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

Paul Kyzivat <> Wed, 09 December 2015 15:43 UTC

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From: Paul Kyzivat <>
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Date: Wed, 09 Dec 2015 10:43:15 -0500
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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Juergen and others,

What you have below is a good start.

I still think we need to at least specify how dcsa impacts IANA 
registration of attributes.

The attribute registration is (going to be) updated so that there is one 
table containing all attributes, with fields indicating for each 
attribute whether it can be used at session level, media level, or 
source level.

Now consider an attribute usable at media and/or source level:

It *might* *also* be usable with dcsa, or it might not. That will at 
least in part be determined by whether the protocol that it applies to 
can run over a data channel.

There was a suggestion (Keith?) that media level attributes be 
implicitly allowed with dcsa if the protocol they apply to is defined 
over a data channel. I find that plausible. But then the instructions 
for registration ought to say that. Also, if there can be exceptions 
(attributes that *can't* be used with dcsa even when the protocol is 
defined over data channel) then there should be a way to register that.

Source level attributes add another complication. They only apply over 
RTP. For now, RTP isn't defined over a data channel. (Though in 
principle it *could* be.) If it were, then I suppose the ussage would 
have to be 'a=dcsa nn source ...'. I don't think we need to deal with 
that right now.

And then there is the possibility of new attributes being defined that 
can be used *only* with dcsa, not directly at media level:

If this is because the protocol is only defined to run over a data 
channel, then it could possibly be registered as a media level attribute 
(which is then allowed by default with dcsa). If the protocol were ever 
defined over another transport the sdp would be ready for it.

But if, for some reason, the protocol can run over a data channel and 
also over some other transport, but for some reason a particular 
attribute only applies with data channels, or only applies without data 
channels, then there ought to be a way to record that in the registry.

The worst case is for every spec that defines an attribute to pick its 
own way to specify all this, and for there to be no hint in the iana 

IMO it is still better for the iana registry to be given another field 
to indicate applicability with data channels.


