Input TDs for the coming SG16 meeting

Sakae OKUBO okubo at
Mon Aug 31 22:23:26 EDT 1998


Thanks for the reply. Now we have the chance to go for the next step to
understand the short comings of the present H.323v2 as well as proposed
extension specified in AT&T's distributed model.

I would like to separate the answer in two parts: 1. General aspects of
H.323 and 2. Specific to Vivian's topology and configuration.

General aspects of H.323:

H.323 is a multimedia application, and is independent of the underlying
transport networks (i.e., LAN, IP, ATM, FR, and/or others - in any
combinations). H.323 considers only H.323 entities (e.g., terminals, MCs,
MCUs, etc) and GKs. The communications occur between H.323 entiries and GKs,
and between GKs. A single GK controls and manages a zone, and a zone may be
independent of network topology and may composed of multiple segments which
are connected using routers, switches, and/or other devices.

H.323 does envision communications between the GKs. For example, LRQ
messages are sent between the GKs. Between the GKs, as mentioned earlier,
there can be many routers, switches, or other devices. This implies an
"abstraction of routing"  for sending the RAS messages between the GKs at
the H.323 level, however, the specific implementation of "routing" of the
RAS messages at the "network level" is not addressed in H.323v2.

[In AT&T's distributed model, it is proposed that all GKs should be capable
to exchanges all RAS signaling messages (not only LRQ as envisions by the
present H.323v2) to solve problems, and the "pathValue" field has been
introduced to formalize the "abstraction of routing" notion between the GKs
at the H.323 level considering the multiple-GK environment. At H.323 level,
AT&T's proposed contribution does not propose any "specific routing schemes"
or "routing interfaces".]

H.323 envisions that all communications shall be done between H.323 entities
such as H.323 terminals, MCUs, MCs, and GKs. At H.323 level, the
communications can take place within a given zone or between multiple zones.
At H.323 level, the communication topology has be covered by the single zone
GK or multiple zone GKs. H.323 cannot offer any help if the communication
topology is not covered by the H.323 GKs.

H.323v2 does not provide mechanisms how the MCUs should be chosen
automatically in view of multiple MCUs. That is, a specific MCU has to be
chosen by the calling party. (The automatic call deflection by the setup
message by a new party to an established multipoint [N-party call]
conference call to a MCU is not supported by H.323v2 unless somehow the
required MCU's known address is put in the setup message.)

At H.323 level, Communications are done between the H.323 entities: H.323
terminals, H.323 MCUs, H.323 GKs, etc., and there is no "notion" of
communications of "local" or "remote".

Specific to Vivian's topology and configuration:

In Vivian's configuration, only one MCU has been shown, and that MCU is
located in a separate zone. There are 4 zones (GK1, GK2, GK3, and GK4) where
H.323 terminals are located, and one MCU in a separate zone that has its own

According to AT&T's proposal, the GKs will be able to know where the MCU is
located through exchanging of RAS messages (e.g., LRQs). The calling party
may select the MCU where the conference will be controlled. The GKs will be
able to send the signaling messages to the particular GK where the MCU is

AT&T's proposal allows the ARQ and other RAS messages between the source and
destination GKs so that all GK s between the source-destination entities
(the word "path" may be mis-understood) can work cooperatively from the
resource management point of view whether a call be accepted or not before
the actual call is setup.

A MCU that will be controlling the multipoint conference call will be the

If a new party (e.g., F in Vivian's example) wants to join the "already"
established "multipoint" conference call, F has to join the conference via
the MCU (most like to know the conference bridge number priori from the
calling party E1 by F). The present H.323v2 does not provide any mechanisms
to deflect the setup message of a new joining member automatically to go the
MCU (until the new calling party [in this case it is  F] knows priori the
address of the MCU).

{RSVP reserves the bandwidth at the network level especially in the IP
network, and RSVP even may not be applicable to reserve the bandwidth at the
LAN. Similarly, there are many mechanisms may be available for the ATM
network level. These BW/QOS schemes are limited to the network level only.
However, H.323 is an application, any the "abstraction" of BW/QOS is
end-to-end that include resources for "network level"  as well as "upper
middleware level" at the H.323 call level.}

I hope that my above answers will clarify all questions that you might have.
If any more questions remain, please let me know.

