Subject: [Sip] PRACK holding the offer

From: "Hisham Khartabil" <[email-masked]>
Date: Tue, 12 Feb 2002 17:17:15 +0200

Hi,

Perhaps this has been discussed earlier, but here is goes anyway:

What is the harm in a PRACK carrying the SDP offer and the answer being
carried in the 200OK for PRACK or even in the 200OK for the initial INVITE?

   INVITE (no SDP)
------------------->

   1xx (no SDP) 100rel
<------------------

   PRACK (SDP)
------------------->

   200 (SDP) for PRACK
<------------------

OR
   200 (SDP) for INVITE
<------------------

Regards,
Hisham

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---

From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 12 Feb 2002 18:28:40 +0200

Hi,

So far we have assumed that if you do not send an offer in the INVITE it
is because you wait for an offer from the UAS. 

Do you have an scenario where the UAC does not introduce an offer in the
INVITE but intends to do it later (only if the first 1xx does not
contain one)??

Gonzalo

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---

From: "Hisham Khartabil" <[email-masked]>
Date: Tue, 12 Feb 2002 19:54:39 +0200

Well, I don't have a specific scenario, just the general one where I invite
you and say "lets do something, what do you have", you send a 1xx saying
"ok, but you let me first what you have".

I just questioned it because the restrictions in section 13.2.1 prohibit
such scenario and I was wondering why should it be prohibited.

Regards,
Hisham

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of Gonzalo
> Camarillo
> Sent: Tuesday, 12 February 2002 6:29 PM
> To: Hisham Khartabil
> Cc: [email-masked]; [email-masked]
> Subject: Re: [Sip] PRACK holding the offer
>
>
> Hi,
>

---

From: Gonzalo Camarillo <[email-masked]>
Date: Tue, 12 Feb 2002 20:48:41 +0200

Hi,

A reson for not allowing this exchange is that it would complicate the
glare prevention mechanisms (if the UAS sends an offer in a 1xx and the
UAC is sending at the same time an offer in an UPDATE we will have a
collision).

But I believe you are thinking of only using the PRACK to a 1xx to send
the offer from the UAC. In my opinion this increases the complexity of
the protocol for no real gain.

If the UAC can send an offer in the first place, it will send it it the
INVITE. If it could not send the offer in the INVITE (maybe because it
is a 3pcc controller), why would it be able to send it now in the PRACK?

On the UAS side, if there is an scenario (I do not know of any) where
the UAS can send an answer, but not an offer, it would not work with
plain SIP (INVITE-200 OK-ACK). Therefore, you would be defining a
mechanism that would only work with certain extensions.

Best regards,

Gonzalo

---

From: Jonathan Rosenberg <[email-masked]>
Date: Wed, 13 Feb 2002 00:53:00 -0500

I agree with Gonzalo on all points. To further his reasoning, with this
proposal, a UAS would not know whether it needs to make the offer, or
whether the UAC will offer it later. This will require additional
signaling from UAC to UAS to say "even though I am not sending the offer
in invite, i will send it later". More needless signaling and
complexity.

-Jonathan R.

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