T.38 potentially broken by recent Corrigendum

Paul E. Jones paulej at PACKETIZER.COM
Sat Mar 15 19:53:39 EST 2003


MessageKevin,

Indeed, it was a typo made by somebody, but whatever effort was made to inform all vendors was insufficient as I have yet to find a single vendors actually confess to be shipping T.38 products with the 2002 syntax.

There are no implementers guides or other public documents available on the ITU web site on the topic.  The first published document available to the public was dated May 2001.  That's 3 years following standardization which, in my opinion, is far too late to be making elementary ASN.1 changes-- even if there was good intention.

I think we have to resolve this issue through some kind of versioning of the protocol.

Paul

  ----- Original Message ----- 
  From: Kevin Boyle 
  To: ITU-SG16 at echo.jf.INTEL.COM 
  Sent: Friday, March 14, 2003 9:15 AM
  Subject: Re: T.38 potentially broken by recent Corrigendum


  All,

  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.

  Kevin
    -----Original Message-----
    From: Klaghofer Karl ICN EN HS D 11 [mailto:karl.klaghofer at SIEMENS.COM] 
    Sent: Friday, March 14, 2003 3:44 AM
    To: ITU-SG16 at echo.jf.INTEL.COM
    Subject: AW: T.38 potentially broken by recent Corrigendum


    Paul,

    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. 

    Regards,
    Karl Klaghofer
    Siemens
      -----Ursprüngliche Nachricht-----
      Von: Paul E. Jones [mailto:paulej at PACKETIZER.COM]
      Gesendet: Montag, 3. März 2003 03:21
      An: ITU-SG16 at echo.jf.INTEL.COM
      Betreff: 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=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
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