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

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

Paul,

Thanks for further feedback. Fully agree now that the subprotocol parameter should stay optional.
Will come back with with a proposal of how the sdpneg text could be enhanced as discussed.

Thanks,
Juergen

On 15.02.2016 17:08, EXT Paul Kyzivat wrote:
> On 2/14/16 5:48 AM, Juergen Stoetzer-Bradler wrote:
>> Paul,
>>
>> In your case 2) I would agree that the usage of dcsa embedded attributes
>> seems questionable. After all, the dcsa attribute is defined in sdpneg
>> as encapsulating a subprotocol specific attribute. If the subprotocol
>> may dynamically change over time without further (SDP offer/answer)
>> negotiation, then a negotiation of dcsa embedded attributes may indeed
>> not be very helpful.
>>
>> In your case 1), I am wondering how would the SDP answerer know which
>> subprotocol to use for which data channel, if the subprotocol id is not
>> added to the data channel's dmap attribute (at least if multiple data
>> channels are negotiated)? Conceivably via some provisioning, or via a
>> mutual understanding of a certain subprotocol to SCTP stream id mapping,
>> or via a certain a priori agreed label usage. I hadn't thought of such
>> cases when raising the question if the subprotocol should actually be
>> mandatory.
>
> IMO it could be any of those. But for this use case it seems more likely that channels will be 
> opened dynamically rather than via SDP.
>
> I think the provision for *not* specifying the subprotocol was primarily to establish equivalence 
> with what is possible via DCEP.
>
> One possibility is that SDP is used to specify how many channels are opened, with the 
> understanding that each of them will use the same application-specific protocol.
>
> One possible use for dcsa with an unnamed protocol would be if the intent is to use a proprietary 
> variant of a standard protocol, where there is still a desire to negotiate options using 
> attributes applicable to that standard base protocol. But I'm really stretching to come up with this.
>
>> But such use cases would certainly go beyond a pure SDP offer/answer
>> negotiation use case and I would argue that such use cases would also go
>> beyond the scope of the sdpneg draft.
>
> Probably.
>
>> I could now imagine the sdpneg draft saying that an SDP offerer should
>> add a subprotocol identifier to an offered data channel's dcmap
>> attribute, if it also adds dcsa embedded subprotocol attributes. And
>> that an implementation would be on its own, if it does not follow that
>> recommendation (for whatever reasons).
>
> That could work.
>
>> Further, similar as for non-dcsa embedded attributes, we could
>> explicitly add text to the sdpneg draft saying that a recipient of an an
>> SDP offer or answer should ignore dcsa embedded attributes not only if
>> they are completely unknown, but also if their semantics related to the
>> subprotocol is not known.
>> (The current draft says in sec 5.2.3 that the SDP answerer "/Parses and
>> applies the SDP offer.  Note that the typical parser normally ignores
>> unknown SDP attributes, which includes data channel related
>> attributes./" I think we could make this clearer and explicitly refer to
>> dcsa embedded subprotocol attributes.)
>
> Yes, that would help.
>
>     Thanks,
>     Paul
>
>> Thanks,
>> Juergen
>>
>> On 12.02.2016 17:58, EXT Paul Kyzivat wrote:
>>> Juergen,
>>>
>>> You bring up a point that perhaps needs further discussion: what does
>>> it mean when the subprotocol is not specified?
>>>
>>> IIUC there are at least a couple of reasons this might come about:
>>>
>>> 1) The protocol to be used is not standardized. There is no globally
>>> unique name for it. The two ends know what protocol to use based on
>>> context.
>>>
>>> 2) The protocol to be used on the channel is not determined via SDP
>>> negotiation. The applications want to establish the channel, and then
>>> dynamically decide what protocol to use with it. (And that protocol
>>> may change over time.)
>>>
>>> I guess that it still might be meaningful to use dcsa with (1), but
>>> there would be no way to determine by examination of the SDP whether
>>> the usage was compatible with the protocol. In this case the usage of
>>> the attributes would be "off label" - being adapted to the proprietary
>>> protocol in a way that hopefully the two ends agree.
>>>
>>> I don't see how dcsa makes much sense for (2). If you don't know what
>>> protocol will be used then how do you know what attributes to use?
>>>
>>>     Thanks,
>>>     Paul
>>>
>>> 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.
>>>>
>>>> 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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