Subject: underspecifying and overspecifying SDP

From: Terry L Anderson <[email-masked]>
Date: Fri, 15 Dec 2000 14:17:45 -0500

We are looking at options for negotiating codecs:
1) MGC presents a list of SDP blocks with different m= lines for codecs
in Local of Add for first (ingress) MG.  The list would have to come
from provisioning or Union of all known to be supported by MGs.  MG
chooses and MGC sends single choice in Local and Remote of other end
(terminating).

? is it permissible for orig to return a (possibly shorter) list of
codecs acceptible to it, have the MGC present this list to other end to
choose?  7.1.8 says that MG is requested to "choose one or more
values".  This would be more like H.245 where the end that makes the
choice knows the list acceptible to other end as well as self.

2. MGC presents a single SDP block with underspecified m= line
    m=audio $ RTP/AVP $
and ingress fills in port AND codec.  This avoids the need for MGC to
have a list of possibles.  Again MGC presents this sole choice to other
end.

? again is it permissible for a MG to return multiple SDP blocks with
multiple acceptible codecs in response to such an underspecified single
block?  It is not clear to me whether "underspecification" can apply to
entire SDP or only to a line of the SDP.  Which is a "property"?

3. MGC presents an empty SDP block consisting of v=0 only.  Is this form
of underspecifying allowed?  MG then fills in all the lines of an SDP or
possibly multiple SDP blocks in reply.

MGC cannot omit Local as MG would not reply with a Local according to
spec.  MGC cannot send an empty Local as that implies release of
reserved resources.

--
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Terry L Anderson              mailto:[email-masked]
Tel:908.582.7013   Fax:908.582.6729
Pager:800.759.8352 pin 1704572   [email-masked]
Lucent Technologies/ Voice Over IP Access Networks/ Applications Grp
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http://its.lucent.com/~tla (Lucent internal) http://www.gti.net/tla

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From: Dan Elbert <[email-masked]>
Date: Mon, 18 Dec 2000 14:41:15 -0500

1. Seems OK if ReserveGroup=True.
What happens after the other end chooses ?
Does the MGC send a Modify releasing all resources but the chosen one?

2. Again, seems like if ReserveGroup=True you should receive all the
alternatives in different SDP blocks.

3. I believe that this is mat not be legal in SDP. In any case the
interpretation would be "a session with no media". This will not do what you
want.

Dan Elbert
RADVISION