Along with the rapid progress of optical technologies, the transmission speed of LANs and public networks has been increased significantly. As a result, it has become possible for computers distributed geographically to communicate with each other with high throughput. However, the current protocols such as TCP/IP have some problems, especially the performance through long distance and wide bandwidth networks. Therefore, new protocols with new data transfer algorithms are required. In this paper, we describe an implementation of a high speed transport protocol by a user level library which interfaces to UDP/IP. This method uses only functions commonly provided by UNIX operating systems, and therefore, allows a new protocol to be developed easily and to be ported to other UNIX workstations easily. Our library called the transport library realizes the coordination of buffer managements in the application and the library, and low overhead and prompt handling of receive and timer interrupts in order to achieve high performance. Our implementation results show 32 Mbits/sec over an ATM network whose effective transmission speed is 36 Mbits/sec, regardless of the propagation delay from 0 to 200 msec. These values of throughput are better than those of the in-kernel TCP programs