Re: [Dart] multiplexing different media types

Eric Rescorla <ekr@rtfm.com> Sun, 15 June 2014 14:21 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 15 Jun 2014 07:20:24 -0700
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To: Harald Alvestrand <harald@alvestrand.no>
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Cc: "Paul E. Jones" <paulej@packetizer.com>, dart@ietf.org
Subject: Re: [Dart] multiplexing different media types
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On Sun, Jun 15, 2014 at 12:18 AM, Harald Alvestrand <harald@alvestrand.no>
wrote:

> (Adding EKR to thread to get a definitive DTLS answer)
>
> On 06/15/2014 06:52 AM, Paul E. Jones wrote:
> > Harald,
> >
> >
> >> "SCTP ... can be multiplexed with one or more RTP sessions". Actually
> >> we can only multiplex SCTP with a single RTP session. There have been
> >> proposals that would allow multiplexing of multiple RTP sessions
> >> (each containing multiple media flows) over a single 5-tuple, but
> >> these were not accepted.
> >
> > Your draft (draft-ietf-rtcweb-transports) says:
> >
> >     RTCWEB implementations MUST support multiplexing of DTLS and RTP over
> >     the same port pair, as described in the DTLS_SRTP specification
> >     [RFC5764], section 5.1.2. All application layer protocol payloads
> >     over this DTLS connection are SCTP packets.
> >
> > I had a question about this as we discussed the DART draft.  I assumed
> > the only DTLS connection would be one used for key negotiation for
> > SRTP.  Is that not the case? Would there be multiple DTLS connections
> > multiplexed?  If so, how would one be differentiated from another?
>
> EKR is the expert here.
>
> As I understand it, the key material for DTLS-SRTP is derived from the
> session keys from the DTLS session. This does not in any way affect the
> usage of the same DTLS session for passing DTLS data.
>

Each transport pair is attached to one DTLS association.

If you mux media and data channels on the same transport pairs,
you have one DTLS association. You derive SRTP keys from the
DTLS keys and send SRTP over the same transport.

-Ekr



> >
> > As for RTP Session multiplexing, it's interesting to hear that
> > proposals are dead.  Is there a proposal for multiplexing different
> > media types (e.g., audio and video) within the same RTP Session,
> > then?  RFC 3550 discourages that, but it was my understanding that
> > browser makers wanted to multiplex the different media types somehow.
> > What's the plan?
>
> draft-ietf-avtcore-multiplex-guidelines covers the RTP aspects.
> draft-ietf-mmusic-sdp-bundle-negotiation has the details on SDP.
>
> The only thing in RTP itself that prevents such multiplexing is the
> words in RFC 3550; technically there is no barrier at the RTP level.
>
> At the SDP level things are a bit more complex, which is why -bundle-
> isn't an RFC yet.
>
> >
> > Paul
> >
>
>
> --
> Surveillance is pervasive. Go Dark.
>
>