## 4 Routing Layer
### Dense Networks  
In this sub-section, the routing protocols mainly dealing with dense networks are discussed. Such networks are characterized by high node density in the geographical area of the network deployment.  
The main goal of focused beam routing (FBR) [104] is to reduce flooding. Every node knows its location and the end destination location. This protocol defines the paths during the traversing of a data packet in a dynamic manner, calculating the next hop at each step. On the other hand, FBR presents some problems, e.g., the possibility that no node will lie within the appropriate area because of nodes becoming sparse owing to water movements. Additionally, when a node is unable to find the next relay node, it needs to send the RTS again (by broadcast), thus increasing the overhead and affecting packet deliveries in sparse zones.  
Another protocol that dynamically calculates routes is multipath routing (MPR) [105], which constructs a route from sender to receiver using several multi-sub-paths, which are sub-paths from the sender to the two-hop neighbors. In this manner, relay nodes check the transmission schedule without delay to verify if a collision has occurred. The forwarder node must defer transmission in the case of collision. Results shows that MPR outperforms other approaches, but its main disadvantage is that at low velocity, it has a higher overhead value.  
A different approach is the one followed by the path-unaware layered routing protocol (PULRP) [106], which is intended for dense 3D USNs. The first step of this algorithm is dedicated to a layering task, where spheres are structured around a sink. In the second step, the selection of the intermediate nodes and normal communication is performed. The main properties of PULRP are that it does not require fixed routing tables, synchronization, or localization. Its main advantages are its gooddelays and message delivery rate. However, its main disadvantage is that the radii of the spheres have a significant impact on message delivery.  
Further improvements were made to this protocol in order to achieve improved energy efficiency. Energy optimized path unaware layered routing protocol (E-PULRP) [107] improves the previous PULRP protocol from a networking energy-balance point of view. Energy balance is performed by selecting the widths of different layers and calculating the minimum consumption and the probability of successful transmissions. The goal of this algorithm is to face the mobility problem without requiring synchronization or localization. The main disadvantages of this approach are that, depending on the scenario, the relation between consumption and throughput varies and impacts the delays.  
Finally, the information-carrying-based routing protocol (ICRP) [108] is a reactive protocol intended for scalable routing and reduced consumption. The routing task is performed by a small number of nodes. The sender initiates the path establishment. The sender must broadcast the message in case there is no established path, carrying the path discovery packet. The nodes that receive this message must broadcast it and save the reverse route. When the final receiver node gets this message, it is possible to obtain the complete reverse route. All routes have timeouts, with it being necessary for a route to be discovered by the defined timeout.  
ICRP evaluation highlights several performance drawbacks. Because decisions are based on saved information, ICRP is not appropriate for underwater networks where nodes are in continuous movement. The other relevant drawback is due to the necessity to broadcast packets if one node does not have a route to reach a destination, which increases power consumption.