## 4 Routing Layer
### Energy Efficiency  
In this sub-section, routing protocols that focus on energy efficiency are presented. Energy is a fundamental aspect in the type of network deployed; hence, it is usually the main topic in research literature in addition to throughput and delay. One widespread technique in radio frequency sensor networks which can be applied underwater is clustering. Hierarchical multi-path routing low-energy adaptive clustering hierarchy (HMR-LEACH) [111] is an enhancement of classic LEACH [112]. In HMR-LEACH, cluster heads must transfer data to the sink using multi-hop, and route selection is performed by assigning probabilities. HMR-LEACH is an appropriate algorithm for large-scale underwater networks because it prevents long-distance transmissions. Experiments show that this approach improves energy efficiency when compared with LEACH and LEACH-M [113]. Another approach that utilizes clusters is the location-based clustering algorithm for data gathering (LCAD) [114]. Its main goal is to improve the behavior of sensor nodes near the sink which rapidly consume their battery power. It is a protocol for 3D UWSNs where sensor nodes are deployed at fixed relative depths, being organized in clusters. An algorithm which takes into account the node location is used for cluster head choice. In LCAD, each cluster will have multiple cluster heads. In order to implement intra-cluster communication, nodes transmit through the horizontal links. The communication protocol during configuration performs the cluster-head designation. Afterwards, LCAD performs intra-cluster communication (nodes send messages to their head) and inter-cluster communication (heads send collected data to the sink being aided by autonomous underwater vehicles). LCAD behavior depends on the USN structure. Specifically relevant for the performance is the location of cluster heads. In USN, nodes can change location within the grid frequently because of underwater node movement, impairing performance. Continuing with the clustering approach, the main goal of the minimum-cost clustering protocol (MCCP) [115] is to enlarge the underwater network life, taking into account the non-uniform node distribution due to continuous movement of ocean currents. MCCP defines a cluster structure, being the clusters established as a function of the energy required by the nodes to transmit packets to the head, the position of the head and the sink, and the energy of the nodes. The heads must calculate a TDMA schedule that must be sent to the nodes inside the cluster. This protocol generates more heads near the sink, avoiding the creation of hot spots. This algorithm does not support multi-hop routing, and the period for the cluster definition is normally very large. This period will influence consumption in target environments where nodes are in continuous movement, moving between various clusters in this interval. The EUROP protocol [116] focuses on energy efficiency and response waiting time, mainly reducing broadcast packets. Each node is equipped with a pressure sensor in order to calculate its depth position, eliminating the necessity for 'hello' packets. The base station located on the water surface can receive information only from shallow water nodes. Detecting the pressure value, sensor nodes determine their corresponding layer and use defined messages to communicate through the acoustic channel, selecting the next hop by applying the rule from deep to shallow nodes. EUROP is not a complex protocol, but the cost per node will increase because of the necessary devices that upon addition will increase consumption. By minimizing the number of transmissions and moving the aggregation point, energy-aware data aggregation via reconfiguration of the aggregation tree (EADA-RAT) [117] improves the consumption and delays. This protocol reconfigures the tree by means of a dynamic pruning procedure to obtain temporal routes.