CTCP: Improving TCP-Friendliness Over Low- Buffered Network Links

Abstract—Compound TCP (CTCP) is a sender-side improve-ment for TCP in high-speed and long distance networks. CTCP is a synergy of delay and loss-based congestion control, effectively combining them to achieve good efficiency, RTT fairness and TCP-friendliness. However, CTCP requires detecting incipient congestion effectively by estimating the backlogged packets and comparing it to a pre-defined parameter gamma. Choosing the appropriate value for gamma could be a problem because this parameter depends on both network configuration and the num-ber of concurrent flows, which are generally unknown to the end-systems. As a consequence, when operating over under-buffered links, CTCP may demonstrate poor fairness to regular TCP flows that may be comparable to HSTCP. In this paper, we present a novel technique that automatically tunes CTCP parameters so that it greatly improves the TCP-friendliness of CTCP over under-buffered links. This new tech-nique, called Tuning-By-Emulation (TUBE), dynamically esti-mates the average queue size for a regular TCP flow, and based on which sets the parameter gamma. This way CTCP can effec-tively lower gamma on under-buffered links to keep good TCP-friendliness, and alternatively increases gamma if the link buffer is sufficient to ensure high throughput. Our extensive packet-level simulations and test-bed experiments on a Windows implementa-tion confirm the effectiveness of CTCP-TUBE. Index Terms — high-speed network, congestion control, com-pound TCP I