Re: draft-ietf-mmusic-sip-session-timer-00.txt

[Jonathan Rosenberg <[e-mail]>]

Dean Willis wrote:
> 
> How about renewing at 1/2 T, where T is the minimum time unit on the
> proxy chain. If T is greater than 1 minute or so, this should preclude
> accidental expirations as the renewal propagates. As T increases, this
> gives us a nice backoff on renewals . . .

I'm not sure we need to specify anything for this. Sending at 1/2T, then
3/2T, 5/2 T, etc., only delays the problem by T. What I mean is lets say
each proxy starts its timer when the response comes down the proxy
chain, at t=0. Now, the client resends at 1/2 T, which is fast enough to
refresh things. Now, each proxy expects to see the next before 3/2 T.
THe client sends at 3/2 T, but because of timing inconsistencies, packet
loss, clock skew, and latencies, it comes too late for some servers.

Instead, the interval should be treated as the frequency the client will
refresh at. Servers really should not time out a session unless no
refresh is received for 3/2 to 2 times this amount of time. Its like the
RTCP refresh interval. A user is not timed out in RTP unless they don't
send an RTCP packet for 5 times the interval. You need this to account
for losses, skew, and so on.

-Jonathan R.



-- 
Jonathan D. Rosenberg                       Lucent Technologies
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