# Ad-BBR: Enhancing Round-Trip Time Fairness and Transmission Stability in TCP-BBR
## 5 Experiment and Evaluation
### RTT Fairness
#### 5.3.1 Static Network Experiments  
The static network experiments test the static competitive fairness of Ad-BBR in NS3 simulation and 5G and Wi-Fi simulation network environments, respectively. In the NS3 simulation experiments, bottleneck buffer sizes of 0.5 BDPs and 5 BDPs were configured. Two streams with round-trip times (RTTs) of 10 ms and 50 ms were introduced simultaneously to compete for a 1 Gbps bottleneck bandwidth. The throughput difference between the two streams was compared under the control of various congestion control algorithms over a duration of 100 s. [PERSON]'s fairness index was employed to quantitatively evaluate RTT fairness. [PERSON]'s fairness index quantifies the RTT fairness based on the throughput achieved by the 10 ms and 50 ms streams. A value closer to 1 indicates that the throughput of the two streams is more balanced, reflecting a higher level of fairness under the control of the evaluated algorithm. Figures 8 and 9 display the real-time throughput trends of each algorithm under different buffer sizes, while Figures 10 and 11 present the corresponding average throughput and fairness index results. These figures clearly indicate that as the buffer size increases, the total throughput of all algorithms also increases. For packet loss-based algorithms such as CUBIC and Reno, the 10 ms stream achieves higher throughput than the 50 ms stream; however, the throughput difference between the short and long streams does not significantly increase with larger buffers. Among the BBR-based algorithms, BBRv1 exhibits a dramatic increase in the throughput of the 50 ms stream as the buffer size grows, while the throughput of the 10 ms stream declines, thereby widening the throughput gap and further deteriorating fairness. In contrast, for BBRv3 and BBQ, the fairness difference between the 10 ms and 50 ms streams is smaller; the 10 ms stream throughput under BBRv3 is even slightly higher than that of the 50 ms stream, and the fairness index differences between BBRv3 and BBQ are minimal. Under a 5 BDP buffer, the fairness index of BBQ is slightly higher than that of BBRv3. The Ad-BBR algorithm consistently maintains a narrow average throughput difference. Although its fairness index may not be the highest in every metric, overall, the throughput difference between the 10 ms and 50 ms streams is much smaller, resulting in more stable transmission and a higher overall fairness index in the static fairness tests. This portion of the static network experiment was repeated in 5G and Wi-Fi simulation environments to evaluate the throughput ratio and fairness index of the 10 ms and 50 ms  
Figure 11: Average throughput and [PERSON]’s fairness index for 10 ms RTT flows and 50 ms RTT flows with 5 BDP buffer. Figure 10: Average throughput and [PERSON]’s fairness index for 10 ms RTT flows and 50 ms RTT flows with 0.5 BDP buffer. Figure 9: Comparison of real-time throughput for 10 ms RTT flows and 50 ms RTT flows with 5 BDP buffer. streams. The throughput ratio is defined as the proportion of the 10 ms stream's throughput to the total throughput. Since only the 10 ms and 50 ms streams are present on the link, a throughput ratio close to 0.5 indicates that the two streams achieve nearly equal throughput, suggesting that their bandwidth acquisition capability is not significantly influenced by their RTT differences. The experimental results are presented in Figures 12 and 13. In the static network experiments, the BBRv1 algorithm exhibited the most pronounced competitive unfairness in both 5G and Wi-Fi simulation environments, with the throughput of 10 ms streams accounting for only one-third of the total throughput. In contrast, the BBQ  
Figure 12: Throughput ratio and [PERSON]’s fairness index with 0.5/5 BDP buffer in 5G network simulation. Figure 13: Throughput ratio and Jain’s fairness index with 0.5/5 BDP buffer in Wi-Fi network simulation.