Re: [rtcweb] Layers in draft-jesup-rtcweb-data-00

[Michael Tüxen <[e-mail]>]

On Nov 1, 2011, at 3:10 PM, Magnus Westerlund wrote:

> On 2011-10-26 22:45, Michael Tüxen wrote:
>> 
>> On Oct 26, 2011, at 10:28 PM, Hadriel Kaplan wrote:
> 
>>> But rather this:
>>> 
>>>       +------+                        +------+
>>>       |WEBAPP|                        |WEBAPP|
>>>       +------+------+------+          +------+------+------+
>>>       | DTLS | Audio| Video|          | SCTP | Audio| Video|
>>> +---------------------------+   +---------------------------+
>>> | STUN | SCTP |S/RTP |S/RTP |   | STUN | DTLS |S/RTP |S/RTP |
>>> +---------------------------+   +---------------------------+
>>> |         Mux/Demux         |   |         Mux/Demux         |
>>> +---------------------------+   +---------------------------+
>>> |            UDP            |   |            UDP            |
>>> +---------------------------+   +---------------------------+
>>> 
>>> [Note: "S/RTP" = SRTP/SRTCP or RTP/RTCP, "Mux/Demux" = tiny logic to mux/demux]
>>> 
>>> Because the audio/video streams may be using the same UDP port, right?
>>> 
>>> And the two "S/RTP" boxes may be just one box depending on how the MMUSIC multiplexing decision turns out.
>>> 
>>> So if we want to choose the left one, because we expect/want that someday the Operating System provides a SCTP/UDP stack in the kernel, and I think we do, could it do so while demuxing and letting STUN, RTP, and DTLS go up to the app layer?  (i.e., given a socket/BIO/FD model)  I have no idea about such things... just asking.
>> This is not possible today. The demultiplexing seems to be specific to this scenario.
>> Not sure it fits. For demuxing you use the first byte to distinguish STUN from DTLS and SRTP.
>> The first byte us the high order byte of the source port. Once could require
>> SCTP to use source ports with the high order byte > 192. That might work.
>> However, you would need to get the Mux/Demux into the kernel. Could be done
>> using a socket option. But I'm not sure it really fits. Maybe Randy has an
>> opinion.
>> 
> 
> Michael,
> 
> I think one of the reasons there is discussion of use land
> implementations of SCTP is so that you can do the above stack diagrams
> with SCTP above UDP that is being shared for several purposes.
> 
> I would also like to correct Hadriel's picture slightly:
> 
>>>       +------+                        +------+
>>>       |WEBAPP|                        |WEBAPP|
>>>       +------+------+------+          +------+------+------+
>>>       | DTLS |Audio & Video|          | SCTP |Audio & Video|
>>> +---------------------------+   +---------------------------+
>>> | STUN | SCTP | DTLS-SRTP   |   | STUN | DTLS | DTLS-SRTP   |
>>> +---------------------------+   +---------------------------+
>>> |         Mux/Demux         |   |         Mux/Demux         |
>>> +---------------------------+   +---------------------------+
>>> |            UDP            |   |            UDP            |
>>> +---------------------------+   +---------------------------+
> 
> I think the above indicating that the common RTP session, potentially
> being a DTLS-SRTP keyed RTP session that can co-exist on the same UDP flow.
> 
> To make the left one work, I think one has to have a source port where
> the first byte value is 192-255 as the de-multiplexing table from
> section 5.1.2 of RFC 5764 shows these to be the only available.
> 
> Thus I think the options are:
> 
> A. Left figure above. (source port must be in 49152-65535)
> 
> B. Right figure above
> 
> C. SCTP in its own UDP flow plus one UDP flow for each RTP session.
> 
> If we would pick SCTP then I think we must have either A+C or B+C working.
And you want one DTLS connection for key derivation of SRTP and a
separate one for SCTP? Or must they be the same?

Best regards
Michael
> 
> cheers
> 
> Magnus Westerlund
> 
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