# Survey on Routing Protocols for Vehicular Ad Hoc Networks Based on Multimetrics
## 5 Discussion  
As we see in the summary of Table 5, several proposals of routing protocols exist for vehicular networks. Recent improvements include metrics that are focused on the characteristics of this kind of network. An assumption of all proposals is the use of GPS due to the mobility present in them;  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
knowing the localization of each node is very important to make the best choice of route. Distance and speed are preferred metrics, because the former can be seen as a progress rate of destination reachability, and the latter accounts for the behavior of the vehicular networks. Then, link life (also called link stability or link quality) is due to a good choice because it follows the route that guarantees perdurable communication to complete the transmission of data. Vehicle density is another metric commonly selected by authors that guarantees the opportunity to have enough options to select the best path. Moreover, Table 5 includes a rough number of operations that the proposed metrics require for a routing decision. Thus, this number of operations, including the \"Metrics used\" column, is presented as a function of the number of neighbors \(n\). The provided number includes the effort to send or receive any additional signalling needed for it. From our results, metrics like mobility, trajectory, or link quality need more operations because they aim to predict future information. Of course, instantaneous values of these metrics are also possible to use, and usually only need one retrieve operation. In almost all protocols, a node uses a linear-dependent number of operations to make a forwarding decision by means of a weighted multimetric value for each neighbor. If there are not enough neighbors (low density), then some of the routing protocols use a well-known technique called carry-and-forwarding (maintain the packet until a good route could be selected). Other several important considered metrics exist that are mentioned in the summary, such as mobility, hops, direction, acceleration, and trajectory. It is important to study the scenarios and applications before selecting a protocol or in order to improve. It is important to highlight that all the protocols in Table 5 have only been evaluated using network simulators in order to review behavior at the moment of selecting the next hop or forwarding path. This is mainly because both in cities and rural areas, communication technology is confronted with challenges, as every car must handle communication in a very short time and ensure quality simultaneously, especially when it comes to a real-time request. Conducting real experimentation about novel proposals could present a risk. The simulation is one of the most often used methods for the performance evaluation of VANETs, but by considering the use of a realistic mobility model for evaluation results to correctly indicate the real-world performance of the system [60; 61]. However, as seen in [62] ETSI ITS-G5 (GeoNetworking protocol) was implemented and validated in a real environment in order to demonstrate the performance of the communication devices in real V2V with good results. In addition, [63] introduced functionality tests in two vehicular use cases using an information-centric networking protocol support in the ETSI ITS station protocol stack. Real experiments were made in the last few years to advance the implementation of these technologies in new vehicles in order to bring many benefits, like increasing road safety, improving traffic efficiency, and offering cloud services. The next step for these novel proposals is their experimentation in testbeds. As we see in Table 5, the majority of the proposals were evaluated in urban areas because, due to the presence of obstacles, it is a greater challenge than highways, as there are quick topology changes.