With the decision to make the sessions functionality mandatory to implement
in NFSv4.1, the minor versioning rules have  been modified.  There needs
to be clear specification of what operations and resultant functionality is
mandatory to implement; this applies to both the client and server.

In the case of the client, the most interesting case relates to the
callback operations used for the pNFS and directory delegations and 
notifications functions.
For these, if the client uses new optional behavior it is a requirement
that some of the callback operations must be available to the server.

It would also be good to have a clear statement about mandatory/optional
to implement operations that were present even in NFSv4.0 to ensure that
new implementors understand the requirements.