Subject: [Sip] changes in session timer

From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 25 Feb 2004 00:45:12 -0500

Folks,

I submitted a revision of the session timer, based on comments received 
from IESG and subsequent list discussion. For some reason, this 
submission was not processed by the I-D editor (I'm working to correct 
that). In the mean time, you can pick it up at:

http://www.jdrosen.net/papers/draft-ietf-sip-session-timer-14.txt

There are a few significant changes:

* Formerly, if a session refresh timed out, the UA waited until 1/3 of 
the session interval remained before sending a BYE. This contradicts 
RFC3261 which basically says to send a BYE immediately. Session timer 
now references that behavior.

* There is now an absolute minimum session timer of 90 seconds. This is 
basically enough time for completion of a SIP BYE transaction after half 
of the session has expired. The Min-SE header field now has a default 
value of 90 seconds when not present. This will preclude usage of 
session timer for really fine grained refreshes, say on the order of 5 
to 10 seconds. I fear that some people may be trying to do some kind of 
nat traversal or something with session timer - this would rule that out 
formally (it was never the intended usage).

* The default for session-expires, when not present, used to be 
"infinity". However, thats not really right. Really, it means 
"undefined", and session expiration is then up to local policy of the 
proxy. Using infinity would mean that a proxy could never clean up state.

Please comment ASAP if you have any problems with these changes. We'd 
like to get session timer moving along.

-Jonathan R.

-- 
Jonathan D. Rosenberg, Ph.D.                600 Lanidex Plaza
Chief Technology Officer                    Parsippany, NJ 07054-2711
dynamicsoft
[email-masked]                     FAX:   (973) 952-5050
http://www.jdrosen.net                      PHONE: (973) 952-5000
http://www.dynamicsoft.com

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---

From: Paul Kyzivat <[email-masked]>
Date: Wed, 25 Feb 2004 09:38:13 -0500

Jonathan Rosenberg wrote:
...
> proxy. Using infinity would mean that a proxy could never clean up state.

I see a problem with this change. What you say makes sense for proxies, 
but it doesn't make sense for a UAC or UAS. For them, infinity is the 
right default, because in this case they don't refresh. In the absence 
of a session-expires the UAC and UAS can't know the proxy's policy. 
Perhaps the wording needs separated out for proxy and UA.

The new wording is:

    The default value of the Session-Expires header field is undefined.
    This means that absence of the Session-Expires header field implies
    no expiration of the session using the mechanism defined in this
    specification. Note that other mechanisms not defined in this
    specification, such as locally configured timers, may apply.

while the old wording was:

    The default value of the Session-Expires header field, when not
    present, is infinity. This means that absence of the Session-Expires
    header field implies no expiration.

How about the following as a compromise:

    The default value of the Session-Expires header field, when not
    present, is infinity. For UAC and UAS this means that absence of
    the Session-Expires header field implies the session timer will
    never expire, so no BYE will be sent on expiration, and no refreshes
    will be performed. This also implies that proxies must make their
    own determination of how long to retain state in the absence of
    any messages within the dialog.

	Paul