## 1 Introduction  
The introduction of fifth-generation (5G) networks has made the deployment of new services and applications feasible. This is achieved via the support of highly demanding features such as ultra-reliable low-latency communications (URLLC), enhanced mobile broadband (eMBB), and massive machine-type communications (mMTC) [1, 2, 3]. In this context, various novel technologies are integrated both in the physical and in the network layer, including millimeter wave (mmWave) transmission, massive multiple input multiple output (mMIMO) deployments, and non-orthogonal multiple access (NOMA), as well as network function virtualization (NFV) and software-defined networking (SDN). mmWave transmission leverages improved data rates over higher-frequency bands [4], mMIMOsystems increase spatial isolation among mobile stations (MSs) and provide flexible interference management [5], and NOMA improves overall spectral efficiency [6]. Finally, NFV and SDN contribute to the decoupling of application deployment from hardware-specific devices [7; 8]. In parallel, novel architectural approaches have been introduced as well, such as the service-based architecture (SBA), which leverages application deployment from various resources [9], and ultra-dense networking [10]. Hence, a variety of new applications can now be supported, including augmented and virtual reality (AR/VR), remote health care, and advanced fleet management in industrial environments [11; 12]. However, the need for advanced services and applications has led to ongoing discussions for sixth-generation (6G) networks [13; 14; 15]. This transition is also dictated by additional advancements in other adjacent fields, such as the Internet of Things (IoT) and the support of advanced applications based on wireless sensor nodes [16]. Hence, there is the need to define a new network paradigm able to interconnect a vast number of heterogeneous resources and protocols and at the same time to effectively manage the huge volume of data generated by IoT devices. Although there is not a concrete standardization yet, the 6G concept will be based both on a number of key-enabling technologies and on architectural concepts. These include distributed mMIMO systems and intelligent reflecting surfaces (IRSs) that can further leverage high transmission rates and improve resilience over physical-layer attacks [17]; software and hardware decoupling during application deployment and execution, along with open access protocols [18]; intent-based networking (IBN) [19]; and the evolution of the SBA in 6G networks [20]. The ultimate goal of 6G infrastructures will be to provide ubiquitous coverage in a wide geographic range, and at the same time support multiple demanding applications in terms of both throughput and latency. Hence, there is a need for a holistic transformation of the underlying architectural design to support more advanced and decentralized applications, such as holoporation and autonomous driving [21]. To this end, various approaches have been studied over the last few years, such as cell-free (CF) systems [22], open radio access networks (O-RANs) [23], the advancement of SBA to an organic concept [24], and decentralized and distributed machine learning (ML) deployments [25; 26]. At the same time, 6G networks should be reconfigurable, able to recover from unwanted situations, and self-optimized. Therefore, various challenges should now be dealt with, such as (a) the ability to collect a vast amount of data from heterogeneous networks and environments in a secure and privacy-preserving way, (b) the ability to effectively train ML models for a variety of use cases and scenarios, (c) dynamic network reconfiguration, and (d) deployment of services and applications according to user needs by properly optimizing related resources. Finally, since 6G networks will rely on the integration of heterogeneous resources, an increased number of potential attacks might now take place [27]. Hence, advanced security protocols and threat mitigation techniques are also required.