* with a response, the object reported will be non-null. If the request's completion
* is implied by a barrier, the object reported will be null.
*
+ * If this request fails on the remote device, {@link FutureCallback#onFailure(Throwable)
+ * will be called with an instance of {@link DeviceRequestFailedException}.
+ *
* @param xid Previously-reserved XID
* @param message Message which should be sent out, or null if the reservation
* should be cancelled.