Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg

Juergen Stoetzer-Bradler <> Sun, 14 February 2016 10:48 UTC

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To: EXT Flemming Andreasen <>, Paul Kyzivat <>,, "DRAGE, Keith (Keith)" <>, "Makaraju, Maridi Raju (Raju)" <>
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From: Juergen Stoetzer-Bradler <>
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Date: Sun, 14 Feb 2016 11:48:03 +0100
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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> It would probably simplify the overall SDP negotiation part, but I don't know if it would 
> constrain the way data channels were envisioned to be used. 

On Friday Paul mentioned in his email two potential use cases, where no subprotocol identifiers 
might be added to a data channel's dcmap attribute. In my last email I hadn't thought of such cases. 
If we don't want to exclude such cases, then requiring the subprotocol ids always to be present 
might indeed be too restrictive.

> Ok. Going back to discussion between Paul and I, do you believe that in for an attribute to be 
> encapsulated in dcsa, the attribute MUST have been explicitly define to support this (Paul's 
> suggestion below) or do you believe that this is overly constraining, and if so, how shoud we 
> relax it ? 

I now think that the sdpneg draft might recommend SDP offerers and answerers to always add a 
subprotocol identifier to a data channel's dcmap attribute if dcsa embedded attributes are also 
negotiated for the data channel's subprotocol. I also think we could explicitly add text to sdpneg 
saying that subprotocol identifiers should be added to the IANA Websocket (and in future combined 
data channel) registry. And that this registration should be done via a document, which then 
explicitly describes the usages and semantics of dcsa embedded subprotocol attributes, _if_ those 
usages or semantics deviate from cases, where these attributes are not dcsa encapsulated. I think 
that in those cases, where the usage and meaning of an attribute (always related to the data 
channel's subprotocol) does not deviate from the non-dcsa encapsulated use cases, such an attribute 
may not explicitly need to be described for data channel usage. But I think it might be helpful to 
explicitly say so in the sdpneg draft.

Somebody inspecting an SDP offer or answer, which was generated by an implementation following these 
two recommendations (and which hence contains an IANA registered subprotocol id if it contains any 
dcsa embedded attributes), could then refer to that IANA table, would then find the reference to the 
document, which defines this subprotocol id for data channel usage, and there could check if any of 
the dcsa embedded attributes has a data channel specific semantic. If yes, that specific semantic 
would be described in that document. If such a specific semantic were not described in that 
document, then the default usage and semantic of the attribute would apply also to that data channel 
transport case.

But in that context I'd have an IANA table related question.
A certain subprotocol might be defined for Websocket usage as well as for data channel usage. MSRP 
is already such a case, where draft-ietf-mmusic-msrp-usage-data-channel defines how to use MSRP over 
data channels, and where draft-pd-dispatch-msrp-websocket defines how to use MSRP over Websockets.
Would the IANA WebSocket Subprotocol Name Registry on then contain two MSRP related rows? Or just 
one row containing references to both the Websocket and data channel documents?
Should we add related text to the IANA registration section of the sdpneg draft?


