## 4 Results
### Routing Performance Evaluation
#### 4.4.1 Impact of Link Lifetime Threshold
Figure 16: Performance evaluation of routing frameworks under various link lifetime threshold values. (**a**) Communication cost fluctuation for various link lifetime thresholds, (**b**) Channel utilization fluctuation for various link lifetime thresholds, (**c**) Latency fluctuation for diverse link lifetime thresholds, (**d**) Packet delivery ratio fluctuation for various link lifetime thresholds. thresholds (e.g., 10 s), as evident from Figure 16c. However, this decrement in latency after the 1.0 s link lifetime threshold cannot be interpreted as a performance gain, as the packet delivery ratio also drops in the same manner for link lifetime thresholds greater than 1 s. Thus, for routing in SDVN architectures (Dijkstra and proposed routing), the latency is lower than AODV, as routes are computed by the controller using the metadata collected from the network. The latency of the proposed method is the lowest under any link lifetime threshold value, as the proposed method has the highest stable least delay mode, which selects low latency paths for packet routing under low contention scenarios. Thus, although the adaptive flow rule installation approach in the proposed method introduces some delay compared to the proactive approach, it was successfully compensated by the selection of low-delay routing paths by the proposed routing technique, resulting in the least latency out of the three routing approaches. According to Figure 16d, it is very clear that the proposed routing framework had the highest PDR for link lifetime thresholds in the range of 0 s to 6 s. However, the average PDR dropped well below that of AODV by the link lifetime threshold of 8 s, and the average PDR dropped below both AODV and Dijkstra by the link lifetime threshold of 10 s. Furthermore, the PDR of AODV was much higher than routing in SDVN using Dijkstra, as evident from Figure 16d. The reason behind Dijkstra's poor PDR compared to AODV is due to the fact that it computes routes using a shortest path algorithm that is unaware of the existence of the links. On the other hand, AODV discovers routes on demand, making sure that routing occurs only on existing links. Results on PDR given in Figure 16d show that a link lifetime threshold for the proposed routing must be carefully selected to maximize the packet delivery ratio. This shows that selecting either a very low or very high threshold can degrade the PDR of the proposed method. Next, we analyzed the overall results of this experiment and decided on the optimum value of the link lifetime threshold for the rest of the experiments. It was clear from Figures 16a-d that communication cost, DSRC utilization, latency, and PDR of the proposed routing increased from link lifetime thresholds increasing from 0 s to 0.20 s, then remained at the highest value in the range of 0.2 s to 1.0 s, and then slowly degraded for link lifetimes greater than 1.0 s. Even though the highest values of communication cost, DSRC utilization, and latency existed at link lifetime thresholds of 0.20 s to 1.0 s, the highest PDR also occurred at the same specified range. We proposed this routing framework for SDVNs to increase the reliability of routing. When analyzing this result on link lifetime threshold, it is very clear that, if a link lifetime threshold lesser than 200 ms is set, those links that are added to the routes using the routing algorithm that have a link lifetime lesser than 200 ms will most probably not be available at routing time. That is the reason for obtaining a lower PDR in the range from 0.0 s to 0.2 s. Thus, the minimum experimental value for the link lifetime threshold can be inferred to be 200 ms. On the other hand, when a higher link lifetime threshold greater than 1.0 s is set for the proposed routing method, links that really exist at the routing time (for example, a link with a lifetime of 600 ms) will not be added to the routes using the hybrid routing algorithm due to the high link lifetime threshold.