## 2 Related Work  
Many routing algorithms have been proposed for VANETs and WSNs, including geographic routing protocols. Geographic routing protocols address several limitations of topology-based routing protocols by using the nodes' geographic position information, the destination position, and the next neighbor's location obtained from the GPS to find the route [7]. The use of beacon messages sent periodically allows for a scalable network adapted to a dynamic environment such as VANET. There are three categories of geographic routing protocols:  
* Delay tolerant networks (DTN): In the dynamic topology, the routing protocols suffer from communication disconnections, latency, and high error rates. The DTN protocols overcome these problems and maintain network connectivity by using a packet forwarding strategy based on storing messages in the cache until a node is suitable for transmitting the packet;
* Non-Delay Tolerant Network (Non-DTN): This category of non-DTN routing protocols aims to overcome the problems encountered with the greedy approach. The greedy approach sends the packet to the nearest node toward the destination. However, it is possible that the forwarding node itself is closest to the destination. Many types of non-DTN protocols have been developed to handle this failure;
* Hybrid routing protocols: The hybrid routing protocols benefit from both position-based routing (DTN and non-DTN) and merge the two routing strategies to resolve network disconnect issues. In VANETs, the frequent changes in topology induce serious problems in scaling up traditional routing protocols. Most of the existing routing protocols are unable to handle frequent network disconnections. However, a few protocols such as Vehicle-Assisted Data Delivery protocol (VADD) [8], the Cross-Layer Weighted, Position-based routing (CLWPR) protocol in Refs. [97 ], and the optimized CLWPR (O-CLWPR) [11] are aimed at improving routing in disconnected vehicular networks in urban scenarios where they benefit from DTN by using a carry-and-forward approach until a relay is found. Thus, it takes a long time to deliver packets in low-density networks. The disconnection phenomenon in VANETs is known as \"local-maximum\" problems that occur when a packet reaches a node that is closer to its destination than any other node within range. For example, node x in Figure 1 is closer to the destination node D than its neighbors y and w, so node x will not choose to forward to y or w using greedy forwarding. Node x is in a local-maximum in its proximity to destination node D. Therefore, the packet delivery is improved by allowing the current node to carry the packet for a while until it encounters some neighbors. Geographic routing has recently gained a lot of popularity because it is incredibly effective and simply needs to know the positions of sensor nodes. However, a fundamental issue with geographic routing in WSNs is the \"local-minimum\" phenomenon, which prevents greedy forwarding. This phenomenon is linked to a region known as a hole that is lacking of active sensors and, as a result, either prevents the packet from being transmitted to a destination node or results in a lengthy detour path [12]. Figure 2 indicates that the WSNs suffer from \"local-minimum\" problems when a packet is forwarded to a node with no direct neighbors closer to the destination than itself. Since there is no direct neighbor closer to the destination than the node A itself, node A receives all destination's traffic packets. This depletes the node A's energy more quickly than other nodes. Uneven energy consumption of sensor nodes can also lead to new routing holes in the WSN [13]. Most existing protocols, such as Refs. [14; 15; 16; 17], allow the holes to be detected prior to routing initialization. Others tend to avoid the holes and solve the local-minimum problem by walking along only one side of the routing hole to recover the route. These protocols cannot guarantee that all packets are delivered in an energy-efficient manner once encountering the routing holes.