Notice of the Berlin meeting

Sakae OKUBO okubo at GITI.OR.JP
Fri Jun 25 02:14:57 EDT 1999

We face a continuing tension between the requirement to ensure
interoperability and the desirability of minimizing the size of the core
protocol, particularly at the MG.  We also have to make sure our protocol
has well-defined procedures for adding both ad hoc (i.e. proprietary or
experimental) and formal extensions.   This note suggests some principles
which we should follow in response to these considerations.

There are really two levels of interoperability to deal with: syntactic (can
the message be parsed?) and semantic (can the message recipient perform the
actions which the sender is requesting?).

At the syntactic level, I believe that we can agree on these principles:

I-1: Every message must be fully parsable down to a certain level of detail,
even if it contains extensions unsupported by the recipient.

I-2: Our protocol document must specify the syntax by means of which
extensions are identified.

I-3: The protocol must provide the means for the recipient to identify in
its response to any message all unsupported extensions it encountered in
that message.

There is an open issue here: what is the minimal parsability level we want
to specify?  Suggestion: all messages are parsable down to the command and
parameter level at a minimum.  Here I use the term "parameter" to imply
either a simple data value or a potentially complex data structure
identified as a syntactic element in our API definitions.  The implication
of this suggestion is that extensions can add no new commands, no new
command parameters, only new possible values for command parameters and new
event packages.  I'm sure others will have comments on this.

Now we get to the interesting part: semantic interoperability.  I'll start
with what I hope are a couple of obvious statements.

I-4: Not all implementations will support all event packages, all possible
parameter values, or whatever else we allow to be extended.

I-5: The protocol definition must include a specification of what the
receiver does if a message contains an unsupported extension.

On this latter point, do we enforce the transaction concept at all levels
and fail the transaction, requiring the sender to reissue the command
without the offending value?  This is definitely a workable solution.  We
could go further and introduce the ability to say in a message whether a
given field is a transaction-stopper, but I'm not sure that capability would
be used in practice  -- it wasn't when we had it in IPDC.

Now for the crucial question: if I-4 is true, what criteria do we use to say
what features MUST be supported by any given implementation?  Can we
distinguish between required support in MGC implementations, versus required
support in MG implementations?  How does each side of the Megaco interface
determine what the other side supports, other than by parsing transaction
rejections?  Here are some propositions to use as a basis for discussion:

I-6: The Megaco/H.GCP specification should be organized into a basic
protocol specification which must be satisfied by all implementations, plus
annexes which are conditionally mandatory.  The conditions under which an
annex is mandatory will be stated within the annex itself, to make it

Some annexes will constitute extensions of the protocol (e.g. event package
specifications) and must therefore be identifiable as such at the syntactic
level.  Other annexes may be more like profiles, stating, for example, what
extensions must be supported by an MG or MGC which supports a given
application.  An extension identifier for the complete profile is a
"nice-to-have", but would be used only as shorthand in Audit responses, not
in other commands.

Looking to the future, extensions could be created:
-- by joint Megaco/ITU action, added as new annexes to H.GCP
-- by action of the ITU on its own, added as new annexes to H.GCP or as new
-- by the IETF on its own, added as new RFCs
In all cases, I suggest that the extensions be registered with IANA in
accordance with procedures and criteria defined in the base protocol

I-7: As a general principle in deciding what each implementation must
support, MGs should be assumed to be more specialized than MGCs.  For
example, we may decide (though I doubt it) that all MGCs must support all
applications documented in the initial Megaco/H.GCP specification, but MGs
may support an arbitrary subset of them.

This is a debatable principle -- some vendors probably have a different view
of the architecture.  I suspect it is the consensus view, however.

I-8: If at all possible, MGs should hide the details of national and
regional variations in channel-associated signalling from the MGC.

This is more debatable than I-7.  I propose it as a principle on the grounds
that MGs are more likely to be serving customers in only one country or
region than are MGCs, so they will be the more logical point of
specialization.  I have a concrete suggestion for our event packages which
may help to support this principle: event descriptor syntax should include
both the abstract event names and specialization identifiers which indicate,
for instance, that the MG should provide busy tone for Country A rather than
Country B to this particular termination.  That way the MGC can pass down
information it has received from signalling, while the physical details are
known only to the MG.

Tom Taylor
Advisor -- IPConnect Standards
E-mail: taylor at (internally, Tom-PT Taylor)
Phone and FAX: +1 613 736 0961

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