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

Paul Kyzivat <> Mon, 29 February 2016 15:16 UTC

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From: Paul Kyzivat <>
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Date: Mon, 29 Feb 2016 10:16:13 -0500
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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Since I am the only one who thinks there is a problem I'm going to just 
stop talking about it. We can deal with the problem if/when it arises.

(I don't like leaving loose ends, but it isn't important enough to 
pursue here.)


On 2/29/16 3:45 AM, Schwarz, Albrecht (Nokia - DE) wrote:
>> Perhaps a document could be created that defined RTP/AVP, RTP/AVPF,
>> RTP/SAVP, RTP/SAVPF as sub-protocols and registered them in the
>> websocket/datachannel registry. And then that document might say that
>> any sdp attribute designed to be used with protos */(each of these
>> subprotocols) may also be used in dcsa for a channel using these
>> protocols.
> [CNG] Yes it "could" be, if someone actually wanted to carry RTP over a
> data channel. That would follow with what is being done for MSRP and
> BFCP. I don't see RTP as being any different.
> There isn't any use case at all for RTP-over-DC, neither for RTP in general nor specific RTP profiles "RTP/<Profile(s)>".
> Thus, I concur to Christian.
> Regards,
> Albrecht
> -----Original Message-----
> From: mmusic [] On Behalf Of EXT Christian Groves
> Sent: Montag, 29. Februar 2016 03:49
> To:
> Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
> Hello Paul and Juergen,
> On 27/02/2016 4:46 AM, Paul Kyzivat wrote:
>> Hi,
>> On 2/26/16 9:17 AM, Juergen Stoetzer-Bradler wrote:
>>> Hello Paul, Christian,
>>> Would propose to add following definition of term "subprotocol" to
>>> the terminology section 3 of the sdpneg draft:
>>>         Data channel subprotocol: The application protocol which is
>>>         transported over a single data channel.  Data channel subprotocol
>>>         messages are sent as data channel payload over an established
>>> data
>>>         channel.  If an SDP offer/answer exchange is used as specified in
>>>         this document to negotiate the establishment of data channels,
>>>         corresponding data channel properties, associated data channel
>>>         subprotocols and data channel subprotocol properties, then the
>>> data
>>>         channel subprotocols may be identified by the values of the
>>>         "subprotocol" parameters of the SDP "a=dcmap" attributes as
>>>         described in Section  Within this document the term
>>> "data
>>>         channel subprotocol" is often abbreviated as just "subprotocol".
>>> This text could explicitly narrow down the notion of "subprotocol"
>>> within the data channel SDP offer/answer context and might especially
>>> be helpful distinguishing it from the usages of "subprotocol" in the
>>> Websocket RFC 6455 (where the term "subprotocol" was taken from, but
>>> where it does not seem to be formally defined). This text may
>>> certainly not be helpful in more general non-data channel contexts.
>>> But it might help to clarify that every occurrence of of the term
>>> "subprotocol" in the sdpneg draft refers to the application protocol
>>> which (typically but not necessarily) is identified via the a=dcmap's "subprotocol"
>>> parameter.
>>> Would such an explicit definition be helpful from your point of view?
>> It might *help*, but it doesn't get at the main problem I see.
>> The question is how does this sub-protocol relate the the proto field
>> used in m-lines? But not to the proto of the m-line for data channel.
>> My point is that many sdp attributes were designed to be used with
>> particular proto fields. For instance, with RTP/AVP, RTP/AVPF,
> [CNG] Does sub-protocol have to relate to the proto field? The proto field in the case of WebRTC-datachannel is UDP/DTLS/SCTP. What ever is carried in the individual data channels is the sub-protocol.
> Draft-ietf-mmusic-data-channel-sdpneg is only considering this.
>> In general they are described as being applicable to RTP. But what is
>> RTP? Is it a subprotocol (of UDP, TCP, DCCP)? Or is it a
