## 5 Discussing Open Issues and Challenges  
Autonomous underwater vehicles have already attracted a lot of attention, especially due to their capabilities to operate in a swarm, significantly minimizing the human factor; UUVs are making civilian and military operations tremendously easier, mitigating any ethical doubts with cost-effective vehicles. Nevertheless, the establishment of a robust and secure underwater network raises several issues. Interoperability between underwater assets is a challenging goal, due to the variety and veracity (quality of data) of data transmitted in the water. Lack of communication protocols' standardization among research institutes and organizations generates even more obstacles, by making difficult the cooperation between them and the establishment of common research foundations. Moreover, a variety of security vulnerabilities arises especially due to the complexity and uncertainty of the IoUT architecture, its vast number of assets and their heterogeneity, as well as the non-formalized domains of IoUT and cybersecurity. Based on an extensive and systematic research of the IoUT and cybersecurity domains, we have identified and presented in this paper the need to further investigate the key issue of interoperability and standardization of communication protocols. Due to the absence of a typical communication channel and the existence of a common IoUT \"language\", the challenges that are emerging are numerous [85]. Critical capabilities of UUVs such as autonomous decision-making, adaptive path-planning, self-management and self-diagnosis, cannot be easily achieved, mainly due to the difficulty in information/data exchange. Furthermore, data related to a key factor of underwater missions, i.e., weather conditions, are unable to be utilized and distributed, which consequently causes the failure of an operation. A commonly agreed formal message encoding and decoding schema can be a solution towards IoUT interoperability. Alternatively, or complementarily, a tool-supported methodology for the automated alignment and translation of exchanged data and messages encoded in different syntax and semantics [123] could be a more realistic and efficient solution in the IoUT domain, allowing different stakeholders (vendors, organizations, research institutes) to keep their own data and semantic models local to their solutions. Furthermore, a consequence which is derived from big data is recurring data, especially in dangerous military and SAR operations. During our research we distinguished the lack of a data \"sieving\" process. During a large-scale operation, two or more UUVs may exchange the same information with the leader, affecting the feedback to the commander and causing networking chaos. Validity of information facilitates communication in the underwater domain, allowing the distribution of high-priority data by not utilizing extra bandwidth. A distinct sensor incorporating machine learning methods, exclusively for the process of information discrimination would be very useful in operations with swarms of UUVs. Emerging from the above issue as a rational result, but also from the principles of military operations, UUVs must act as a role-based asset. This means that every unit should be assigned its own tasks, and its sensors are dedicated to these tasks according to their capabilities. This strategy allows for the elimination of recurring information, but mainly supports the quality of information which is transmitted by the leader of the swarm to the commander. Nevertheless, non-standardization of communication protocols remains a critical obstacle, due to the heterogeneous devices every role-based UUV would possess. Sharing knowledge between UUVs in a common machine-understandable language can be achieved through semantic knowledge presentation. Additionally, the impact of fast and trustworthy decision-making in critical situations can be easily predicted and the exchange of important information, as well as its representation, can be efficiently achieved.