Subject: RE: [Sip] The Problem with PRACK - offerless invite

From: "Christian Jansson" <[email-masked]>
Date: Thu, 16 Mar 2006 17:13:42 +0100

>There is certainly no implied bias towards audio in the offerless
>invite. I expect that a UA should offer that which has the capability
to
>do, and that it thinks are appropriate to offer according to its *own*
>current state and context. Ideally in a UA with the capability for more
>than one type of media, the ones to use in this manner would be
>configurable. (E.g. whether to offer video as well as audio.)
>
>> But how can Bob know that? Why does Bob not include audio +
>> video + application/whiteboard, etc, etc, and even worse, start all
>> those apps in receiving mode as Bob now has generated the SDP offer?
>
>Depending on the capabilities of the UA, it may well want to do all of
>that.

So completely depending on Bob's UA, Alice may get _any_ kind of call
back, msrp+audio, video+application/whiteboard, 2 video streams? Even
though Alice initially invited Bob! Why would Alice invite Bob at all if
she does not have _any_ clue of what session Bob will offer? Offerless
invites are useless in practice for anything else than audio, that is a
fact, and I have seen the problems when trying to get things to
interoperate.

>> Is it possible to build a 3pcc service that sets up a video+voice
call
>> using this offerless technique?
>
>Certainly it is possible.

Possible, but not in a controllable way. It would be impossible for the
3pcc to indicate that it should be a video+voice call. 

The only way to get this to work is to create a new extension to let
offerless invites indicate the media that should be included in the
session. But that would also mean that only new UAs that chose to
support that extension would behave well in 3pcc scenarios.

(I would say that the sane answer to "I invite you" would be "to what?",
not to start guessing.)

/ Christian Jansson

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---

From: "Tom-PT Taylor" <[email-masked]>
Date: Thu, 16 Mar 2006 13:45:02 -0500

I don't understand why there should be a problem, unless it is careless 
programming. RFC 3264 is totally clear on what the UAC has to do to pick 
out the session it is interested in. If it doesn't like any of them it 
ACKs and then does a BYE -- no big deal.

>>invite. I expect that a UA should offer that which has the capability
...

---

From: Paul Kyzivat <[email-masked]>
Date: Thu, 16 Mar 2006 14:54:20 -0500

Christian Jansson wrote:
> she does not have _any_ clue of what session Bob will offer?

Presumably bob wouldn't do this unless this is what he wants.

> Offerless

In practice, offerless invites are useful in 3pcc scenarios, where there 
is a controller acting as an intermediary in connecting two other 
parties. It may not know a priori what either of them support. It just 
asks one, and takes whatever is offered and offers it to the other. It 
doesn't care what that is as long as both parties are happy with it.

> support that extension would behave well in 3pcc scenarios.

This is true only if you care. Why would a ua that is capable of voice 
plus video not offer that?

	Paul