# Ad-BBR: Enhancing Round-Trip Time Fairness and Transmission Stability in TCP-BBR
## 2 Related Work  
In 2016, Google introduced the BBR congestion control algorithm, which redefined the concept of congestion control [3]. It introduced the delayed bandwidth product (BDP) and calculated the BDP value by alternately measuring the link's BitBw and RTprop, ensuring the amount of inflight data remains close to the BDP value. This approach also solved the problem of identifying Kleinrock's optimal operating point [16]. BBR's operational method fundamentally differs from traditional algorithms that rely on packet loss, marking a shift in congestion control strategies. As a policy-based algorithm, BBR uses a feedback-driven mechanism that dynamically adjusts the congestion window in real time, optimizing the use of available bandwidth to improve performance. However, many studies have shown that BBR seems to have some unfairness problems. [PERSON] et al. [17] tested its fairness by building a testbed and observed that long RTT streams consistently outperform short RTT streams, especially during bandwidth allocation. To address this, they proposed a bottleneck queue buildup suppression algorithm that limits the flight cap to below 2 BDPs when a bottleneck queue forms, thereby suppressing excessive data transfers and minimizing the transmission queue. If no bottleneck is detected, the flight cap is set to 1 BDP. This was the first attempt to address the RTT fairness problem in the BBR algorithm. Although it improves RTT fairness to some extent, the convergence remains slow and unstable, and setting the upper limit to 1 BDP significantly impacts the total throughput. [PERSON] et al. [12] found, through extensive experiments, that the probing duration of long RTT streams is always longer than that of short RTT streams. This results in long RTT streams dominating the transmission queue and increasing the likelihood of data pileup. To address this, they proposed an algorithm that limits the probing duration of long RTT streams, reducing their bandwidth pre-emption and consumption while improving the competition fairness between long and short RTT streams. However, this algorithm is only effective when a small number of streams compete for bandwidth. Its performance significantly degrades when multiple streams compete simultaneously. The BBQ algorithm improves RTT fairness and reduces latency by reconfiguring the _pacing_gain_ cycle's first phase with additional conditional constraints [12]. Meanwhile, [PERSON] et al. [18] identified high packet loss, high retransmission rates, and throughput unfairness in the BBR algorithm. To address these issues, they developed an algorithm that reduces the congestion window (CWND) by half and decreases the transmission rate by 25% under high network congestion and packet loss. However, this approach performs poorly for delay-sensitive applications and services. [PERSON] et al. observed that BBR algorithms often transmit excessive data, causing an imbalance in throughput across different RTT streams. To address this, they proposed the delay-aware BBR algorithm (DA-BBR), which improves RTT fairness and throughput by controlling the client's transmission rate and congestion window (CWND) based on the relationship between the RTT and RTprop [19]. [PERSON] et al. noted that many existing algorithms use a fixed congestion window gain of 2, which results in smaller congestion windows for short RTT streams compared to long RTT ones. To overcome this, they developed the BBR adaptive congestion window (BBR-ace) algorithm, which dynamically adjusts the congestion window gain based on the sending rate and RTT [20]. [PERSON] et al. [21] proposed the BBR-EFRA algorithm to address competitive fairness issues between long and short RTT streams, as well as the potential for the malicious exploitation of fairness loopholes in buffer queue states. [PERSON] [22] introduced an algorithm for dynamically adjusting pacer_gain based on packet loss rates and congestion levels in wireless networks. [PERSON] et al.