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Top1. Introduction
Mobile Ad hoc Networks (MANETs) are an infrastructure-less wireless networks which consist of wireless portable devices such as Laptops and smart phones with wireless technology such as blue tooth and IEEE 802.11/Wi-Fi, that allow nodes to communicate with one another by establishing a connected path between source and destination before transmitting the message. However, there are a number of situations where the node exhibits intermittent connectivity among them due to the node mobility and the short transmission range. The Network with these properties, called Intermittent Connected Networks/Opportunistic Networks, cannot guarantee that a fully connected path exists between two nodes during the session of message transmission. Hence, Ad hoc Network’s (Crow et al., 1997) routing protocols such as Dynamic Source Routing (DSR) (Johnson et al., 2001), Adaptive On Demand Vector (AODV) (Perkins et al., 2003), and Fish Eye State (Pei et al., 2000) are unable to deliver messages in such a kind of scenarios. Because, they employ route discovery process before starting the data transmission, assuming an end-to-end connected path exists between any pair of nodes. However, it fails to continue the forwarding process when the discovered route becomes stale; i.e., when the topology of the network changes dynamically while the message is in transit, the established path is no longer useful. If the network exhibits intermittent connectivity among nodes, it cannot guarantee a complete path between source and destination during the message transmission. Extensive improvement in wireless technology and wireless portable devices, makes data transmission possible between any pair of devices located under decentralized control networks. Therefore, different kinds of application scenarios such as Emergency search and rescue operation, Military mission, Wild Life’s Activity monitoring system, Interplanetary Networks, and Underwater Acoustic System employ Intermittent Connected Networks/Opportunistic Networks to full fill their determined goals even though they make routing a challenging issue. So, there is a need to find a way to route the message exploiting the contacts which would occur between nodes by node’s mobility opportunistically.
Recent research works (Chen et al., 2016; Lin et at., 2018) focus on developing opportunistic routing protocols which use ‘store-carry-forward’ routing paradigm [Fall Kevin, 2003]. The 'store-carry-forward' routing allows a node with a message for another node, to store and carry the same until it gets an opportunity to forward the message to another candidate node which has the capability of routing the message towards the destination. By opportunistically forwarding the message from one node to another, the message gets the chance of reaching the intended node. However, the problem before the nodes is whether the node should replicate the message and which message it should replicate whenever contact occurs. With all the constraints of mobile devices such as limited buffer, bandwidth and energy, routing is one of the challenging issues (Dalcy and Haahr, 2010; Krifa et al., 2008; Wang et al., 2015).
Routing protocols for opportunistic networks are broadly classified into three types (Ajaleely et al., 2017): Forwarding-based (single-copy), Replication-based (multi-copy) and Hybrid. Forwarding- based protocols keeps only one copy of the message in the network at any point of time; therefore, each node with a message either sends it to destination directly or forwards it to only one neighbor according to relay selection criteria and it incurs high delay in message delivery with the least overhead (e.g., Direct Delivery (),First Contact algorithm (Jain et al., 2004)). Replication based protocols, on the other hand, create duplicate copies of the message in order to increase the chance to deliver the message towards the intended destination; therefore, it achieves great delivery rate with high overhead. In hybrid routing, both replication and forwarding mechanisms are combined; some nodes take responsibility for replication and others do the forwarding. In this paper, a hybrid routing protocol named Replica Reduced Energy Based Routing (REB) is proposed which employs two types of forwarding conditions such as Replica reduced forwarding and energy-aware forwarding. Replica reduced forwarding avoids forwarding huge amount of duplicate copies of the message and energy-aware forwarding makes a message to be forwarded from a source before the source node is down due to lack of energy.