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

[Paul Kyzivat <[e-mail]>]

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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