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

Brett Tate <> Wed, 08 August 2012 11:53 UTC

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From: Brett Tate <>
To: "Ali C. Begen (abegen)" <>, "" <>
Date: Wed, 08 Aug 2012 04:53:41 -0700
Thread-Topic: Draft-ietf-mmusic-rfc4566bis-05: session-level attributes
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Subject: Re: [MMUSIC] Draft-ietf-mmusic-rfc4566bis-05: session-level attributes
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> > 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,