AW: H.450.1 ASN.1 interoperability issue

Horvath Ernst ernst.horvath at SIEMENS.COM
Mon May 13 08:48:45 EDT 2002


Mark,

there was no intention to change any ASN.1 definition in H.450.1. The note
was added just to make implementers aware that the InvokeID is encoded
differently in the "invoke" APDU and in the "returnResult/returnError"
APDUs: in the first case it is constrained by InvokeIdSet, in the second
case it is not constrained. This is not an incosistency, just a consequence
of the way Packed Encoding Rules were designed.

Please also note that "InvokeIdSet" is used in two different meanings: in
table 4 it is the name of a parameter of the parameterized structure ROS
(i.e. a placeholder), whereas in table 3 it is an "actual" value-set of
integers. 

The brackets in the new note (InvokeIdSet = 0..65535) are not an ASN.1
definition, just an indication what the effective constraint is, namely
0..65535. Table 3 is still the (unchanged) formal definition of the actual
"InvokeIdSet".

Ernst Horvath
Siemens AG

> -----Ursprüngliche Nachricht-----
> Von: Mark Brown [mailto:Mark.Brown at ED.ACULAB.COM]
> Gesendet am: Freitag, 10. Mai 2002 17:29
> An: ITU-SG16 at echo.jf.INTEL.COM
> Betreff: H.450.1 ASN.1 interoperability issue
> 
> We've been experiencing some fairly serious interoperability problems
> with some systems due to confusion over what exactly the ASN.1 for
> H.450.1 is and were hoping someone could clarify things for us.  The
> problem stems from section 6.6.1.3 of the V4 implementors guide which
> adds a note below table 4:
> 
>    In the Invoke APDU, the invokeID is an INTEGER constrained by a
>    PER-visible constraint (InvokeIdSet = 0..65535) and is therefore
>    encoded as a *constrained* INTEGER (16 bits, no length field).  In
>    the ReturnResult and ReturnError APDUs, however, the invokeID is
>    encoded as an *unconstrained* integer (with explicit length field)
>    because the applicable constraint... is not PER-visible.
> 
> However, in H.450.1 table 3 (on page 8) defines InvokeIdSet to be:
> 
>    InvokeIdSet     INTEGER ::= {InvokeIds,...}
>    InvokeIDs   ::= INTEGER (0..65535)
> 
> which does not gel with the text in the IG and causes interoperability
> issues since the two versions of the ASN.1 for InvokeIdSet are not
> encoded identically.  This text appears to be unchanged from the
> prepublished edition on Packetizer.
> 
> Our understanding of the text added in the IG is that the 
> intent was to
> emphasise that the encoding of the invokeID is not consistent between
> the various APDUs and that the no change to the ASN.1 was intended.
> This is particularly the case since the IG doesn't modify table 3 as
> would be expected if it were correcting the ASN.1 there.  Others read
> things differently and are using the definition of 
> InvokeIdSet given in
> the IG.
> 
> What is the intention here?  If the intention is to redefine the ASN.1
> then some clarification as to why this was done would be very much
> appreciated.
> 
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