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

Juergen Stoetzer-Bradler <juergen.stoetzer-bradler@nokia.com> Tue, 23 February 2016 13:29 UTC

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From: Juergen Stoetzer-Bradler <juergen.stoetzer-bradler@nokia.com>
To: Flemming Andreasen <fandreas@cisco.com>, Paul Kyzivat <pkyzivat@alum.mit.edu>, DRAGE Keith <keith.drage@nokia.com>, "Makaraju, Maridi Raju (Raju)" <Raju.Makaraju@nokia.com>, "mmusic@ietf.org" <mmusic@ietf.org>
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Date: Tue, 23 Feb 2016 14:28:59 +0100
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Subject: Re: [MMUSIC] WGLC for draft-ietf-mmusic-data-channel-sdpneg
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Flemming,

Thanks for further feedback. Regarding your comments:
> So far so good, however the case I had in mind is one where the new attribute would apply to one 
> or more existing subprotocols.
and
> While that is certainly a valid use case, I wasn't thinking of restricting it to that and hence 
> the request for an actual sub-registry for each of these subprotocols to register attributes that 
> can be used with each of them (some attributes may apply to multiple). 

Ok, as per my understanding such a new attribute would be introduced in a (let's assume) future 
document. I think that such a future document should describe the new attribute's semantic with 
respect to all protocols for which it is intended to be used. And if one of these protocols can be 
transported over data channels, and if the new attribute's meaning then also has data channel 
specific aspects, this future document should also describe those aspects. If this new attribute is 
then added to the IANA SDP attribute registry (presumably as media level only or media and session 
level attribute), then this document could be found when searching for the new attribute.

I am not yet sure I fully understand how such subprotocol specific registries could be used. Right 
now there are already drafts describing MSRP, BFCP and T.140 as data channel subprotocols. Do you 
think IANA registry tables for each of these subprotocols should be created (and in future for any 
further protocol, which might be used as data channel subprotocol)?
Where then a new attribute might be listed in multiple of these subprotocol specific registries, 
depending on which subprotocol it might be used for?
And all already existing attributes, which might also be used for a data channel subprotocol - do 
you think that these should also be added to such subprotocol specific registry tables?

Coming back to MSRP as an example - RFC 5547 defines several file transfer related attributes. 
Conceivably, a future document could define another file transfer protocol unequal to MSRP (and 
based on data channel transport, or based on a transport protocol unequal to data channel). And that 
potential future document might then describe how (some of) the file transfer related SDP attributes 
might be re-used for this new file transfer protocol. Would you then expect these file transfer 
related SDP attributes to be added to such a new registry associated with the new file transfer 
protocol? Also if data channel transport of this potential new file transfer protocol were not 
defined in that assumed future document?

Right now I'm not sure if this topic is only data channel specific. E.g. the a=setup attribute was 
originally introduced in RFC 4145 for TCP connection negotiation. Later on, this attribute was 
reused for TLS connection negotiation (RFC 4572) and DTLS association negotiation (RFC 5763, 
draft-ietf-mmusic-sctp-sdp, draft-ietf-mmusic-dtls-sdp). As far as I see the IANA SDP parameters 
registry for the a=setup attribute only refers to RFC 4145. And the two sctp-sdp and dtls-sdp drafts 
don't seem to request updating this registry. However, all these documents have setup attribute 
related texts describing the extended semantics of the setup attribute.

Somebody seeing an SDP offer like the UDP/DTLS/SCTP related one in section 13.1 of 
draft-ietf-mmusic-sctp-sdp-15, when looking for the setup attribute's usage in this SDP offer and 
when therefore searching the related reference in the IANA SDP attribute registry, would only find 
the reference to RFC 4145. However, he/she could look up the UDP/DTLS/SCTP m-line proto value in the 
IANA SDP proto registry, and there would find the reference to the future RFC of 
draft-ietf-mmusic-sctp-sdp, where the semantic of the setup attribute within this SDP offer instance 
is specified.

