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

[Paul Long]

Frank,

It's not that an EP must send TCS _every time_ it receives a TCS, only the
first time if it has not already sent it for this session. It just needs to
satisfy the TCS-first rule (before any other message, including acks).
That's all.

"Capability exchange" may be overloaded. Strictly speaking, I think it
refers to the true exchange at the beginning of the H.245 session where both
EPs send their caps to the other EP. This may be referred to more
specifically as "THE Capability Exchange." When an EP unilaterally sends a
subsequent TCS, it could also be said to be performing a capability
exchange, but this is misleading. It is not "exchanging" capabilities, per
se, because the remote EP is not required to also send its caps.

Paul Long
ipDialog, Inc.

-----Original Message-----
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[mailto:ITU-SG16 at mailbag.cps.INTEL.COM]On Behalf Of Frank Derks
Sent: Tuesday, April 03, 2001 1:15 PM
To: ITU-SG16 at mailbag.cps.INTEL.COM
Subject: Re: What does 'exchange' mean and which H.245 message gets to
be sent first?


Paul,

H.323v4 is indeed where I got the text from, but that still leaves me
clueless about what these procedures could be. You say that the sending
of the TCS can force the remote EP into sending a TCS itself. I didn't
think it is also the case that upon reception of a TCS, the receiving
EP also has to send a TCS. It is only in this first occasion (or after
an "empty TCS"), that this situation arises. What is your interpretation
of "capability exchange" (I.e. the sending of the capability set only or
the mutual exchange of capabilities)?



Regards,

Frank




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Frank,

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

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?

Frank

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