# A Distance-Vector-Based Multi-Path Routing Scheme for Static-Node-Assisted Vehicular Networks
## 3 RDV: The Base Scheme  
### Overview  
RDV assumes that static unwired nodes are sparsely placed at major intersections to help forwarding packets among static nodes. In RDV, static nodes behave as routers and vehicles are the carriers of packets. RDV is a routing scheme that maintains routing tables at each static node in the manner of distance-vector routing scheme. When a vehicle comes into the communication range of a static node, the node exchanges control messages and data packets with the vehicle. Packets are delivered from source static nodes to their destination static nodes via several relay static nodes according to the routing table entries. Since static nodes are located to cover a certain area, RDV supports the packet delivery from and to any locations in the area. Note that, although we describe packet delivery between two static nodes, the sources and the destinations of packets in RDV could not be limited to static nodes but moving vehicles. To support this, we can combine several methods with RDV that provide packet delivery between static nodes (RSUs) and vehicles such as D-Greedy [19], PROMPT [20], or TSF [18].  
Vehicles as carriers deliver packets between two static nodes with the _store-and-forward_ behavior. When vehicles are in the communication range of static nodes, they switch to _intersection mode_. They pass packets to the node to relay them to other vehicles, and receive packets from the node that should be forwarded toward the vehicle's on-going direction. Otherwise, when vehicles are not in the communication range of any static node, they work in _road mode_, in which they pass packets to the vehicles ahead of them whenever being possible. In combination of those two modes, RDV provides efficient packet delivery in both urban and rural scenarios.  
One of the novel characteristics of RDV is providing reliable packet delivery in that packets are delivered with the pre-configured expected delivery ratio such as 99%. This is done by the function APD (Adaptive Packet Duplication), which creates several duplicated copies of a packet at every relay node to improve the expected delivery ratio to the value pre-configured by the administrators. Note that APD controls the number of duplicated packets to the minimum as long as the expected delivery ratio exceeds the preconfigured value. This significantly reduces the overhead of packet copies and enhances the capacity of networks.  
We define the units in distances specific to RDV as follows. See Figure 1. We define the unit _hop_ as the number of static nodes that a path involves. Consequently, since a vehicle carries packets from node A to B without visiting any other static node, the distance from A to B is 1-_hop_. Since a vehicle visits two nodes between A and D, the path is 3-hop long. On the other side, we define the unit _carry_ as the number of vehicles required to forward packets. Since a packet can be forwarded from node A to D with a single vehicle, the distance from A to D is 1-_carry_ even if it takes several-_hops_. Since a packet is not possible to be forwarded from A to E or F with a single vehicle, the packet must be relayed by static nodes. Since a packet is reachable from A to F via two relay nodes, it must be forwarded by at least three vehicles, so the distance from A to F is 3-_carries_.