Somebody seeing an SDP offer like the DTLS-SRTP related one in section 7.1 of RFC 5763, again when 
trying to find the semantic of the setup attribute in this SDP offer, would similarly find the 
reference to RFC 4145. When searching the proto registry for UDP/TLS/RTP/SAVP he/she would find the 
reference to RFC 5764. RFC 5764 does not mention the setup attribute at all, but RFC 5763 does and 
describes its usage in the DTLS-SRTP case. [But then it might not be clear if this setup attribute 
is used in the original sense of RFCs 5763 / 5764, or with the modified semantic described in 
draft-ietf-mmusic-dtls-sdp (which excludes value 'holdconn').]

In these cases, do you think that subprotocol (TLS and DTLS as "subprotocols" of TCP and UDP) 
specific sub-registries should be created? And if yes, how do you envision those would be used?

Thanks,
Juergen


On 19.02.2016 18:24, EXT Flemming Andreasen wrote:
>
>
> On 2/19/16 8:56 AM, Juergen Stoetzer-Bradler wrote:
>> Flemming,
>>
>> Regarding
>>> Sounds reasonable with the caveat that you may have attributes defined for use with a particular 
>>> sub-protocol outside the subprotocol draft itself (e.g. because such an attribute was either 
>>> defined subsequently or its use with a specific subprotocol is defined subsequently). If so, we 
>>> would need an IANA registry structure (under SDP) that for each subprotocol defines the set of 
>>> attributes that have been defined with specific semantics for that subprotocol. 
>>
>> Let's assume a future document defines how to establish MSRP sessions over a transport protocol 
>> unequal to TCP, TLS, or data channel, and that this document also defines a new MSRP related 
>> attribute, whose semantic is only defined for this new transport case (like a maximal MSRP chunk 
>> size specific for the new transport). I assume that this future document would request to add 
>> this new attribute to one of the existing IANA SDP attribute tables (assumingly to the "att-field 
>> (media level only)" table).
> So far so good, however the case I had in mind is one where the new attribute would apply to one 
> or more existing subprotocols .
>> Let's further assume that an implementation creates an SDP offer for an MSRP session over a data 
>> channel and adds this new attribute as dcsa embedded MSRP subprotocol attribute (for whatever 
>> reasons).
>>
>> Somebody seeing and inspecting this SDP offer could use the dcmap attribute's subprotocol value 
>> "MSRP" to get a reference to the msrp-usage-data-channel document. That document would not 
>> mention that new attribute at all. The next step could then be to consult the IANA SDP tables, 
>> and there this new attribute would be found together with a reference to this future document.
>> As assumingly this new MSRP related attribute would only be defined for this new transport,
> While that is certainly a valid use case, I wasn't thinking of restricting it to that and hence 
> the request for an actual sub-registry for each of these subprotocols to register attributes that 
> can be used with each of them (some attributes may apply to multiple).
>
>> I think this new attribute should then be ignored by the recipient of this SDP offer.
>> Section 5.2.5 of most recent draft-ietf-mmusic-data-channel-sdpneg-08 now contains two related 
>> recommendations:
>>
>> 'SDP offer or answer has an "a=dcsa" attribute, whose subprotocol attribute is unknown.
>>  *  The receiver of such an SDP offer or answer SHOULD ignore this entire "a=dcsa" attribute line.
>>
>> SDP offer or answer has an "a=dcsa" attribute, whose subprotocol attribute is known, but whose 
>> subprotocol attribute semantic is not known for the data channel transport case.
>>  *  The receiver of such an SDP offer or answer SHOULD ignore this entire "a=dcsa" attribute line.'
>>
>> Would these recommendations address the case you are describing?
