## 2 Related Work  
Achieving reliable delivery of data and energy efficiency are two crucial tasks in WSNs. As the sensor nodes are mostly deployed in an unattended environment and the likelihood of any node going out of order is high, the maintenance and management of topology is a rigorous task. Therefore, the routing protocol should accommodate the dynamic nature of the WSNs. Opportunistic routing protocols developed in the recent past years provided trustworthy data delivery but they are still deficient in providing energy-efficient data transmission between the sensor nodes. Some latest research on OR, experimented by using the formerly suggested routing metrics and they concentrated on mutual cooperation among nodes. GeRaF [10] (Geographic Random Forwarding) described a novel forwarding technique based on the geographical location of the nodes involved and random selection of the relaying node via contention among receivers. Exclusive Opportunistic multi-hop routing for wireless networks [11] (ExOR) is an integrated routing and MAC protocol for multi-hop wireless networks, in which the best of multiple receivers forwards each packet. This protocol is based on the Expected Transmission Count (ETX) metric. The ETX was measured by hop count from the source to the destination and the data packet traveled through the minimum number of hops. ExOR achieves higher throughput than traditional routing algorithms but it still has few limitations. ExOR contemplates the information accessible at the period of transmission only, and any unfitting information because of recent updates could worsen its performance and could lead to packet duplication. Other than this, there is another limitation with ExOR, as it always seeks coordination among nodes that causes overhead, in case of large networks. Minimum Transmission Scheme-Optimal Forwarder List Selection in Opportunistic Routing [12] (MTS) is another routing protocol that uses MTS instead of ETX as in ExOR. The MTS-based algorithm gives fewer transmissions as compared to ETX-based ExOR. Simple, practical, and effective opportunistic routing for short-haul multi-hop wireless networks [13]. In this protocol, the packet duplication rate was decreased. This is a simple algorithm and can be combined with other opportunistic routing algorithms. Spectrum Aware Opportunistic Routing [14] (SAOR) is another routing protocol for the cognitive radio network. It uses optimal link transmission (OLT) as a cost metric for positioning the nodes in the forwarder list. SAOR gives better QoS, reduced end-to-end delay, and improved throughput. Energy-Efficient Opportunistic Routing [15] (EEOR) calculates the cost for each node to transfer the data packets. The EEOR takes less time than ExOR for sending and receiving the data packets. Trusted opportunistic routing algorithm for Vanet [16] (TMCOR) gives a trust mechanism for opportunistic routing algorithm. It also defines the trade-off between the cost metric and the safety factor. A novel socially aware opportunistic routing algorithm in mobile social networks [17] considered three parameters, namely social profile matching, social connectivity matching, and social interaction. This gives a high probability of packet delivery and routing efficiency. ENSO-opportunistic routing algorithm for relay node selection in WSNs is another algorithm where the concept of an energy-efficient node is implemented [18]. The packet delivery rate of ENSO is better than GeRaF. Economy--a duplicate free [19] is the only OR protocol that uses token-based coordination. This algorithm ensures the absence of duplicate packet transmissions. With the advent of the latest network technologies, the virtualization of networks along with its related resources has made networks more reliable and efficient. The virtual network functions are used to solve the problems related to service function chains in cloud-fog computing [20]. Further, IoT works with multiple network domains, and the possibility of compromising the security and confidentiality of data is always inevitable.