With the widespread arrival of bandwidth-intensive applications such as bulk-data transfer, multi-media web streaming and computational grids for high-performance computing, networking performance over the wide-area network has become a critical component in the infrastructure. Tragically, operating systems are still tuned for yesterday's WAN speeds and network applications. As a result, a painstaking process of manually tuning system buffers must be undertaken to make TCP flow-control scale to meet the needs of today's bandwidth-rich networks. Consequently, we propose an operating system technique called dynamic right-sizing that eliminates the need for this manual process. Previous work has also attacked this problem, but with less than complete solutions. Our solution is more efficient, more transparent, and applies to a wider set of applications, including those that require strict flow-control semantics because of performance disparities between the sender and receiver.