Thanks for reply,
>
> Hi,
> If a SEQUENCE is extended then I understand that
> implication/impact of the OPTIONAL/DEFAULT keyword is for the
> encoder only. For that particular spec, the encoder will
> expect the mandatory elements following the extension
> (otherwise the specified value may be considered invalid -
> strictly).
I agree.
> It is possible that the sender complies to the
> same SEQUENCE but with no elements following the extension
> marker (the sender having a previous version of the
> specification). Hence as far as the decoder is concerned, it
> would have to consider all elements following the extension
> as OPTIONAL (of course not the individual elements of an
> extension group, when only the group itself would be
> optional).
Right. I don't know yet how to deal with this, but that's in the future:-)
> As for whether to decode the tag or not (for
> trailing elements after the last mandatory element- NOTE that
> elements following the extensions are considered non
> mandatory) will depend of the remaining length of the
> SEQUENCE value after decoding till the last mandatory
> element.
Yes, I didn't realize this.
> If in the spec an element of the extension addition
> is mandatory, then the decoder must assume the possibility of
> the absence of the same (remember that this mandatory nature
> applied to the encoder only). The remaining length (if kept
> tab on) will ward off the factor of no. of tags to be
> considered that was mentioned, or atleast limit it to the
> SEQUENCE at hand, which is leaves the problem bounded.
Yes, the problem is bounded in the SEQUENCE. But still... Look at the example:
If the decoder will have this structures:
S ::= SEQUENCE {
a INTEGER,
b BOOLEAN OPTIONAL, --version 1
...
c C OPTIONAL
}
C ::= CHOICE {
ca INTEGER,
cb OCTET STRING,
}
Then, when it recieves new version:
S ::= SEQUENCE {
a INTEGER,
b BOOLEAN OPTIONAL,
...
e ENUMERATED -- version2
...
c C OPTIONAL
}
Then before skipping e, it must check the tag with tags of both - ca and cb.
And what if C could have extensions too?
C ::= CHOICE {
ca INTEGER,
cb OCTET STRING,
...
}
Is this incorrect for Asn1 or is there a way to handle such cases?
Thanks
Vit Novak
