# Fault Tolerant DHT-Based Routing in MANET
## 3 Related Work  
Much research exists over the classical fault-tolerance problem in MANETs but is not applicable in DHT networks (for detail please see Section 2). E2 FT [19] addresses the problem using multipath source routing philosophy in MANET. The protocol maintains multiple paths between a source and destination pair. The protocol iteratively estimates and selects specific reliable paths among the available paths. For this purpose, a source node estimates the quality of a route by sending packets and measuring the delivery ratio (the destination node acknowledges the packets received). Specifically, the protocol uses mechanisms such as route estimation (a source node estimates the path quality, i.e., delivery ratio probability), and route selection (best paths are selected for routing with the help of two additional procedures known as confirmation and dropping). The confirmation and dropping procedures are used to refine the selection process progressively. However, the protocol incurs extra overhead to maintain a certain delivery ratio. BFTR [20] performs routing in the presence of faulty nodes using a best-effort approach. BFT's key idea is to tolerate the fault to a certain extent and maintain maximal network performance in an adversarial environment. BFT classifies existing routes between a source and destination into two classes, i.e., most feasible and non-feasible paths. For this purpose, the protocol maintains immediate records (i.e., delivery ratio and delay) of a path. The paths with the highest delivery ratio are marked as feasible and are selected for routing. Similarly, the paths having poor past statistics are declared as infeasible and are discarded. BFT does not detect faulty nodes on a routing path, but evaluates a path's routing feasibility based on its past end-to-end performance statistics. An enhancement over E2 FT [19] was proposed known as weak-estimation learning-based fault-tolerant routing (WEFTR) [21; 22]. WEFTR exploits stochastic learning procedures and obtains better path estimation and selection. The protocol achieves a certain packet delivery ratio with relatively low overhead as compared to E2 FT. WEFTR also argued that weak estimation procedures best suits mobile environments. FTAR [23] proposed a bio-inspired solution to the problem of fault-tolerant routing in MANETS. FTR exploits the ideas of foraging in natural ants [24]. A source routing mechanism is used to find routes between a source and destination pair. The proposed solution ranks the available paths using pheromone value/metric. In this manner, FTR selects optimal routes (i.e., having minimum faulty nodes or high pheromone valued routes) among the existing routes. However, existing DHT based routing protocols in MANETs [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15] assume ideal network environments or mainly focus on addressing schemes and LS resilience in the perspective of routing. There exist no straightforward solutions, and the problem is not yet been explored in the DHT context. However, some closely related work partially targets and solves the problem (for certain specific scenarios); the subsequent paragraphs provide an overview of the closely related work. VCP [13] exploits chord-shaped structures for DHT-based routing in MANETs. In network bootstrapping, nodes compute LIDs, in addition to UIDs, using a pre-defined LS that ranges [S = 0, E = 1]. In this manner, nodes build a logical chord-shaped structured network over MANET's ad hoc physical topology. The LIDs computed based on adjacent physical neighbors' LIDs show relative positions of nodes in the logical chord. Each node proactively maintains logical neighbors (predecessor/successor) and physical neighbors recorded in the routing table. Forwarding decisions consider physical and logical neighbors, where routing processes assume a uniformly distributed (i.e., dead-ends free) chord. Furthermore, in case of AN failure, VCP resolves lookup requests using redundancy.