Re: [rtcweb] Proposal for a JS API for NoPlan (adding multiple sources without encoding them in SDP)

Emil Ivov <> Tue, 18 June 2013 06:17 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 00A0A21F9ADA for <>; Mon, 17 Jun 2013 23:17:14 -0700 (PDT)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -2.57
X-Spam-Status: No, score=-2.57 tagged_above=-999 required=5 tests=[AWL=0.030, BAYES_00=-2.599, NO_RELAYS=-0.001]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id fsU2e0Iy3v0s for <>; Mon, 17 Jun 2013 23:17:13 -0700 (PDT)
Received: from ( [IPv6:2a00:1450:400c:c00::234]) by (Postfix) with ESMTP id 2F73621F9A42 for <>; Mon, 17 Jun 2013 23:17:12 -0700 (PDT)
Received: by with SMTP id b12so3082583wgh.19 for <>; Mon, 17 Jun 2013 23:17:11 -0700 (PDT)
X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=20120113; h=message-id:date:from:organization:user-agent:mime-version:to:cc :subject:references:in-reply-to:content-type :content-transfer-encoding:x-gm-message-state; bh=Us2hfQdYkQEa2tDb6JoWxJV813FiRPInMlPn7+lBm68=; b=eWTPIgIRCv9ggneOhO5e0zkIrZtxb05z+ZQuWL6UFmCYwtQa/v0VffJ2JIDh7g9sl9 HXIrv7y1LmqmyU1ZXa4dN2F+hzesYZqMhaTj+t0aZCSYo3wlTd48pw31e9MffIkNoM2L 2BEYMg+4RE1t7qKJOh2VQCawHRo0L2sOL/Tq5qVAbeCu3atwGMF3CBNg6f8AAOAYPBdq p9OUZYj0ADR7DNDpjTUE+W3n3VlOqT/2nKkwFsuMYfmYkZfvJoKZYRKVG66vheJ2oKHu 8yb/HU35gU1ArGZaX8ooAg96KxLdkfpKkVUnQlk8qHKSkMbNCw2tdkapkrodI4iMpBNX ZZRQ==
X-Received: by with SMTP id b14mr6729349wiw.6.1371536231926; Mon, 17 Jun 2013 23:17:11 -0700 (PDT)
Received: from camionet.local ([2a01:e35:8a55:abc0:898c:abd3:25f1:23ca]) by with ESMTPSA id ay7sm26388337wib.9.2013. for <multiple recipients> (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Mon, 17 Jun 2013 23:17:11 -0700 (PDT)
Message-ID: <>
Date: Tue, 18 Jun 2013 08:17:09 +0200
From: Emil Ivov <>
Organization: Jitsi
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:17.0) Gecko/20130509 Thunderbird/17.0.6
MIME-Version: 1.0
To: Silvia Pfeiffer <>
References: <> <>
In-Reply-To: <>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
X-Gm-Message-State: ALoCoQlWIIvh44p0W82Ip86+VHt72ik3AgXw1Xi+UVVbuno2bJG01o4CjMbcMr1aPrtZG8Z5iGtC
Cc: "<>" <>
Subject: Re: [rtcweb] Proposal for a JS API for NoPlan (adding multiple sources without encoding them in SDP)
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: Tue, 18 Jun 2013 06:17:14 -0000

Hey Silvia,

On 18.06.13, 03:00, Silvia Pfeiffer wrote:
> What I would like to see, though, is a bit different from what you've
> proposed. In particular, the MediaFlowDescription object is the one
> object that I believe is supposed to enable JS developers to  do "SDP
> hacking" without having to understand SDP. Unfortunately, the way in
> which it is currently written, this API doesn't help a JS developer
> much. There are properties in that object like "ssrc" that mean
> nothing unless you understand SDP.

SSRC is really just a flow identifier and it actually comes from RTP, 
not SDP.

Using SSRCs is important because they are the only way to refer to an 
RTP flow. Any other identifier (such as an MSID) would have to be mapped 
to an SSRC. Such a mapping could only happen in SDP and would have to be 
updated when new streams are added which brings us back to over-reliance 
on SDP Offer/Answer, which is what No Plan is trying to alleviate.