Cross-purpose SEQUENCE/CHOICE protocol extension fields

[Hallvard B Furuseth <h.b.furuseth@usit.uio.no>]

There are two different ways to extend BindResponse and ModifyRequest.
I imagine this may lead to some sort of conflict or at least a rather
messy situation, in particular in which order the fields should occur:

1) If one defines a new BindRequest.AuthenticationChoice or
   ModifyRequest.changes.change.operation, one may need a new matching
   sequence element in BindResponse or ModifyRequest.changes.change.

2) One may define trailing SEQUENCE elements; [Protocol] section 4 says
   implementation must ignore unrecognized trailing SEQUENCE elements.

I don't know that works when there are several extra SEQUENCE elements
present:  Will an implementation pick up a known element following an
unknown element?  Can elements from different specifications be present
in any order?  Or must all specifications of such elements define where
in the SEQUENCE the element occurs in relation to all previous
specifications of other elements?

If this is not a problem, ignore the rest of this message.  Otherwise:
One fix might be to define (not in LDAPbis) two extensions to be used
with future specifications of Bind authentication choices and Modify
operation types:

        -- appended to BindResponse
        BindAuthData ::= [something] CHOICE {
             -- currently no choices are defined
        }

        -- appended to ModifyRequest.changes.change
        ModifyChangeData ::= [something] CHOICE {
             -- currently no choices are defined
        }

where each CHOICE tag number is reserved for the same
BindRequest.AuthenticationChoice tag number and
ModifyRequest.changes.change.operation value.

If this is desirable, I don't know if such a draft could - or should -
be rushed out quickly enough that [protocol] and [ldapiana] can add a
(maybe informative) reference to it?

For reference, here are the current ASN.1 definitions:

        BindResponse ::= [APPLICATION 1] SEQUENCE { 
             COMPONENTS OF LDAPResult, 
             serverSaslCreds    [7] OCTET STRING OPTIONAL }

        ModifyRequest ::= [APPLICATION 6] SEQUENCE { 
             object          LDAPDN, 
             changes         SEQUENCE OF change SEQUENCE { 
                  operation       ENUMERATED { 
                       add     (0), 
                       delete  (1), 
                       replace (2) }, 
                  modification    PartialAttribute } }

As far as I can tell there are no other protocol fields with similar
troubles, but I make no promises:-)

-- 
Hallvard