RE: Last Call: draft-lawrence-sipforum-user-agent-config (Session Initiation Protocol (SIP) User Agent Configuration) to Informational RFC

[Scott Lawrence <[e-mail]>]

On Thu, 2010-04-08 at 15:15 -0400, Hadriel Kaplan wrote:
> 
> > -----Original Message-----
> > From: Scott Lawrence [mailto:[e-mail]]
> > Sent: Thursday, April 08, 2010 9:37 AM
> > To: Hadriel Kaplan
> > 
> > Well, one could argue that a provider could cause the returned SIP url
> > for the change notice subscription to be one for which there is no
> > routing (return 'Link: <sip:devnull.example.org>').  By the rules, the
> > UA would periodically make a DNS request to try to find it, but would be
> > allowed to use the configuration data.  Silly, but allowed.
> 
> Right, but the since that would make it an "unknown validity" config,
> and the requirements do not mandate any UA to *use* an "unknown
> validity" config... do you see a problem?

The requirements explicitly allow the UA to use an "unknown validity"
configuration.  I don't think it would be appropriate to put in a MUST
that says the UA should use any configuration data response - the data
may be in the wrong format, corrupt, or have any other problem, so that
would just lead to a different argument.

> Instead of getting into an unknown-behavior state, why don't you
> simply allow the HTTP response to NOT have a Link header, or define a
> NULL URI to use - and then state that it means there is no
> Subscription service and the UA MUST consider the HTTP-based config
> valid?

> > No one is going to be forced to use any of this specification.  If you
> > don't want the features it provides (automatic initial configuration
> > with prompt updates), then don't use it.
> 
> So we should go define another profile which is a textual copy of this
> one, but changes two sentences??  Is that really good for SIP or the
> SIP-Forum?

> > At the risk of repeating myself, I want to make sure that one reason for
> > using SUBSCRIBE/NOTIFY for the change notices is clear:  there is no
> > other existing standard way to address a specific User Agent.  
> 
> Right, I understand that you have no other way to do X.  Fine, so
> specify how to do X.  Don't mandate that X be used with Y, when Y does
> not depend on X to function properly, and X is not trivial.

Perhaps our fundamental disagreement is whether or not having a prompt
way to reconfigure a UA is a requirement.  When the SIP Forum chartered
this work, it was agreed that that was requirement (and I certainly
think it is).  I think that a configuration mechanism that does not
allow for updates under the control of the service won't be successful.

Could we allow the Configuration Service to omit the Link?  Obviously,
we could.  I think that would materially reduce the utility of the
protocol and would be a bad idea.  Clearly we differ on that.