H.323 Fast Connect and Versioning

Wed Sep 10 03:26:43 EDT 2003

Paul wrote
"Perhaps we can require the calling device to not transmit any data until it
receives at least one IFP packet from the called side and determines the
ASN.1 version used to encode the message."
Unfortunately this won't work - although typically the called endpoint will
provide the first IFP (Probably a CED) this doesn't work when you poll for a
fax - in that case the calling endpoint will probably want to send the first
The only way I can see out of this is to add a new data application (say,
t38faxV2) to DataApplicationCapability etc in the H.245 ASN.1.    t38fax
would use the 1998 ASN.1 and t38faxV2 would use the 2002 ASN.1 - and future
carefully checked modifications ;-).  Now there's no problem, a 2002 aware
endpoint can offer both versions and a 1998 aware endpoint can only accept
the ASN.1 it understands.
Pete Price
Vegastream Limited

-----Original Message-----
From: paulej at PACKETIZER.COM [mailto:paulej at PACKETIZER.COM]
Sent: 09 September 2003 20:32
To: itu-sg16 at external.cisco.com
Subject: H.323 Fast Connect and Versioning

Today, I was exchanging e-mail with somebody over the fax version number
issue and the different syntax that is used (1998 vs 2002).
If we open H.245 and exchange a full set of capabilities, and H.323 endpoint
could determine the version supported by the other side and open a channel
supporting that particular version.  However, I don't think any text is
explicitly clear on that.
Another scenario-- and one I have more trouble with-- is Fast Connect.  If a
calling endpoint populates the fastStart element with "version 2" proposals,
for example, the called side (say, a version 0 device) might accept the
proposal and return the response.  However, it is not allowed to modify the
version field.  The reason is that Fast Connect proposals are not ordered in
a way such that replies must be ordered the same way.  Rather, the calling
device determines which proposals are accepted based on characteristics of
the proposals returned (e.g., codec type, samples per packet, or other
information).  In some cases, a calling endpoint will actually not try to
"match" the proposal returned, but just accept it as a proposal and run with
The problem is that if a calling device proposes version 2 and the called
device returns version 2 (but is actually a v0 device), then the wrong
syntax will be transmitted on the wire.  Thus, the text needs to state
somewhere one of these options (or something similar):

1.	The calling device must offer a proposal for each version it wants
to potentially use and the called device must accept the first proposal it
can accept (in order of the proposals) and the called device must not accept
any proposal for a version it does not support 

2.	The calling device must wait for capability exchange to complete to
determine the actual supported version of the other device

Alternatively, we could make an allowance for the endpoint to change the
version number in the Fast Connect proposal, but I don't think that's a good
idea, as it would quite possibly break interoperability with some devices.
What would a version 0 device do today if it received a Fast Connect
proposal advertising version 2?  Would it accept it?  I suspect so and I'm
afraid that we might have some interop problems regardless of the direction
we go.
Perhaps we can require the calling device to not transmit any data until it
receives at least one IFP packet from the called side and determines the
ASN.1 version used to encode the message.  As much as we can push onto the
shoulders of a v2 device, the better, as I don't think we have any real
deployments in the field (yet)... might be wrong, but I think it would be a
far less significant impact on that side.
I'm open to suggestions.  Perhaps this issue is even addressed and I've
simply overlooked it.

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