ICMP Type and Code Numbers
The Internet Control Message Protocol (ICMP) is used to transport error or control messages between routers and other devices. An ICMP message is encapsulated as the payload in an IP packet. Figure H-1 shows the ICMP message format. Notice that in the case of an error condition, the first 8 bytes (64 bits) of the original datagram causing the error are included in the ICMP message. This provides the protocol and port numbers of the original message to be seen, making troubleshooting easier.
Figure H-1. ICMP Message Format
ICMP type codes are registered with the Internet Assigned Numbers Authority (IANA). ...