## 2 Related Works  
As mentioned before, UWSNs are different from WSNs on land. Many routing protocols used in WSNs are either inapplicable, or have low efficiency in UWSNs. Recently, some researchers have been designing routing protocols to support the application of UWSNs. In vector based forwarding (VBF) [6], the sensor nodes need not acquire the state information. From the source node to the destination node, VBF constructs a pipe using a vector. The nodes in the pipe may be selected as the forwarding nodes; the nodes outside the pipe do not participate in forwarding. However, VBF is a location-based routing approach. It needs information on the location of all the nodes, which is another problem. According to the pressure of sensor nodes under water, in [7; 8; 9] the authors proposed routing protocols based on pressure. [PERSON] et al. proposed depth-based routing (DBR) in [10], which needs only local depth information. To acquire the depth, each underwater sensor is equipped with a pressure sensor. After receiving a packet, the sensor node judges whether it is a candidate as forwarding node by the difference between its depth and the sender's. Maybe there are many candidate nodes; so, to avoid all the qualified nodes forwarding packets redundantly, a technique of suppressing redundant forwarding is introduced in DBR. The location information of nodes is not necessary in DBR. However, DBR has some defects; for example, link quality and residual energy are not considered, and essential redundant forwarding in DBR leads to considerable energy consumption. Therefore, many improved protocols have been proposed based on DBR. EEDBR considers the depth and residual energy in [11]. Due to the complex underwater environment, considering only residual energy is not enough. The depth-controlled routing (DCR) protocol was proposed in [12]. In DCR, the underwater sensor nodes are equipped with network topology controllers, which are used to adjust their depth when the greedy geographic routing fails. Clearly, DCR is a geographical routing protocol, and it requires geographical location information in order to control the topology structure, which is so far an unsolved problem. In [13], Tairq proposed the L2-ABF protocol. In L2-ABF, the underwater 3D-space is divided into layers. To save energy and decrease redundant forwarding, a node first calculates its flooding angle. The underwater sensor nodes forward data only to the nodes in the shallower layers. If a node fails to find a qualified next-hop after broadcasting a control packet at initial power and angle, then it increases the transmitting power as well as the broadcasting angle. L2-ABF is a location-based routing protocol. However, it does not provide a specific scheme to determine the initial flood angle. After all, the initial angle is related to node density. In addition, the priority of receiver is set according to the residual energy threshold, which leads to redundant forwarding. In [14], the link expiration time-aware routing protocol (LETA) was proposed. In LETA, the priority of candidate nodes is calculated based on the Bayes theory, in which the depth, residual energy and distance are used as the parameters for calculating forwarding probability. However, LETA needs to acquire location information, and to calculate new location when nodes are mobile, which is complex and energy intensive. The channel-aware routing protocol (CARP) was presented in [15], which is location-free. In CARP, the depth is considered as well as the link quality. The node broadcasts a \"ping\" packet to search a route, which consumes a lot of energy. To improve the energy efficiency, [PERSON] et al. proposed E-CARP [16]. In E-CARP, the most recently used node is preferentially selected as the next-hop. Due to the dynamic topology, the most recently used node may be not the best next-hop. The directional flooding-based routing protocol (DFR) was proposed in [17]. DFR increases the directional angle in flooding.