Subject: Re: Washington Issue 9 - Wildcards in SDP

From: Tom-PT Taylor <[email-masked]>
Date: Fri, 3 Dec 1999 14:57:39 -0500

There is an issue concerning the respective contents of LocalControl and
Local which was nearing resolution before I lost my E-mail contact with the
world.  To make the nature of this issue clear I will review how we do media
negotiation now, then make a proposal for how it can be simplified.

As stated in section 7.1.7 of the H.248v12 document,

"The LocalControl Descriptor contains the Mode parameter and properties of a
termination (defined
in Packages) that are stream specific, and are of interest between the MG
and the MGC. Values of
properties may be underspecified as in Section 7.1.5."

In the next section, Local is defined as follows:

"The Local and Remote Descriptors contain the parameters describing the
flows sent to and received
from the MG, and are of interest between two MGs....Local is the capability
of the local MG and is typically sent from MG to MGC, and subsequently used
as part of a capability negotiation between two MGCs."

In the MGCP tradition, we have assumed that LocalControl provides guidance
on codec selection, amongst other things.  That is, the MGC sends down
policy in LocalControl, the MG returns capabilities within the confines of
that policy, and the MGC passes these to the far end via whatever signalling
system it is using.

Unfortunately, even ignoring the binary vs. text issue this seems to imply a
lot of double defining of codepoints.  Here are two concrete examples based
on the "Network" package in Annex E, section E.9:

(1) The Network package specifies a "Network type" property which can be
used to indicate the transport stack to be used.  This information is echoed
in the network type as given on the c= line of Local, plus further stack
details as given on the m= line.  Two encodings for the same information.

(2) The Network package defines "Preferred Encoding" and "Preferred
Decoding" properties.  These are echoed in the RTP payload type in the m=
line, plus, possibly, some a=rtpmap entries.  Two encodings for the same
information.

We should not be doing everything twice over (plus the binary encoding).
This is going to be inefficient to administer and confusing to use.  I
propose two alternatives:

Either we use the same encoding for any information which will be common to
LocalControl and Local.

or

We make things even simpler and eliminate the redundancy.  The only
information which would go into LocalControl would be properties of strictly
local interest (i.e. which would not be signalled to the far end).

The implication of this second alternative is that Local as sent down from
the MGC to the MG will be underspecified (not really something new).  What
will be new is the additional information that Local will contain compared
with MGCP practice.  As an example, there will be a c= line indicating the
transport network type and possibly even the address type, but no actual
address.  There will be an m= line specifying the medium for that stream,
the rest of the transport stack, and the coding preferences, but that line
won't have a port number.  Thus we will need placeholders (wildcards?)
within the SDP to indicate the information to be filled in by the MG.

Tom Taylor
Advisor -- IPConnect Standards
E-mail: [email-masked] (internally, Tom-PT Taylor)
Phone and FAX: +1 613 736 0961

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From: Christian Groves <[email-masked]>
Date: Mon, 6 Dec 1999 22:01:37 +0100

Hello Tom,

We had some discussion about what the Local and Remote descriptor
actually means and I guess the email below is trying to solve this
issue. I was under the impression that local described the properties of
the "received media" and that remote described the properties of the
"transmitted media". This is how I interpret the text in the Whitepaper.

Attached is a PDF on how I view the configuration.

After discussion with people that have been working with H248 for quite
awhile they were under the same impression, however other see it
differently. I continuously get questions regarding what local and
remote mean therefore I propose the addition of the diagram to the H.248
protocol.

I agree with your sentiments that data is being duplicated in the
current situation. Local should have the properites/characteristics of
the "received media". Remote should have the properties/characteristics
of "sent media". This would also align the text and binary versions.

Cheers, Christian