# On Cross-Layer Interactions for Congestion Control in the Internet
## 5 Performance Evaluation
### Access Link, Mixed TCPs  
In the last scenario, the Access Link topology with varied RTTs is used along with all TCP versions, i.e., every node implements a different congestion control mechanism. Table 12 gathers the average throughput, bandwidth sharing, and fairness. The average throughput and standard deviation of the throughput (column AvgThr (SD)) are calculated for each node, thus for different TCP flavors. Column BD presents the percentage of the overall throughput achieved by the streams of a given TCP version. Finally, column JI contains the [PERSON]'s index calculated either for flows using the same TCP version (inter-fairness) or for all connections (RTT-fairness). The results are presented for three different congestion levels and three evaluated buffer management strategies. Preserving acceptable inter- and RTT-fairness, when all TCPs are used simultaneously, is much harder in Access Link scenario than in the Data Center scenario. In combination with DT, connections using TCP Compound can take only a disproportionately small percentage of the accessible resources. Thus, TCP New Reno and TCP Cubic consume almost all available bandwidth. This is because Cubic is more aggressive than the other two TCPs and the links dedicated to New Reno have the shortest average RTT values. TCP Cubic and C-TCP provide significantly better inter-fairness than New Reno. However, in all load scenarios, the overall RTT-fairness is rather poor. Applying PIE or CoDel does not significantly affect the inter-fairness, actually C-TCP provides better results in combination with DT. Nonetheless, both AQMs improve the fair bandwidth distribution. From the conducted simulations, it cannot be clearly stated which version of TCP behaves the most aggressively or how RTT differentiation influences the achieved throughput. To fully understand and verify these dependencies, additional simulation studies would be required. Figure 8: The queue length of DT, PIE and CoDel in combination with mix TCP flows in the Data Center scenario (**a**) in uncongested network, (**b**) during mild congestion, (**c**) during heavy congestion. Table 13 shows the mean queue length and the standard deviation of queue length, as well as the drop ratio for each scenario case. The queue length variation over time is presented in Figure 9.  
The observed queue behavior is similar to previous scenarios. DT allows filling the queue, which results in a large average queue size and, consequently, long delays. In the uncongested network, the queue oscillations, therefore delay variations, are the highest (see Figure 9a). In the uncongested and mildly congested network, PIE and CoDel provide similar results, i.e., rather short and stable queues. CoDel, as depicted in Figure 9c, fluctuates between the maximum and the target value of queue size in the heavily congested network, which results in unacceptably high queue size variations. PIE acts similarly in all load scenarios and provides comparable average queue size and rather small variations. However, during heavy congestion, it causes considerably more packet losses than the other two approaches. A similar problem with fairness in the uncongested and moderately congested networks is observed in the Access Link scenario with common TCPs. Once again, the RTT fairness could be improved by deploying the dropping function, which would more accurately penalize the faster flows. A summary of our findings is given below. * Varied RTT significantly worsens the results in terms of achievable RTT- and inter-fairness. * Applying PIE or CoDel improves the RTT-fairness among competing flows using different TCP versions, however, unsatisfactorily.