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

Christian Groves <> Thu, 25 February 2016 00:24 UTC

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From: Christian Groves <>
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Date: Thu, 25 Feb 2016 11:24:33 +1100
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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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
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.

Regards, Christian

>     Thanks,
>     Paul
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