Re: [rtcweb] *MUST* DCEP?

Paul Kyzivat <> Tue, 04 March 2014 16:38 UTC

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Date: Tue, 04 Mar 2014 16:36:34 +0000
From: Paul Kyzivat <>
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To: "Makaraju, Maridi Raju (Raju)" <>, Ted Hardie <>
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Subject: Re: [rtcweb] *MUST* DCEP?
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On 3/4/14 3:39 PM, Makaraju, Maridi Raju (Raju) wrote:
>> -----Original Message-----
>> From: Paul Kyzivat []
>> Sent: Tuesday, March 04, 2014 8:59 AM
>> On 3/4/14 12:08 PM, Makaraju, Maridi Raju (Raju) wrote:
>>> While a=sctpmap assumes use of DCEP, indication of external negotiation
>>> (instead of DCEP) also requires subprotocol and it's parameters to be
>>> conveyed. This has to be done per data channel stream.
>>> Are you looking at more than what is documented in
>>> [I-D.ejzak-mmusic-data-channel-sdpneg]?
>>> Or are you looking for an indication of this (for all streams) directly
>>> in a=sctpmap?
>> I'm ok as things currently stand, though the documents are confusing.
>> The 'a=sctpmap:nnn websocket-datachannel' means that DCEP is supported.
>> My concern was a proposal I think I heard that would make support for
>> DCEP to be optional. In that case, IMO it is important that the O/A
>> exchange establish whether DCEP is supported or not.
> [Raju]
> That's the reason why I proposed to add 'protocol' parameter to a=dcmap.
> Having this parameter at a=sctmap level may not make that much sense due to:
> sctpmap is for entire association and protocol defaults to DCEP. To make use of external negotiation, which is per data channel stream, one would need additional a= line(s) (e.g. ejzak's draft) to convey subprotocol info. IMO adding data channel protocol option to a=dcamp, which is per subprotocol, makes sense.
> [/Raju]

I have no problem with a=dcmap.

It is the 'app' parameter of the sctpmap attribute that I am talking about.

If I want to use DCEP, the dcmap attribute is irrelevant to me. What is 
it that tells me that DCEP is supported. The only datum available that 
could imply that is the 'app' parameter of sctpmap.

Right now, if app is webrtc-datachannel, that implies DCEP is MTI. That 
serves the purpose.

But as of today there was a suggestion that you could have cases where 
there is an SCTP association that supports Data Channels, but not DCEP. 
I don't know how the two would be distinguishable. Possibly by a 
different 'app' value.

A related question is what is the applicability of a=dcmap. Surely it 
doesn't apply for all uses of SCTP - those that don't use data channels 
at all. Do you propose to couple this to particular 'app' values? Or is 
support for this negotiated independently of a=sctpmap?