# Performance Evaluation of TCP BBRv3 in Networks with Multiple Round Trip Times
## 5 Performance Evaluation
### Intraprotocol Fairness  
The purpose of this group of simulations was to verify if BBRv3 showed similarly strong dependency on the bottleneck buffer size, as well as unfairness towards flows with different RTTs. Our objective was to investigate whether the impact on results stems solely from the ratio of RTT times or if it was the relation of the RTT to buffer size that played a significant role for a part of the bandwidth that the flow achieves. Across both sets of simulations, the connection between node N1 and N4 consistently maintained an RTT of 40 ms (_baseRTT_), while the throughput of all the links remained fixed at 100 Mbps. Buffer sizes were determined as multiples of BDP, relative to the baseRTT, and were calculated as the \(BDP=baseRTT*100\ Mbps\). This work varied the RTT between node N2 and N5, thus ensuring that the delay on the N2-N5 path spanned increments of 0.1, 0.2, 0.5, 0.8, 1, 2, and 5 times the baseRTT. It employed two distinct settings for the bottleneck delay: 2 ms and 10 ms, depending on the required RTT ratio. Simulations were conducted across a spectrum of buffer sizes ranging from 0.1 to 10 times BDP. The overall throughput is depicted in Figures 7-9. Each bar is divided to represent the throughput experienced by individual flows, with the lower section consistently indicating the throughput of the flow with \(baseRTT\). Given the substantial volume of results acquired,  
Figure 5: The throughput trend over time and the steady state behavior. Figure 6: The amount of inflight data for each BBR version while varying bandwidth capacity. this study restricted the presented results to configurations where differences were evident; in certain instances, the outermost buffer sizes did not yield significant alterations in outcomes. The Jain's index data is collated in Tables 2 and 3. For smaller buffers, BBRv1 failed to fully utilize the available bandwidth, thus requiring a buffer size of at least 2*BDP to achieve 90% bandwidth utilization. In contrast, both BBRv2 and BBRv3 did not exhibit these limitations and consistently delivered high throughputs across various buffer sizes. This is attributed to their ability to respond to packet losses by dynamically adjusting the sending rate. This work has not presented the results of the drop ratio, as they merely confirm previous findings that BBRv1 exhibited a packet loss ratio of up to 5% for shallow buffers. The observation of a high loss ratio (and consequently, a high retransmission rate) is in line with the findings of prior work [6, 9, 11]. When RTTs are equal, the fairness depends on the buffer size. BBRv1 exhibited the most aggressive probing behavior and facilitated rapid synchronization; thus, even with very shallow buffers, it provided high fairness. However, this came at the expense of bandwidth underutilization and a high loss/retransmission ratio. BBRv2 and BBRv3 required a buffer size of at least 1-2*BDP to ensure equitable operation. BBRv2 struggled the most to achieve fairness when the buffer was shallow. This issue may be attributed to a known bug in this iteration, which was fixed with BBRv3 (the details are provided in Section 3.3). Figure 8: The flow throughputs (bottom—baseRTT, top—x*baseRTT) for different buffer sizes and different RTT ratios; bottleneck link delay = 2 ms.  
Figure 7: The flow throughputs (bottom—baseRTT, top—x*baseRTT) for different buffer sizes and different RTT ratios; bottleneck link delay = 2 ms.  
Figure 9: The flow throughputs (bottom—baseRTT, top—x*baseRTT) for different buffer sizes and different RTT ratios; bottleneck link delay = 10 ms. The buffer size and disparities in the RTT significantly influence bandwidth allocation between flows. In the case of BBRv1, buffers smaller than 1*BDP typically resulted in shorter RTT streams obtaining more bandwidth. However, it was only when the buffer exceeded 2*BDP that a notable disparity emerged in favor of streams with longer RTT times. These findings present a partial contradiction to the assertions found in the existing literature.