Re: [Dart] multiplexing different media types

Harald Alvestrand <> Sun, 15 June 2014 07:18 UTC

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Date: Sun, 15 Jun 2014 09:18:09 +0200
From: Harald Alvestrand <>
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Subject: Re: [Dart] multiplexing different media types
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(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.

> 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

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