# A Geographical Heuristic Routing Protocol for VANETs
## 1 Introduction  
Vehicular ad hoc networks (VANETs) [1] are a special case of mobile ad hoc networks (MANETs), where nodes are vehicles that interchange data to establish and maintain communication. Vehicular applications can be divided into safety, vehicular traffic efficiency and infotainment applications [2]. Traffic flow control or environmental conditions monitoring are some aims of such applications. The efficiency-oriented applications require a continuous monitoring phase of the streets and city conditions. Vehicles can gather this information and feed the monitoring centers through the VANET. Vehicles can reach the watching center using Vehicle-to-Infrastructure (V2I) or Vehicle-to-Vehicle (V2V) communications, in a direct or multi-hop fashion, respectively. Challenges in VANETs, such as fast topology changes, low link lifetime or a potentially high number of nodes taking part in the network,have encouraged researches to propose geographical routing protocols for multi-hop communication in VANETs as an alternative to the classical topology-based routing approach. Geographical protocols, also known as position-based protocols, make their routing decision using local information, mainly nodes' positions. Two procedures can be recognized in the operation of geographical routing protocols: the forwarding mechanism, which defines the rules that a node follows to choose the next forwarding hop; and the recovery strategy, used by a node when it does not find any neighbor that meets the forwarding criteria. One of the first and widely used classifications of VANET routing protocols presented in [3] distinguishes two types of recovery strategy: the so-called \"carry and forwarding\" that consists of storing a packet until the node finds a suitable next forwarding node. This \"carry and forwarding\" strategy introduces in general high delay in data transmission. Hence, it is adequate for applications for Delay-Tolerant Networks (DTN), which are the target applications in this paper. The second recovery method is for Non-DTN protocols. There are two most common alternatives: (1) the use of the right hand rule; and (2) the construction of a recovery path through request/reply signaling messages. As the DTN recovery approach has increased its use in VANET routing protocols, a recent classification [4] of VDTN (Vehicular Delay-Tolerant Network) uses geographical knowledge needed by the routing protocols to differentiate them. This information can range from only the necessity of get the geographic location of nodes, to use road maps or even to use on-line information. Regarding the forwarding mechanisms in geographical routing, there are some classifications depending on the factor used to differentiate them. According to [3], geographical protocols can be classified in non-overlay, in which all of the nodes of the network make routing decisions; and overlay, if only some nodes of the network are allowed to change the routing decision of a packet. A recent classification of geographical routing protocols that consider traffic and network status in their routing metrics [5] identifies protocols that construct the full path using information like distance between nodes and vehicles' density, among others, which is not the common approach. Other protocols make routing decisions in junctions or the anchor or in each node. Updated surveys of geographical routing protocols [5; 6; 7] show that the forwarding criteria of these protocols have evolved from considering only the geographical distance between nodes as in Greedy Perimeter Stateless Routing (GPSR) [8] to other routing proposals that require detailed geographical information and include other additional metrics, like the speed and direction of the vehicle. Nevertheless, despite the enhancements in the routing decisions, the selection of the best node as the next hop maintains as predominant in the routing criterion among most of the geographical routing protocols.