Internet Control Message Protocol Packets

ICMP packets (see RFC792[83]) are used to send diagnostic information and notifications for other protocol types. Logically considered part of layer three, ICMP packets are carried as a payload of IP packets and, as such, are no different from the layer four payload. ICMP does not carry any new user-space data between endpoints and provides a trivial signaling method for IP instead. Figure 9-8 shows the ICMP header structure.

The ICMP header structure

Figure 9-8. The ICMP header structure

A variety of messages are sent using ICMP in response to TCP or UDP traffic, usually indicating that a particular packet cannot be delivered, expired ...

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