Message

Glenn Parsons, Dave Duehren and I have all pointed out that the ellipsis was supposed to be

there but was dropped by the TSB during publication. We had a Rapporteur's/ Working Party meeting in

in Fall 1999 that confirmed the need to reinstate it. The driver for the reinstatement was based on

the need we foresaw to extend T.38 to support new features such as V.34 fax.

And yes, we all tried to let people know about the issue at the time. But, it did take a while

for the ITU to document the change, which has created today's issue. In a related vein,

the fax question also saw that there were variations emerging as new T.38 features emerged, such

as the TPKT support for TCP/IP, so we agreed to start adding a version to the T.38.

New implementations will need to recognize that there are different versions of T.38 in order to gain

proper interoperation. IP call control will also need to expose versions during negotiations if

interoperability is to be assured.

Given where we are, the relevant questions will need to sort this out at the next SG16 meeting

in May. Gaining a rough consensus on how to resolve it before then would be a good thing.

It is my understanding that the ellipsis was intended to be there, and that it wasn't until folks started implementing that it was noticed that it got dropped somewhere between the editor's hands and publication. There was a TD in the Feb 2000 meeting noting this typo. I can produce it if anyone wants to see it.

It is also my understanding that care was taken to ensure that all known vendors were made aware of the change at the time.

I fully agree with you, that the T.38 (1998) asn.1 and the Revision of T.38 (2002) asn.1 needs to be backwards compatible.

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=T-REC-T.38

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,

Paul E. Jones