Subject: Annex C Descriptor Assignments

From: "Tom-PT Taylor" <[email-masked]>
Date: Thu, 10 May 2001 20:05:32 -0400

Here is the contribution to SG 16 I've drafted on the subject of descriptor
assignments for the Annex C properties.

TITLE:	Descriptors For Megaco/H.248 Annex C Properties
___________________

INTRODUCTION

Megaco/H.248 Annex C defines a set of properties relating to media streams.
As such, these properties may appear:
·	in the LocalControl descriptor, if they are local interest only, or
·	in the Local and Remote descriptors, if they must be coordinated
between the local and remote Media Gateways.

Megaco/H.248 Annex C currently fails to indicate which alternative applies
to each of the properties specified within the Annex.  This contribution
argues that the H.248 Implementor's Guide should be updated to reflect such
an assignment.  The contribution goes on to derive principles for making the
assignment decision, and then applies those principles to derive a detailed
set of  descriptor assignments for all of the properties in the Annex.

NEED FOR A STANDARDIZED PROPERTY ASSIGNMENT

This section presents a semantic and a syntactic argument for standardizing
the assignment of properties to descriptors in Annex C.  At the semantic
level, it is essential that the assignment to descriptors be the same for a
given property, regardless of the H.248 protocol encoding.  Otherwise one
encounters a semantic mismatch between encodings, such that with respect to
a given property asymmetric flow descriptions are possible in one encoding
and not the other.  Such a mismatch may be introduced by general usage or by
explicit adoption of Annex C properties in standards created by bodies other
than the IETF Megaco Working Group or Study Group 16.

At the syntactic level, assignment of specific properties to specific
descriptors simplifies parser construction by adding predictability to
message construction.
A final argument in favour of adding explicit descriptor assignments is that
Annex C, as "Package 0", should conform as much as possible to the
documentation rules for packages.  This includes documentation of the
descriptor to which each property should be assigned.

ASSIGNMENT PRINCIPLES
 
The basic criterion for descriptor assignment was indicated in the
introduction: whether or not the property concerned must be coordinated with
the remote Media Gateway.
However, some borderline cases will require the use of subsidiary criteria.
The first such criterion was suggested in the previous section: assurance
that the assignment is the same for both text and binary (Annex C)
encodings.  Because the text encoding for the Local and Remote descriptors
is tied to SDP (RFC 2327), this means that at the very least the mandatory
properties found in RFC 2327 should  be assigned to the Local and Remote
descriptors rather than LocalControl.  Properties which are optional in SDP
may be placed in LocalControl if this makes sense, but the assignment must
be the same in both the text and the binary encodings.

There is a further possible criterion: is there a potential requirement  for
a given property to have a different value for each direction of flow, and
will it be technically feasible to meet that requirement?  If so, the
property should be assigned to Local/Remote rather than LocalControl.

DETAILED ASSIGNMENTS

The following table proposes detailed assignments for the Annex C
properties.  The "Remarks" column indicates criteria used where the
assignment was not immediately obvious.

[Rather than repeat the tables, let me summarize by saying that almost
everything goes into Local/Remote.  Here are the exceptional cases:

Table C.1: Transmission mode (tag 1002) should perhaps be deprecated because
the Mode property in LocalControl carries the same information.  This is
equivalent to saying that the SDP in Local and Remote should not include any
of the directionality attributes.  People may want to comment.

Table C.1: Gain (tag 100C) and Jitterbuff (tag 100D) are placed in
LocalControl (of local interest only)

Table C.9: Modem (tag 901B) goes into the Modem Descriptor.  Actually it
should be deprecated because the necessary codepoints have already been
defined in the Modem descriptor.

Table C.9: DialledN (tag 901F) and DiallingN (tag 9020) go into LocalControl
because they do not require end-to-end coordination at the media level.]

Tom Taylor
Multimedia Control and Applications Standards
Ph.  +1 613 736 0961
[email-masked]

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From: "Bemmel, Jeroen van (Jeroen)" <[email-masked]>
Date: Fri, 11 May 2001 09:13:45 +0200

That is what I had in mind too. But isn't it cumbersome to have such a large
'package 0'?
Is it likely for a Media Gateway to support all those properties?

I would suggest to split the attributes in Annex C over multiple packages,
eg put IPaddress/port in the RTP package (or perhaps some other newly
defined package), ATM related properties in another package, etc

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From: "Tom-PT Taylor" <[email-masked]>
Date: Fri, 11 May 2001 08:21:39 -0400

This has been discussed.  The agreement is that an implementation can pick
and choose amongst the elements of Annex C that it supports.

The peculiar aspect of Annex C is that (as my submission indicates)
practically all of it must be realized on the text side as SDP extensions
rather than Megaco/H.248 packages.

> -----Original Message-----
> From: Bemmel, Jeroen van (Jeroen) [mailto:[email-masked]]
> Sent: Friday, May 11, 2001 3:14 AM
> To: Taylor, Tom-PT [NORSE:B881:EXCH]; 'megaco'
> Subject: RE: Annex C Descriptor Assignments
> 
>

---

From: "Rosen, Brian" <[email-masked]>
Date: Fri, 11 May 2001 09:58:11 -0400

Before we deal with the details of this, I question the premise:
  I think that Annex C ONLY is used in 
	binary encoding  and
	in local and remote (NOT localControl or TerminationState)
I think the I-G might state that better.
I think that if there are some things that don't belong in remote
  and local, they should be removed and possibly made part of a package,
  because you don't use Annex C in LocalControl

Brian

INTRODUCTION

ASSIGNMENT PRINCIPLES

DETAILED ASSIGNMENTS

Tom Taylor