# Congestion Control Mechanism Based on Backpressure Feedback in Data Center Networks
## 5 Evaluation
### Evaluation of Single-Receiver Topology
#### 5.1.2 Experiment Results  
Based on the findings in Section 3.1, it is learned that Switch 3 is the most prone to congestion when three groups of flows compete at the same time. To evaluate the simultaneous competition of three groups of flows, we set the link bandwidth to 10 Mbps in the network topology shown in Figure 1, and each group of flows had a size of 1 MB (with a packet size of 128 B) and concurrently sent traffic to receiver. Figure 9 shows the comparison between BFCC and the other schemes in the metric of throughput. It can be observed from the graph that the throughput of the congestion control scheme can reach up to 600 kbps, and the average throughput was around 450 kbps, which is better than the other three schemes. For the other three schemes combined with the rate control at the endpoints, the throughput fluctuations were more obvious. BFCC achieved rate adjustment between the switches, there was no sender rate reduction strategy, and the overall throughput level was maintained at a high level. In the metric of buffer queue length, we focused on observing buffer changes under congestion point number 3. We sampled the queue lengths in the switches and analyzed their cumulative distribution. Figure 10 shows the CDF under the four schemes. The differences between these schemes can be seen, where it can be observed that the BFCC  
Figure 9: Throughput comparison of four schemes. mechanism is at a lower buffer queue length and guarantees a high throughput while maintaining the low buffer characteristics. It can also be shown that the switch direct action approach is slightly better than the endpoint control scheme. In order to test the FCT under different scenarios, the link bandwidth was adjusted to analyze the differences between different schemes. The link bandwidth was adjusted from 10 Mbps to 50 Mbps, 100 Mbps, and 200 Mbps, and the FCT was observed for three groups of flows sent at the same time. As shown in Figure 11, with different link bandwidths, the BFCC maintained the lowest FCT, which shows that the switch-to-switch congestion control mechanism ensures high throughput and low latency with low buffer. In terms of the overall FCT, BFCC achieves completion times 1.2-2\(\times\) faster than other ECN schemes. Because of the limitation of BMV2, the test could not simulate the high link bandwidth in the real network, and the different bandwidths of 10 Mbps, 50 Mbps, 100 Mbps, and 200 Mbps were set for the metric of FCT in order to simulate the state of the link bandwidth constantly changing in DCN. Through the results, it was observed that BFCC demonstrated better experimental performance. Figure 11: FCT in the Switch 3 congestion scenario. Four different bandwidth settings are used to compare the FCT of the four schemes. Figure 10: Buffer queue length comparison. In addition to this, the RTT per packet was also compared, as shown in Figure 12. As mentioned in Section 4.2, the design of the BFCC mechanism involves the recirculation of packets, which can lead to slightly higher RTT for some packets, but overall, it is still low and stable. From this, it can be seen that using collaboration between switches to solve the congestion problem is faster than two-point architecture (endpoint-switch) and three-point architecture (endpoint-switch-endpoint). According to the tests conducted in Section 3.1, it is known that in addition to Switch 3 being the most congested when three groups of flows compete at the same time, Switch 1 has two groups of flows sharing the buffer. Other scenarios were set up to evaluate the competition between Group 1 flow and Group 2 flow at Switch 1. At a bandwidth of 10 Mbps, the Group 2 flow changed from the previous 1 MB to 2 MB, 4 Mb, and 8 MB, and the Group 1 flow remained at 1 MB. Analyzing the FCT of Group 1 flows under the influence of Group 2 flows, Figure 13 illustrates that BFCC of Group 1 flows experienced the shortest completion time during the competition, significantly enhancing the flow completion rate. Figure 12: Comparison the RTT per packet.