# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 3 Review on Opportunistic Routing Void Avoidance Protocols for UWSNs
In order to achieve this, the void-flag field in the packet header is set to \"0,\" indicating that only nodes whose depth is less than the current forwarder are eligible to be chosen as candidates. However, the protocol switches to void recovery mode when the packet is stuck in the void node as, illustrated in Figure 6:4  
The current node will retransmit the data packet in a void recovery mode, where the value \"1\" is entered in the void-flag field, allowing the neighbor nodes with the greatest depth to be chosen as candidate nodes. A node should only forward packets with the same ID once for a predetermined period of time in order to reduce duplicate transmissions, comparable to the DBR. As a result, the new candidate set of current nodes would continue the forwarding process. The next best packet forwarder from the current node will then transmit packets in a greedy manner toward the sink if no other the void node is reached. Figure 6:5 Network Topology with Two Embedded Sinks [54]. RPSOR  
In [56], another novel OR protocol called Reliable Path Selection and Opportunistic Routing (RPSOR) for UWSNs is presented to address the void area problem in UWSNs. It is an improved version of the WDFAD-DBR protocol. RPSOR operates in two stages: knowledge acquisition and packet forwarding. In the knowledge acquisition stage, nodes exchange their information through hello packets sent from surface sinks, neighbor request packets, and ACK packets generated by each sensor node. In addition, the node maintains three different tables, which are the source info table, the first hop info table, and the routing table. Furthermore, in the packet forwarding stage, the decision for PFN selection will be made based on the priority function, which is defined by three elements: the reliability index, the advancement factor, and the shortest path index. RPSOR only selects the nodes of the upper hemisphere as the forwarding neighbors. Therefore, nodes having higher a depth than the current node simply drop the packet. In RPSOR, two sinks are mobile, as can be seen in Figure 6:6 Mobile sinks are utilized to travel to denser network areas that experience high traffic. At the beginning of each simulation round, the network uses hello messages to assess the node density at various hops, and it then permits the sink to travel to any hops with a high node density. The nodes located at the following hop must transmit a large amount of load created by denser network locations. The majority of the packets are lost when this high traffic enters the network's sparse area since the network cannot handle such high traffic levels. Utilizing the position data of the denser hop, which was acquired by the greeting message, the sink determines the vertical trajectory. **PCR**  
Recently, a novel power control-based opportunistic (PCR) routing protocol for the Internet of Underwater Things (IoUTs) was proposed in [57]. They develop opportunistic routing and transmission power control methods in order to send data in IoUTs with the least amount of energy possible. Each node in PCR checks many transmission power levels before selecting its candidate set for the next-hop. The PCR protocol implements a periodic beaconing technique during the neighbor discovery phase for each transmission power level in order to gather information from neighbors and update the neighbors table. The candidate set will be expanded to include the neighbor node exhibiting positive packet  
Figure 6:7 EDORQ void recovery mode [55]. progress. The appropriate transmission power level and the next-hop forwarding set are then computed based on the energy waste for each candidate set. Hence, the set of candidate nodes with the least energy waste is chosen as the best candidate set to continue forwarding the packet to the next hop until the packet reaches the destination. The nodes in the candidate set will then be sorted based on their normalized packet advancement to define the node's priority.