On 12.02.2016 15:52, EXT Flemming Andreasen wrote:
> On 2/12/16 5:45 AM, Juergen Stoetzer-Bradler wrote:
>> Flemming, Paul,
>> The current a=dcmap related text in draft-ietf-mmusic-data-channel-sdpneg doesn't require that 
>> the 'subprotocol' parameter must always be present - rather it is specified as an optional 
>> parameter. Thus, current sdpneg text would allow to create an SDP offer for a data channel, which 
>> contains one a=dcmap attribute and potentially multiple a=dcsa attributes without the subprotocol 
>> actually being given. Based on this discussion I am wondering if the subprotocol parameter should 
>> actually be mandatory.
> It would probably simplify the overall SDP negotiation part, but I don't know if it would 
> constrain the way data channels were envisioned to be used.
>> In the specific case of MSRP, the msrp-usage-data-channel draft says in that the dcmap 
>> attribute includes the label and subprotocol parameters. The current text could possible be made 
>> more explicit by saying that the 'subprotocol="MSRP"' parameter must always be present.
>> Have just submitted version 04 of the msrp-usage-data-channel draft, which proposes to add 
>> subprotocol identifier "MSRP" to the WebSocket Subprotocol Name registry. This registry would 
>> then associate subprotocol id "MSRP" with the msrp-usage-data-channel document.
>> There, in section the MSRP specific usages of the a=dcsa attribute are specified. And 
>> there the MSRP specific SDP attributes, which can be dcsa embedded, are described.
>> 'setup' is an attribute, whose semantic changes when being dcsa embedded and associated with 
>> subprotocol MSRP, as compared to the meaning of an "a=setup" media level attribute of a TCP/MSRP 
>> m-line. Hence these semantical differences are explicitly addressed in the 
>> msrp-usage-data-channel draft.
>> Regarding sdpneg, I also think that the current text in sdpneg seems to be sufficient regarding 
>> the usage of dcsa encapsulated SDP attributes as being bound to the data channel's subprotocol. 
>> But as the semantic of a dcsa encapsulated attribute may be subprotocol specific (like 'setup'), 
>> I'd now tend to consider the subprotocol parameter in the dcmap attribute as being mandatory, as 
>> mentioned above. As already discussed, the Websocket subprotocol registry would then refer to the 
>> document, which specifies the subprotocol specific usage of dcsa encapsulated parameters.
> Ok. Going back to discussion between Paul and I, do you believe that in for an attribute to be 
> encapsulated in dcsa, the attribute MUST have been explicitly define to support this (Paul's 
> suggestion below) or do you believe that this is overly constraining, and if so, how shoud we 
> relax it ?
> Thanks
> -- Flemming
>> Thanks,
>> Juergen
>> On 11.02.2016 21:52, Flemming Andreasen wrote:
>>> On 2/8/16 12:09 PM, Paul Kyzivat wrote:
>>>> On 2/8/16 11:09 AM, Flemming Andreasen wrote:
>>>>> On 2/6/16 1:44 PM, Paul Kyzivat wrote:
>>>>>> On 2/4/16 10:43 PM, Christian Groves wrote:
>>>>>>> Isn't this the approach we're taking today?
>>>>>>> draft-ietf-mmusic-data-channel-sdpneg has general text and specific
>>>>>>> drafts are used to describe protocols that use the mechanism (i.e.
>>>>>>> draft-ietf-mmusic-msrp-usage-data-channel &
>>>>>>> draft-ietf-clue-datachannel).
>>>>>> It remains to be seen if that will be enough. E.g., there currently
>>>>>> aren't any iana considerations in
>>>>>> draft-ietf-mmusic-msrp-usage-data-channel.
>>>>>> Suppose I encounter some sdp that uses msrp over a data channel, but
>>>>>> that usage is unknown to me. How do I find the spec (the reference to
>>>>>> draft-ietf-mmusic-msrp-usage-data-channel) that defines that usage?
>>>>>> I would like to think that the iana registries will allow me to trace
>>>>>> back to the relevant specs.
>>>>> No disagreement on that part, however having taken another look at both
>>>>> sdpneg and the msrp-usage documents, I still don't agree with your
>>>>> original request for all (existing and new) attributes to specify how
>>>>> they may or may not be used with the dcsa attribute defined by sdpneg.
>>>>> As Christian noted, the sub-protocol specifics are defined in individual
>>>>> documents (like msrp-usage), which calls your the parameters that are at
>>>>> least needed to be supported for that usage. Taking MSRP as an example,
>>>>> why isn't that enough, and how do you see the resulting set of
>>>>> attributes that may or may not be used with MSRP differ between use in a
>>>>> data-channel (and hence encapsulated in dcsa) or as a regular media
>>>>> stream ?
>>>> Based on this discussion, I conclude that it should be sufficient for this draft to say that 
>>>> before an attribute can be used with dcsa, such usage must be defined somewhere. This could be 
>>>> either:
>>>> - as part of the definition of the attribute, OR
>>>> - as part of the definition of the protocol referenced on the m-line.
>>> We are getting closer, but it's still not obvious to me that you cannot use an attribute with 
>>> dcsa if it has not been explicitly defined for the attribute in question. Clearly, there are 
>>> attributes that wouldn't make sense over data channels, just like there are attributes that 
>>> don't make sense over particular media descriptions.
>>> Again, I'd like to hear from more people on this, including the authors.
>>> Thanks
>>> -- Flemming
>>>>     Thanks,
>>>>     Paul
>>>>> Also, it would be good to hear from more people on this, including the
>>>>> document authors.
>>>>> Thanks
>>>>> -- Flemming
>>>>>>     Thanks,
>>>>>>     Paul
>>>>>>> Regards, Christian
>>>>>>> On 4/02/2016 3:58 PM, Paul Kyzivat wrote:
>>>>>>>> On 2/3/16 5:42 PM, Flemming Andreasen wrote:
>>>>>>>>> I'm not concerned about the IANA part. I agree that *if* we need to
>>>>>>>>> expliclty specify attribute interactions for "dcsa", then it should be
>>>>>>>>> part of the IANA registry. What I am not agreeing with at this
>>>>>>>>> point is
>>>>>>>>> that there is indeed a need to explicitly speficy these
>>>>>>>>> interactions as
>>>>>>>>> opposed to relying on a more general algorithmic approach (plus the
>>>>>>>>> offerer being responsible for generating a valid offer if he wants to
>>>>>>>>> establish a data channel).
>>>>>>>> Well, an obvious one is that the protocol(s) the attribute pertains to
>>>>>>>> need to be defined to work over data channels.
>>>>>>>>     Thanks,
>>>>>>>>     Paul