>>
> They only address the case(s) partally per the above.
>
> Thanks
>
> -- Flemming
>
>
>> Thanks,
>> Juergen
>>
>>
>>
>> On 18.02.2016 20:03, EXT Flemming Andreasen wrote:
>>>
>>>
>>> On 2/14/16 5:48 AM, Juergen Stoetzer-Bradler wrote:
>>>> Flemming,
>>>>
>>>> Regarding
>>>>> It would probably simplify the overall SDP negotiation part, but I don't know if it would 
>>>>> constrain the way data channels were envisioned to be used. 
>>>>
>>>> On Friday Paul mentioned in his email two potential use cases, where no subprotocol identifiers 
>>>> might be added to a data channel's dcmap attribute. In my last email I hadn't thought of such 
>>>> cases. If we don't want to exclude such cases, then requiring the subprotocol ids always to be 
>>>> present might indeed be too restrictive.
>>>>
>>>> Regarding
>>>>> Ok. Going back to discussion between Paul and I, do you believe that in for an attribute to be 
>>>>> encapsulated in dcsa, the attribute MUST have been explicitly define to support this (Paul's 
>>>>> suggestion below) or do you believe that this is overly constraining, and if so, how shoud we 
>>>>> relax it ? 
>>>>
>>>> I now think that the sdpneg draft might recommend SDP offerers and answerers to always add a 
>>>> subprotocol identifier to a data channel's dcmap attribute if dcsa embedded attributes are also 
>>>> negotiated for the data channel's subprotocol. I also think we could explicitly add text to 
>>>> sdpneg saying that subprotocol identifiers should be added to the IANA Websocket (and in future 
>>>> combined data channel) registry. And that this registration should be done via a document, 
>>>> which then explicitly describes the usages and semantics of dcsa embedded subprotocol 
>>>> attributes, _if_ those usages or semantics deviate from cases, where these attributes are not 
>>>> dcsa encapsulated. I think that in those cases, where the usage and meaning of an attribute 
>>>> (always related to the data channel's subprotocol) does not deviate from the non-dcsa 
>>>> encapsulated use cases, such an attribute may not explicitly need to be described for data 
>>>> channel usage. But I think it might be helpful to explicitly say so in the sdpneg draft.
>>>>
>>>> Somebody inspecting an SDP offer or answer, which was generated by an implementation following 
>>>> these two recommendations (and which hence contains an IANA registered subprotocol id if it 
>>>> contains any dcsa embedded attributes), could then refer to that IANA table, would then find 
>>>> the reference to the document, which defines this subprotocol id for data channel usage, and 
>>>> there could check if any of the dcsa embedded attributes has a data channel specific semantic. 
>>>> If yes, that specific semantic would be described in that document. If such a specific semantic 
>>>> were not described in that document, then the default usage and semantic of the attribute would 
>>>> apply also to that data channel transport case.
>>>>
>>> Sounds reasonable with the caveat that you may have attributes defined for use with a particular 
>>> sub-protocol outside the subprotocol draft itself (e.g. because such an attribute was either 
>>> defined subsequently or its use with a specific subprotocol is defined subsequently). If so, we 
>>> would need an IANA registry structure (under SDP) that for each subprotocol defines the set of 
>>> attributes that have been defined with specific semantics for that subprotocol.
>>>
>>>
>>>> But in that context I'd have an IANA table related question.
>>>> A certain subprotocol might be defined for Websocket usage as well as for data channel usage. 
>>>> MSRP is already such a case, where draft-ietf-mmusic-msrp-usage-data-channel defines how to use 
>>>> MSRP over data channels, and where draft-pd-dispatch-msrp-websocket defines how to use MSRP 
>>>> over Websockets.
