Re: [MMUSIC] Draft-ietf-mmusic-rfc4566bis-05: session-level attributes

[Flemming Andreasen <[e-mail]>]

We clearly need an escape hatch as there are several examples of 
attributes with a given name that may occur multiple times to provide 
different values by use of additional qualifiers inside the attribute, 
and some of those attributes may appear at both the session and media 
level (capability negotiation for example uses this approach, as does 
"rtpmap", "fmtp", etc. inside a media description). In fact, I'd argue 
it's more than an escape hatch and probably more of a "default rule" in 
the absence of anything else specified for the attribute.

In terms of the "choice" wording below, I think it's underspecified what 
that means, and I'm also not sure we really need a general rule here. Do 
we have other examples than the direction attributes currently ? If not 
(or we can't think of those), I'd rather just call those out explicitly.

Thanks

-- Flemming (as Individual)



On 12/29/12 6:03 PM, Ali C. Begen (abegen) wrote:
> I don¹t remember us reaching a consensus regarding the text proposed by
> Brett. One thing certain is that we don't wanna break anything that
> complies with 4566 but we should also make the text as clear as possible.
>
> I am OK with Brett's suggestion as I don't recall an attribute like what
> Paul might have seen. But, even if such an attribute exists, the current
> text would be incorrect, too (This would indicate a problem with that
> particular attribute not 4566 or 4566bis).
>
> Brett's proposal does not make the text any worse. So, I am planning to
> incorporate this change.
>
> Any objections?
>
> -acbegen
>
> -----Original Message-----
> From: Paul Kyzivat <[e-mail]>
> Date: Monday, August 27, 2012 12:03 PM
> To: "[e-mail]" <[e-mail]>
> Subject: Re: [MMUSIC] Draft-ietf-mmusic-rfc4566bis-05:
> session-level	attributes
>
>> I've always thought this should be the way things work, for all
>> attributes. But somewhere I came across an attribute (sorry, don't
>> remember which one) where the attribute at session level means something
>> different than the attribute at media level, and so can't be treated as
>> a default for media level. (E.g. the session level is an aggregate limit
>> for something, while the media level is a limit for a single m-line.)
>>
>> Am I just dreaming this? Or can someone identify specific attributes
>> that behave this way?
>>
>> If there are existing attributes that behave this way, then we can't
>> make a blanket statement like this. There will need to be an escape
>> hatch. For example:
>>
>> "The connection ("c=") information in the
>>   session-level section applies to all the media of that session unless
>>   overridden by connection information
>>   in the media description.  For instance, in the example
>>   below, TBD..."
>>
>> "The attribute ("a=") information in the
>>   session-level section applies to all the media of that session unless
>>   overridden by connection information or an attribute of the same name
>>   or choice in the media description, or the specification of that
>>   attribute calls for a different treatment.
>>   For instance, in the example
>>   below, each media behaves as if it were given a "recvonly" attribute.
>>   If the example had additionally contained a "sendrecv" media-level
>>   attribute for audio, only the video media would behave as if it were
>>   given a "recvonly" attribute."
>>
>> Even this could have some backward compatibility consequences. I think
>> there are some attributes that are only defined for media level. This
>> revision would mean that a session level value could override a default
>> for media level. Implementations that aren't prepared for that might
>> break.
>>
>> 	Thanks,
>> 	Paul
>>
>> On 8/27/12 12:04 PM, Ali C. Begen (abegen) wrote:
>>> Brett suggested the following change in the SDP draft. Any objections?
>>>
>>> -acbegen
>>>
>>> Current text:
>>>
>>> "The connection ("c=") and attribute ("a=") information in the
>>>    session-level section applies to all the media of that session unless
>>>    overridden by connection information or an attribute of the same name
>>>    in the media description.  For instance, in the example below, each
>>>    media behaves as if it were given a "recvonly" attribute."
>>>
>>> Proposed text:
>>>
>>> "The connection ("c=") and attribute ("a=") information in the
>>>    session-level section applies to all the media of that session unless
>>>    overridden by connection information or an attribute of the same name
>>>    or choice in the media description.  For instance, in the example
>>>    below, each media behaves as if it were given a "recvonly" attribute.
>>>    If the example had additionally contained a "sendrecv" media-level
>>>    attribute for audio, only the video media would behave as if it were
>>>    given a "recvonly" attribute."
>>>
>>>> -----Original Message-----
>>>> From: Brett Tate [mailto:[e-mail]]
>>>> Sent: Wednesday, August 08, 2012 7:54 AM
>>>> To: Ali C. Begen (abegen); [e-mail]
>>>> Subject: RE: Draft-ietf-mmusic-rfc4566bis-05: session-level attributes
>>>>
>>>>>> Draft-ietf-mmusic-rfc4566bis-05 section 5 indicates the
>>>>>> following.
>>>>>>
>>>>>> "In general, session-level values are the default for all
>>>>>> media unless overridden by an equivalent media-level value."
>>>>>>
>>>>>> "The connection ("c=") and attribute ("a=") information
>>>>>> in the session-level section applies to all the media of
>>>>>> that session unless overridden by connection information
>>>>>> or an attribute of the same name in the media description.
>>>>>> For instance, in the example below, each media behaves
>>>>>> as if it were given a "recvonly" attribute."
>>>>>>
>>>>>> If the attributes are related but with different names
>>>>>> (such as "inactive" and "sendrecv"), does the attribute
>>>>>> at the media-level have precedence over the value at the
>>>>>> session-level?  If so, I recommend that the draft be
>>>>>> updated to clarify the topic.
>>>>> I believe so. at least that is my understanding from the
>>>>> text. Does it really need clarification? Thoughts?
>>>> I think that it should be clarified because it is unclear if "an
>>>> attribute of the same name" indicates the typical situation or only
>>>> situation concerning application of "unless overridden by an
>>>> equivalent media-level value".  For instance if I sent "a=inactive"
>>>> at session-level and "a=sendrecv" at media-level, I'd be concerned
>>>> that the receiver may interpret the SDP as being
>>>> abnormal (such as including both values at media-level) instead of
>>>> allowing "a=sendrecv" to override "a=inactive".
>>>>
>>>> In case it matters from an extendibility perspective, requiring the
>>>> same name might make it easier to recognize the override.
>>>> However since section 6 already introduces complications with
>>>> defaulting to "sendrecv" if a new option is defined/used and
>>>> unknown, allowing the override even when the names are not the same
>>>> doesn't make things too much worse; it actually
>>>> allows communication of a backup attribute (such as "a=inactive") if
>>>> they don't understand the preferred new attribute (such
>>>> as "a=foo").
>>>>
>>>> "If none of the attributes "sendonly", "recvonly", "inactive",
>>>> and "sendrecv" is present, "sendrecv" SHOULD be assumed as the
>>>> default for sessions that are not of the conference type
>>>> "broadcast" or "H332" (see below)."
>>>>
>>>> Thanks for the response,
>>>> Brett
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