## 2 Related Work
The location service includes a retransmission mechanism, which consists of retransmitting the broadcast packet if the original sender does not hear the transmission of the same packet from a forward neighbor. Although this retransmission mechanism helps overcome the loss of packets, it could also lead to unnecessary retransmissions. Furthermore, route replay is performed by a unicast strategy, which has one main drawback: other nodes may not be warned about the search. This may lead to unnecessary searches, with the consequent waste of resources. The GPSR-L protocol was introduced in [49] with the aim of overcoming some of the problems shown by GPSR when used in VANETs. In GPSR-L, a lifetime threshold for each link is introduced in the neighbors table in order to overcome the lack of a prediction mechanism in GPSR. With the introduction of this threshold, GPSR-L aims to overcome erroneous next-hop selections caused by old information stored in the neighbors table. Nevertheless, for highway scenarios, erroneous next-hop selections caused by the channel can still occur as a deterministic radio channel was assumed in [49]. The LORA-CBF protocol is a position- based routing algorithm introduced in [54]. This protocol implements a clustering algorithm where only the gateway nodes are allowed to retransmit packets. Nevertheless, since freespace propagation conditions are assumed in LORA-CBF, the clustering algorithm does not guarantee cluster stability in highway scenarios. This may lead to packet losses and/or delays, because it may be difficult to form clusters or an excessive number of clusters may be formed (consisting of a single node) depending on the particular channel conditions at any given moment. Another problem arising with LORA-CBF in highway scenarios is that the most forward within radius neighbor is selected as the next-hop when disseminating data. As it will be explained in the following sections, this assumption may lead to significant packet losses caused by the constraints imposed by the highway V2V channel. In [39], the authors introduced a routing protocol specifically developed for VANETs deployment in highway scenarios. This protocol was named Destination Discovery Oriented Routing (DDOR). Compared to routing protocols previously introduced for VANETs, the DDOR protocol offers advantages such as a built-in location service and a low complexity position prediction algorithm. The inclusion of these mechanisms is important, because they help to overcome erroneous next-hop selections. Additionally, DDOR proposes enabling the IEEE 802.11 RTS/CTS mechanism as a redundancy strategy for its location service and for the data dissemination stages. The LS included in DDOR is an important feature that makes DDOR more suitable for highway scenarios. Nevertheless, one weakness of DDOR is that its redundancy strategy is not controlled by itself. A consequence of this lack of control is that erroneous next-hop selections may lead to a significant overhead increase in the MAC layer. Despite this weakness, it seems that for highway scenarios DDOR overcomes several of the problems shown by the routing protocols previously mentioned. In fact, in [39] several performance metrics were provided for DDOR, showing that this protocol outperforms AODV, DSR and GPSR. It has been suggested that GPSR-L may offer good performance in highway scenarios compared to CAR, GPSR and GyTAR [26]. However, GPSR-L was not developed with the highway scenario in mind. Furthermore, for this scenario GPSR-L has the drawback that a packet retransmission strategy, a discovery service, and a strategy to handle the acceleration of vehicles were not implemented. The inclusion of these strategies in a routing protocol for highway VANETs is very important, because of the wireless channel constraints imposed by these scenarios may lead to significant packet losses. Furthermore, a self-dependent algorithm is more suitable for this kind of scenarios [26,39]. In contrast, DDOR was explicitly developed for highway scenarios.