## 4 Next Forwarding Node Selection in UWSNs
### Taxonomy of Next Forwarding Node Selection in UWSNs
#### 4.2.1 Localization Routing Protocols  
In the localization category, to select the next forwarding nodes towards the sink, it is supposed that each node already has location information about itself and sinks using GPS. The main idea of all localization routing protocols is to employ location information for opportunistic routing, even though they have major differences in their ways of finding neighboring nodes toward the sinks. The localization protocols take into account specific shapes between the sink node and the forwarder node to select the next forwarding nodes towards the sink such as layer [39], cone [40], zone [41], and virtual pipeline [25; 31; 42]. Based on the selection mechanism of forwarding nodes, localization routing protocols can be divided into three categories: virtual shape, energy-efficient, and void-aware. The following paragraphs discuss these subcategories in detail regarding the selection of the next forwarding nodes. Virtual Shape  
In localization routing protocols, selecting the next forwarding nodes can be done based on designing specific shapes between the sink and forwarder node such as layer [39], cone [40], zone [41], and virtual pipeline [25; 31; 42]. These shapes have a direct impact on the performance of the protocol. More precisely, in big shapes, the packet delivery ratio is totally increased, and the chance of void areas decreases with an increase in the number of forwarding nodes. Moreover, the energy consumption increases when increasing the number of nodes, which has a direct impact on reducing the performance of overhearing algorithms. On the other hand, in small shapes, the performance and path selection are improved, the packet delivery ratio is totally decreased, and the chance of void areas increases with the decrease in the number of forwarding nodes, which can reduce the possibility of selecting the best forwarding nodes. To summarize, the inapplicability of GPS in an underwater environment causes designing the virtual shapes to be very costly. Moreover, all of the proposed routing protocols in the location-based category are designed based on virtual shapes, and the performance can be directly affected by a shape's size and type. The following paragraphs discuss the localization routing protocols belonging to this subcategory in detail. Figure 1: Taxonomy of next forwarding node selection in UWSNs. Vector-Based Forwarding (VBF) Routing Protocol provides a solution for one fundamental problem in UWSNS: robust, scalable, and energy-efficient routing [42]. A Location-Based Routing Approach, named Vector-Based Forwarding Protocol for Underwater Sensor Networks (VBF), was proposed. VBF is the first localization routing protocol by which each node knows its sink, neighbors, and own location information. In order to select the next forwarding node, each sender node uses a fixed virtual pipeline from source nodes to the destination/sink (distance to the sink) with a threshold value equal to the predefined radius value, as shown in Figure 2. Only nodes that are located inside the pipeline, taking into account the threshold value, can be selected as forwarding nodes. Figure 2 illustrates an example of how to select the next forwarding nodes. Assume that nodes A, B, and C are source nodes. Therefore, they create a virtual pipeline toward the sink and then each source node embeds its sink location and its own location in the header of the data packet and broadcasts it. Each receiver node calculates its distance to the created vector. If the computed distance is less than the threshold, it considers itself a candidate to forward the data packet because it is located inside the virtual pipeline. Thus, it accepts the data packet, updates the data packet's header information, and then broadcast it; otherwise, it discards the data packet. In order to improve the network traffic and energy consumption in dense deployments, a desirable factor and a time interval delay are calculated by each eligible node to locally identify the density and decrease the number of forwarder nodes.