Interoperability Problems due to Open Type encoding of Null types (fwd)

Bancroft Scott baos at OSS.COM
Wed Jan 17 23:57:44 EST 2001


On Wed, 17 Jan 2001, Manoj Paul wrote:

> Experts,
>
>   We have come across a specific problem which has been the root
> cause of interoperability failures across different implementations
> of H.323 stack. The problem is related to the
>  1) Encoding and decoding of NULL data types when they appear as
>    extension IEs.
>  2) Encoding and decoding of extension IEs which have NULL as the
>    outermost ASN type.
>
> There have been different interpretations of the PER rules that apply
> to the above conditions causing interop failures at encode/decode level.
> Although the implementations would encode/decode (1) and (2) according
> to Open Type elements, some subtle differences come as mentioned below:
>
>   In case of (1), some implementations would not encode an extra 0x00
> octet for NULL data type and would just encode the length of the open type
> element as 0x00. This idea probably stems from the fact that a NULL
> data type has no encoding when they appear in the root of the ASN.1 type.
> However, as per 10.1.3 and 10.2.1 of X.691, the correct encoding for
> such a case would be  0x01 0x00, where 0x01 is the uncosntrained length
> of the NULL data type encoding and 0x00 is the value of NULL data type in
> extension.

Your understanding of the encoding is correct.

>  In case of (2), some implementations would encode the extension IE as per
> it's rules (e.g as a Choice/Sequence etc.), but would not add a 0x00 for
> the outermost NULL data type in it's encode tree. This would change the
> value of length field to be appended to this open type. Whereas some
> implementations do add 0x00 octet for the outermost NULL data type.
>
> There appears to be a confusion as to what is meant by the "outermost"
> value in 10.1.3 of X.691. I would appreciate clarification on this
> clause as it would save a lot of interop overhead at encode/decode level.

"outermost" value refers to a value of a type that is either directly
nested in an open type or is a complete application message unto itself.
For example, if you had:

        Foo ::= SEQUENCE {
                b       BOOLEAN,
                v       TYPE-IDENTIFIER.&Type,
                ...,
                i       NULL
        }

        bar  Foo ::= {b TRUE, v NULL : NULL, i NULL}

The value of Foo, bar, is an outermost value. The value of v (NULL : NULL)
is encoded as an outermost value, so in this case since NULL encodes to 0
bits it is replaced with eight 0-bits, and this is prefixed with a length
of 1.  Similarly, the value of i is encoded as an outermost value, so the
resulting encoding in this case will have a length of 1 followed by eight
0-bits.

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