# Fault Tolerant DHT-Based Routing in MANET
## 5 Simulation and Results Analysis
### End-to-End Delay  
Resolution of lookup queries with minimum possible E2E delay is imperative for successful routing in DHT networks and defines overall network performance, especially in the adversarial environment offered by MANETs. Similarly, network size and mobility are the influencing factors. Therefore, for a fair analysis, network size and mobility should be considered while analyzing E2E delay.  
Figure 7 shows E2E delay encountered by FTDN, DART, and VCP with varying network size and nodes moving speed (the Figure 7a-d show the results at speed 0.5 m/s, 1.0 m/s, 1.5 m/s and 2 m/s respectively). FTDN gains are significant as compared to DART and VCP. Since, in FTDN, the lookup queries are resolved by the nearby replicas despite being far from actual AN. Similarly, in case of actual AN failure or partition, the mapping information remains accessible comparatively at shorter distances than the original AN. Moreover, resolution of lookup requests by the nearby replicas instead of the designated AN reduces transmission overhead in FTDN. Eventually, this decreases contention at the MAC layer (i.e., in IEEE 202.11); these factors further reduce E2E lookup delay in FTDN compared to VCP and DART. Similarly, high mobility compromises network connectivity (links) and increases the probability of nodes/links failure and the existence of more critical nodes in the network. Therefore, FTDN deploys more replicas (i.e., since, in FTDN, replication decisions consider network dynamics and adaptively replicate mapping info across the critical regions) and, the lookup queries would get resolved by the nearby replicas, even in highly mobile environments. So, FTDN is a better choice for highly mobile environments. However, a slight increase in overhead and delay due to recurrent partition detection and merging schemes. However, the case is different in VCP and DART since VCP does not consider physical network dynamics in the replication process and offers no resistance to mobility. Specifically, in highly mobile environments, the restricted ring search used in VCP not only fails to discover replicas but introduces massive delay due to the false attempts by locating the replicas. Similarly, in DART, due to frequent failures (nodes/links, partition, etc.), more and more lookup queries are needed to be buffered; this causes a buffer overflow, and the forwarded lookup quarries get lost before reaching the new selected AN. This worsens with an increase in network size (i.e., an increase in network size also increases average shortest path length; therefore, SMI packets have to cover longer distances in  
Figure 6: Lookup success ratio results, as boxplots, against different node moving speeds with varying network sizes.  
periodic updates. Therefore, VCP and DART encounter considerable delays in the lookup process as mobility and network size increase. However, for small networks with low mobility, DART outperforms VCP.  
Figure 8 provides E2E delay comparison using boxplots against different nodes speed with varying network size. The data distribution further endorses FTDN's significant gains.  
Figure 7: Average E2E lookup delay as a function of network size and speed.