# Performance Evaluation of TCP BBRv3 in Networks with Multiple Round Trip Times
## 5 Performance Evaluation
### RTT Fairness and Access Link Scenario
This phenomenon occurs because these streams possess relatively larger buffer shares. With an increase in buffer size, the distribution of bandwidth shifts towards connections characterized by longer RTTs. For a buffer size of 10*BDP, streams with extended RTTS emerged as dominant, with the three longest-duration streams collectively occupying over 70% of the available bandwidth. This outcome aligns with findings from prior studies, thus affirming that in scenarios featuring sufficiently large buffers, streams with longer RTTs tend to consume the majority of bandwidth, which is attributable to an upper limit of inflight data calculated at twice the estimated BDP. The throughput results for BBRv2 are presented in Table 7. The performance of BBRv2 significantly degraded in terms of fairness. In scenarios involving smaller buffers, there persistently existed a single connection with an RTT below the average, thus consuming approximately 30% of the available bandwidth. Unlike BBRv1, no evident trend emerged regarding the excessive allocation of bandwidth by streams with shorter RTTs for small buffer sizes. This discrepancy can be attributed to the absence of a limiting factor calculated using the estimated bandwidth and propagation time. As the buffer size increased, there was an observable trend towards a fairer distribution of bandwidth. However, in scenarios with the largest buffer, BBRv2 exhibited the same behavior as the previous version: the three streams with the longest RTTs collectively consumed a large part of the bandwidth. It is also worth mentioning that in an analogous scenario, CUBIC achieved a JI of 0.7 when the buffer size was equal to the BDP [5]. The results for BBRv3 (see Table 8) are the most inconclusive. This version demonstrated optimal performance when the buffer size equaled 2*BDP, although the flow with \begin{table}
\begin{tabular}{c c c c c} \hline
**Buffer Size** & **0.5*BDP** & **BDP** & **2*BDP** & **10*BDP** \\ \hline
**RTT [ms] | JI** & 0.84 & 0.95 & 0.98 & 0.49 \\ \hline
8 & 19 & 15 & 12 & 5 \\
32 & 14 & 12 & 10 & 2 \\
58 & 11 & 12 & 12 & 2 \\
78 & 10 & 11 & 10 & 2 \\
102 & 9 & 10 & 9 & 3 \\
128 & 8 & 9 & 12 & 8 \\
154 & 7 & 10 & 10 & 13 \\
178 & 5 & 6 & 9 & 32 \\
204 & 5 & 8 & 9 & 26 \\ \hline \end{tabular}
\end{table} 
Table 6: Throughput of BBRv1 flows [Mbps] and overall Jain’s index (JI) for access link scenario. \begin{table}
\begin{tabular}{c c c c c} \hline
**Buffer Size** & **0.5*BDP** & **BDP** & **2*BDP** & **10*BDP** \\ \hline
**RTT [ms] | JI** & 0.51 & 0.54 & 0.70 & 0.80 \\ \hline
8 & 34 & 10 & 14 & 5 \\
32 & 6 & 11 & 10 & 10 \\
58 & 12 & 8 & 8 & 7 \\
78 & 8 & 36 & 24 & 1 \\
102 & 4 & 6 & 7 & 13 \\
128 & 1 & 6 & 9 & 14 \\
154 & 23 & 12 & 19 & 16 \\
178 & 5 & 1 & 2 & 10 \\
204 & 2 & 4 & 2 & 19 \\ \hline \end{tabular}
\end{table} 
Table 7: Throughput of BBRv2 flows [Mbps] and overall Jain’s index (JI) for access link scenario. the shortest RTT allocated 25% of the bandwidth, and the Jain's index was much lower than for the first version. Worse fairness can result from less aggressive probing parameters compared to the first version. Notably, only the average queue size and standard deviation exhibited significantly lower values compared to previous iterations. On the other hand, we can observe that for the deepest buffer, the average queue size became notably large. To delve deeper into this phenomenon, this study presents the queue length trends across different iterations of the BBR algorithm for the two largest buffer settings. The results of this analysis are presented in the Figures 10 and 11. BBRv1, despite providing a high fairness index, induced significant queue oscillations. Only for a very deep buffer did the queue achieve stability, thus remaining below an occupancy threshold of 5*BDP. During the PROBE RTT phases of BBR, synchronization occurs, thus leading to buffer depletion during these intervals.