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

Paul Kyzivat <> Mon, 03 December 2012 20:00 UTC

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Date: Mon, 03 Dec 2012 15:00:18 -0500
From: Paul Kyzivat <>
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Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
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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. 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