# mmS-TCP: Scalable TCP for Improving Throughput and Fairness in 5G mmWave Networks
## 4 Evaluations
### Inter-Protocol Fairness (vs. CUBIC)  
In mmWave networks, an aggressive increase in cwnd can improve TCP throughput, but it can cause fairness issues for other flows. Therefore, an inter-protocol fairness experiment was performed to investigate whether S-TCP and mmS-TCP, which provide high throughput, compete fairly with the existing CCAs. The fairness with CUBIC, which was adopted as the default congestion control in all representative operating systems such as Linux, Windows, and MAC, was evaluated.  
In a scenario in which three NLoS events occur, it was assumed that a single CUBIC flow competes one-to-one on the mmWave link provided by the same base station as CUBIC, S-TCP, and mmS-TCP. In addition, evaluation of the competition situation between S-TCP and mmS-TCP was also performed.  
Figure 11 shows the change of cwnd over time, and Figure 12 shows the average throughput of each flow as stacked bars. When two CUBIC flows compete, the flow that reduces cwnd first results in lower throughput. This is not because of the aggressiveness of a specific flow, but because cwnd increases very slowly in the congestion avoidance of the CUBIC algorithm. Two CUBIC flows that do not fully utilize the capacity of the mmWave link cannot reduce the difference in cwnd until a new packet drop occurs. The cwnd of the two flows become similar due to continuous packet drops occurring in the CUBIC_2 flow, but the cwnd after 11 s is of a size that cannot fully utilize the link.  
Figure 11: Cwnd variation in competition between two flows.  
When S-TCP competes with CUBIC flow, many packet losses occur at the beginning of the connection with two slowstart phases. S-TCP immediately rises based on the ssthresh value set in the second slowstart, while CUBIC does not rise high because it is set lower than S-TCP. The gap between the two flows, which occurs immediately after the slowstart, is temporarily narrowed at the time the RTO occurs (about 8 s), but the gap widens again due to S-TCP's aggressiveness.  
Unlike S-TCP, mmS-TCP is not dominant from the beginning of the connection because it sets the ssthresh similar to that of CUBIC after one slowstart. After 11 s of an RTO event, mmS-TCP dominates, but if an NLoS situation triggers an RTO, mmS-TCP halves the ssthresh, giving the CUBIC flow a chance to reverse. At the bottom of Figure 11, we can see the cwnd fluctuations when S-TCP and mmS-TCP compete. Due to the aggressiveness of S-TCP, mmS-TCP showed relatively low cwnd size than S-TCP. However, mmS-TCP can compete with S-TCP to provide better throughput compared to CUBIC.  
Figure 12 shows the average throughput of each flow over 15 s of competition. In the competition between the two CUBIC flows, one flow has the upper hand and the total throughput is low, as it does not fully use the available capacity of the link. Total throughput is higher when S-TCP and mmS-TCP compete with CUBIC, but S-TCP shows approximately 2.8 times higher throughput than CUBIC flow. mmS-TCP also has higher throughput than the competing CUBIC flow, but the throughput of the CUBIC flow increases about 1.6-fold compared with the case of CUBIC vs. CUBIC. mmS-TCP had a throughput of 344.43 Mbps when competing with S-TCP and slightly overwhelmed by S-TCP but showed approximately 50% higher throughput than when CUBIC competed with S-TCP.  
In a situation where the CUBIC, S-TCP, and mmS-TCP algorithms compete with each other, S-TCP occupies a lot of bandwidth and it was confirmed that S-TCP harms inter-protocol fairness (average throughput; S-TCP: 494 Mbps, mmS-TCP: 264 Mbps, CUBIC: 88 Mbps).  
For [PERSON]'s fairness index [40], CUBIC, S-TCP, and mmS-TCP were 0.9768, 0.819, and 0.9773, respectively, and mmS-TCP showed the highest fairness index. The redesigned mmS-TCP did not ideally share the link with CUBIC TCP, but mmS-TCP improves fairness with CUBIC than S-TCP without compromising total throughput. In addition, mmS-TCP competed with S-TCP and provided a relatively high fairness index of 0.9614.