## 4 Implementation and Discussion
### The Scalability Challenges  
In the design and implementation of a Kerberos-blockchain VANET authentication system, scalability is a critical consideration, especially when applied across diverse network scenarios such as suburban and urban environments. As the number of vehicles and number of TASs increases, the system must efficiently manage the load distribution of the TASs. Furthermore, while the blockchain component of the system is designed to store authentication messages, the block size limitation becomes a significant factor.  
As the number of vehicles and TAS increases within the network, ensuring an even and efficient load distribution among TASs becomes a crucial challenge. In the suburban scenario, the number of vehicles is set to 100, whereas it increases to 200 in the urban environment for scenario 2, with the same number of TASs. Since scenario 2 did not meet the VANET requirements concerning the maximum authentication delay, we increased the number of TASs to two in scenario 3. This increase resulted in a reduction in the authentication delay, bringing the delay below the 100 ms threshold, thereby fulfilling the VANET criteria. However, the reduction in delay from scenario 3 is not exactly 50%, despite the addition of a second TAS, but is instead 40.3%. This discrepancy is influenced by the Ad-Hoc On-Demand Distance Vector (AODV) routing protocol, which selects the shortest path based on the node count between the source and the destination. When the network includes two TAS units, the load may become imbalanced, as each TAS may have different node counts between itself and the vehicles. This imbalance can lead to one TAS becoming more heavily loaded than the other, resulting in higher authentication delays for the vehicles managed by that TAS. As the vehicle density increases and one TAS handles more load than the other, there is a potential risk of bottlenecks and performance degradation if the load is not managed properly. To address the load imbalance issue, several protocols could be considered for future work, such as the Dynamic Load Balancing Protocol (DLBP), Least Loaded Server Protocol, Round-Robin Load Balancing, or other load balancing methods. However, the results of this study indicate that the proposed method did not cause any bottlenecks, as the performance of scenario 3 satisfied the VANET delay requirements.  
Another scalability challenge in the proposed method is the block size limitation for storing authentication messages. The maximum block size in Ethereum ranges from 1 to 2 MB. It is critical that the messages stored within each block do not exceed this limit, as doing so may increase the risk of blockchain forking and result in inconsistencies across the network. To assess the impact of vehicle density, we evaluated the effects of increasing the number of vehicles from 100 to 200 in our experiments. The maximum storage required for 200 authentication messages was calculated to be only 29.894 KB, which is significantly below the maximum block size threshold. Therefore, even as scalability expands, the three scenarios presented in this study remain well within the acceptable block size limits.