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

Salvatore Loreto <> Sat, 01 December 2012 11:03 UTC

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Date: Sat, 01 Dec 2012 13:03:09 +0200
From: Salvatore Loreto <>
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Subject: Re: [MMUSIC] SCTP question: Where does it multiplex?
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Hi Harald,

sorry to be late on this
and thanks a lot for raising the question

IMO how to multiplex DTLS/SCTP traffic with te rest of the "WebRTC" 
traffic is something that belongs
to the BUNDLE discussion, and there won't be anything about how to 
bundle in this draft.
To answer your question at present I don't think there should be 
anything special for DTLS/SCTP

However, what I have in my working copy on the draft (I will publish it 
at some point next week)
and it can be interesting to you is the possibility to have multiple 
SCTP associations on top of the same
DTLS connection (for future WebRTC scenarios, or non WebRTC scenarios):

        Running SCTP over DTLS make possible to have multiple SCTP
        associations on top of the same DTLS connection; each SCTP
        association make use of a distinct port number that is mainly
    used to
        demultiplex the associations.  If the <proto> sub-field is 'DTLS/
        SCTP' the <fmt> sub-fields contain SCTP association port numbers.
        When a list of port number identifiers is given, this implies that
        all of these associations MUST run on top of the DTLS connection.
        For the payload type assignments the "a=sctpmap:" attribute
        SHOULD be used to map from a port number to a media
        encoding name that identifies the payload format transported by the
        association or the actual application protocol running on top of it.

                   m=application 54111 DTLS/SCTP 5000 5001 5002
                   c=IN IP4
                   a=sctpmap:5000 protocol=webrtc-DataChannel;streams=16
                   a=sctpmap:5001 protocol=XXX;streams=2
                   a=sctpmap:5002 protocol=YYY;streams=1

best regards

On 11/2/12 12:39 PM, Harald Alvestrand wrote:
> As currently specified, the DTLS/SCTP protocol is carefully specified 
> in such a way that it can ride on the same 5-tuple as an SRTP session. 
> I'd like to keep that property when we SDP-negotiate it.
> The proposed syntax for DTLS/SCTP seems to be
> m=<something, likely application> <port number> DTLS/SCTP <fmt data, 
> mostly meaningless>
> I see two ways to define this as a multiplex:
> - Give the same port number as another m= line in the same SDP session
> - Give a meaningless port number, and use the a=group:BUNDLE (of 
> whatever flavor) to give which port number it's actually multiplexed 
> together with.
> The best way to do this may be obvious after the BUNDLE discussion, 
> but in case there are other considerations .... are there thoughts?
>                Harald
> _______________________________________________
> mmusic mailing list

Salvatore Loreto, PhD