## 7 Future Research Directions  
Sixth-generation networks will be highly distributed in nature. Network control will be shifted to far edge nodes, such as micro-edge [56], which may arise with new terminologies and technologies. Since centralized control will not be suitable for latency critical services, distribution of network management and control functions will be inevitable. However, such distribution will complicate the dependability of future networks. The challenges to dependability will be from many dimensions. For example, reliable network function or service transfer from a centralized cloud infrastructure to far edge or user node will be challenging and require more research in terms of resource discovery, alongside reliable functioning or service transfer. The distributed control, apparently, can increase the availability. However, such control functions can also be targeted by security attacks, such as DoS attacks and resource exhaustion attacks, to block access to legitimate users. The success of such attacks usually depends on the availability of resources at the receiving end(s). Hence, distributing network control functions into small units may decrease the overall dependability. The resilience of the networks also must increase, which will require proper resilience strategies. Network resilience means that a network operates in the presence of difference challenges, such as security attacks, operational mistakes, configuration errors, and equipment failures [87]. This will require network mechanisms that are capable of protecting the network from failures through flexible means of configurations, cooperative techniques that enable network devices and segments to provide alternative routes, and efficient anomaly detection and traffic-shaping techniques. The notion of network abstraction and the simplicity of control put forward by the concepts of software defined networking (SDN) [88] can really help in this context, for instance, in the run-time mobility of traffic [89], traffic management [90], or load balancing among congested nodes [91]. Whether the technologies of SDN will prevail in 6G or not, research will show, however, the concepts and philosophy of SDN can surely help with increasing the dependability of 6G networks, and thus must be researched further. Safety and security are highly intertwined. For example, if equipment is not physically safe, it cannot be considered to be secure. Similarly, if a device can be compromised through cyber attacks, its physical security can be meaningless in most use cases. Safety must be researched from the technical perspectives to obtain key performance indicators for safety, much like security. We do have security-by-design, but safety-by-design is rarely discussed. The security in 6G is highly complicated. New technologies are emerging, and it is possible that security requirements of different technology do not match, for instance, in the debate of privacy vs. accountability. Furthermore, the more the network control becomes distributed and modular, the more complicated the security of the whole eco-system comprising diverse services, users, and even network functions, will become. Dependability, therefore, will be highly complicated to comprehend and must be researched from this perspective. One of the key questions in this regard will be to take an evolutionary incremental approach, based on existing 5G standards or a revolutionary approach, and start thinking of redesigning from scratch. In 6G, AI will be used on a much higher scale than 5G; thus, the real threats that AI can pose will be more visible. Furthermore, existing AI systems use huge amounts of resources [92], ranging from energy and computing to bandwidth and spectrum. Therefore, research is needed on sustainable AI-based security approaches for AI-based security threats in 6G.