# Fault Tolerant DHT-Based Routing in MANET
## 3 Related Work
For this purpose, nodes forward and store mapping information on ANs in the address publication phase. Further, the designated ANs replicate and store copies at directly connected neighbors (i.e., 1-hop physical neighbors or logical neighbors predecessor/successor). VCP periodically updates the mapping information stored at actual ANs and the neighbors (i.e., replicas). So that, in case of an actual AN failure, lookup requests are routed to the vicinity of the failing node, and a ring search is employed to locate the replicas. The ring search is restricted to search replicas among 1-hop physical neighbors, i.e., using broadcast. There are several serious concerns regarding the fault tolerance capability of VCP. The replicas deployment, maintenance, and locating replicas introduce massive delays, overhead, and failures. Since VCP replicates and maintains (i.e., periodically updates) mapping information blindly and in a static manner (i.e., network dynamics are not considered in the replication decisions). This introduces extra overhead and delay. For example, VCP suffers from the mismatch problem [12; 14]. Therefore, neighbors in the logical network might be distant nodes in the physical network and vice versa. So that replicating mapping information on predecessors/successors needs several transmissions in the physical network. This makes the replication process more expansive, mainly when VCP updates the replicas periodically. This also limits the applicability of the restricted ring search in the lookup process. The employed ring search, where the search space is restricted to 1-hop physical neighbors, misleads the lookup requests and introduces huge delay and overhead with false attempts to solve the lookup queries. Similarly, if the designated AN is critical and fails, then logical and physical neighbors would remain unreachable, or the network would get partitioned into disjoint parts. In these cases, the lookup resolution process would fail. Therefore, the replication decisions should be adaptive using knowledge about network dynamics. Furthermore, VCP does not consider network partition, merging, and its impact on network availability. An entirely fault-tolerant system should consider these practical considerations, especially in MANETs. DART [14], and M-DART [15] embed the ad hoc physical topology of MANETs into a logical tree shape structured network. In network bootstrapping, joining nodes compute LIDs and form a logical tree-structured network (i.e., address tree). After computing LIDs, nodes store mapping information on specific ANs. A source node retrieves mapping information from the designated AN before starting the communication session with the destination node to route data packets. M-DART [15] is an enhanced version of DART. M-DART's sole purpose was to explore and pro-actively maintain multiple paths in the address tree (i.e., logical network) between a source and destination pair. M-DART pro-actively and blindly (i.e., do not consider physical network connectivity) maintains multiple paths between a source and destination pair. Therefore, in case of a route failure, alternate paths, if available, can be exploited. This is an inherent feature of multi-path routing, i.e., redundancy. M-DART completely fails in situations where a single path exists. In other words, MDART does not consider network partition. Similarly, the impact of failures (i.e., AN failure, lookup failures, and address tree partitioning/merging) on DHT routing is not addressed. Moreover, M-DART does not claim fault tolerance capability and does not fit in the problem scope (please see Section 2). However, DART solves the problem partially by targeting the ANs failure problem. DART keeps updating mapping information on the ANs so that the information remains available after every update. The idea is to keep updating or selecting new ANs blindly and repeatedly. By doing so, the information will be available after every refresh period despite the failure.