# On Cross-Layer Interactions for Congestion Control in the Internet
## 5 Performance Evaluation
### Data Center, Mixed TCPs  
In the third scenario, the combination of all TCP versions is used simultaneously, i.e., every node implements a different congestion control mechanism. The results are shown in Tables 10 and 11. Table 10 shows 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, depending on the congestion load, it is the average of three, thirty, or three-hundred values. Column BD presents the percentage of the overall throughput achieved by the streams of a given TCP version. Column JI contains the Jain's index calculated either for flows using the same TCP  
Figure 5: The queue length of DT, PIE and CoDel in combination with (**a**) New Reno, (**b**) Cubic, and (**c**) C-TCP in the Access Link scenario with common TCPs in uncongested network. Figure 6: The queue length of DT, PIE and CoDel in combination with (**a**) New Reno, (**b**) Cubic, and (**c**) C-TCP in the Access Link scenario with common TCPs during mild congestion. Figure 7: The queue length of DT, PIE and CoDel in combination with (**a**) New Reno, (**b**) Cubic, and (**c**) C-TCP in the Access Link scenario with common TCPs during heavy congestion. version (inter-fairness) or for all connections (intra-fairness). The results are presented for three different congestion levels and three evaluated buffer management strategies. The most unfair bandwidth allocation is observed when DT is applied at the buffer. Even in the uncongested network, although the inter-fairness is preserved, the intra-fairness is rather poor, and it drops significantly as the congestion grows. In the uncongested and mildly congested network, New Reno acts most aggressively and utilizes more than 60% of the available bandwidth. On the other hand, C-TCP is the least competitive and suffers significant performance degradation. During heavy congestion, TCP Cubic overtakes the whole bandwidth. Replacing DT with PIE or CoDel noticeably improves the inter- and intra-fairness in all scenarios, even during heavy congestion state. However, there are minor differences in terms of the average throughput; in most cases, TCP Cubic acts slightly more aggressively towards New Reno and C-TCP connections. Table 11 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 8. As far as queue behavior is concerned, similar dependencies can be observed as in the DC scenario with common TCP. CoDel provides a rather low and stable queue in the uncongested and mildly congested network. However, as the network load increases, CoDel's queue oscillates between the maximum and the target value, which causes high latency variability. The problem is discussed in the Data Center scenario with common TCPs (Section 5.1). PIE results in a comparable average queue size and queue size variation, regardless of the congestion level. During mild and heavy congestion, PIE causes 10% more packet losses than CoDel.