## 5 Evaluation
### Network Evaluation
#### 5.3.1 Virtual Network Evaluation
As the size of the network increases, the success rate of the reachable and unreachable node lookups may be reduced by more network packets being sent and the node k-bucket refreshing. The significant decrease in the success rate of unreachable nodes at 4000 nodes may be caused by the interference generated when querying unreachable node addresses to each reachable node due to network congestion. As the ratio of unreachable nodes increases,  
Figure 16: Average packet numbers for building overlay networks at different network size. the success rate of reachable nodes slightly increases probably because fewer reachable nodes are involved in DHT construction, while unreachable nodes do not have a significant impact on reachable nodes. Starting from 60% of unreachable nodes, the success rate of reachable nodes decreases probably because a large number of unreachable nodes occupy network resources and have an impact on the refreshing of DHT of reachable nodes, which can be confirmed by the rapid decrease of the success rate of unreachable nodes after 60%. We then explore the effect of the proportion of unreachable nodes on the network. First, we randomly generate 5000 node addresses, without distinguishing between reachable and unreachable nodes. Then, with a constant total number of nodes and a constant number of long connections of unreachable nodes of 1, we investigate the effect of the ratio of unreachable nodes at 10%, 20%, 30%,..., 90%, and converging to 100% of the proportion of unreachable nodes in the overlay network were experimented with separately. In each group of experiments, when the nodes reach the steady state of the DHT, no less than ten reachable nodes are selected, and no less than five reachable nodes and no less than five unreachable nodes are selected for each reachable node respectively, and the success rate of the lookup are shown in Figure 18.  
Figure 17: Lookup success rate of reachable and unreachable nodes at different network size. Figure 18: Lookup success rate of reachable and unreachable nodes with different ratio of unreachable nodes. In the figure we can see that the reachable nodes show a stable state when the proportion of unreachable nodes is low, with a tendency to increase as the proportion increases, and the success rate starts to drop when the proportion is high. The success rate of unreachable nodes fluctuates when the proportion is below 60%, fluctuates around 85%, and decreases rapidly when the proportion is above 60%, until finally the success rate is only 20%. Finally, we explore the effect of the size of the number N of unreachable nodes maintaining long connections on the success rate of the lookup. First, we randomly generate 5000 node addresses, without distinguishing between reachable and unreachable nodes. Then, we randomly select 30% of these 1500 nodes as unreachable nodes and the remaining 3500 nodes as reachable nodes, with the bootstrap nodes included in the reachable nodes. We take N as 1, 2, 3, 4, 5 to construct the DHT stable overlay network respectively. For each group of experiments, as before, no less than 10 reachable nodes are selected, and no less than five reachable nodes and no less than five unreachable nodes are selected for each reachable node respectively, and the success rate of the lookup is shown in Figure 19.  
In the figure we can see that the success rate of finding reachable nodes decreases slightly as N increases. When N is 2, the success rate of unreachable nodes rises significantly compared to when N is 1, and fluctuates when N is greater than 2. As the number of long connections N increases, redundancy is created in finding unreachable nodes in the network, avoiding the failure of finding them due to the network or some nodes, so the success rate increases significantly at N of 2 compared to 1.