Improving TCP congestion control over Internets with heterogeneous transmission media

We present a new implementation of TCP that is better suited to today's Internet than TCP Reno or Tahoe. Our implementation of TCP, which we call TCP Santa Cruz, is designed to work with path asymmetries, out-of-order packet delivery, and networks with lossy links, limited bandwidth and dynamic changes in delay. The new congestion-control and error-recovery mechanisms in TCP Santa Cruz are based on: using estimates of delay along the forward path, rather than the round-trip delay; reaching a target operating point for the number of packets in the bottleneck of the connection, without congesting the network; and making resilient use of any acknowledgments received over a window, rather than increasing the congestion window by counting the number of returned acknowledgments. We compare TCP Santa Cruz with the Reno and Vegas implementations using the ns2 simulator. The simulation experiments show that TCP Santa Cruz achieves significantly higher throughput, smaller, delays, and smaller delay variances than Reno and Vegas. TCP Santa Cruz is also shown to prevent the swings in the size of the congestion window that typify TCP Reno and Tahoe traffic, and to determine the direction of congestion in the network and isolate the forward throughput from events on the reverse path.