6.6 4.4BSD Processing of ICMP Messages

Since ICMP covers such a wide range of conditions, from fatal errors to informational messages, each ICMP message is handled differently, even within a given implementation. Figure 6.12 is a redo of Figure 6.3, showing the handling performed by 4.4BSD for each of the possible ICMP messages.

Figure 6.12. Handling of the ICMP message types by 4.4BSD.

If the final column specifies the kernel, that ICMP message is handled by the kernel. If the final column specifies "user process", then that message is passed to all user processes that have registered with the kernel to read received ICMP messages. If there are none of these user processes, the message is silently discarded. (These user processes also receive a copy of all the other ICMP messages, even those handled by the kernel, but only after the kernel has processed the message.) Some messages are completely ignored. Finally, if the final column is a string in quotes, that is the Unix error message corresponding to that condition. Some of these errors, such as TCP's handling of a source quench, we'll cover in later chapters.