>>>> Would the IANA WebSocket Subprotocol Name Registry on 
>>>> http://www.iana.org/assignments/websocket/websocket.xml then contain two MSRP related rows? Or 
>>>> just one row containing references to both the Websocket and data channel documents?
>>>> Should we add related text to the IANA registration section of the sdpneg draft?
>>> See above.
>>>
>>> Thanks
>>>
>>> -- Flemming
>>>>
>>>> Thanks,
>>>> Juergen
>>>>
>>>>
>>>> On 12.02.2016 15:52, EXT Flemming Andreasen wrote:
>>>>>
>>>>>
>>>>> On 2/12/16 5:45 AM, Juergen Stoetzer-Bradler wrote:
>>>>>> Flemming, Paul,
>>>>>>
>>>>>> The current a=dcmap related text in draft-ietf-mmusic-data-channel-sdpneg doesn't require 
>>>>>> that the 'subprotocol' parameter must always be present - rather it is specified as an 
>>>>>> optional parameter. Thus, current sdpneg text would allow to create an SDP offer for a data 
>>>>>> channel, which contains one a=dcmap attribute and potentially multiple a=dcsa attributes 
>>>>>> without the subprotocol actually being given. Based on this discussion I am wondering if the 
>>>>>> subprotocol parameter should actually be mandatory.
>>>>>>
>>>>> It would probably simplify the overall SDP negotiation part, but I don't know if it would 
>>>>> constrain the way data channels were envisioned to be used.
>>>>>
>>>>>
>>>>>> In the specific case of MSRP, the msrp-usage-data-channel draft says in 5.1.1.1 that the 
>>>>>> dcmap attribute includes the label and subprotocol parameters. The current text could 
>>>>>> possible be made more explicit by saying that the 'subprotocol="MSRP"' parameter must always 
>>>>>> be present.
>>>>>> Have just submitted version 04 of the msrp-usage-data-channel draft, which proposes to add 
>>>>>> subprotocol identifier "MSRP" to the WebSocket Subprotocol Name registry. This registry would 
>>>>>> then associate subprotocol id "MSRP" with the msrp-usage-data-channel document.
>>>>>> There, in section 5.1.1.2 the MSRP specific usages of the a=dcsa attribute are specified. And 
>>>>>> there the MSRP specific SDP attributes, which can be dcsa embedded, are described.
>>>>>> 'setup' is an attribute, whose semantic changes when being dcsa embedded and associated with 
>>>>>> subprotocol MSRP, as compared to the meaning of an "a=setup" media level attribute of a 
>>>>>> TCP/MSRP m-line. Hence these semantical differences are explicitly addressed in the 
>>>>>> msrp-usage-data-channel draft.
>>>>>>
>>>>>> Regarding sdpneg, I also think that the current text in sdpneg seems to be sufficient 
>>>>>> regarding the usage of dcsa encapsulated SDP attributes as being bound to the data channel's 
>>>>>> subprotocol. But as the semantic of a dcsa encapsulated attribute may be subprotocol specific 
>>>>>> (like 'setup'), I'd now tend to consider the subprotocol parameter in the dcmap attribute as 
>>>>>> being mandatory, as mentioned above. As already discussed, the Websocket subprotocol registry 
>>>>>> would then refer to the document, which specifies the subprotocol specific usage of dcsa 
>>>>>> encapsulated parameters.
>>>>>>
>>>>> Ok. Going back to discussion between Paul and I, do you believe that in for an attribute to be 
>>>>> encapsulated in dcsa, the attribute MUST have been explicitly define to support this (Paul's 
>>>>> suggestion below) or do you believe that this is overly constraining, and if so, how shoud we 
>>>>> relax it ?
>>>>>
>>>>> Thanks
>>>>>
>>>>> -- Flemming
>>>>>
>>>>>
>>>>>> Thanks,
>>>>>> Juergen
>>>>>>
>>>>>>
>>>>>> On 11.02.2016 21:52, Flemming Andreasen wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 2/8/16 12:09 PM, Paul Kyzivat wrote:
>>>>>>>> On 2/8/16 11:09 AM, Flemming Andreasen wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 2/6/16 1:44 PM, Paul Kyzivat wrote:
>>>>>>>>>> On 2/4/16 10:43 PM, Christian Groves wrote:
>>>>>>>>>>> Isn't this the approach we're taking today?
