Re: What's a reasonable time-to-live?


Lixia Zhang (lixia@COMET.LCS.MIT.EDU)
Fri, 21 Mar 86 17:30:36 est


Jeff,

You asked a good question, but I may not have a good answer. As far as I know,
currently the TTL (time-to-live) field in IP header is used, at best, as a
gateway hop count, i.e. each gateway is required to decrease TTL at least
by 1. (I'm not aware of any gateway implementation that measures the staying
time of the packet and decreases TOS accordingly. Some people may tell us if
they know.)

But even if gateways do decrease TTL by real time, it is not of much meaning
anyway, because TTL does not count the network packet transfer delay between
gateways. (To see how long the net delay might be in worst case, consider
the fact that Arpanet source-IMP retransmission timer is 30 seconds!)

Sum up the above:
- TTL value is not the expiration life time of IP packets, since it does not
  take network transmission delays into account.
- TTL is now used as gateway hop count, it prevents packets from looping
  indefinitely inside the internet. So setting TTL to a large value is not
  a good idea.

Lixia



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