extending existing object classes vs deriving new classes

["Luis P. Caamano" <[e-mail]>]

hello.  We need some opinions, pointers, lessons, hints, whatever
you have. :)

We are defining a new X.500 schema that among other things will
include unix administration support.  We still haven't decided about
the DSA implementation, that is, we still have the option to implement
our own DSA which could have our view of the directory schema. Of
course, whatever we do will be X.500 compliant.

While discussing the information that we would need to store/retrieve
in the directory, we have hit an impasse in deciding if we should
extend standard object classes with new attributes or derive new
objects from the standard object classes.

For instance, if we need to store some special attribute in say, the
Application Process object class, do we simply define in our private
schema the extra attributes as MAY CONTAIN or do we derive
MyApplication Process as a subclass of Application Process?

One of our goals is for our DSA to support as many DUAs as possible.

If we extend, other DUAs will query our DSA and get an ASN.1 encoded
object that will have strange attributes.  However, the DUA will know
about the object class.  Thus, it's possible that the DUA will simply
skip the unknown attribute while displaying the other ones.  However,
we are not sure if this is allowed by the standard.  After all, 
Application Process has an object identifier and that in itself might
carry the object semantics for some DUAs.  

On the other hand, if we derive, we are afraid that the DUA might not
be able to find out that MyApplication Process is just a derivation of
Application Process and then reject the whole object class.  We are
not sure how polymorphism works in the directory or how DUAs find
about new object semantics when using the 1993 extensions. 

Does anybody know about any other sources that talk about the
new extensions to X.500 (1993) for schema support other than
the X.500 document itself or Sara Radicati's X.500 book?

Are there any X.500 implementations that support subtrees, operational
attributes, and the like?

thanks a lot for your time on this, any help will be greatly
appreciated.

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Luis P. Caamano  (LC2385)             |           [e-mail]
SecureWare, Inc. Atlanta, GA          |           [phone]