Network Troubleshooting Video Series
Episode 22, Segment 05 of 07
The traceroute utility then generates a - ping - message addressed to the
destination computer and sends it on its way in the usual manner. However,
instead of setting the - "Time To Live" - field to some conservative value (like
64), it is set to 1.
This, of course, means that the first router along the data path decrements the Time To Live field to zero and thinks to itself: "Whoa! This must be a really old packet. I'd better discard it and alert the sender that it didn't get beyond me!", and a courtesy message is sent back to the traceroute process back on the originating computer.
Accordingly, the traceroute utility can then display the IP address of the first router on the path to the network destination.
Traceroute then makes another ping packet as before, but this time the -
"Time To Live" - value starts out at 2.
The first router along the outbound path decrements it to 1, and the value is decremented to zero by the - second - router, which discards it and sends back another alert message disclosing it's disposition. When that alert message returns back to the source, the traceroute utility can display the IP address of the second router on the path to the destination.
This process repeats for the next router, and the next and the next, with the initial - Time To Live - field increasing step by step until the ping packet actually arrives all the way to the destination, whereupon a normal - Ping Response - is sent back.
Episode 22, Segment 06 of 07