Re: [MMUSIC] Some issues with draft-ietf-mmusic-sctp-sdp-02

Paul Kyzivat <> Fri, 11 January 2013 18:05 UTC

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Date: Fri, 11 Jan 2013 13:05:24 -0500
From: Paul Kyzivat <>
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Subject: Re: [MMUSIC] Some issues with draft-ietf-mmusic-sctp-sdp-02
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Replying to myself, since nobody else did.

There has been more discussion recently (subject: SCTP question: Where 
does it multiplex?) on the same issue I raise below.

More inline.

On 12/3/12 3:29 PM, Paul Kyzivat wrote:
> While commenting on some related discussions I had occasion to look at
> this draft again and I found a few issues:
> First, some very specific ones::
> Section 4.1 says:
>     If the <port> sub-field is 'SCTP' or ...
> This is a typo. s/port/proto/
> But the major issue is that this draft doesn't explain the semantics of
> the <port> field. The only things I can find that touch on this are:
> - section 10.1 refers to "SCTP port 54111" and then shows an m-line
>    with: m=application 54111 SCTP 0 1 2
>    But this is just an example, not normative text.
> - section 4.1 says:
>     [OPEN ISSUE 3] do we need also an attribute to specify the SCTP port
>     number for the SCTP over DTLS scenario? that can be useful in the
>     case where multiple association are running on top of the same DTLS
>     session.
>    This implies that the SCTP port number is *not* what is in the port
>    field in this case. Presumably because the UDP port number is in
>    that field.
> ISTM that there needs to be a clear discussion of all the required port
> numbers for each case, and how they are specified. When there is more
> than one (namely in the SCTP/DTLS and DTLS/SCTP cases), *one* of them
> can be in the m-line and the other one must be somewhere else. And if
> the one that is not in the m-line is optional, then I guess there must
> be a default.

For *native* SCTP (directly over IPv* - <proto>=SCTP) it seems obvious 
that the port in the m-line should be the SCTP port.

For <proto> of SCTP/DTLS (RFC6083) there is again only the SCTP port to 
consider, so it should presumably go in the m-line. (My earlier comment 
about this one was wrong.)

For <proto> of DTLS/SCTP (draft-tuexen-sctp-udp-encaps) we need both the 
SCTP port and the UDP port, so we must pick which one goes in the 
m-line. What arguments are there for which should go in the m-line?

- Consistency with the other SCTP protos argues for the SCTP port.
- The port in the m-line is mandatory. AFAIK there is no default
   SCTP port. But there *is* a default UDP port for DTLS/SCTP
   (9899, defined in draft-tuexen-sctp-udp-encaps). This is also
   an argument for the SCTP port in the m-line.
- are there any arguments in favor of the UDP port in the m-line?
   (I haven't thought of any.)

Absent some arguments on the other side, I propose that this document 
specify that the <port> field of the m-line denote the SCTP port for all 
of the SCTP <proto>s, and specify an attribute to carry the UDP port, 
with a default of 9899.