ICMPv4 Traceroute Messages
The Echo and Echo Reply messages you saw earlier in this chapter are used for the most basic type of test that can be conducted between two devices: checking if they can communicate. A more sophisticated test can also be performed in order to see not only if the devices are able to talk, but also to discover the exact sequence of routers used to move datagrams between them. In TCP/IP, this diagnostic is performed using the traceroute (or tracert) utility.
The first implementation of traceroute used a clever application of Time Exceeded error messages, as described in the previous chapter. By sending a test message to a destination first with a Time to Live (TTL) value of 1, then 2, then 3, and so on, each router in the ...
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