## 1 Introduction  
In the context of Intelligent Transport Systems (ITS), Vehicular _Ad Hoc_ Networks (VANETs) are considered the key technology required to radically improve safety [1,2]. Their importance can also be inferred from the growing interest that VANETs have drawn from entities directly involved in ITS research and development, [3, 4, 5]. In fact, according with the National Highway Traffic Safety Administration, VANETs (also called vehicle-to-vehicle communications or V2V) potentially address 79% of all pre-crash scenarios involving unimpaired drivers [1]. Furthermore, it is also expected that a wide range of applications will become available by enabling V2V communication or vehicle-to-infrastructure communications (V2I). Some examples of important applications include safety [6], Internet connectivity [7], entertainment through multiplayer games [8], file sharing support [9], lane change assistance [2] and traffic control [10]. For several years, consortiums and car manufacturers around the World have focused their efforts on enhancing driver safety, comfort and efficiency [11, 12, 13, 14, 15, 16]. These efforts have already provided meaningful results, which range from simulation tools [17] to real prototypes of vehicular safety applications [18]. A significant result is a family of international standards that shares a common architecture, networking protocols and air interface definitions for VANET wireless communications. This family of standards is called Communication Access for Land Mobile (CALM) [19], which was adopted by the European community in 2006. Additionally, the Institute of Electrical and Electronic Engineers (IEEE) has significantly contributed to VANET development by providing a series of standards for Dedicated Short-Range Communications (DSRC), such as the IEEE 802.11p standard [20]. The research community has also been actively contributing in different research areas related to the development of VANETs. Examples of active research topics in VANETs include medium access control methods [21], novel applications [8,9], security [22,23] and routing protocols [24, 25, 26]. In VANETs the network topology is _ad hoc_ in nature. Hence, no infrastructure beyond the network adapter inside the vehicles is necessary. Importantly, the self-dependence of VANETs offers two main advantages: ubiquitous information sharing and low implementation costs. Although there are several communication standards that can be used to establish a radio link between vehicles in a VANET such as mobile WiMAX [27], IEEE 802.11b radios [28, 29, 30] and Bluetooth [31, 32, 33], currently the IEEE 802.11p standard is emerging as the most prominent option. The IEEE 802.11p standard is based on the physical layer (PHY) of the IEEE 802.11a standard. The main difference is that IEEE 802.11p limits the channel bandwidth to 10 MHz and sets the operating frequency to 5.9 GHz [34]. Importantly, the Medium Access Control (MAC) of IEEE 802.11p is still based on the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) method. Despite the advantages offered by CSMA/CA, a significant issue arising when using this MAC mechanism is that it cannot guarantee message dissemination by itself. Another issue with CSMA/CA is that it cannot provide a time frame for packet delivery because of the exponential back off mechanism. Lastly, it is worth mentioning that IEEE 802.11p adapts the enhanced distributed channel access (EDCA) mechanism of the IEEE 802.11e standard (with some modifications to the transmission parameters). Shortly, the EDCA mechanism defines different distributed coordination function (DCF)-CSMA/CA parameters for four different traffic classes: background, best effort, video and voice, [20, 35, 36]. Currently, the design of routing strategies represents one of the most important research topics for VANETs. In fact, routing is one of the most critical and challenging components that must be solved to enable V2V communications.