>>>>>>>>>>> draft-ietf-mmusic-data-channel-sdpneg has general text and specific
>>>>>>>>>>> drafts are used to describe protocols that use the mechanism (i.e.
>>>>>>>>>>> draft-ietf-mmusic-msrp-usage-data-channel &
>>>>>>>>>>> draft-ietf-clue-datachannel).
>>>>>>>>>>
>>>>>>>>>> It remains to be seen if that will be enough. E.g., there currently
>>>>>>>>>> aren't any iana considerations in
>>>>>>>>>> draft-ietf-mmusic-msrp-usage-data-channel.
>>>>>>>>>>
>>>>>>>>>> Suppose I encounter some sdp that uses msrp over a data channel, but
>>>>>>>>>> that usage is unknown to me. How do I find the spec (the reference to
>>>>>>>>>> draft-ietf-mmusic-msrp-usage-data-channel) that defines that usage?
>>>>>>>>>>
>>>>>>>>>> I would like to think that the iana registries will allow me to trace
>>>>>>>>>> back to the relevant specs.
>>>>>>>>>>
>>>>>>>>> No disagreement on that part, however having taken another look at both
>>>>>>>>> sdpneg and the msrp-usage documents, I still don't agree with your
>>>>>>>>> original request for all (existing and new) attributes to specify how
>>>>>>>>> they may or may not be used with the dcsa attribute defined by sdpneg.
>>>>>>>>>
>>>>>>>>> As Christian noted, the sub-protocol specifics are defined in individual
>>>>>>>>> documents (like msrp-usage), which calls your the parameters that are at
>>>>>>>>> least needed to be supported for that usage. Taking MSRP as an example,
>>>>>>>>> why isn't that enough, and how do you see the resulting set of
>>>>>>>>> attributes that may or may not be used with MSRP differ between use in a
>>>>>>>>> data-channel (and hence encapsulated in dcsa) or as a regular media
>>>>>>>>> stream ?
>>>>>>>>
>>>>>>>> Based on this discussion, I conclude that it should be sufficient for this draft to say 
>>>>>>>> that before an attribute can be used with dcsa, such usage must be defined somewhere. This 
>>>>>>>> could be either:
>>>>>>>> - as part of the definition of the attribute, OR
>>>>>>>> - as part of the definition of the protocol referenced on the m-line.
>>>>>>>>
>>>>>>> We are getting closer, but it's still not obvious to me that you cannot use an attribute 
>>>>>>> with dcsa if it has not been explicitly defined for the attribute in question. Clearly, 
>>>>>>> there are attributes that wouldn't make sense over data channels, just like there are 
>>>>>>> attributes that don't make sense over particular media descriptions.
>>>>>>>
>>>>>>> Again, I'd like to hear from more people on this, including the authors.
>>>>>>>
>>>>>>> Thanks
>>>>>>>
>>>>>>> -- Flemming
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>>     Thanks,
>>>>>>>>     Paul
>>>>>>>>
>>>>>>>>> Also, it would be good to hear from more people on this, including the
>>>>>>>>> document authors.
>>>>>>>>>
>>>>>>>>> Thanks
>>>>>>>>>
>>>>>>>>> -- Flemming
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>     Thanks,
>>>>>>>>>>     Paul
>>>>>>>>>>
>>>>>>>>>>> Regards, Christian
>>>>>>>>>>>
>>>>>>>>>>> On 4/02/2016 3:58 PM, Paul Kyzivat wrote:
>>>>>>>>>>>> On 2/3/16 5:42 PM, Flemming Andreasen wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> I'm not concerned about the IANA part. I agree that *if* we need to
>>>>>>>>>>>>> expliclty specify attribute interactions for "dcsa", then it should be
>>>>>>>>>>>>> part of the IANA registry. What I am not agreeing with at this
>>>>>>>>>>>>> point is
>>>>>>>>>>>>> that there is indeed a need to explicitly speficy these
>>>>>>>>>>>>> interactions as
>>>>>>>>>>>>> opposed to relying on a more general algorithmic approach (plus the
>>>>>>>>>>>>> offerer being responsible for generating a valid offer if he wants to
>>>>>>>>>>>>> establish a data channel).
>>>>>>>>>>>>
>>>>>>>>>>>> Well, an obvious one is that the protocol(s) the attribute pertains to
>>>>>>>>>>>> need to be defined to work over data channels.
>>>>>>>>>>>>
>>>>>>>>>>>>     Thanks,
>>>>>>>>>>>>     Paul
>>>>>>>>>>>>