6.7 Summary

This chapter has been a look at the Internet Control Message Protocol, a required part of every implementation. Figure 6.3 lists all the ICMP message types, most of which we'll discuss later in the text.

We looked at the ICMP address mask request and reply and the timestamp request and reply in detail. These are typical of the request-reply messages. Both have an identifier and sequence number in the ICMP message. The sending application stores a unique value in the identifier field, to distinguish between replies for itself and replies for other processes. The sequence number field lets the client match replies with requests .

We also saw the ICMP port unreachable error, a common ICMP error. This let us examine the information returned in an ICMP error: the IP header and the next 8 bytes of the IP datagram that caused the error. This information is required by the receiver of the ICMP error, to know more about the cause of the error. Both TCP and UDP store the source and destination port numbers in the first 8 bytes of their headers for this reason.

Finally, we presented our first time line of tcpdump output, a presentation format we'll use in later chapters.