Re: [rtcweb] JSEP: Relaxing SDP O/A rules?

Kaiduan Xie <kaiduanx@gmail.com> Wed, 03 October 2012 13:42 UTC

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Date: Wed, 03 Oct 2012 09:42:51 -0400
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From: Kaiduan Xie <kaiduanx@gmail.com>
To: Christer Holmberg <christer.holmberg@ericsson.com>
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Subject: Re: [rtcweb] JSEP: Relaxing SDP O/A rules?
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There was a discussion on this,

http://www.ietf.org/mail-archive/web/rtcweb/current/msg05141.html

However I have not seen how the IETF webrtc work group address this issue.

/Kaiduan

On Wed, Oct 3, 2012 at 9:03 AM, Christer Holmberg
<christer.holmberg@ericsson.com> wrote:
> Hi,
>
> I've been looking at the O/A state machine, and the associated procedure
> text, in section 4.2 of draft-jsep, and a couple of issues.
>
> First, the text says:
>
>         "As in [RFC3264], an offerer can send an offer, and update it as
> long as it has not been answered."
>
> That is not true. In order to update an offer (before it has been answered),
> one needs to cancel the previous offer.
>
>
> Second, the text says:
>
>         "A description used as a "pranswer" may be applied as a response to
> an "offer", or an update to a previously sent "answer"."
>
> I don't understand the update-to-a-previously-sent-answer thing. I don't
> understand why one would update an "answer" with a "preanswer".
>
> And, the state machine doesn't seem to support it either. So, if done,  what
> state will I be in after such action? Can I then also send a new "answer",
> to update the "preanswer" that updated the "answer"?
>
>
> Third, is there a reason why a new "offer" can't be sent once in the
> "Preanswer" state?
>
>
> Regards,
>
> Christer
>
>
>
>
>
> -----Original Message-----
> From: Martin Thomson [mailto:martin.thomson@gmail.com]
> Sent: 2. lokakuuta 2012 22:00
> To: Christer Holmberg
> Cc: rtcweb@ietf.org
> Subject: Re: [rtcweb] JSEP: Relaxing SDP O/A rules?
>
> On 2 October 2012 11:00, Christer Holmberg <christer.holmberg@ericsson.com>
> wrote:
>> There has also been ideas about JSEP "relaxing" the SDP O/A rules.
>
> JSEP already does that.
>
> PRANSWER is probably the most obvious diversion, but it goes deeper.
>
> There's a simple view of what JSEP is and it goes a little like this:
>
> - SDP provides a description of the session that you are going to create [1]
>
> - You need two SDP blobs so that you can get all the non-negotiated
> parameters like crypto and candidate attributes
>
> - setLocalDescription and setRemoteDescription provide a way of matching up
> two SDP blobs into a complete session description
>
> - negotiation is performed outside this machine, and the browser can help
> you with that in two ways: providing createOffer and createAnswer to build
> the SDP; and by nudging you with a negotiatedneeded event when it learns of
> things that necessitate changes.
>
> Obviously, it's a little more nuanced than that, but if you take this view,
> you are more likely to get something to work.
>
> --Martin
>
> [1] Caveat: PRANSWER adds a layer of complexity to this by shifting part of
> the session description to the offer rather than the answer
>
>
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