# Fault Tolerant DHT-Based Routing in MANET
###### Abstract  
In Distributed Hash Table (DHT)-based Mobile Ad Hoc Networks (MANETs), a logical structured network (i.e., follows a tree, ring, chord, 3D, etc., structure) is built over the ad hoc physical topology in a distributed manner. The logical structures guide routing processes and eliminate flooding at the control and the data plans, thus making the system scalable. However, limited radio range, mobility, and lack of infrastructure introduce frequent and unpredictable changes to network topology, i.e., connectivity/dis-connectivity, node/link failure, network partition, and frequent merging. Moreover, every single change in the physical topology has an associated impact on the logical structured network and results in unevenly distributed and disrupted logical structures. This completely halts communication in the logical network, even physically connected nodes would not remain reachable due to disrupted logical structure, and unavailability of index information maintained at anchor nodes (ANs) in DHT networks. Therefore, distributed solutions are needed to tolerate faults in the logical network and provide end-to-end connectivity in such an adversarial environment. This paper defines the scope of the problem in the context of DHT networks and contributes a Fault-Tolerant DHT-based routing protocol (FTDN). FTDN, using a cross-layer design approach, investigates network dynamics in the physical network and adaptively makes arrangements to tolerate faults in the logically structured DHT network. In particular, FTDN ensures network availability (i.e., maintains connected and evenly distributed logical structures and ensures access to index information) in the face of failures and significantly improves performance. Analysis and simulation results show the effectiveness of the proposed solutions. 20  
manner, the transient addresses build a logical, structured network that guides the routing process. This eliminates flooding in routing and makes the system scalable. DHT provides a scalable way to decouple node location from its identity and facilitate general mapping between them. In the last few years, several DHT-based routing protocols for MANETs have been proposed that eliminate flooding in the route discovery, thus making the network more scalable [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. In DHT-based routing protocols, a node is assigned a logical identifier (LID) in addition to its permanent identifier (i.e., MAC/IP address also known as user id, i.e., UID) based on the LIDs of its physical neighbors. The LID is drawn from a predefined logical identifier space (LS) (i.e., the LS forms a logical, structured network following a tree, ring, chord, etc., structure). A node must maintain a disjoint portion of the whole LS, referred to as Logical Space Portion (LSP) (i.e., the LSP acts as the LID for a node). Moreover, each node keeps track of its logical neighbor nodes with LIDs close to its LID by following a ring, chord, or multidimensional structure. Thus, a logical, structured network is built over the ad hoc physical topology. Routing is performed in the logical, structured network using the transient addresses (i.e., LIDs). The logical structure guides routing processes and eliminates flooding at the control plane and the data plane. Furthermore, a connected and evenly distributed LS is always needed for successful routing. DHT networks can be defined with the help of three entities: (i) Logical Network construction, (ii) Address Publication phase, also called Anchor Request, and (iii) Lookup phase, also known as Address Resolution, and actual data forwarding. 1. Logical network construction: In network bootstrapping, nodes compute LIDs and form a logical, structured network (i.e., LS) over the ad hoc topology of MANET. For this purpose, a periodic hello message containing necessary information (depends on protocol specification) is exchanged among the neighboring nodes. 2.