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

Salvatore Loreto <> Tue, 11 December 2012 09:08 UTC

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Date: Tue, 11 Dec 2012 11:08:16 +0200
From: Salvatore Loreto <>
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Cc: Randell Jesup <>
Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
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Hi Paul,

see in line!

On 12/3/12 10:00 PM, Paul Kyzivat wrote:
> Commenting on a different point
> On 12/1/12 9:53 AM, Harald Alvestrand wrote:
>> The interesting difference is that the multiplexing between DTLS/SCTP
>> traffic and BUNDLE multiplexing is that DTLS/SCTP traffic is not carried
>> in SSRCs, which means:
>> - There can be only one DTLS/SCTP stream in a bundle (which may have
>> multiple associations, as you state below); you can't have multiple
>> lines with proto DTLS/SCTP in a bundle.
> I am not an SCTP expert. But IIUC, SCTP was designed to run directly 
> over IP. It has its own notion of port used to demux multiple SCTP 
> associations over the same IP address.
> I presume that that same mechanism is still there when SCTP is run 
> over DTLS over UDP.
> So, the traffic coming over DTLS must first be demuxed into RTP 
> traffic and SCTP traffic. 
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.

> Then the RTP traffic can be demuxed based on SSRC, and SCTP traffic 
> can be demuxed based on SCTP port. And once the traffic for a single 
> SCTP port is identified, it can be demuxed based on stream number.
> Representing this in SDP is a challenge. Some variant of the bundle 
> proposal might allow bundling together several RTP m-lines and some 
> DTLS/SCTP m-lines. This would require a mechanism for specifying the 
> SCTP port number - already an open issue (#3) in 
> draft-ietf-mmusic-sctp-sdp-02.
I agree that it is a challange how then to bundle everything together


Salvatore Loreto, PhD