# Performance Evaluation of TCP BBRv3 in Networks with Multiple Round Trip Times
## 5 Performance Evaluation
### Interprotocol Fairness  
In the final scenario, this study investigated the interaction between BBR and the TCP CUBIC. A CUBIC sender was deployed on node N1, thus initiating two flows denoted as F1 and F2, while on node N2, a BBR sender transmitted data via two flows labeled as F3 and F4. The recipient for the data streams F1 and F2 was implemented at node N4, whereas the recipient for the F3 and F4 flows was located at node N5. The transmission performance was evaluated under two distinct scenarios: one where all paths were set to an identical RTT of 40 ms and another where the path N1-N4 had an RTT of 40 ms, while the path N2-N5 had an RTT of 80 ms. The start times for flows F1 and F3 were randomized around the beginning of the simulation, while flows F2 and F4 were randomized to initiate transmission approximately at the 5 th second. The simulations were executed with two different buffer sizes, namely 0.5 times and 5 times the BDP. The BDP was computed based on the _baseRTT_ of the N1-N4 path. The results are compiled in Table 9. The throughput for each TCP version was calculated for individual flows, thus considering their start and stop times. The throughput for individual flows is depicted in Figures 12 and 13 for flows sharing equal RTT, while for two different RTTs, the throughput is presented in Figures 14 and 15. Similarly to the previous scenarios, BBRv1, characterized by significant throughput oscillations, impaired the full utilization of the available bandwidth and induced the highest drop ratio in scenarios with shallow buffers. Notably, it monopolized nearly 80% of the bandwidth. The additional buffer capacity allowed CUBIC to claim more bandwidth; however, the BBRv1 flows continued to control over two-thirds of the bandwidth share. The results confirm the widely accepted assertion regarding BBRv1's unfairness toward CUBIC. As the buffer size increased, CUBIC transmitted more data before encountering loss, thus consequently achieving a higher throughput. BBRv2 and BBRv3 significantly modified the allocation of bandwidth resources. BBRv3 exhibited a nearly equitable distribution of bandwidth capacity between congestion controls in almost all cases. In several experiments, even when the bandwidth was distributed fairly among the CUBIC and BBR flows, the Jain's Index (JI) indicated suboptimal fairness. The high or low values of the Jain's Index cannot be solely attributed to BBR, as CUBIC also played a significant role in this regard. However, upon analyzing the throughputs of the individual flows, it becomes apparent that BBR struggled more to equitably distribute its allocated bandwidth share, particularly in scenarios with deep buffers where earlier-starting flows consumed a disproportionate share of the bandwidth. These findings may suggest the prolonged convergence time of BBRv3 towards achieving fair bandwidth allocation. For shallow buffers, BBRv1's indifference to packet losses provided it with an advantage over CUBIC. BBRv3 operated around the estimated BDP without causing additional losses, thereby facilitating the swift loss recovery performed by CUBIC. In the case of deep buffers, CUBIC tended to fill the entire available buffer space, whereas BBRv3 maintained  
Figure 14: The flow throughputs: F1 and F2—CUBIC; RTT=40 ms; F3 and F4—BBR; RTT = 80 ms; shallow buffer (0.5°BDP). F1 and F3 start at 0; F2 and F4 start at 5 s.  
Figure 13: The flow throughputs: F1 and F2—CUBIC; RTT = 40 ms; F3 and F4—BBR; RTT = 40 ms; deep buffer (5°BDP). F1 and F3 start at 0; F2 and F4 start at 5 s.  
Figure 15: The flow throughputs: F1 and F2—CUBIC; RTT = 40 ms; F3 and F4—BBR; RTT = 80 ms; deep buffer (5°BDP). F1 and F3 start at 0; F2 and F4 start at 5 s.  
the volume of inflight data close to the BDP. The bufferbloat caused by CUBIC impaired the proper operation of BBR. Summary: BBRv3 improved fair bandwidth sharing in the presence of CUBIC, especially when all flows shared identical RTTs.