T.38 potentially broken by recent Corrigendum

[Robert Jongbloed]

On Sun, 2 Mar 2003 21:20:46 -0500
"Paul E. Jones" <paulej at PACKETIZER.COM> wrote:

> I just recently discovered that the ITU-T published a corrigendum to
> T.38 (1998) that apparently breaks backward compatibility for all T.38
> devices currently deployed that follow the original 1998 specifications.
...
> What I would like to know is whether this change impacts your product or
> not.  I am aware of many deployed devices on the market today that
> employs the 1998 syntax without this ellipsis.  I would like to find out
> to what extent this change is going to present problems for companies
> represented on various lists and, if I get sufficient support, I want to
> take a contribution to the ITU-T SG16 meeting in May to try to resolve
> this problem in such a way as to not break backward compatibility and
> interoperability.

>From the point of view of OpenH323 this was most assuredly a nasty and
annoying problem! Which version is used seems to vary from product to
product and even what version of product.

And more importantly, as far as I could tell, it is impossible to
determine at run time which version is in use. You must establish this a
priory and decode PDU's accordingly. And if a stack does not identify
itself sufficiently you get into all sorts of trouble. Not very good for
interoperability!

I am not sure what they could have done so you could tell at run time.
The only thing I could think of was to add a new Type-Of-Message data
enumeration type (eg t38-version2) which must be sent before any other
data PDU's. The receiving stack can then assume if it is there that it
uses the new syntax and if not the old.

This is not perfect though as the PDU's are sent by UDP and even with
some redundancy you cannot GUARANTEE it will work!



----------
Robert Jongbloed                        Quicknet Technologies, Inc.
Architect H.323 Open Source Library, Remote Worker (Research Group)
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