Applications, such as grid computing, process their data over wide-area networks. The schedule of these applications is influenced by the duration of data transfer. Guaranteeing the performance of data transfer is helpful to optimize the schedule of these applications. Diffserv AF is an efficient QoS control method to guarantee the duration of data transfer, but the resource is limited in many network environments. Conventional data transfer methods require much greater network resource than the target rate of data transfer, or it is hard to implement their algorithms into applications. We propose a data transfer middleware that is easy to use and improve the utilization of network bandwidth using plugin for Linux kernel. We evaluate the performance of the algorithm of our middleware with simulation. The algorithm of our middleware requires much smaller network bandwidth to guarantee the duration of data transfer.