FaiRTT: An Empirical Approach for Enhanced RTT Fairness and Bottleneck Throughput in BBR

TL;DR

The paper tackles RTT fairness between elephant and mice flows under BBR v2 in beyond-5G networks by identifying intra-protocol fairness gaps that persist despite improvements over BBRv1. It introduces FaiRTT, an RTT-driven algorithm that dynamically adjusts the BDP-based sending rate using flow-specific RTT measurements and an adjustment coefficient derived from lastRTT and minRTT, formalized through expressions such as $\text{Inflight}_t = \text{BDP}_t \times \text{cwnd\_gain}$ and $\alpha_t = Wfcount_t \cdot \frac{\sum_{j\in W_t} WminRTT_j}{W_t}$ with $\beta=0.8$ and $\gamma=0.99$. The authors validate FaiRTT with NS-3 simulations, showing an average elephant/mice throughput ratio of $1.08$, a Jain's fairness index around $0.98$, and bottleneck utilization near $98.78\%$, outperforming BBRv2 across diverse queue sizes and RTT pairs. These results suggest a practical pathway to improved fairness and network utilization in NGN environments without sacrificing bottleneck throughput. The work contributes a concrete, per-flow BDP adjustment mechanism grounded in RTT measurements and demonstrates its effectiveness for equitable bandwidth sharing in complex, heterogeneous traffic scenarios.

Abstract

In next-generation networks, achieving Round-trip Time (RTT) fairness is essential for ensuring fair bandwidth distribution among diverse flow types, enhancing overall network utilization. The TCP congestion control algorithm -- BBR, was proposed by Google to dynamically adjust sending rates in response to changing network conditions. While BBRv2 was implemented to overcome the unfairness limitation of BBRv1, it still faces intra-protocol fairness challenges in balancing the demands of high-bandwidth, long-RTT elephant flows and more frequent short-RTT mice flows. These issues lead to throughput imbalances and queue buildup, resulting in elephant flow dominance and mice flow starvation. In this paper, we first investigate the limitations of Google's BBR algorithm, specifically in the context of intra-protocol RTT fairness in beyond 5G (B5G) networks. While existing works address this limitation by adjusting the pacing rate, it eventually leads to low throughput. We hence develop the FaiRTT algorithm to resolve the problem by dynamically estimating the Bandwidth Delay Product (BDP) sending rate based on RTT measurements, focusing on equitable bandwidth allocation. By modeling the Inf light dependency on the BDP, bottleneck bandwidth, and packet departure time after every ACK, we can resolve the intra-protocol fairness while not compromising the throughput on the bottleneck link. Through extensive simulations on NS-3 and comprehensive performance evaluations, FaiRTT is shown to significantly improve the fairness index and network throughput, significantly outperforming BBRv2, for diverse flow types. FaiRTT achieves an average throughput ratio of 1.08 between elephant and mice flows, an average fairness index of 0.98, and an average utilization of the bottleneck link of 98.78%.

Figures (5)

• Figure 1: BBRv2 Phases and Flow Diagram.
• Figure 2: Experimental network topology.
• Figure 3: Throughput Comparison.
• Figure 4: Fairness Index.
• Figure 5: Bottleneck Link Utilization of FaiRTT vs. BBRv2.