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

Paul Kyzivat <> Tue, 11 December 2012 15:43 UTC

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Date: Tue, 11 Dec 2012 10:43:08 -0500
From: Paul Kyzivat <>
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Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
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More inline.

On 12/11/12 4:08 AM, Salvatore Loreto wrote:
> 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.

Yes, that is what I thought. But Harald has been asking about 
multiplexing this with RTP traffic. (Actually I think it would be 
DTLS/SRTP traffic that it would be multiplexed with.)

> 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.

IIUC, SCTP (having been designed as a transport layer protocol) defines 
its own notion of port, and has fields in its protocol to carry the 
local and remote port number. Presumably those fields are still there 
when run over UDP or DTLS. So it should be possible to support multiple 
SCTP associations over the same DTLS connection, each distinguished by 
its own port pair.

That of course depends on having a signaling mechanism to set it up.

> 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.

AFAIK WebRTC is just one possible user of this mechanism. The SDP 
mechanism shouldn't be limited by the constraints of WebRTC. It would be 
very difficult to define the SDP so that it was impossible to set up 
multiple SCTP associations over different 5-tuples.

>> 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

But it is a challenge that needs to be tackled if we are to realize 
Harald's dream.