On 12/9/15 8:24 AM, Juergen Stoetzer-Bradler wrote:
> Paul, Christian, Keith,
> Would propose to add following two new paragraphs to the end of current
> section (dcsa Attribute) in order to more clearly describe the
> relationship of a subprotocol's attribute and the subprotocol's
> transport protocol, especially data channel.
>     It is assumed that in general the usages of subprotocol related media
>     level attributes are independent from the subprotocol's transport
>     protocol.  Such transport protocol independent subprotocol related
>     attributes are used in the same way as defined in the original
>     subprotocol specification, also if the subprotocol is transported
>     over a data channel and if the attribute is correspondingly embedded
>     in a "a=dcsa" attribute.
>     There may be cases, where the usage of a subprotocol related media
>     level attribute depends on the subprotocol's transport protocol.  In
>     such cases the subprotocol related usage of the attribute is expected
>     to be described for the data channel transport, e.g. in an extension
>     of the original subprotocol specification.
> Section 5.1.2 already explicitly mentions that (subprotocol related)
> media level attributes may follow a data channel declaration (the
> a=dcmap line). Thus the restriction to subprotocol related media level
> attributes only  seems to be covered already.
> Thanks,
> Juergen
> On 01.12.2015 02:12, DRAGE, Keith (Keith) wrote:
>> I think we need to be a little bit careful here.
>> In a large number of cases, the use of the attribute will be unaltered
>> by the transport that is being used, and I do not think we should have
>> a need of saying anything if this is the case - it should be the default.
>> So for example for MSRP, there are a number of attributes associated
>> with MSRP - for file transfer and so on, and as far as I can tell,
>> they are used when MSRP is sent over a data channel in exactly the
>> same way as when MSRP is sent over TCP or SCTP. We don't write special
>> rules for TCP, so why write special rules for datachannel.
>> We do need to identify how we document any special cases, and yes when
>> we find them, drafts like the msrp draft are the way to do that.
>> Additionally, if we find when defining a new attribute, that it is
>> inappropriate for datachannel usage in some way, then we can write
>> that in the defining specification for the new attribute (although I
>> suspect in this case it will be inappropriate to other transports than
>> just datachannel).
>> I do assume this entire discussion is limited to attributes that can
>> be applied to media lines. We might find it useful to say that in the
>> general draft, if we have not done so already.
>> Regards
>> Keith
>> -----Original Message-----
>> From: mmusic [] On Behalf Of Christian
>> Groves
>> Sent: 01 December 2015 00:53
>> To:
>> Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
>> Hello Paul,
>> I actually meant to post the reply to the list, sorry for the confusion.
>> I do understand your concerns. For multiplexing it makes sense to
>> check all the existing attributes to see whether they're applicable
>> due to the fact that quite alot may be used when multiplexing occurs.
>> I think the data channel usage is quite a bit more narrow in scope.
>> Whilst this draft does introduce a general purpose mechanism the
>> actual use of the dcsa is being defined by other drafts. E.g.
>> draft-ietf-mmusic-msrp-usage-data-channel on MSRP discusses what
>> happens when both a=setup and a=dcsa:setup are present. msrp-cema is
>> another one discussed, etc.
>> I think you need to understand the application protocol being carried
>> in a data channel before you can say whether particular attributes
>> should be carried in dcsa and what that actually means. Going through
>> the IANA registry wouldn't be trivial and unless the attributes are
>> put in context of an application protocol I don't think it will be
>> terribly helpful.
>> draft-ietf-mmusic-data-channel-sdpneg in cl. does indicate that
>> the dsca parameters follow the procedures defined in the subprotocol
>> specific documents. Maybe we could strengthen this to indicate that
>> the use of dsca by an application protocol should be documented in a
>> specification?
>> Regards, Christian
>> On 1/12/2015 11:30 AM, Paul Kyzivat wrote:
>>> Christian,
>>> On 11/30/15 6:45 PM, Christian Groves wrote:
>>>> Is this level of detail really needed? There are several drafts for
>>>> CLUE, MSRP and BFCP related to the use of this sdpneg mechanism. E.g.
>>>> CLUE doesn't use dcsa and MSRP provides guidelines on the use of dcsa.
>>>> Can't we just follow the principle that the use of the dcsa attribute
>>>> should be detailed in a specification and leave it at that? I'm not
>>>> sure what we gain by analysing every existing attribute and forcing
>>>> every new attribute to do this analyses when the set of possible
>>>> parameters in likely to be very small.
>>> I guess it is arguable both ways. Originally there was only session
>>> level and media level. That was always supposed to be identified, but
>>> it wasn't always followed.
>>> Then we got source level. That has complicated the registry and the
>>> usage. I am not convinced that two SDP experts would come up with the
>>> same list of which attributes can be used at source level.
>>> The dcsa attribute becomes the 4th "level" for attributes. It at least
>>> makes sense to me that it ought to be managed analogously to the
>>> source attribute. But I agree it doesn't provide the same degree of
>>> confusion as the others. It only makes sense if the usage the
>>> attribute applies to works over a data channel. And that should be
>>> known to whatever is processing the SDP.
>>> I see this was a private message. Why don't you repost it on the list
>>> and lets see what other people think? (Feel free to repost this reply
>>> if you wish.)
>>>      Thanks,
>>>      Paul
>>>> Regards, Christian
>>>> On 1/12/2015 2:36 AM, Paul Kyzivat wrote:
>>>>> One thing occurred to me that we may have forgotten about:
>>>>> This draft introduces the dcsa attribute, which allows other
>>>>> attributes to be used in this new scope. It notes that not all
>>>>> attributes are suitable for use this way.
>>>>> ISTM that in the future other attribute definitions should include
>>>>> an indication of whether they can be used with dcsa, and the IANA
>>>>> registration of the attribute ought to include this. And then there
>>>>> is a need to update the iana registry for all the existing attributes.
>>>>>      Thanks,
>>>>>      Paul
>>>>> On 11/25/15 4:52 AM, Bo Burman wrote:
>>>>>> This is to announce a 4 week Working Group Last Call for
>>>>>>                   draft-ietf-mmusic-data-channel-sdpneg-06
>>>>>> as proposed standard.
>>>>>> Please review and provide any comments you may have on the document
>>>>>> by Wednesday, December 23, 2015. Comments should be sent to the
>>>>>> document authors and the MMUSIC WG list. If you review the document
>>>>>> but do not have any comments, please send a note to that effect as
>>>>>> well.
>>>>>> Please also forward this WGLC call to any other interested parties
>>>>>> who may be able to review the draft, asking them to also direct
>>>>>> their comments to the authors and the list as above.
>>>>>> Thank you!
>>>>>>           Bo Burman (MMUSIC co-chair)
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