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       dd_driver - Data descriptors globus_xio uses data descriptors to
       associate meta data with the data being writen or the data read.

       Data descriptors flow into the drivers read and write interface
       functions by way of the operation structure. If the driver is
       interested in viewing the data decriptor it can request it from the
       operation structure via a call to
       globus_xio_driver_operation_get_data_descriptor() and it can view any
       driver specific data descriptor via a call to
       globus_xio_driver_data_descriptor_get_specific(). The driver can modify
       values in the data descriptor by setting values before passing the
       request down the stack. Several functions are available to modify the
       data descriptors. There is no need to ’set()’ the data descriptors back
       into the operation. The functions for manipluating the values in a DD
       affect the values xio has directly.

       Data descriptors flow back to the driver in the callbacks for the data
       operations. When calling finished operation on a data operation the
       driver must pass in a data descriptor. It should get this data
       descriptor from the io operation callback.

       Life Cycle:

       Passing in a data descriptor: A data descriptor is first created by the
       globus_xio user. The user can add driver specific data descriptors to
       it. Once the usre has created and set the attributes on its data
       descriptor to their liking they pass it into a globus_xio data
       operation (either read or write). When the data descriptor is passed on
       globus_xio will make an internal copy of it. It does this by first
       coping the user the level data descriptor and then walkinging through
       the list of driver specific data descriptor contianed in to and
       requesting the the driver make a copy of the driver specific data
       descriptor. If ever a driver specific data descriptor is NULL
       globus_xio need not call into its drivers dd_copy function. If ever the
       user level data descriptor is NULL globus_xio need not deal with the
       data descriptor functionality at all.

       A data descriptor coming back up the stack Once an io operation reachs
       the transport driver (the bottom of the stack) it takes on a slightly
       different role. On the way in it is describing what is requested to be
       done with the data, on the way out it is describing what has actually
       been done. Once the transport driver performs the operation it should
       adjust the data descriptor to reflect what has actually happened (few
       drivers will need to worry about this). Each driver on the way up can
       adjust the data descriptor and its driver specific data decriptor. When
       xio reachs the the top of the stack it calls a user callback. When that
       callback returns all memory associated with the data descriptor is
       cleaned up. The interface function
       globus_xio_driver_data_descriptor_free() is used for this.