What does 'exchange' mean and which H.245 message gets to be sent first?

Paul Long plong at IPDIALOG.COM
Tue Apr 3 13:04:55 EDT 2001


1. I have no idea what those failing procedures would be. Maybe it's a v4

2. Yes, that is correct. Because the TCS must be the first message, if an EP
receives a TCS, it must sends its own TCS (if it has not already) before it
responds to the TCS with, typically, TCSAck. This is how an EP can force the
remote EP into performing cap exchange.

Paul Long
ipDialog, Inc.

-----Original Message-----
From: Mailing list for parties associated with ITU-T Study Group 16
[mailto:ITU-SG16 at mailbag.cps.INTEL.COM]On Behalf Of Frank Derks
Sent: Tuesday, April 03, 2001 10:33 AM
To: ITU-SG16 at mailbag.cps.INTEL.COM
Subject: What does 'exchange' mean and which H.245 message gets to be
sent first?

H.323 talks about capability exchange, but it is not clear
to me whether exchange really means that communicating endpoints
send their capability sets to _each other_ or that one endpoint
sends its capability set. An example of this can be found in the
below text. It talks about succesfully completing capability
exchange, but does this means that the initiating endpoint has
sent its capability set and that this was acknowledge by the
receiver or does it mean that it has also received the capability
set of the other party?

8.2/H.323 states:

"Endpoint system capabilities are exchanged by transmission
of the H.245 terminalCapabilitySet message. This capability
message shall be the first H.245 message sent unless the
endpoint is indicating that it understands the
parallelH245Control field (see section 8.2.4). If prior to
successful completion of terminal capability exchange, any
other procedure fails, (i.e. rejected, not understood, not
supported) then the initiating endpoint should initiate and
successfully complete terminal capability exchange before
attempting any other procedure. "

The second sentence mandates that the terminalCapabilitySet
message be the first message sent (given that
"parallelH245Control" is not used). Which "failing procedures"
are then being referred to in the third sentence?

If terminalCapabilitySet has to be the first message to be
sent, does this mean that an endpoint that has not sent a
terminalCapabilitySet message yet and receives this message,
it first has to send a terminalCapabilitySet message before
acknowledging the received capability set?


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