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

From: "Christian Jansson" <[email-masked]>
Date: Thu, 16 Mar 2006 14:10:13 +0100

>Consider Alice sends offerless invite to Bob. But Bob is only willing
to
>do secure calls. So Bob returns an offer in a 183 with security
>preconditions. Bob needs an answer back that releases those
>preconditions before he is willing to ring, much less answer.

What does an offerless invite mean? In practice in the implementations
that exist I guess that Bob will send back an offer for a single audio
stream. 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?

Probably Alice/Alice's UA would be very confused after receiving such a
major SDP offer, given that most UAs of some reason expect an offerless
invite to setup an "audio call".

Have I missed something? (In that case flame me)

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

/ Christian Jansson

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---

From: Paul Kyzivat <[email-masked]>
Date: Thu, 16 Mar 2006 09:13:19 -0500

Christian Jansson wrote:
> stream.

I certainly hope the reason you see this as the common practice is 
because the majority of UAs only support audio.

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 +

Depending on the capabilities of the UA, it may well want to do all of that.

> invite to setup an "audio call".

If they do, then they ought to do more thinking about the future. This 
is no different than receiving an invite offering many media.

> using this offerless technique?

Certainly it is possible.

In spite of the latent capabilities, actual usage of sip for multimedia 
applications seems very limited. As this becomes more common these 
issues will come into view. I consider this an area that needs a lot of 
attention if it is to work in a reasonable way. There are many 
possibilities for interoperability problems.

For instance I get the impression that some people think that multimedia 
means establishing a separate sip dialog for each medium and then 
correlating those dialogs. Such an approach won't interoperate well with 
one that includes multiple media sessions in a single sip dialog.

	Paul