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Top1. Introduction
Due to the rapid growth of communications and computing resources that leads to exchange the data among wireless mobile devices, like laptops, cell phones, portable devices, and the tablets, regardless of guaranteed any end-to-end connection exists. Thus, the DTN faced the challenges on the communication between the devices that loss connectivity continuously because of mobility (Wan, et al., 2015). However, the DTN is the network paradigm in which the connectivity between mobile nodes are not guaranteed continuously. In the network, each node participates in hop by hop, message routing among the destination and source of message. In addition, the DTN nodes containing less capacity of buffer and battery. Additionally, the mobility of the nodes are increased in network that communicates complex from the end to end among the nodes (Fall, 2003; Ababou, et al., 2018). Thus, the DTN is introduced for solving the challenging situations in restricted networks with intermittent disruption, less energy, and the sparse density. Thus, the DTN having frequent disconnections due to low transmission range, nodes mobility, and the dynamic topology, along with several features, self-organizations, without central administrations, and self-organizations (Fall & Farrell, 2008; Guo, et al., 2017). The messages that are delivered through forward paradigm and store carry to reached their destinations (Ayub & Rashid, 2019).
The DTNs is introduced for the communication purpose in several fields, such as post-disaster scenario, interplanetary communication in the space, rural communication and so on. Unlike conventional wireless or wired networks, the communication is performed dynamically at the node pairs where the node movements are unpredictable in the network (Ababou, et al., 2018; Roy, et al., 2018). The DTN application are as follows: underwater communication, urban areas, military purposes, and deep space communication. In the recent years, the NASA works with DTN in the Deep space project (Pathak, et al., 2017), but still the DTN faces several limitations. For instance, due to the absence of synchronous end-to-end connectivity, and the problem in forwarding the messages to destinations that results to limited message delivery rate and long transmission delay. The constraints parameters, like communication bandwidth, battery power, and the storage capacity are considered to increase the delivery rate of message. The DTN refer to the specific routing schemes for forwarding messages for delivering the messages based on probability-based (Musolesi & Mascolo, 2009; Dini & Duca, 2012) and epidemic-based (Lindgren, et al., 2003; Dini & Duca, 2012). In probability-based approaches, the sender sends the message to the node with highest probability of the message delivery (Dini & Duca, 2012) whereas, in epidemic, the several messages are transmitted and one reaches the receiver. Hence, the routing is very challenging due to the DTN characteristics (Voyiatzis, 2012; Guo, et al., 2017). DTN routing protocols (Guo, et al., 2017) are partitioned into multi copy and single copy protocols. In single (Spyropoulos, et al., 2004), the node forwards the unique message copy to their neighbors. Therefore, the message containing one carrier for reaching their destination. However, the single copy protocols needless resources with the long delivery delay. Thus, the multi copy protocols (Spyropoulos, et al., 2007) forward and create various copies of every message. For example, the epidemic protocol produces several copies of the buffered messages and passes to the connected nodes, and thus results higher buffer space consumption, bandwidth and energy. Here, the mobile nodes are not equipped with large resources and the different solution is needed for managing resource requirement (Ayub, et al., 2017). The performance of routing is significantly improved by exploiting the utilization of regular patterns. Another scheme, named 3R (Vu, et al., 2011) is fine-grained history-enabled routing where the pair-wise encounters contact time is recorded accurately. When maintaining fine-grained encounter history, the predictability of encounter in the 3R is the time-dependent. Consequently, the 3R is also the forwarding-enabled approach (Wan, et al., 2015).