With the advent of computational grids, networking performance over the wide-area network (WAN) has become a critical component in the grid infrastructure. Unfortunately, many high-performance grid applications only use a small fraction of the available bandwidth because operating systems and their associated protocol stacks are still tuned for yesterdays WAN speeds. As a result, network gurus undertake the tedious process of manually, tuning system buffers to allow TCP flow control to scale to today's WAN grid environments. Although recent research has shown how to set the size of these system buffers automatically at connection set-up, the buffer sizes are only appropriate at the beginning of the connection's lifetime. To address these problems, we describe an automated and scalable technique called dynamic right-sizing. We implement this technique in user space (in particular for bulk-data transfer) so that end users do not have to modify the kernel to achieve a significant increase in throughput.