# A Distance-Vector-Based Multi-Path Routing Scheme for Static-Node-Assisted Vehicular Networks
## 1 Introduction  
Vehicles are regarded as a kind of mobile sensor nodes and vehicular ad hoc networks are regarded as a potential infrastructure to help collecting or delivering various data that may enhance our life. Vehicular Ad hoc NETworks (VANETs) have been deeply studied to realize the dependable data infrastructure that consists of vehicles by means of multi-hop packet forwarding. Although VANETs are expected to support both metropolitan and rural scenarios, especially rural (or suburban) scenarios in which vehicles are not dense enough have a significant problem on packet delivery ratio. If vehicle density is low, VANET is regarded as a kind of Delay-Tolerant Networks (DTNs), and the past contributions on DTN routing have proved that high packet delivery ratio is hardly achievable with sparsely located nodes; with simple schemes such as the epidemic routing or the spray-and-wait approach, a significant load for duplicated packets is required to improve delivery ratio of packets [1; 2; 3]. A large number of studies have been dedicated to overcoming this sparsity problem in VANET [4; 5; 6; 7; 8; 9; 10; 11]. From the early stage of VANET, geographic routing is regarded as one of the best-suited routing strategies, and several techniques have been developed based on it. In DTN-based vehicular networks, vehicle selection to pass packets is a key decision to improve packet delivery ratio. Thus, _carry-and-forward_ behavior [8] that affords vehicles to have time to choose the next-hop vehicle has been deployed with techniques for selecting good next-hop vehicles such as utilizing vehicles' trajectories (e.g., [9]) or estimating vehicle traffic amount of roads (e.g., [10]). However, although those techniques certainly improve the delivery ratio of packets in sparse scenarios of VANETs, the ratio is still too low in order to support practical applications over VANETs. The idea to utilize static, but not wire-connected wireless nodes located at intersections is introduced in SADV [12]. In SADV, static nodes are located at each intersection and they work as a link-state router; they retrieve the network topology from the roadmap, measure the packet forwarding delay of each link between two neighboring static nodes, and compute the shortest-path to each destination. At intersections, static nodes receive packets from vehicles and pass them to other vehicles that are likely to carry packets along the shortest paths to the destinations. SADV shows that inexpensive unwired static nodes located at each intersection can significantly improve the packet reachability to their destinations. In addition, by forwarding duplicated copies of a packet to the primary and the secondary next-hops at every relay node, i.e., by using multiple paths, SADV further improved the packet delivery ratio. However, since SADV has no specific mechanism to control the expected ratio on end-to-end packet delivery, there are cases in which packet delivery ratio is not sufficiently high, which is inconvenient in practice as a reliable infrastructure. In addition, since every node always forwards packets to the two next-hops, the overhead of communications is high so that the network capacity is limited. [PERSON] et al. proposed a distance-vector-based routing scheme RDV that provides the pre-configured expectation on packet delivery ratio by creating the sufficient number of packet copies at static nodes [13]. Namely, if administrators configure to expect 99% end-to-end packet delivery, RDV provides the 99% expected packet delivery ratio. Since RDV creates the minimum number of packet copies to achieve the preconfigured expectation, RDV requires the lowest overhead of duplicated packets. However, since RDV uses a single shortest path to forward packets to a destination, data traffic easily concentrates on the shortest paths and excesses the capacity. In addition, since the paths are selected based on vehicle density rather than delivery delay, the forwarding paths provide far larger delay compared to SADV.