Re: [MMUSIC] Single SCTP usage per SDP session?

["Makaraju, Maridi Raju (Raju)" <[e-mail]>]

> 
> >>> Hopefully there are some SCTP experts in da hause that can clarify the
> following:
> >>>
> >>> Previously, in the SCTP-SDP discussions, we have agreed that for a given
> SCTP/SCTPoDTLS/DTLSoSCTP association, we will only allow a single usage
> (e.g. a webrtc-datachannel).
> >>>
> >>> So, if one wants multiple usages, one needs to establish multiple
> associations, using multiple m- lines.
> >>>
> >>> BUT, according to RFC 4960, two SCTP endpoints MUST NOT have more than
> one SCTP association between them at any given time. Not exactly sure why,
> but that is what the text says.
> >>>
> >>>      "SCTP association: A protocol relationship between SCTP endpoints,
> >>>                 composed of the two SCTP endpoints and protocol state
> information
> >>>                 including Verification Tags and the currently active set
> of
> >>>                 Transmission Sequence Numbers (TSNs), etc.  An
> association can be
> >>>                 uniquely identified by the transport addresses used by
> the
> >>>                 endpoints in the association.  Two SCTP endpoints MUST
> NOT have
> >>>                 more than one SCTP association between them at any given
> time."
> >>>
> >>> <Sal>
> >>> That is not a problem.
> >>> As it has also already suggested it is enough to use a different port
> number on one side.
> >>> </Sal>
> >>
> >> After discussing with an SCTP expert, my understanding is that it is
> enough to have a unique 4-tuple for each SCTP association.
> >>
> >> So, that means that SDP can contain multiple m- lines describing an SCTP
> association, as long as each use unique 4-tuples.
> >>
> >> However, afaik, that means that a BUNDLE group CANNOT contain multiple
> SCTP associations, as they would all share the same 4-tuple.
> >>
> >>
> >>> <Sal>
> >>> note also that in
> >>> https://tools.ietf.org/html/draft-ietf-tsvwg-sctp-dtls-encaps-06
> >>> we already have the following text that implies that is possible to
> >>> have multiple SCTP associations between two endpoints
> >>>
> >>>  o  Multiple SCTP associations MAY be multiplexed over a single DTLS
> >>>     connection.  The SCTP port numbers are used for multiplexing and
> >>>     demultiplexing the SCTP associations carried over a single DTLS
> >>>     connection.
> >>
> >> Based on the one-SCTP-assocation-per-4-tuple assumption, isn't that text
> wrong?
> >
> > <Sal>
> > the text is correct!
> > because it implies that different SCTP associations use different ports
> (i.e. each of them use an unique 4-tuples)
> >	-> "The SCTP port numbers are used for multiplexing and
> demultiplexing..."
> > </Sal>
> 
> Well, in case of SCTPoDTLS one could claim that the SCTP port is not really
> part of the 4-tuple. But, if the tsvwg folks are ok to multiplex multiple
> SCTP associations using a single UDP/TCP port, then fine... I don't have any
> strong opinion - I just want to clarify :)
> 
> However, in the SCTP-SDP discussions in Honolulu it was agreed to only allow
> a single SCTP association in the case of SCTPoDTLS (even if draft-ietf-
> tsvwg-sctp-dtls-encaps allows multiple). The reason is that you would
> otherwise have multiple m- lines with the same port value (even if you don't
> use BUNDLE), as the port indicates the UDP/TCP port - not the SCTP port.

<Raju>
Per my understanding, draft-ietf-tsvwg-sctp-dtls-encaps just talks about SCTP over DTLS and allowing multiple SCTP associations, which is perfectly fine.
Then it is possible to come up with SCTP-SDP semantics to use that capability.
One m= line allowing one SCTP over DTLS limit is fine. Using BUNDLE you can replicate same 4-tuple in multiple m= lines. Then there is only one DTLS association established and multiple SCTP associations over that is possible (one per m= line).
If BUNDLE is not used then I believe same 4-tuple on multiple m= lines are NOT possible. This is because, per my understanding, unlike TCP/native-SCTP used under non-ICE environments, in ICE-environments the TCP/UDP port specified is used both as client and server port for DTLS. 

I am not sure if draft-ietf-mmusic-sctp-sdp makes any distinction between use of ICE or not. I believe the semantics for port in m= line are different when ICE is used vs. ICE is not used.
- When ICE is used it is the client as well as the server port.
- When ICE is not used, the port is the server port and not the client port. For a=setup:active cases discard port 9 is used per RFC 4145, which draft-ietf-mmusic-sctp-sdp refers.

</Raju>

BR
Raju