## 1 Introduction  
Vehicular Ad-hoc Network (VANET), as one of the core technologies of the Intelligent Transportation System (ITS), has huge application potential and commercial value. Based on the Vehicle-to-Vehicle (V2V) communications and the vehicle-to-roadside communications (V2R) [1, 2], VANETs are aimed at the applications including traffic information dissemination, traffic safety, and Internet access services, etc. Furthermore, with the emergence of the Internet of everything in 5G [3, 4], VANET has been given a new functional role, as a natural extension of the mobile network in the road travel scenario. The inherent characteristics present in VANETs i.e., the diverse node velocities, severe channel fading, dynamic network topology, and limit transmission coverage of vehicles, etc., impose a lot of challenges in designing an efficient routing algorithm with both short delivery delay time and high reliability. Conventional routing protocols in VANETs are usually topology-based routing or geographical-based routing. The topology-based protocols (e.g., Ad hoc On-Demand Distance Vector Routing (AODV) and Dynamic Source Routing(DSR)) which utilize the link information in the networks are not suited for vehicular ad hoc networks because links between vehicles can break down so frequently that it is hard to discover a multi-hop route from the source to the destination [5]. Even though the authors in [6] have proposed an efficient AODV with backbone based routing for message dissemination to increase packet delivery ratio, the performance of this protocol is still limited during low vehicle density due to frequent link breaks. In addition, geographic-based protocols (e.g., Greedy Perimeter Stateless Routing (GPSR), Geographic Source Routing (GSR)and Geographical and Energy Aware Routing (GEAR) [7; 8] only need local neighborhood information to perform packet forwarding and require quite low routing overhead, but this type of routing can lead to local optimization and causes a long end-to-end packet delay. All these routing protocols above used in traditional networks can hardly meet the requirements of efficient data exchanging on internet of vehicles (IOVs), such as data transmission efficiency, network resource utilization rate, etc. Software-defined network (SDN) [9; 10], as a model of the next generation network, could be rationally adopted for the data exchanging network of IOVs, and it will bring some benefits of introducing SDN technology to VANET. In SDN-based VANET, it is not necessary for vehicles to exchange routing messages with each other which causes the routing overhead and end-to-end packet delay to be greatly improved. So far, researchers have investigated more how to take advantage of SDN benefits to improve the performance of current VANETs. Some existing works [11; 12; 13] propose fully SDN-based architectures to extend SDN to operate in wireless vehicular network scenarios. The resulting Software Defined Vehicular Network (SDVN) provides a bird's eye view over the network, and thus, the link stability can be better scrutinized. The authors in [14] propose a SDVN architecture and relative services, they also demonstrate in simulation the advantage of a SDVN routing on the reliability, flexibility, and programmability. The authors in [15] present a SDN-based geographic routing (SDGR) protocol to address the local maximum problem in sparse networks. The controller computes the shortest connected path with probability of high connectivity, and with this, a packet forwarding algorithm is proposed to select the next hop to solve the load balance problem at junctions. SDN-based vehicular ad-hoc on-demand (SVAO) routing protocol [16] separates the control layer and the data forwarding layer to improve network performance in VANETs. The protocol is an amendment of the ad hoc on-demand distance vector (AODV) protocol, in which both global and local controllers are used to improve the packet reception rate.