One More Bit Is Enough

Abstract — Achieving efficient and fair bandwidth allocation while minimizing bottleneck queue length and congestion-induced packet loss rate for high Bandwidth-Delay Product (BDP) networks has long been a daunting challenge. The current endto-end congestion control schemes including TCP and its many variants and the TCP+AQM/ECN proposals have significant limitations in achieving this goal. XCP, while achieving the goal, requires multiple bits (not available in the IP header) to exchange the congestion information between network and endhosts. Adding these bits to the IP header is a non-trivial and lengthy process. In this paper, we design and implement a simple, lowcomplexity algorithm called Variable-structure congestion Control Protocol (VCP) that leverages only the existing two ECN bits for network congestion feedback and yet achieves comparable performance as XCP, i.e., exponential convergence onto high utilization, low persistent queue length, negligible packet loss rate and reasonable fairness. However, VCP has significantly slower fairness convergence speed in comparison to XCP. Extensive ns2 simulations validate the performance of VCP for a wide range of high BDP scenarios. Additionally, VCP can also be extended to provide bandwidth differentiation. I