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

[Juergen Stoetzer-Bradler <[e-mail]>]

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.

In the specific case of MSRP, the msrp-usage-data-channel draft says in 5.1.1.1 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 5.1.1.2 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.

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
>>>>>>
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