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

Christian Groves <> Mon, 29 February 2016 02:48 UTC

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From: Christian Groves <>
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Date: Mon, 29 Feb 2016 13:48:46 +1100
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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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 

> 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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