Re: [MMUSIC] RESTART: SCTP question: Where does it multiplex?

Paul Kyzivat <> Fri, 11 January 2013 16:57 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 7A4BB21F8808 for <>; Fri, 11 Jan 2013 08:57:06 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -0.099
X-Spam-Status: No, score=-0.099 tagged_above=-999 required=5 tests=[AWL=0.338, BAYES_00=-2.599, FH_RELAY_NODNS=1.451, HELO_MISMATCH_NET=0.611, RDNS_NONE=0.1]
Received: from ([]) by localhost ( []) (amavisd-new, port 10024) with ESMTP id i21YyJl+E3lv for <>; Fri, 11 Jan 2013 08:57:05 -0800 (PST)
Received: from ( [IPv6:2001:558:fe14:43:76:96:62:24]) by (Postfix) with ESMTP id 1176521F87CE for <>; Fri, 11 Jan 2013 08:57:04 -0800 (PST)
Received: from ([]) by with comcast id mbHh1k0091c6gX851gx4NR; Fri, 11 Jan 2013 16:57:04 +0000
Received: from Paul-Kyzivats-MacBook-Pro.local ([]) by with comcast id mgx41k0013ZTu2S3jgx499; Fri, 11 Jan 2013 16:57:04 +0000
Message-ID: <>
Date: Fri, 11 Jan 2013 11:57:02 -0500
From: Paul Kyzivat <>
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:17.0) Gecko/20130107 Thunderbird/17.0.2
MIME-Version: 1.0
To: Dan Wing <>
References: <><> <> <><> <><010401cdd7d0$d006d310$70147930$> <> <01f801cde962$f1f6c2c0$d5e44840$> <> <01da01cdef7e$b424c9c0$1c6e5d40$> <> <02e201cdefbf$ae035b60$0a0a1220$>
In-Reply-To: <02e201cdefbf$ae035b60$0a0a1220$>
Content-Type: text/plain; charset="ISO-8859-1"; format="flowed"
Content-Transfer-Encoding: 7bit
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=q20121106; t=1357923424; bh=sWe+OQkLkK8FJs6+YH6phOE3vwzkoqxvwzDDfpSupHo=; h=Received:Received:Message-ID:Date:From:MIME-Version:To:Subject: Content-Type; b=AB10xp3mkSbFf6tpMzFiT/1pIvqmEXrr1YdVG5afF9Mn46CtJ2okF2DtJ+PSAt29l xQpSKeBU35n7FeSE2+xxgSMoCpKoChm5/NbZSJTvS1XRIOxCfTy3NGQN+/BlLgdhaz jbtKbtSRQGWNh+5nfWTAQRkR+DWRFMx0O82aIQUZ3emKwTY8Cmy73R+EiwmZhgIQt4 Iqzb7tLG55ZiF1iaSgBJjBHKvj4tq8Sgs2FWBE30WcmfJwwEz8D87VbCylMUpX0TP+ 2d3NENlUowA+DUYIcb/WANmb26W30/VZMdzJPZmWQxCu40Q0uG4LihM7/NEc99Q14B U/ynJoot9CQ8A==
Subject: Re: [MMUSIC] RESTART: SCTP question: Where does it multiplex?
X-Mailman-Version: 2.1.12
Precedence: list
List-Id: Multiparty Multimedia Session Control Working Group <>
List-Unsubscribe: <>, <>
List-Archive: <>
List-Post: <>
List-Help: <>
List-Subscribe: <>, <>
X-List-Received-Date: Fri, 11 Jan 2013 16:57:06 -0000

Commenting on one specific point...

On 1/11/13 12:51 AM, Dan Wing wrote:

>>> We need a way for SDP to signal:
>>>     1. the UDP port for SCTP.  This feels like an SDP media ("m") line
>> to me.
>>>     2. SCTP native (native SCTP over IPv6 should be tenable, whereas
>>> native SCTP over IPv4 is pretty much untenable),
>>>     3. the data carried over each SCTP port (e.g., port NNN for XMPP,
>>> port NNNN for file sharing, port NNNNN for screen sharing, and so on).
>>> This feels like an SDP media ("m") line to me.
>> I don't disagree with any of your points, but you have missed the point
>> I was making.
>> Everything you list applies for SCTP on its own 5-tuple. But then there
>> is the question of how to describe how this shares a 5-tuple with some
>> audio and video streams.
> (1), from my list, is the UDP port.  This could be the same UDP port
> as the audio/video media.

Yes. But SDP doesn't permit two m-lines with the same address and port. 
Bundle has been proposed to work around that. But currently bundle 
requires all the bundled m-lines to have the same <proto>. And that 
won't work for bundling SCTP and RTP.

The restriction to the same <proto> is dictated by the need for a way to 
demux. Bundle specifies rules for multiple RTP m-lines that ensures they 
can be demuxed. The demuxing of RTP from SCTP on the same UDP port uses 
a different mechanism. So for this to work bundle needs:

- more elaborate rules for what <proto> values may be bundled,
   and which may not

- explicit specification for the demuxing of each of the different
   m-lines that have been bundled.

This is not rocket science. Once you think about it a bit it is fairly 
evident which combinations work, and how to demux them. But it needs to 
be written down, directly or by reference.