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

"Cullen Jennings (fluffy)" <> Mon, 08 October 2012 22:51 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 8768711E80CC for <>; Mon, 8 Oct 2012 15:51:47 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -110.449
X-Spam-Status: No, score=-110.449 tagged_above=-999 required=5 tests=[AWL=0.150, BAYES_00=-2.599, RCVD_IN_DNSWL_HI=-8, USER_IN_WHITELIST=-100]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id 4loF1cApG6qF for <>; Mon, 8 Oct 2012 15:51:46 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id 492EF11E809B for <>; Mon, 8 Oct 2012 15:51:33 -0700 (PDT)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple;;; l=3972; q=dns/txt; s=iport; t=1349736693; x=1350946293; h=from:to:cc:subject:date:message-id:references: in-reply-to:content-id:content-transfer-encoding: mime-version; bh=DYsKXV8oByP6fJYKodPQ1PkzwJhnNoIcSzPDKCZszwg=; b=Gjpn3P6kyODvxMJPuMX9AnEWidZCJ7xZasXb7lHF3y3AppymGdCrGyaR vZgdUGEGanulha5kmjNG36dDcdKz1QR9ZtCy+/akErGuvlNJq41zMqxnL mGlj0kANDRtcl4Tj81dGY7tr6PYlFJzHRBnSNM77Zzm+xQaY4Xr6WyYzH Y=;
X-IronPort-Anti-Spam-Filtered: true
X-IronPort-AV: E=Sophos;i="4.80,556,1344211200"; d="scan'208";a="129528132"
Received: from ([]) by with ESMTP; 08 Oct 2012 22:51:32 +0000
Received: from ( []) by (8.14.5/8.14.5) with ESMTP id q98MpWQ3022383 (version=TLSv1/SSLv3 cipher=AES128-SHA bits=128 verify=FAIL); Mon, 8 Oct 2012 22:51:32 GMT
Received: from ([]) by ([]) with mapi id 14.02.0318.001; Mon, 8 Oct 2012 17:51:32 -0500
From: "Cullen Jennings (fluffy)" <>
To: Christer Holmberg <>
Thread-Topic: [rtcweb] JSEP: Relaxing SDP O/A rules?
Thread-Index: AQHNpad1aenXLuIV20+kkVz1nsB6Xg==
Date: Mon, 8 Oct 2012 22:51:31 +0000
Message-ID: <>
References: <> <> <> <> <> <> <> <>, <> <>
In-Reply-To: <>
Accept-Language: en-US
Content-Language: en-US
x-originating-ip: []
x-tm-as-product-ver: SMEX-
x-tm-as-result: No--39.592400-8.000000-31
x-tm-as-user-approved-sender: No
x-tm-as-user-blocked-sender: No
Content-Type: text/plain; charset="us-ascii"
Content-ID: <>
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
Cc: "" <>
Subject: Re: [rtcweb] JSEP: Relaxing SDP O/A rules?
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Real-Time Communication in WEB-browsers working group list <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Mon, 08 Oct 2012 22:51:47 -0000

Uh, sort of  - if you send an offer that says you could get early media, you may get early media from two places even thought you have not received any answer. Yes, I realize you can't do ICE till you get the answer but I think the above is still the case in non ICE case an with ICE case you may end up with two dialogs formed but dialogs are really a Sip level concept not an RTP concept. 

On Oct 5, 2012, at 14:26 , Christer Holmberg <> wrote:

> Hi,
> Roman is correct.
> The SDP O/A doesn't consider forking, because each forked SIP leg creates a unique early dialog, and each early dialog has its own O/A state machine.
> Regards,
> Christer
> ________________________________
> From: Roman Shpount []
> Sent: Friday, October 05, 2012 8:46 PM
> To: Harald Alvestrand
> Cc: Christer Holmberg;
> Subject: Re: [rtcweb] JSEP: Relaxing SDP O/A rules?
> On Fri, Oct 5, 2012 at 1:13 PM, Harald Alvestrand <<>> wrote:
> I've asked that question before, but I don't remember seeing an answer from people who know what they're talking about:
> Does serial forking, as practiced by SIP UAs, violate the SDP O/A model, or does it not violate the SDP O/A model?
> I don't understand how it, in the form that has been described as "what we have to support", can be SDP O/A compatible, but then there are many things about SIP that I don't understand, so I may understand it when it's explained to me.
> O/A does not describe multiple answers to a single offer. Nothing explicitly prohibits it there but I would argue this is not part of this specification. No standard document that I am aware of describes how serial O/A is supposed to work. Serial forking as practiced by SIP UA does violate SIP RFC 3261 which states that each dialog can only have one answer to each offer. If answer is sent in both provisional and final response, it should be the same. You can, however, create multiple dialogs with the same offer. This normally implies parallel forking, but the most common use case is with early media, where you end up with multiple early dialogs. For instance you call a phone number, phone network sends you SDP in early media and plays a dial tone, then the called number does not answer, and you get connected to a voice mail which uses a completely different SDP in final answer. According to SIP these answers should come with different to-tags and technically would be parts of
>  two different dialogs. What makes this a bit tricky is when the phone network and the voice mail are behind SBC (or some other sort of B2BUA) you only see one dialog which send you multiple answers to the same offer. This scenario is so common that it is more likely to be implemented then parallel forking. This is why PRANSWER was suggested.
> If cloning can be implemented in WebRTC it would be more standard compliant then PRANSWER and would allow for a lot more use cases. Typical difficulty in parallel forking implementation in SIP is due to RTP from multiple sources coming to the same IP and port with no consent or notification to the receiving side. This makes it very difficult to present this media to the user. There is no way to tie media to actual dialogs since RTP can come from different IP and ports then specified in answer SDPs. This is not an issue with WebRTC where consent to receive media is required. I would argue let's implement cloning and not waste any more time on PRANSWER which in my opinion will always stay a half working hack.
> _____________
> Roman Shpount
> _______________________________________________
> rtcweb mailing list