# A Survey of Secure Time Synchronization
## 5 Discussions for Future Work
After adjusting their own clock according to the two parameters, cluster heads send broadcast messages to all the intra-node clusters which will be used as a reference point by the intra-node clusters in order to achieve synchronization within the clusters by using the RBS mechanism. In this way, in order to achieve synchronization, a node has to exchange information only with the nodes in the same cluster instead of exchanging information with all the nodes in the network. This process eliminates a large number of information message exchanges and provides high accuracy with efficient energy consumption. The FISP is widely known for its high precision and robustness against node and topology failures. The FISP runs periodic flooding synchronization messages to achieve robustness and implicit topology updates. MAC layer timestamps are used to achieve high precision, whereas sources of delays and uncertainties are eliminated by providing mitigation techniques for them. All these functions provide high accuracy by keeping the error within one microsecond and achieve better robustness than the the TPSN and the RBS protocol; however, these processes consume a large amount of energy, whereas energy is very limited in sensors. One possible solution for this is to reduce the overhead of synchronization messages by not transmitting the rooID and seqNum in synchronization messages, since they are the same as those transmitted before for the first-time synchronization. This would reduce the bandwidth usage and energy consumption. By eliminating these errors in these protocols, a secure time synchronization protocol with low overhead can be proposed which will be able to maintain high accuracy while reducing energy consumption. In the era of Industry 4.0, Internet of Things (IoT) devices are growing rapidly. Industry 4.0 is related to the increasing digitization and linkage of products and value chains. People and devices are all connected globally in Industry 4.0 [62]. Industry 4.0 also extends to the healthcare domain, named Healthcare 4.0, to assist doctors and patients [63]. There is  
Figure 15: RBS protocol clustering topology. a need for time synchronization between industry IoT devices or healthcare devices and applications to improve communication and resource availability and to appropriately allocate resources [64]. The issues associated with secure time synchronization should be addressed via authentication and encryption, suspicious packet removal, and the design of new secure time synchronization protocols [65]. Some researchers have proposed a time synchronization scheme based on the NTP, trust management, and blockchain techniques [66]. Blockchain techniques have been used to record time synchronization requests and responses, and the experimental results have illustrated that the proposed approach could achieve security goals. Time synchronization plays a crucial role in Industry 4.0 and Healthcare 4.0, and secure time synchronization is an essential component. For Industry 4.0 or Healthcare 4.0 applications, the deployment of secure time synchronization should guarantee high, precise time synchronization to satisfy the real-time communication needs of the applications. In addition, it should prevent attacks and keep communication safe. A very promising approach is to integrate time synchronization with blockchain techniques. Based on the survey results, we answer RQ1 in Section 3 by conducting a detailed analysis of the time synchronization protocols. For RQ2, we answer it in Section 4, and we answer RQ3 in Section 5. The future of secure time synchronization should be developed with blockchain techniques for Industry 4.0 and Healthcare 4.0.