# Survey on Routing Protocols for Vehicular Ad Hoc Networks Based on Multimetrics
###### Abstract
The former can be seen as a special case of V2V routing protocols, so almost no survey distinguishes between them. A typical classification of routing protocols is presented in [14] and it is  
Figure 1: Example of a vehicular ad hoc network (VANET) scenario: accident-information dissemination using vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) to send data to emergencies services. based on transmission strategies in which a protocol can be unicast, multicast, broadcast, or geocast. This well-accepted classification of routing protocols was used in the survey of [7; 8]. Moreover, unicast protocols were further split based on routing information in topology or position-based in [15; 16], and cluster-based in [13]. According to a recent survey [17], unicast routing protocols are organized in the categories of topology, geographic, hybrid, clustering, opportunistic, and data fusion. [PERSON] et al. [9] presented a routing-protocol survey with both, based on transmission strategies and routing-information classification. It is clear that the second assortment is a subclassification of unicast protocols. Geographical routing protocols for VANETs are an important subset of unicast protocols into protocol classification based on routing information because they make their forwarding decision by using local information; therefore, this kind of protocol can react fast to frequent topology changes. As a consequence, many surveys specifically target geographical (also known as position-based) protocols in the literature. In [18], the authors explained the importance of a geographic protocol in VANETs and provided an updated survey. More specific reviews of geographical-routing proposals have been published over the years. In [19] only greedy approaches for geographical routing were presented, while [20] reviewed geographical routing for Vehicular Delay-Tolerant Networks (VDTNs) and group proposals according to the geographical knowledge needed for their operation. The traffic-aware classification of geographical-routing protocols proposed in [21] identified protocols that use surrounding information to improve communication performance. Surveys of VANET routing protocols along years show that the forwarding criteria, especially in geographical protocols, have evolved from using only one metric to more novel proposals that employ several metrics, like vehicle speed and direction. In this paper, we concentrated on unicast routing protocols designed for VANETs using different metrics in hop-by-hop selection to improve vehicular communications. We present the principal metrics, their importance in vehicular scenarios, and which of them are selected by the proposals explained below. The rest of this paper is organized as follows: Section 2 presents the main characteristics of VANETs that are relevant in the context of the paper. After that, Section 3 presents the systematic literature-review process used in this survey. Section 4 presents the most common metrics used by researchers in their routing protocols and their multimetric proposals. Next, Section 5 discusses our findings, and Section 6 presents conclusions.