Less Is Fair: Reducing RTT Unfairness Through Buffer Sizing

Highlights: What are the main findings? Smaller buffers can substantially improve RTT fairness without degrading throughput. Bandwidth sharing is primarily governed by buffer size and RTT. What is the implication of the main finding? Buffer sizing is not just about utilization, but fairness. Smarter buffer provisioning strategies can reduce the need for RTT-aware congestion control algorithms. Sharing bottleneck bandwidth among TCP flows with diverse round-trip times (RTTs) remains a persistent challenge. This study investigates RTT unfairness and evaluates the behavior of two widely deployed congestion control algorithms, TCP Cubic and TCP BBR, under a variety of scenarios. The main objective is to better understand the underlying causes of RTT-based throughput disparity and to identify network configurations that promote fair bandwidth sharing. Using the Mininet emulation platform, extensive experiments were conducted to examine the effects of buffer size, sender distribution, and delay asymmetry on transmission performance metrics. The results show that while TCP BBR achieves high utilization with minimal buffering, its fairness depends on the interaction between RTT and buffer size. On the other hand, TCP Cubic achieves better fairness when moderate buffer sizes are provisioned and bandwidth imbalance is driven mostly by RTT ratio. These findings suggest that careful buffer sizing can reduce RTT unfairness and highlight the broader impact of queuing strategies on network performance.