Network Troubleshooting Video Series
Episode 22, Segment 03 of 07
When the computer originating a message builds the outgoing packet, it makes
a conservative estimate of the number of router - "hops" - that will be needed
to traverse the Internet to the destination, and the "Time To Live" field is
initialized accordingly. A typical trip across the Internet may need to pass
through about 15 router - "hops" - but because this value is always estimated
conservatively, the originating Time To Live (TTL) field is generally set to 32
or more. (Most operating systems take the easy way out, always setting all
outgoing TTL fields to the same high starting value).
(You can learn a lot more about Internet Protocol Packets and the reasons why they are organized as they are by watching the AskMisterWizard.com Movie entitled: "Ethernet Delivers The Internet").
Normally, as each router forwards a packet toward it's destination, the process of decrementing the "Time To Live" field is not very interesting. However, if a packet has been bouncing around between routers for long enough to use up it's planned "Time To Live", the router that decrements it to zero is expected to generate a - "courtesy" - message back to the originator, as if to say:
"Excuse me: I'm the router with IP address 188.8.131.52, and I found a packet that you were trying to send through me to mit.edu. It must be really old, because it's - Time To Live - has expired. I've discarded the packet, but I figured you ought to be alerted, because the recipient isn't ever going to receive it. Sorry..."
It's rare for a computer to receive an unexpected message like that, because misconfigured routers tend to be quickly detected and corrected in today's modern, Internet-dependent world.
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Episode 22, Segment 04 of 07