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

Paul Kyzivat <> Tue, 23 February 2016 21:26 UTC

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From: Paul Kyzivat <>
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Date: Tue, 23 Feb 2016 16:26:07 -0500
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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Hi Juergen,

See response at end.

On 2/23/16 8:29 AM, Juergen Stoetzer-Bradler wrote:
> Hi Paul,
> Thanks for your feedback to the updates in version 08.
> Please find my remarks inserted below your comments.
> Thanks,
> Juergen
> On 19.02.2016 17:42, EXT Paul Kyzivat wrote:
>> On 2/17/16 8:12 AM, Juergen Stoetzer-Bradler wrote:
>>> Hi Paul, Christian, Flemming, Bo,
>>> Have just submitted version 08 of draft-ietf-mmusic-data-channel-sdpneg.
>>> The changes compared to version 07 are essentially as follows.
>>> *   Two new paragraphs in section (dcsa Attribute) regarding the
>>> relationship of subprotocols and their attributes.
>>> *   Two new SDP offer/answer considerations in section 5.2.5 (Various
>>> SDP Offer/Answer Scenarios and Considerations)  regarding unknown
>>> subprotocol attributes or known subprotocol attributes, whose data
>>> channel transport specific semantic is not known.
>>> *   A new paragraph in section 8.1 (IANA Considerations / Subprotocol
>>> Identifiers) related to cases, where a subprotocol is defined for data
>>> channel and Websocket transport.
>>> These changes should address the points discussed in this email thread.
>> This is an improvement. But I think things could still be made clearer.
>> Consider the following addition to
>>    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.  A data channel
>>    specific usage of a subprotocol attribute is expected to be specified
>>    in the same document, which registers the subprotocol's identifier
>>    for data channel usage as described in Section 8.1.
>> This text makes sense when there is a clear distinction between
>> subprotocol and protocol. Unfortunately, the way SDP has evolved there
>> is no such clear distinction in many cases, such as RTP over UDP or
>> TCP, etc. Those are combined into a single protocol value. While that
>> can usually be parsed apart at slashes, there isn't good terminology
>> for it.
>> My point is that when I read the above, I don't know how it applies
>> to, say, RTP attributes. Or does it only apply for attributes that are
>> clearly defined for a *sub*protocol?
>> I think this is primarily that we lack well defined vocabulary for all
>> of this. But I think it would be too much to expect this draft to
>> *solve* the vocabulary problem. In lieu of doing so, maybe it would be
>> sufficient to give some concrete examples, even if they have to be
>> hypothetical ones.
> [Juergen] Agree that it would be helpful to have more precise
> definitions of the differences of the terms protocol and subprotocol,
> especially when those terms are used outside the scope of data channels
> (or Websockets). When only focusing on data channels the notion of a
> "subprotocol" seems to be clearer - at least
> draft-ietf-rtcweb-data-protocol explicitly refers to the "WebSocket
> Subprotocol Name Registry" when specifying DCEP's "Protocol" parameter.
> (But draft-ietf-rtcweb-data-channel does not define what a data
> channel's "subprotocol" is.) So far the sdpneg draft relatively
> informally starts using the term "subprotocol" in the introduction and
> there refers to Websocket "subprotocols". Perhaps we should add the term
> "subprotocol" to the list of used terminology in section 3.
> The sdpneg document, together with the data channel subprotocol specific
> document (which defines the value of the a=dcmap attribute's
> "subprotocol" parameter), should certainly give clear guidance on how to
> interpret SDP offers or answers like e.g.:
>       m=application 10001 UDP/DTLS/SCTP webrtc-datachannel
>       c=IN IP4
>       a=max-message-size:100000
>       a=sctp-port:5000
>       ...
>       a=dcmap:0 subprotocol="MSRP"
>       a=dcsa:0 accept-types:message/cpim text/plain
>       a=dcsa:0 framerate:...
>       a=dcsa:0 lang:...
> An implementation receiving such an offer would need to decide what to
> do with the dcsa embedded framerate and lang attributes. Or, someone
> implementing MSRP over data channel based services may need to decide
> whether or not to use these attributes, and if yes, how.
> (I am using these two attributes just as hypothetical examples - don't
> want to suggest that those may indeed be used for MSRP over data channel
> transport).
> The msrp-usage-data-channel document doesn't mention these two
> attributes. When looking at the IANA SDP attribute registry tables, I
> would find both attributes specified in RFC 4566. There, "framerate" is
> explicitly said to be defined only "for video media". Just to be sure I
> could additionally have a look at the MSRP specifying documents, RFC
> 4975 and RFC 4976, but there would not find any text at all related to
> "framerate". So this case seems pretty clear and I would therefore
> conclude that the "framerate" attribute should not be used for MSRP, and
> that a receiver of such an offer or answer should ignore it.
> When looking at the definition of the "lang" attribute in RFC 4566 I
> would not see any explicit hint of what protocols this attribute might
> be used with, especially if "lang" could be used when negotiating an
> MSRP session. When then looking at RFC 4975 I would indeed find "lang" -
> but not as SDP attribute, rather as XML tag parameter within an example
> MSRP message payload. Thus, the case of the "lang" attribute might not
> be as unambiguous as the one with the "framerate" attribute, but here
> too I think the typical choice would be to ignore that attribute when
> receiving such an offer or answer.
> It seems to me that the two new "ignore" rules in section 5.2.5 of
> sdpneg-08 may also be applied in these cases.
> Admittedly, these examples may seem a bit far-fetched, but would those
> go into the direction you had in mind?

Yes. Note that using examples is just me grasping at straws, since a 
real solution looks like to big a problem for this draft to tackle by 
itself. I am entirely open to other ideas for how to deal with this.