To insure that changes to the protocol are backward-consistent with
pre-existing servers/clients, the following rules should be followed:

a) Any new status/request codes need to be added to the end of the list of 
   existing codes.

b) Only reserved fields may contain new information (i.e., we can't change
   the definition of existing fields except to add new status bits to an
   existing flag).

c) By extension, structure sizes or layout may not change for any existing
   structure passed to the server.

Any other addition is fair game since the server doesn't really care.
However, additions to the protocol defined here may conflict with future
versions should a future version use an added definition for some other
purpose. To prevent this from happening, please feedback your changes to
the XRootD collaboration via https://github.com/xrootd so that your changes,
upon acceptance, are reflected in the official protocol definition.
