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

Harald Alvestrand <> Fri, 05 October 2012 17:13 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 134BB21F87BA for <>; Fri, 5 Oct 2012 10:13:14 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -110.419
X-Spam-Status: No, score=-110.419 tagged_above=-999 required=5 tests=[AWL=0.180, BAYES_00=-2.599, RCVD_IN_DNSWL_HI=-8, USER_IN_WHITELIST=-100]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id r3oR66u3fs5k for <>; Fri, 5 Oct 2012 10:13:13 -0700 (PDT)
Received: from ( []) by (Postfix) with ESMTP id 4992821F87B8 for <>; Fri, 5 Oct 2012 10:13:13 -0700 (PDT)
Received: from localhost (localhost []) by (Postfix) with ESMTP id 55B6639E1D0; Fri, 5 Oct 2012 19:13:12 +0200 (CEST)
X-Virus-Scanned: Debian amavisd-new at
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id CY9DDStXKhdA; Fri, 5 Oct 2012 19:13:11 +0200 (CEST)
Received: from (unknown [IPv6:2620:0:1043:1:be30:5bff:fede:bcdc]) by (Postfix) with ESMTPSA id 7CEC939E1BD; Fri, 5 Oct 2012 19:13:11 +0200 (CEST)
Message-ID: <>
Date: Fri, 05 Oct 2012 19:13:10 +0200
From: Harald Alvestrand <>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120827 Thunderbird/15.0
MIME-Version: 1.0
To: Christer Holmberg <>
References: <> <> <> <> <>, <> <>
In-Reply-To: <>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
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: Fri, 05 Oct 2012 17:13:14 -0000

On 10/04/2012 08:21 PM, Christer Holmberg wrote:
> Hi,
>>> For the Web use case, I think PRANSWER is an unnecessary distraction; if
>>> we're writing both initiator and responder, multiple O/A rounds, or use of
>>> multiple PeerConnections, is a much cleaner solution for the use cases I can
>>> wrap my head around.
>> I'd remove that qualifier.  We currently have a draft that describes
>> both cloning AND PRANSWER, both of which are more cleanly addressed by
>> either multiple O/A rounds or multiple independent sessions.
>> The ability to describe a session that includes receipt capabilities
>> that are a superset of send capabilities.  SDP O/A doesn't really
>> provide a way for me to describe this because the default assumption
>> is that items not appearing in the answer are no longer needed.
>> With something like bundle involved, we could split m= lines into
>> sendonly and recvonly portions to allow for this case to be provided.
> I don't think the reason for PRANSWER comes from SDP O/A as such. It's more about supporting serial forking.
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.