# Ad-BBR: Enhancing Round-Trip Time Fairness and Transmission Stability in TCP-BBR
## 1 Introduction
Although Google's official versions--BBRv2 and BBRv3--aim to improve the adaptivity and fairness of the BBR algorithm, they suffer from significant throughput degradation compared to BBRv1 [10; 11]. While these modifications do enhance congestion control, they also deteriorate overall link performance. Similarly, several unofficial variants, such as BBQ--which suppresses the sending rate of long RTT streams [12]--have been shown, through analysis and experiments [13; 14], to possess inherent limitations. Moreover, BBR-PG [15] improves overall fairness but lacks fine-grained adjustment, resulting in persistent bandwidth wastage. To the best of our knowledge, few studies have successfully balanced RTT fairness with throughput performance, achieving significant fairness enhancements without substantially compromising throughput. This paper analyzes the limitations of BBR's design mechanisms from a theoretical perspective and proposes the Ad-BBR algorithm. This algorithm incorporates real-time RTT and link queue-state monitoring, as well as introduces a new RTprop determination mechanism and finer-grained adaptive rate adjustment based on the RTT to mitigate over-injection and improve RTT fair ness. Moreover, the ProbeRTT phase trigger mechanism is updated to enhance the stability and smoothness of data transmission. The proposed approach aims to offer a practical optimization solution for congestion control in high-speed wireless network environments. Inspired by these motivations, this paper presents the following contributions:  
1. This paper analyzes the data over-injection phenomenon, the long- and short-flow RTT unfairness, and some of the unnecessary ProbeRTT phase triggers in the BBR congestion control algorithm through formula derivation and theoretical proofs. 2. The Ad-BBR algorithm is proposed to mitigate the data over-injection issue and enhance RTT fairness in multi-stream contention scenarios. This is achieved by replacing the RTprop with a dynamically varying \(nRTT\) and adaptively adjusting the sending rate based on the real-time RTT and \(nRTT\). Additionally, a new ProbeRTT phase entry logic is introduced to reduce unnecessary send-rate degradation and bandwidth waste. 3. The function and performance of the Ad-BBR algorithm are verified through NS3 and 5G and Wi-Fi simulation network experiments, and the proposed algorithm in this paper is evaluated in terms of the throughput, RTT fairness, retransmission rate, and latency, respectively, and the results show that Ad-BBR achieves higher throughput, better fairness, more stable data transmission, and improves the multi-stream contention in most cases of congestion control performance in multi-stream competition scenarios. The remaining sections of this paper are organized as follows: Section 2 reviews existing research on optimizing the BBR congestion control algorithm. Section 3 discusses the principles of the BBR algorithm and the motivation for its improvement. Section 4 provides a detailed description of the design and structure of Ad-BBR. Section 5 presents experimental results and analysis, while Section 6 concludes the paper.