## 2 Related Work
### Position-Based Routing Algorithms without Adopting Reinforcement Learning  
(1) DPPR [23] assumes that each vehicle knows its own driving trajectory in advance, and the driving trajectory is a sequence formed by the number of intersections that will be passed through. Due to the traffic information, each road can be assigned a weight value, which is the transmission delay time of this road, and the shortest delay path can be calculated via the Dijkstra algorithm [24]. DPPR contains two different modes: the straightway mode and intersection mode. When the vehicle wants to forward the packet, it first calculates the shortest delay path to determine the next expected intersection arrival time, and detects whether its neighbor vehicles will pass the destination of the packet. If so, the vehicle carrying the packet will forward the packet to this vehicle; if not, the neighbor vehicle that is driving toward the target intersection and is faster than the vehicle carrying the packet is selected, and the packet is copied to the neighbor vehicle; if both vehicles cannot be detected, the vehicle carrying the packet will place the packet into its routing queue and schedule. The disadvantage of this method is that it needs to know the completedriving trajectory, and then calculate the shortest delay path, which is not suitable for vehicles without fixed routes.  
(2) The NTR (Novel Trajectory Routing) [22] algorithm uses the vehicle GPS trajectory to explore all the possible movement patterns of each vehicle, establish its own trajectory tree, and establish a vehicle encounter tree by comparing the trajectory trees of the vehicles. Compared with the vehicle trajectory representation used in the previous paper, only the location where the vehicle passed by is recorded, and the NTR also records the elapsed time in the trajectory. The vehicle encounter tree records all possible paths in which the packet is forwarded from the source to the destination, and records the road segment between the vehicle carrying the packet and the expected vehicle to be forwarded the packet. After building the vehicle encounter tree, NTR converts the vehicle encounter tree into a predicted transit vehicle sequence diagram. The packet sender deletes the key of the vehicle child nodes that will meet along the road; changes them into the parent node of the vehicle that the packet sender will meet directly; and adds two parameters, which are the expected packet arrival rate (expected delivery ratio, EDR) and token (token value indicates the number of packet replicas). If the expected packet arrival rate of the destination node is 1, the expected packet arrival rate of each node toward the root node in the direction of the parent node is continuously calculated. When the expected packet-forwarding sequence diagram and related information are calculated, the packet transmitting end can use the sequence diagram and related information to transfer the packet. The advantage of the NTR algorithm is that it can calculate the transfer vehicle sequence between any two vehicles and achieve a transfer process close to the optimal solution. The disadvantage of this is that the calculations of establishing a trajectory tree, comparing the trajectory tree of vehicles, and establishing a vehicle encounter tree are complicated. They require a large amount of computing time and storage space for the pre-operation.