Subject: [Sip] Help: About Early-Session and Session

From: [email-masked]
Date: Thu, 26 Oct 2006 11:35:36 +0800

Dear Sip Group,

     There encounters a question when I read RFC 3959 and RFC 3960 which 
refer to the Early-Session , that is
 
       "Does Offer/Answer Model defined in RFC 3264 apply to Early-Session 
and Session independently?"

     In RFC 3959 section 7, there is a example as below:
      A                           B
      |                           |
      |--------(1) INVITE-------->|
      |            offer          |
      |                           |
      |<--(2) Session Progress----|
      |       early-offer         |
      |       answer              |
      |                           |
      |---------(3) PRACK-------->|
      |             early-answer  |
      |                           |
      |<--------(4) 200 OK--------|
      |                           |
      |  *                     *  |
      | ************************* |
      |*       Early Media       *|
      | ************************* |
      |  *                     *  |
      |                           |
      |<--------(5) 200 OK--------|
      |                           |
      |----------(6) ACK--------->|
      |                           |

          Figure 1: Message flow

    Hope to have a letter from you soon.

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---

From: Paul Kyzivat <[email-masked]>
Date: Thu, 26 Oct 2006 09:40:00 -0400

[email-masked] wrote:
>        "Does Offer/Answer Model defined in RFC 3264 apply to 
> Early-Session and Session independently?"

There has been little to no discussion of this. All the extensive and 
detailed investigation of offer/answer, including the offeranswer draft, 
have avoided this subject.

But IMO if the application server model is followed, using the 
early-session disposition, then indeed the session and early-session 
must independently follow the offer/answer model.

This can potentially lead to some very complex interactions. For 
instance, the caller could make a offer for the session, and the callee 
could make an early-session offer in the other direction. Then they both 
could use preconditions that result in further offers and answers for 
both session and early-session as those are resolved. Each would 
presumably be required to follow the rules about only one offer or 
answer per transaction, but a single transaction might carry an offer or 
answer for each of session and early-session.

I'm not aware of anyone having attempted an implementation of this. If 
somebody has, please speak up. (Copying Gonzalo since he is author and 
might know of an implementation, and Robert in case this has ever come 
up at SIPit.)

IMO its extremely unlikely that this will ever be implemented well 
enough (if at all) to achieve interoperability. There is enough trouble 
getting basic offer/answer working.

	Paul

> ------------------------------------------------------------------------
>

---

From: Gonzalo Camarillo <[email-masked]>
Date: Sat, 28 Oct 2006 12:26:34 +0300

Hi,

some folks told me offline that they were going to implement this, but I 
did not hear from them after that.

Giving the level of early-media-related discussions we are having 
lately, it is obvious that people are still looking for new mechanisms 
to handle early media.

Cheers,

Gonzalo

Paul Kyzivat wrote:
>>      There encounters a question when I read RFC 3959 and RFC 3960 
>> which refer to the Early-Session , that is
>> ZTE Information Security Notice: The information contained in this 
>> mail is solely property of the sender's organization. This mail 
>> communication is confidential. Recipients named above are obligated to 
>> maintain secrecy and are not permitted to disclose the contents of 
>> this communication to others.
>> This email and any files transmitted with it are confidential and 
>> intended solely for the use of the individual or entity to whom they 
>> are addressed. If you have received this email in error please notify 
>> the originator of the message. Any views expressed in this message are 
>> those of the individual sender.