## 2 Exploring SDN, NFV, Policies, ML, Network Slicing and Telecom Security
### Network Functions Virtualization  
Network Functions Virtualization (NFV) revolutionizes telecommunications infrastructure by decoupling network functions from proprietary hardware and migrating them to software running on general-purpose servers. NFV is seen as complementary to SDN, and it involves implementing network functions in software that can run on a range of industry-standard server hardware. These functions can be moved to or instantiated in different network locations as needed, without installing new equipment [31]. While offering agility and cost-efficiency, this shift fundamentally alters the security landscape. NFV's acting as a support for cloud computing, Software-Defined Networking (SDN), and open architectures introduce new vulnerabilities absent in traditional hardware-centric networks [32]. Furthermore, the increased network complexity in NFV architectures, with dynamic interactions between VNFs, makes it challenging to enforce consistent security policies across the entire infrastructure [33]. NFV is a transformative approach introduced to enhance the agility and flexibility of network service provisioning. Initially proposed to address the escalating complexity of traditional networks filled with proprietary hardware appliances, NFV leverages virtualization technologies to decouple network functions from physical hardware. This decoupling allows network services like firewalls, switches, and routers to be hosted on standard commercial off-the-shelf (COTS) hardware. The essence of NFV lies in its ability to instantiate these virtual network functions (VNFs) dynamically, thus enabling on-demand deployment without additional physical equipment [34]. NFV offers a wide range of benefits that promise to revolutionise the telecommunications industry associated with 5G [35]. For network carriers, it potentially reduces the capital and operational expenses by consolidating network appliances into virtual functions that can be managed and scaled as required. This transition also shortens the time-to-market for new services and facilitates more rapid deployment of network services tailored to specific user needs. However, the adoption of NFV is not devoid of challenges. One significant issue is ensuring that the network performance of virtual appliances meets or exceeds that of traditional hardware-based solutions. Furthermore, the dynamic nature of NFV introduces complexities in managing virtual appliances, including their efficient placement, instantiation, and migration across the network. These challenges necessitate ongoing research and development to ensure that NFV can reliably meet the performance and reliability expectations of modern network environments, as explored in the survey by [13]. Similarly to the rise of other technologies, new solutions and advances also come with new challenges, and keeping with the theme of security, the following items showcase a few issues found within NFV security, as well as a few possible solutions disseminated in the literature:  
* **Virtualization Layer Vulnerabilities**: NFV relies heavily on virtualization technologies, exposing networks to vulnerabilities inherent in hypervisors and virtual machine managers (VMMs). These vulnerabilities can lead to escalated privileges or escape attacks, where an attacker gains control over the host machine or other virtual machines [36]. \(\bigcirc\) _Proposed solution--Security Reference Architecture (SRA):_ This solution was proposed in [35]. It includes specific security patterns and reference architectures to mitigate identified threats based on these patterns, which can be reused for continuous monitoring of the virtualized layer. * **Isolation Failures**: Proper isolation of network functions is crucial to prevent cross-VM attacks. Any failure to maintain strict isolation can lead to information leakage, unauthorised data access, or denial of service (DoS) attacks.