Re: [Dart] multiplexing different media types

Eric Rescorla <ekr@rtfm.com> Sun, 15 June 2014 14:31 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Sun, 15 Jun 2014 07:30:54 -0700
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To: "Paul E. Jones" <paulej@packetizer.com>
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Cc: Harald Alvestrand <harald@alvestrand.no>, dart@ietf.org
Subject: Re: [Dart] multiplexing different media types
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On Sun, Jun 15, 2014 at 7:24 AM, Paul E. Jones <paulej@packetizer.com>
wrote:

> Harald,
>
> Thanks for the clarification. If you'll indulge, allow me to ask one more
> basic question...
>
> In draft-ietf-avtcore-multiplex-guidelines, it goes to great length to
> explain that multiplexing is on the SSRC. It also says no two media sources
> shall use the same PT value. Thus, a payload type could be used when
> demultiplexing, though the text says otherwise. Perhaps this language
> exists to support multipoint?  Thus, a receiver first looks at the SSRC to
> identify the source, then the PT to determine what the packet contains? Is
> SSRC demuxing there only for multipoint?
>
> Why not allow the PT number space to be distinct per SSRC?
>
This is an inconsistency in the drafts. See:
http://tools.ietf.org/html/draft-roach-mmusic-unified-plan-00

for extensive documentation of the issues at hand and the
current plan of record.

-Ekr

Paul
>
>
> ------------------------------
> *From:* Harald Alvestrand <harald@alvestrand.no>
> *Sent:* June 15, 2014 3:18:09 AM EDT
> *To:* "Paul E. Jones" <paulej@packetizer.com>, dart@ietf.org, Eric
> Rescorla <ekr@rtfm.com>
> *Subject:* Re: multiplexing different media types
>
> (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. A!
>>  ll
>> 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 sa!
>>  me 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
>
>
>
>