Subject: Re: [Sip] Re: Offer by the callee

From: [email-masked]
Date: Thu, 21 Mar 2002 13:56:16 +0100

Hi Gonzalo,

An example.
Alice calls her bank, using an audio stream only; Alice is a good customer.
Bob, the Alice's account manager, receives the call, and his terminal is
pre-programmed to offer automatically a video stream also and high-quality
voice codecs, for good customers.
Therefore:
- Offer-1 from Alice to Bob: one audio stream with normal voice codec
- Offer-2 from Bob to Alice: one audio stream with high-quality codec (not
necessarily present in Offer-1, but including supported codecs from Offer-1
to give a fallback possibility), and one video stream

As Offer-2 is pre-programmed in Bob's terminal, it is useless for A to
start resource reservation for Offer-1.
So in the scenario, I was using:
- 183 response, to allow Bob to tell Alice "I'm handling your session
invitation", so it is not a final response carrying an error message
- all port numbers equal to zero, to tell Alice "wait a minute, don't start
resource reservation"
- and the UPDATE message from Bob to Alice will follow immediatly after 200
Prack with the new offer Offer-2 (this is suggested by the scenarios in the
Update draft)

Obviously, there is a charging model behind that, but this is another
story. Anyhow, Alice has always the possibility to reject the Offer.

Best regards
Juan Carlos

Gonzalo Camarillo <[email-masked]> on 21/03/2002
05:30:42

To:   Juan-Carlos ROJAS/FR/ALCATEL@ALCATEL
cc:   Jonathan Rosenberg <[email-masked]>, [email-masked]
Subject:  Re: [Sip] Re: Offer by the callee

Hi,

what do you want to put in the offer-2 to be sent that you cannot put in
the answer-1? Understanding the scenario you have in mind would be
helpful...

Gonzalo

[email-masked] wrote:
>
> Hi Gonzalo,
>
> Thank you for your answer
> You are right, but sending a held SDP does not prevent the calling party
to
> start resource reservation, but only to send media.
> I agree with you that normally, a port number to zero indicates refusal
of
> the media stream. Nevertheless, according to Offer/Answer draft, all port
> numbers equal to zero is used also to announce capabilities (section 9),
> and very often it is used to say just "don't use this media".
>
> What I need here is to give to the answerer the possibility to say
"Mr.the
> Offerer, I'm not rejecting your Invite, but I'd like to send you a new
> offer right now, so it is useless to start resource reservation before
> receiving this new offer"
>
> Best regards
> Juan Carlos

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---

From: Gonzalo Camarillo <[email-masked]>
Date: Thu, 21 Mar 2002 16:55:02 +0200

Hi,

The situation you are describing does not sound very sensible, because
if Alice offers a set of codecs, it is because she wants to use that set
of codecs. Therefore, even if Bob sends her an offer with a bunch of
extra codecs, the possibilities that Alice sticks to her first offer are
very high. Well, if she is using some kind of capability announcement
Bob could know that she supports the high quality codec...

About the video stream, the QoS reservation will begin when the second
offer is sent, so it does not represent any problem.

So, even if Alice uses simcap, for instance, and Bob will offer a new
codec in an extra offer, it is a good idea to start resource reservation
as soon as possible. Just in case Alice refuses the second offer.

I understand you scenario, but it does not fit very well with the
offer/answer model. In the offer/answer model, I send you an offer, and
if you accept it, we have a session. If not, we don't. You are now
saying that you do not want to accept the session, but that if the offer
was different, you would be willing to accept it... that's why you may
have to wait a RTT before you send the second offer.

Anyway, if you really want to express that, you can always set your
ports to zero, although I do not think it is a very good idea.

Regards,

Gonzalo

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