Versions of Annex E

Chip Sharp chsharp at CISCO.COM
Tue Apr 4 13:48:41 EDT 2000


Chris,

Hey, we're in luck! Only one side is required to initiate MSD:

        8.2/H.323v3: "Master-slave determination shall be advanced (by sending
either MasterSlaveDetermination or MasterSlaveDeterminationAck as
appropriate) in the first H.245 message after Terminal Capability Exchange
has been initiated."

So we're all set. If an EP initiates TCS, regardless of the transport, the
remote EP is required to respond in kind if it has not previously done so.
Since the remote EP is not required to initiate its own MSD, the EP that
initiated TCS should also then initiate MSD. Once the two MSDAcks are done,
the H.245 session is up.

Note, however, that section 8.2 indicates that an EP cannot respond to the
TCS message without first having sent its own TCS. Guessing how most stacks
are probably written, it would probably not cause interoperability problems
to first respond to the TCS then send your own TCS.

Paul Long
Smith Micro Software, Inc.

-----Original Message-----
From: Chris Wayman Purvis [mailto:cwp at ISDN-COMMS.CO.UK]
Sent: Tuesday, April 04, 2000 4:40 AM
To: ITU-SG16 at MAILBAG.INTEL.COM
Subject: Re: Force remote to start H.245 session?


Paul,

> I think you see my point, then. Now that I think about it, an EP _is_
> required to at least send its TCS, if it has not already, if it
receives an
> H.245 request message. Specifically, it cannot send the TCSAck until
it has
> sent its own TCS, according to the following:
>
> 8.2/H.323v3: "Endpoint system capabilities are exchanged by
transmission of
> the H.245 terminalCapabilitySet message. This capability message shall
be
> the first H.245 message sent."

That's what I thought, although I didn't have the reference to hand...

> And then there's the following. I think it means that if an EP
receives TCS
> without already having sent TCS itself, it shall do so now. Is that
your
> understanding?
>
> 8.2/H.323v3: "An endpoint which receives a terminalCapabilitySet
message
> from a peer prior to initiating capabilities exchange shall  respond
as
> required by 6.2.8.1, and should initiate and successfully complete
> capabilities exchange with that peer prior to initiating any other
> procedure."

Let's just say that your interpretation is not so much my
"understanding" of
that text as its "intention" when Dave Walker and I wrote that
passage...

> So if an EP initiates H.245 procedures, the remote is forced to also
perform
> H.245 procedures. Okay, that's what I was looking for. Maybe it just
means
> that it has to perform the cap exchange. You know, both sides don't
really
> need to initiate the MSD. It's enough that the side that first wanted
to use
> H.245 procedures has performed it, so TCS alone is enough of a prelude
to
> get the H.245 session up and running. Do you think this needs
clarification
> text?

As long as MSD is completed that's fine.  I agree there's no point in it
being
initiated by each side.  Is that required somewhere?  The intention was
that
one should always do one's best to get MSD out of the way "as soon as
possible", but TCS must happen (or at least start...!) first.

Regards,
Chris
--
Dr Chris Purvis -- Development Manager
ISDN Communications Ltd, The Stable Block, Ronans, Chavey Down Road
Winkfield Row, Berkshire.  RG42 6LY  ENGLAND
Phone: +44 1344 899 007
Fax:   +44 1344 899 001



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