## 4 Next Forwarding Node Selection in UWSNs
### Impact of Next Forwarding Node Selection on the Design of Routing Techniques  
Next forwarding node selection is one of the most common issues in routing protocols that must be taken into consideration [4; 6; 7; 35]. This issue attracts researchers to design a next forwarding node algorithm. In routing, the general structure of this algorithm is as follows. Each source node selects a group of its neighbors. These nodes have been chosen based on different metrics such as residual energy, physical distance, and link quality. The nodes then hold the data packet for a certain length of time based on various criteria such as distance, sound propagation speed, transmission range, and residual energy to avoid collision and redundant packet transmission. The best node has the lowest holding time, so it has the highest chance of forwarding the data packets. If a node overhears the forwarded data packet, it simply removes the data packet from its buffer. Otherwise, it waits until its holding time finishes to forward the packets. Therefore, these algorithms have a direct impact on performance indicators such as packet delivery ratio and total energy consumption.  
The use of different metrics for selecting the next forwarding node has a direct impact on the overall performance of routing protocols. The use of a residual energy metric helps in balancing the energy between nodes. Link quality is another important metric that has a direct impact on improving the packet delivery ratio and reducing energy consumption [36]. The use of a depth metric helps in reducing the energy consumption because each node calculates the depth locally, whereas the physical distance can be calculated using beaconing messages provided by the sink. The use of GPS for identifying the location of the node is costly due to the inability to use GPS in underwater environments. Therefore, it is essential to design a forwarding node selection algorithm based on multi-metrics, which selects energy-efficient and reliable forwarding nodes to reduce energy consumption, reduce network traffic, and ensure data delivery.  
Communication void is one of the significant issues that occurred while selecting the next forwarding nodes. This problem occurs if the sender node does not find any neighbor node in its transmission range [11; 25; 37; 38]. Since UWSNs suffer from dynamic and sparse network topology, these type of topologies impose a low packet delivery ratio and further minimize throughput due to a lack of algorithms that handle the communication void problem. Moreover, the use of weak void avoidance algorithms entails high energy consumption. In TWSNs, void-handling algorithms are not efficient to use in harsh environments due to the specific characteristics of UWSNs [37]. Therefore, avoiding the void nodes in the process of selecting the next forwarding set is necessary to improve the packet delivery ratio and ensure data delivery.  
Selecting the shortest path is one of the fundamental issues that has a direct impact on the number of transmissions, energy consumption, and network lifetime [7]. Selecting a node that has less depth than the sender in transmission range is a common way of handling the problem of reducing the number of transmissions and the number of nodes selected to forward the data packets. The main disadvantage of this algorithm is that it totally depends on the distance between the sender node and the receiver node, without taking into account different metrics such as residual energy and link quality. Consequently, it is necessary to enhance path selection while selecting the next forwarding nodes, which will help by reducing the total number of data packets forwarded, suppressing unnecessary forwarding, and reducing the total energy consumption.