[Megaco] Package Dependencies

Tom-PT Taylor taylor at nortelnetworks.com
Mon Jul 23 08:24:47 EDT 2001

I have an interesting situation with the Advanced Audio Server packages
(H.248 Annex M.1), and I wanted to check out my proposed solution with the

The "basic" package currently consists of three functions: play, play and
collect digits, and play and collect a recording.  It also includes an
extensive description of a syntax for specifying individual announcements,
including variable elements.  This syntax appears to the Megaco/H.248
grammar as a parameter VALUE in quotedString form.

One "advanced" package defined in the current draft Annex doesn't change any
of the functions, but extends the announcement-specifying syntax.  Using
past practice as a guide, the package was modelled as an extension of the
basic one.  However, note that the new functionality is actually hidden
below the level that would normally distinguish an extension -- no new
signals or events are added, and no new parameters are added.  (That last
wouldn't be legal in any event.)

However, this approach doesn't really work.  The first sign of trouble was
when I thought to split up the play, play-and-collect, and play-and-record
functions, both to clarify the functional definitions and because it would
provide more flexibility for product implementation.  How would I introduce
the "advanced" syntax for each function?  Do I need to define an "advanced"
package for each, so that the two packages become six?

It becomes worse now that more than one advanced syntax feature has been
proposed.  I now need a package for the basic announcement syntax, one for
that plus feature A, one for that plus feature B, and a fourth one for all
three features together.  You get the idea: an exponential explosion,
multiplied by the number of functions if we decide to split them.

I have an answer to this, but I thought it might be a good idea to
legitimize it.  It could even lead to a v2 formalization of the process if
accepted (with no change in what goes out over the wire!).  My proposal is

1) We define the announcement specification syntax itself in a package of
its own.  Note that this package will have none of the usual content -- no
properties, signals, events, or statistics.

2) We define each additional feature of the announcement specification
syntax in another package which is an extension of the basic syntax package.

3) Within the functional packages (play, play-and-collect, etc.) we say
first that the package USES the syntax of the basic syntax package and
secondly that if any extension of the basic syntax package is supported the
functional package also USES that extension.  An implication is that if a
device supports an advanced syntactical feature (e.g. sets) for one function
it MUST support that feature for all of the functions it implements.

Comments?  Is this a legitimate use of packaging?  Should we add the USES
relationship to our section 12 package description requirements in v2?

Please copy follow-ups to the Megaco list, now at megaco at ietf.org.

Tom Taylor
taylor at nortelnetworks.com
Ph. +1 613 736 0961 (ESN 396 1490)

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