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

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

> > 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.
> 
> Even with BUNDLE, you would have to use the same m= line port value from the
> beginning, which is not allowed. So, you basically would have to add the
> additional SCTPoDTLS m= lines once the usage of BUNDLE has been negotiated.

<Raju>
You are right. Section 8.2.1 of draft-ietf-mmusic-sdp-bundle-negotiation
says the m= line should have unique 4-tuple to allow possibility of 
BUNDLE rejection. Very valid.
However, to facilitate multiple SCTP over same bundled SCTP, 
won't the following work?
- Offerer adds one m= line for each SCTP association needed.
- If the answerer selects BUNDLE then all the SCTP m= lines now map
  and use the same DTLS association.
- If the answerer does not accept BUNDLE then we don't have the option
  of negotiating multiple SCTPs over same DTLS anyway; so it will result
  in multiple DTLS, which still gives you a way to get multiple SCTPs.

With this approach a new subsequent SDP offer is not needed to 
add SCTP m= lines.
</Raju>

> 
> > 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.
> 
> Per the agreement in Honolulu, there will be no port 9 for SCTP, as the
> agreement was that both endpoints will initiate the SCTP association.
> 
> And, in order to prevent two separate SCTP associations to be established,
> the m= line port needs to indicate both the client and server port also
> without ICE.

<Raju>
So, these 2 clarifications are needed in draft-ietf-mmusic-sctp-sdp as
it deviates from referred RFC 4145.
</Raju>

BR
Raju