RE: [SIP] draft-ietf-sip-session-timer-04: INVITE with Require and Support timer and Session-Expires

["Brett Tate" <brett@broadsoft.com>]

Jonathan, 

Thanks for the response.  Comments inline.

> > What should happen if a UAC sends an INVITE 
> > with Session-Expires and both Supported timer 
> > and Require timer?
> 
> The Require implies Supported, so the behavior would be 
> identical to the
> case where just Require were present. This is probably an issue best
> addressed in the Supported header specification, since its 
> more general than
> session timer.

The "Require implies Supported" needs to be 
reflected in the timer draft since Table 3 
is otherwise misleading if UAC includes 
Require timer with session-expires without 
Supported timer.

If a UAC both requires and supports an option-tag, 
should it include it in both headers?  

Isn't the "require implies supported" going to limit 
devices that want to require others to support an 
extension without them actually supporting it?

> > 
> > What should happen if the UAS responds to such 
> > a request and includes the same the three 
> > headers and values?  Are both now effectively 
> > responsible for sending refreshes?  Or is the 
> > UAS forbidden from sending such a response?
> 
> These are good questions. A lot of it depends on what we mean 
> when Require
> is placed in the request. In principal, its quite useless. 
> Require normally
> means "don't process the request unless you understand the 
> extension". But,
> session timer works just fine even if the UAS doesn't support 
> the extension.
> So, based on this definition, inserting it has zero meaning. 
> However, one
> might think that Require in the INVITE means, "UAS, you should do the
> refreshes". 
> 
> I'm inclined to be consistent with normal Require usage. In 
> this case, the
> presence of Require in the INVITE has no impact on processing 
> at the UAS if
> it supports the extension. UAC will do the refreshes. If the 
> UAC wants the
> UAS to do them, the UAC can insert the Session-Timer header, but not a
> Supported header. This will result in the UAS doing them if 
> it supports it,
> otherwise there won't be session timers. 

Sounds fine.

It has some restrictions since the Supported 
and Require headers are still being used to indicate 
who is refreshing instead of what is supported 
and required.  However without an extra parameter 
in Session-Expires to indicate who is refreshing, 
I can't think of a better solution.

Actually the same parameter could be used to allow 
for smaller Session-Expires values which are not to 
be refreshed.

Session-Expires  =  
  ("Session-Expires" | "x") ":" 
              ( SIP-date | delta-seconds ) [refresher]

refresher = ";" "refresher" "=" "uas"|"uac"|"none"


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