A Compound TCP Approach for High-speed and Long Distance Networks

Kun Tan, Jingmin Song, Qian Zhang, Murari Sridharan

MSR-TR-2005-86 |

Many applications require fast data transfer over high speed and long distance networks. However, standard TCP fails to fully utilize the network capacity due to the limitation in its conser-vative congestion control (CC) algorithm. Some works have been proposed to improve the connection’s throughput by adopting more aggressive loss-based CC algorithms. These algorithms, although can effectively improve the link utilization, have the weakness of poor RTT fairness. Further, they may severely de-crease the performance of regular TCP flows that traverse the same network path. On the other hand, pure delay-based ap-proaches that improve the throughput in high-speed networks may not work well in an environment, where the background traffic is mixed with both delay-based and greedy loss-based flows. In this paper, we propose a novel Compound TCP (CTCP) approach, which is a synergy of delay-based and loss-based approach. In CTCP, we add a scalable delay-based component into the standard TCP Reno congestion avoidance algorithm (i.e., the loss-based component). The sending rate of CTCP is controlled by both com-ponents. This new delay-based component can rapidly increase sending rate when network path is under utilized, but gracefully retreat in a busy network when bottleneck queue is built. Aug-mented with this delay-based component, CTCP provides very good bandwidth scalability with improved RTT fairness, and at the same time achieves good TCP-fairness, irrelevant to the win-dows size. We developed an analytical model of CTCP and imple-mented it on the Windows operating system. Our analysis and experiment results verify the properties of CTCP.