Subject: [Sip] Questions on draft-ietf-sip-session-timer-13 and refresh timers

From: Dean Willis <[email-masked]>
Date: Fri, 23 Jan 2004 13:13:07 -0600

I'me reviewing some IESG comments on this draft, and I've found
something I don't understand.

The original comment from Ted was something like "Why do we recommend
refresh at 1/2 the session timer in sections 7 and 8, but require a BYE
at a recommended 1/3St-10s in section 10?"

I understand why we require refresh, and why we suggested refreshing at
1/2 of the session timer value. What I don't understand is the BYE logic
in Section 10.

>From Section 10:

   If no 2xx response to a session refresh request is received before
   the session expiration, the UA SHOULD send a BYE request to terminate
   the session. It SHOULD send this BYE slightly before session
   expiration. The minimum of ten seconds and one third the session
   interval is RECOMMENDED.

   For example, if the session interval is 120 seconds, one third of
   this is 40 seconds. Since the minimum of 10 seconds and 40 seconds
   is 10 seconds, the BYE would be sent 10 seconds before the session
   expires.

I read this as:

A UA has issues a session refresh request, and is waiting around for
that transaction to complete. Unfortunately, it either 1) didn't issue
the request sufficiently early for the transaction to complete before
the session is going to expire, so it needs to abandon the refreshing
transaction and terminate the session before the session expires, 
or 2) the session refresh request failed, but we're going to continue to
use this session for a while before tearing it down before it expires.

It seems more reasonable that the UA should issue the session refresh
request far enough in advance of session expiration that the session
refresh request will complete (or fail with timeout) before the session
expires, and that if the session refresh request fails, the UA should
immediately terminate the session with a BYE.

This implies that the lowest allowable threshold for the negotiated
session timer should be larger than the largest timeout value for a
session refresh request (which could be an INVITE transaction). It also
may place requirements on UAs that have implemented non-standard timer
values to calculate this lowest allowable threshold for the negotiated
session timer value and include this calculated value in their
negotiation by initializing the Min-SE value.

--
Dean

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