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

Paul Kyzivat <> Tue, 11 December 2012 19:26 UTC

Return-Path: <>
Received: from localhost (localhost []) by (Postfix) with ESMTP id 5242F21F84F3 for <>; Tue, 11 Dec 2012 11:26:57 -0800 (PST)
X-Virus-Scanned: amavisd-new at
X-Spam-Flag: NO
X-Spam-Score: -0.401
X-Spam-Status: No, score=-0.401 tagged_above=-999 required=5 tests=[AWL=0.036, 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 9hTpKDSx1ZDT for <>; Tue, 11 Dec 2012 11:26:56 -0800 (PST)
Received: from ( [IPv6:2001:558:fe14:44:76:96:59:212]) by (Postfix) with ESMTP id 9869521F8482 for <>; Tue, 11 Dec 2012 11:26:56 -0800 (PST)
Received: from ([]) by with comcast id aKQ81k0050ldTLk5EKSwKT; Tue, 11 Dec 2012 19:26:56 +0000
Received: from Paul-Kyzivats-MacBook-Pro.local ([]) by with comcast id aKSv1k00o3ZTu2S3QKSvew; Tue, 11 Dec 2012 19:26:55 +0000
Message-ID: <>
Date: Tue, 11 Dec 2012 14:26:54 -0500
From: Paul Kyzivat <>
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:17.0) Gecko/17.0 Thunderbird/17.0
MIME-Version: 1.0
References: <> <> <> <> <> <> <010501cdd7d0$d02dbc00$70893400$>
In-Reply-To: <010501cdd7d0$d02dbc00$70893400$>
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=1355254016; bh=K6sgng+92qFZMgp0a2vCfa7AgaD+z9oALGzdYGINjgw=; h=Received:Received:Message-ID:Date:From:MIME-Version:To:Subject: Content-Type; b=NC/oqo0RMIggujGavgYb0anQ+/TQGlOmM0ptNDntgIkW6w9BTwcuQn1Ypo+KHg0vU U9dHWsMXlGTJZjQC3uakYtiAzS2tR+l7d8QgGheCarspgbNgXJZ4bZqOo7rtstYFs9 sOjYRVXLzF+YS3wmBoOkrzZLArgSnckp3pE8GU+V95J7tQxh6fFLi5bGX46q5iXZxq zDXXt7E67X8AO8jQjVEhnextihaUub9Ljt4Paykkyvv3xdT6O07Rb3sn5VAA1pRG4L tJS9rbMFaIRq+G3fupCE4BXnYNMVoRh2p1uNVEuXEZuXjkcKchrBJtx1Ymqq7Afiv8 IAsCIyDUQSEcQ==
Subject: Re: [MMUSIC] 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: Tue, 11 Dec 2012 19:26:57 -0000

On 12/11/12 1:53 PM, Dan Wing wrote:
>> -----Original Message-----
>> From: [] On Behalf
>> Of
>> Sent: Tuesday, December 11, 2012 2:34 AM
>> To:;
>> Cc:
>> Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
>> Hi Sal,
>> Salvatore Loreto wrote:
>>> based on the current stack the SCTP traffic is the only traffic that
>>> runs directly over the DTLS stack.
>>> What I am trying to do is to include the Randell Jesup (I am including
>>> him in CC as I am not sure he is subscribed to this mailing list)
>>>   suggestion to give the possibility to have multiple SCTP
>>> *associations* running  on top of the same DTLS session and of course
>>> providing a way to signal it in SDP.
>>> to be clear: at moment WebRTC allows only one SCTP association per PC,
>>> so this is something that would be nice to define just to be ready for
>> the future.
>> It's a bit unclear to me what the benefit of this would be. SCTP itself
>> allows multiple streams to be transported in parallel within a single
>> SCTP association. So, in what type of situations would we need multiple
>> parallel SCTP associations? I thought the main point of SCTP is that it
>> provides the parallelism and multiplexing by itself.
>> In other words: I do understand the desire to multiplex RTP and SCTP
>> within the same UDP flow, so we can for instance reduce the number of
>> needed NAT/FW bindings.
> Sending bulk data (SCTP) and interactive audio/video over the same port
> will break prioritization of the audio/video flow above the bulk data.

Yeah, probably. In theory you could use DSCP to mark the packets 

This indeed might be a reason to *not* multiplex the SCTP association 
for data channels with the media. Assuming we find a way to do that 
multiplexing, this would be an argument for making that multiplexing 

>> I also understand why multiple independent non-
>> HOL blocking streams within SCTP are useful for applications. But I
>> don't yet understand the additional benefit of multiple parallel SCTP
>> associations. No doubt it can be technically done, but to what purposes?
> One example:  transferring a big file while, at the same time, doing
> screen sharing or an instant message chat.

When using DTLS/SCTP this is an argument for using SCTP associations 
over separate DTLS connections. Two SCTP associations (differing by SCTP 
port) over the same DTLS connection wouldn't solve this problem.