## 4 Next Forwarding Node Selection in UWSNs
### Energy Efficiency  
As mentioned before, energy efficiency is one of the main goals that attracts researchers designing routing protocols. In this subcategory, the main aim is to reduce the energy consumption and improve the network lifetime by using residual energy factor or some of the factors that help balance the energy consumption between sensors. The following paragraphs discuss the localization routing protocols belonging to this subcategory in detail. The Depth-Based Routing (DBR) Protocol is considered as a first pressure routing that provides scalable and efficient routing services in UWSNs [37]. This protocol does not need full location information for the sensor nodes; it only requires local depth data. In this protocol, in order to select the next forwarding nodes, the sender node tries to find the best neighboring nodes regarding depth information to forward the packet. In other words, neighboring nodes with less depth than the sender will be candidates for forwarding the packet, as shown in Figure 10. DBR follows a specific routing procedure, starting with the sender node broadcasting the packets, including its depth information, to its one-hop neighbors. After receiving the packets, the neighboring node compares its depth with the sender's depth, which is embedded in the packet. This node will be a candidate for forwarding the packet if its local depth is less than the sender depth. Otherwise, the node directly discards the packet. After that, the candidate nodes broadcast the packet, including their depth, to their one-hop neighbor  
Figure 9: Layer assignment phase in APCR. nodes, and so on. This technique uses hop-by-hop. In each hop, the data packet is sent to a node with a lower depth than the sender until it is delivered the sink. Moreover, for each received packet, each forwarder node calculates the holding time based on the depth information. The candidate nodes start forwarding the packets if the holding time finishes. Therefore, the node with the lowest depth is the best sender node. Upon overhearing the packet, the candidate node removes the packet from its buffer. Otherwise, it will forward the packet after its holding time is finished. DBR has some advantages. The employment of multi-sink helps reduce the energy consumption for nodes placed closer to the sink. Moreover, it does not require any location information as it calculates the depth locally. However, DBR does not provide efficient next forwarding node selection due to the lack of energy and reliable metrics. It uses only depth information in terms of forwarding and holding the data packet. Moreover, it still does not provide any solution to the communication void problem. Furthermore, it selects the shortest path based on depth information only, which has a direct impact on reducing the energy consumption due to always selecting the node with the lowest depth. Based on DBR, a new routing protocol called Depth-Based Multi-hop Routing Protocol (DBMR) has been proposed [47]. This protocol tackles an energy-efficient routing problem in an underwater sensor network by using the multi-hop mode of each node to send packets, thereby reducing the communication cost. In addition, this protocol takes advantage of multiple sinks without introducing extra cost. DBMR consists of two phases, namely the route discovery phase and sending packets. In the route discovery phase, the process of selecting the next forwarding nodes has been designed as follows: each node discovers the next hop node by calculating its depth. Then, it broadcasts its ID and depth information as a control message. Next, it waits for a specific length of time to receive the reply message. After receiving the control message, the neighboring node will compare its depth with the depth included in the control message.