T.38 potentially broken by recent Corrigendum
Folks,
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. The particular document in question is Corrigendum 1, which was approved in 2000 and subsequently published in May 2001. You can download the corrigendum document for free from this site: http://www.itu.int/rec/recommendation.asp?type=folders&lang=e&parent...
The issue at hand is a change in the ASN.1 syntax found in Annex A/T.38. The particular data structure is shown here:
Data-Field ::= SEQUENCE OF SEQUENCE { field-type ENUMERATED { hdlc-data, hdlc-sig-end, hdlc-fcs-OK, hdlc-fcs-BAD, hdlc-fcs-OK-sig-end, hdlc-fcs-BAD-sig-end, t4-non-ecm-data, t4-non-ecm-sig-end, ... }, field-data OCTET STRING (SIZE(1..65535)) OPTIONAL }
The syntactical change was the addition of an ellipsis at the end of the ENUMERATED "field-type". In the published 1998 specification, this ellipsis did not exist.
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.
I am looking forward to your replies.
Kindest Regards,
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ For help on this mail list, send "HELP ITU-SG16" in a message to listserv@lists.intel.com Paul E. Jones
On Sun, 2 Mar 2003 21:20:46 -0500 "Paul E. Jones" paulej@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!
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participants (2)
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Paul E. Jones -
Robert Jongbloed