[Csci551-talk] TTL handling : Just FYI

[John Heidemann]

On Sat, 13 Mar 2004 10:17:46 PST, "Devarshi Shah" wrote: 
>This is just FYI... thought I’d share it.
>
>One thing that the project does not mention is how and if the TTL field is
>to be handled… I had a talk with Dr. Heidemann and he said that we do need
>to consider the TTL field and decrement TTL every hop  - and stop
>propagating the packets if the TTL goes to 0. The TTL field can be
>initialized to an arbitrary initial value - he (Dr. Heidemann) suggested 10.

THanks for bringing this up.  This is the clarification I added to the
assignment:

The TTL field should be initialized to 16 when a message is sent
and decremented by one each hop.
Packets with a TTL less than 1 should be dropped.
We will not have topologies bigger than this in our test cases---the purpose
is that if you have bugs in your code that cause packet looping, you 
don't want the packets going around infinitely!


Another student asked in private e-mail:
}In my understanding about link state routing, each router will send out LS
}message about its links to the other router, and the message will be
}propagated to all the other router such that each router will know about the
}whole network link state. But the TTL implies that this is not 100% true,
}the LS message will be dropped after forwarded for a certain number of hops.
}And the router far far away, say > 10 hops, will not know about my links
}information.
}...
}...should I assume that all the router can be reached < 10 hops. And the
}TTL field is to avoid the flooding going non-stop?

Yes, you should assume that we will NOT have topologies where the TTL
causes routing to be incomplete.

The TTL is only there to catch bugs!

   -John Heidemann