# Fault Tolerant DHT-Based Routing in MANET
## 5 Simulation and Results Analysis
### Lookup Success Ratio  
The lookup success ratio defines the overall fault tolerance capability of a DHT network. In the lookup process, a source node forwards MREQ (mapping info request) packets towards the AN and obtains mapping information (by receiving MREP (mapping reply) packet from the AN) to start a communication session with the destination node. Therefore, successful resolution of lookup requests is imperative to initiate actual data forwarding in DHT networks. However, network variations (node(s)/link(s) failures, network partition, MAC layer collisions, etc.) prohibit successful resolution of lookup. Similarly, the lookup resolution is directly associated with network size and mobility. The lookup success ratio decreases by increasing the network size. Because an increase in the network size increases the average hop count between a source and destination pair and the network traffic, this eventually causes packets collisions at the MAC layer (i.e., IEEE 802.11) and MREQ/MREP are likely to be delayed in a queue or dropped. Likewise, the lookup success ratio decreases as mobility increases because high nodes' move speed compromises nodes connecting links (network connectivity) and introduces frequent network partition, merging, leaving/joining nodes, and failures (AN failures, etc.). These frequent and unpredictable changes introduce extra transmission overhead (due to the recurrent execution of joining, partition detection, and merging procedures), congestion at the MAC layer (losses of MREQ/MREP due to collision), and eventually, decrease lookup success ratio in DHT networks. Figure 5 shows lookup success ratio comparison against different network sizes and nodes moving speeds (the Figure 5a-d show the results at speed 0.5 m/s, 1.0 m/s, 1.5 m/s and 2 m/s respectively). In Figure 5, FTDN outperforms its counterparts and shows significant gains. Moreover, the impact of network size and mobility is lower in FTDN than VCP and DART; this shows the effectiveness of FTDN in highly mobile environments (i.e., in the presence of faulty nodes, partition, etc.) with an increasing number of nodes. In FTDN, the main factors in performance gain are dynamic replication (adaptively replicating mapping data across the critical regions in-network) and finding alternate paths to avoid failure. These measures ensure access to mapping information despite AN failure, routes breakages due to faulty nodes, and network partition. Moreover, these measures are fully distributed and rely on the local knowledge available to nodes. However, due to the recurrent execution of merging, LIDs reassignment, and increasing mobility and network size, a slight decrease in lookup success ratio is reported in Figure 5.  
However, VCP and DART degrade lookup success ratio significantly as network size and mobility increase. Besides that, VCP maintains replicas (i.e., at predecessor/successor or physical 1-hop neighbors) and exploits a restricted ring search (restricted to 1-hop physical neighbors) to locate the replicas. However, due to mismatch problem [4; 6] and network partitioning, the ring search not only fails, but introduces huge overhead (causes more packet collisions at MAC) and delay in the false attempts to locate replicas. Likewise, an increase in mobility and network size makes it worse and, eventually, the lookup success ratio drops. In DART, mapping data stored at the ANs are updated periodically; therefore, in case of failure or partition, the mapping data remains accessible after every refresh period, but if a lookup request reaches before the arrival of fresh copy, then the requests are buffered at the neighboring nodes (i.e., neighbors of the failed AN). This technique works well in small networks with lower mobility and provides access to mapping data despite network partition and actual AN failure.