>> super-protocol (of AVP, AVPF, SAVP, SAVPF)?
> [CNG] I'm not sure it matters? If someone wants to run a particular RTP
> profile over a data channel then it is a sub-protocol of UDP/DTLS/SCTP.
> All the other above examples have already been defined so I don't see we
> gain anything by trying to label them as a sub-protocol etc.
>> If we wanted to define use of RTP over a data channel, what
>> sub-protocol(s) would we have to define? I *think* we would have to
>> define as many of RTP/AVP, RTP/AVPF, RTP/SAVP, RTP/SAVPF as we wanted
>> to support over a data channel.
> [CNG] Yes I would agree.
>> And then how would we do it, and where would we specify which
>> attributes could be used with dcsa? Would we have to update the
>> documents that define those attributes?
> [CNG] I assume we'd follow the example of
> draft-ietf-mmusic-msrp-usage-data-channel. It discusses the use of
> existing attributes. I don't see that RTP would need to do anything
> different.
>> ISTM there are similar (though not so complex) issues for pretty much
>> any attribute that we might want to reuse over a data channel.
> [CNG] Some will be complex, some will be a no brainer. What
> draft-ietf-mmusic-data-channel-sdpneg provides is a syntax for re-using
> attributes. It doesn't provide semantics for every attribute.
> Implementors need to understand that some attributes have been defined
> with a particular protocol stack in mind and that they the behaviour may
> need to be modified when using a data channel.
>> Perhaps a document could be created that defined RTP/AVP, RTP/AVPF,
>> RTP/SAVP, RTP/SAVPF as sub-protocols and registered them in the
>> websocket/datachannel registry. And then that document might say that
>> any sdp attribute designed to be used with protos */(each of these
>> subprotocols) may also be used in dcsa for a channel using these
>> protocols.
> [CNG] Yes it "could" be, if someone actually wanted to carry RTP over a
> data channel. That would follow with what is being done for MSRP and
> BFCP. I don't see RTP as being any different.
>>      Thanks,
>>      Paul
>>> Thanks,
>>> Juergen
>>> On 25.02.2016 22:38, EXT Paul Kyzivat wrote:
>>>> On 2/24/16 7:24 PM, Christian Groves wrote:
>>>>> Hello Juergen and Paul,
>>>>> Please see at end.
>>>>> Regards, Christian
>>>>> On 24/02/2016 8:26 AM, Paul Kyzivat wrote:
>>>>>> ..snip..
>>>>>>> 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.
>>>>> [CNG] I don't see what the example buys? I don't see that the
>>>>> behaviour
>>>>> is any different between using additional attributes in the
>>>>> datachannel
>>>>> vs. the non data channel case. E.g. for
>>>>>       c=IN IP4
>>>>>       m=message 7394 TCP/MSRP *
>>>>>       a=accept-types:message/cpim text/plain text/html
>>>>>       a=lang:....
>>>>>       a=framerate:...
>>>>> The ignore behaviour would be the same.
>>>>> In the above example the attributes are scoped by the m= line. In the
>>>>> data channel case the attributes are scoped by the relevant a=dcmap:
>>>>> line.
>>>> My concern is that SDP has no notion of subprotocol, even though in
>>>> practice it shows up lots of places. It only has a notion of the
>>>> protocol field in the m-line. Beyond that a *convention* has developed
>>>> to denote a layering within the protocol through use of "/". AFAIK
>>>> this isn't formally written down anywhere.
>>>> So, in principle we could define an RTP sub-protocol for use over a
>>>> data channel. And then we could talk about using the attributes that
>>>> apply to RTP in dcsa for a channel using RTP. But note there is no
>>>> formal definition of the *protocol*s where RTP attributes are relevant.
>>>> A lot of the very old stuff was just sloppy. To be fair, it was
>>>> probably good enough for the cases in front of them at the time, and
>>>> they weren't yet in a position to foresee how things would evolve. It
>>>> is just another example of how old stuff rots and has to be refreshed
>>>> from time to time.
>>>> But I don't think *this* draft is the place to fix it. So, in lieu of
>>>> doing that I'm just looking for some way to clarify things.
>>>>      Thanks,
>>>>      Paul
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