# On the Fairness of Internet Congestion Control over WiFi with Deep Reinforcement Learning
## 5 Experimental Methodology, Results, and Discussion
### Scenario 3: BBR vs. PCC-BBR  
This analysis explores Scenario 3, where PCC and BBR compete for bandwidth. While PCC initially offered higher throughput for Client 2 (infer throughout fairness \(Th_{\textit{ratio}}\)), significant fluctuations indicated potential instability. Switching Client 2 to BBR resulted in more stable performance for both clients across key metrics: throughput, RTT, jitter, and packet loss. Initially, the competing CCA, PCC, provided Client 2 with significantly higher throughput, exceeding 44 Mbps on average, as seen in Figure 13a. However, this advantage came with substantial fluctuations in PCC's throughput, hinting at potential instability in its congestion control mechanism. Figure 11: Jitter comparison of Client 1 (PCC) and Client 2 (BBR\(>\)PCC) in milliseconds over time before and after switching at 25 s. Few fluctuations were observed for Client 1 (PCC) before switching (**a**). Median jitter remained near 1 ms for Client 1 after Client 2 switched to BBR\(>\)PCC (**b**). Figure 12: Packet loss ratio comparison of Client 1 (PCC) vs. Client 2 (BBR\(>\)PCC) before and after switching at 25 s. The packet loss ratio for Client 1 (PCC) decreased from 48.97% to 9.22% after switching to Client 2 (PCC) in (**a**). The packet loss distribution for Client 2 (BBR), later PCC, had a median value of around 50% before dropping to nearly 10% (**b**). Client 2 switched from PCC to BBR, yielding positive impacts on network performance for both clients across several key metrics. Notably, Client 2 (BBR) maintained a stable throughput above 40 Mbps, demonstrating the effectiveness of BBR in achieving good throughput while ensuring stability. Interestingly, considering Figure 13b, even Client 1 (BBR) experienced improvement, with its median throughput rising to 40 Mbps. This suggests that the overall network congestion was better managed after the switch, potentially due to BBR's identical strategy to adapt network variation and optimize resource allocation. Prior to the switch, Client 2 (PCC) exhibited significant spikes in RTT in Figure 14a, reaching as high as 104 ms at times. These fluctuations likely stemmed from the instability in PCC's throughput. After the switch, both clients displayed consistently lower and more stable RTs under 15 ms. Similarly, jitter in Figure 15, which measures the variation in RTT, decreased significantly for both clients, with Client 2 (BBR) dropping to below 2 ms. This jitter reduction highlighted the benefit of stable throughput in maintaining consistent data flow and minimizing delays. Figure 14: RTT variation over time: Client 2 (PCC) peaks at 104 ms at 3 s, dropping to 4 ms; Client 2 (PCC->BBR) ranges from 155 ms to 4 ms. Both clients maintained RTTs below 15 ms (**a**). Post-switch to Client 2 (BBR), RTT variation and median values decreased (**b**). Figure 13: Comparison of throughput and sending rate in Mbps over time before and after switching Client 2 (PCC->BBR) for an understanding of throughout fairness \(Th_{ratio}\). Before switching to BBR at 25 s, Client 2 (PCC) had an average throughput of 44 Mbps, double that of the competing Client 1 (BBR) flow. After switching, Client 2 (BBR) received over 40 Mbps (**a**). The median value for Client 1 increased from 15 Mbps to 40 Mbps (**b**). The observed improvements can be explained by considering the interrelationships between network performance metrics. High and unstable throughput in PCC could lead to buffer overflows, causing packet drops in Figure 16 and increased jitter in Figure 15. BBR's focus on congestion avoidance likely contributed to its lower initial throughput but resulted in greater stability across all metrics. This aligns with the established principle that stable throughput, even if slightly lower than the peak offered by an unstable CCA, leads to more efficient data flow and minimizes negative impacts on other metrics. Client 2 (PCC) also experienced a higher packet loss ratio compared to Client 1 (BBR) before the switch.