NAME

       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.