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

Randell Jesup <> Tue, 11 December 2012 22:50 UTC

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Date: Tue, 11 Dec 2012 17:49:01 -0500
From: Randell Jesup <>
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Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
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On 12/11/2012 1:53 PM, Dan Wing wrote:
>> 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.

Right.  We're defining SDP for SCTP here, not *just* for WebRTC. (and 
direct use of rtcweb without WebRTC could allow for multiple 
associations in theory).

>> 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.

For the general case, you could provide different DSCP markings, etc, 
per Dan's suggestion.

And also you could run different protocols on top of SCTP, each in an 
association.  One could run DataChannels, another could run BFCP, etc.  
That way they don't conflict (in how they allocate streams, etc).

>> 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.

Right - until we create a new draft, there are buffer-hogging issues 
with large transfers (transfer a 270MB file, like I did at IETF, and all 
other streams are starved until the transfer finishes). Separate 
associations would also deal with the case without needing draft.  
(We'll want the draft for rtcweb).  They might use different congestion 
priorities, etc.

Randell Jesup