# Improving Congestion Control of TCP for Constrained IoT Networks
## 6 Performance Evaluation
We note that the number of retransmissions of the original uIP TCP in the \(5\times 5\) grid topology network is very large. It is even larger than the number of successfully received segments. In Figure 4(a),b, the 95% confidence intervals for the means of our proposed scheme do not overlap those of the original uIP TCP for both topologies. This suggests that our proposed scheme significantly outperforms the original uIP TCP in terms of goodput and the number of retransmissions, because it can adapt the actual retransmission timer to the large RTT fluctuations. Moreover, we observe from Figure 4(a),b that, for the \(5\times 5\) grid topology, the 95% confidence intervals for the means of our proposed scheme and the uIP TCP with dithering do not overlap. This implies that a more severe hidden terminal problem results in a more effective retransmission timer adjustment based on RTT estimates. It is notable that, when ContikiMAC is used, TCP connection failures are often observed. In particular, the original uIP TCP suffers from more TCP connection failures than the others, resulting in a lower fairness index. Figure 4: Performance in \(4\times 4\) grid and \(5\times 5\) grid topology networks when no RDC mechanism is used. Figure 5: Performance in \(4\times 4\) grid and \(5\times 5\) grid topology networks when ContikiMAC is used. To investigate the impact of the queue size, we compare performance with the number of packets in the link-layer queue set to four packets and eight packets in the \(4\times 4\) grid topology network. As shown in Figure 6, the increase in the link-layer queue size from four packets to eight packets does not significantly impact goodput and the number of retransmissions. In Figure 5(a), there is no overlap of the 95% confidence intervals for the means between the original uIP TCP and our proposed scheme, which shows that our proposed scheme can improve goodput for both queue sizes. Figure 5(b) shows that, when ContikiMAC is used, the 95% confidence intervals for the means of the original uIP TCP are relatively large, but do not overlap those of our proposed scheme. This implies that, when ContikiMAC is used, our proposed scheme reduces retransmissions for both queue sizes. To figure out the impact of the random loss rates, we compared performance between the three schemes with the random loss rates of 0%, 5%, 10%, and 15%. In this test, the number of packets in the link layer queue was set to 4. We used the \(4\times 4\) grid topology because too many connection failures occurred in the \(5\times 5\) grid topology. Figure 7 depicts the performance when no RDC mechanism is used. As the random loss rate increases, the total goodputs and the fairness indexes of all the schemes converge. However, in Figure 6(b), showing the number of retransmissions, the 95% confidence intervals for the means of our proposed scheme do not overlap those of the original uIP TCP with the loss rates of 10% and 15%. This implies that our proposed scheme is effective for reducing the number of retransmissions with high loss rates. Figure 8 shows the performance when ContikiMAC is used. We can see that, for all the schemes, the performance is less sensitive to random loss rates than when no RDC mechanism is used. In Figure 7(a),b, the 95% confidence intervals for the means of the original uIP TCP do not overlap those of our proposed scheme at all loss rates of 0% through 15%, which indicates that our proposed scheme significantly improves goodput and the number of retransmissions. However, we observe from Figure 7(c), depicting fairness, that there are overlaps between the confidence intervals of the original uIP TCP and our proposed scheme at the loss rates of 5% and 15%. As the random loss rate increases, the frequency of connection failure also tends to increase for all three protocols, which remains as an issue to be addressed in the future work. Figure 6: Effects of the link-layer queue size.