Thanks and regards,


        From:   Douglas Clowes[SMTP:dclowes at OZEMAIL.COM.AU]
        Reply To:       Mailing list for parties associated with ITU-T Study
Group 16
        Sent:   Sunday, August 30, 1998 7:30 PM
        To:     ITU-SG16 at MAILBAG.INTEL.COM
         Subject:       Re: ARQ/ACF


In common with others, I cannot see in H.323v2 generally, nor section 8.1.6
specifically, as quoted in your other response, where ARQ is being sent
between gatekeepers. I'll let their comments rest.

I am not too clear on conference calls, and how the MCU is determined.
presumably, that is done by the first party, or between the first two
parties. For the n'th party to join a conference, do they join by reference
to the conference, the MCU (not knowing which MCU was selected) or by
reference to one of the endpoints currently participating in the
conference? I suspect the latter, and that the call will be deflected to
the MC by a Facility response to the Setup.

Section 8.1.9 allows for an ad-hoc conference to be connected with "an MC
associated with the Gatekeeper". It also allows for "The master-slave
determination procedure is used to determine which MC will be the active MC
for the conference." So, if the MC is dynamic, how is the new endpoint
expected to "know" of it?

At 17:18 28/08/98 -0400, Radhika wrote:
>I am extremely happy in seeing Mr. Clowes answers. I am also adding a
>text in Clowes' answers to clarify further in relation to the proposed
>AT&T's distributed model.
>I am using one of your assumptions is that MCU is located in zone that has
>its own GK. In H.323, each GK manages resources in its own zone.
>Now MCU will be acting as the central point that will be controlling the
>call. That is, a calling H.323 endpoint will set up a call with the MCU,
>in turn MCU will set up the call with the called H.323 endpoints. If a new
>H.323 endpoint located in new zone wants to join the conference, it will
>also contact the MCU.
>Now the RAS signaling messages between the GKs will be following the
>logic. That is, from the calling H.323 endpoint's GK to the MCU's GK, and
>then from MCU's GK to the called H.323 endpoints' respective GKs, etc.
>1. Current H.323v2 (e.g., Section 8 with explicit Figures) DOES envisions
>the RAS signaling messages including ARQ can be sent between the GKs in
>addition between the H.323 entities and the GK. (Please note the difference
>with Mr. Purvis reply).
>2. In "logical" zone environment, the resource management is also extremely
>important especially in the context of large network where million of users
>share the same physical network (e.g., in carrier networks, in virtual
>private networking environment, the network reosources are partioned
>logically between the corporate customers, and resource managemnt is also
>done accordingly). [Please note the difference with Mr. Purvis reply].
>> >   Should GK4 forwards ARQ to GK that has the MC?? F should not use any
>> >   resources in the zone of GK is responsible for.
>> Not normally.
>[Radhika: As I have mentioned in the begining, the communication will be
>done between the MCU and the H.323 endpoint. So, the call will span at
>two zones: Zone of MCU and Zone 4 of F. Each GK manages resources in each
>zone. As a result, GK of Zone 4 will not be able to confirm the ARQ message
>because it does not know about the resources of Zone of the MCU. Accordance
>to AT&T's proposed distributed model, GK4 of Zone 4 will send the ARQ
>message to the GK of (MCU) zone after cinfirming its own resources. As soon
>as the (MCU) GK confirms the resources of its zone, ACF message will be
>from the (MCU) GK to the GK4 of Zone 4. GK4 then sends the ACF message to

Where this initial connection is to one of the endpoints involved in the
conference, this gets to be a little confusing. How should the GK in the
destination zone respond, if a deflection to an out-of-zone MC is the
(unknown by GK at this time) future response, and yet local resources are

>> >or F has to, because H.245 control message exchanges also consume the
>> >   reources?
>> Endpoint F should receive an ACF from its own gatekeeper, relating to
>> resource and policy in its own zone. It should then establish a call
>> signalling channel and send a SETUP to the MC. The MC will send an ARQ to
>> its own gatekeeper, to request resourse allocation in its own zone, for
>> this part of the conference.
>[Radhika: According to AT&T's proposed distributed model, it will be
>preferable to follow the ARQ/ACF signaling paths as mentioned above by me.
>Then, the call signaling SETUP message should be sent to the MCU (because
>one of the main puposes of the ARQ message is to confirm the resource
>between the source-destination path).]

I thought that the ARQ bandwidth management was only local, and not path
related. Each endpoint (including the MCU) may send an ARQ to its own GK
for local bandwidth management. Gatekeepers in unrelated zones are not able
to do much about resources on shared backbone path segments. Mechanisms
such as RSVP, executed by the endpoints, would seem to be appropriate for
path resource reservation and confirmation.



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