## 4 New Virtualization Solutions for Network Functions  
The deployment of mMTC solutions in softwarized networks, like 5G and 6G networks, is expected to benefit from the dynamic management of VNFs and network slicing capabilities [24; 25]. Network softwarization indicates new network architectures and technologies that decouple the software implementing network functions, protocols and services from the hardware used to run them, and can be considered the evolution of technologies like NFV and SDN. VNFs used in different IoT applications are deployed in the scope of the corresponding slice. If edge nodes are involved in service provisioning, some of the VNFs used in that situation are expected to be executed in the relevant multi-access edge computing (MEC) section of the slice. Typical examples include VNFs for event management and data exchange in sensor networks [26] and for implementing IoT gateways. In particular, the latter implements strategic functions characterizing individual scenarios, such as data pre-processing and adaptation, load balancing, flow aggregation and, sometimes, slice termination. Its operation must be characterized by scalability and very high availability. We illustrate new VNF deployment solutions supporting the IoT scenarios presented in Section 3. The general architecture of the IoT system is shown in Figure 2, where the proposed NFV infrastructure consists of a lightweight, security-focused virtualization solution. It aims to smoothly deploy the softwarized network functions, needed by different applications related to the aforementioned scenarios, in a serverless way. The reason for introducing new solutions is that the traditional approach based on VMs is notoriously slow and over-provisioned. Cloud-native network function (CNF) provisioning, based on containers to implement VNF, offers improved performance. However, it is affected by some security and isolation issues, caused by the use of a shared kernel, along with a long cold startup latency [5]. Unikernels, which are minimalist runtime environments running on top of virtual hardware abstractions, are proposed as an alternative to containers, since they are more secure and often faster. However, removing the concept of process from its monolithic appliances, Unikernels gain less flexibility and applicability since dynamic forking, which is a basis for the common multi-process abstraction of conventional UNIX applications, is not supported. Moreover, runtime management, such as online library updating and address space randomization, is not allowed, which complicates debugging. The Unikernels peculiarity of running their applications directly in the kernel ring, in thesame address space, could also entail some security issues, since privilege separation is impractical and dangerous kernel functions could be invoked from applications [27; 28]. The latest European Telecommunications Standard Institute (ETSI) report on the enhancement of the NFV architecture towards a cloud-native and platform-as-a-service (PaaS) approach presents some examples about the utilization of CNFs [29]. They include the use case of VNF components (VNFCs) implemented in single operating system (OS) containers, potentially wrapped in VMs. This way, they provide a nested virtualization environment and address the aforementioned security issues of CNFs. A new architecture for NFV management and orchestration (MANO), able to manage containerized VNFs, is proposed in Ref. [30] and is shown in Figure 3. It emerges that some new entities are necessary in the control plane for managing containers, as follows:  
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* CISM (container infrastructure service management) is the function that manages the CIS (container infrastructure service), a service offered by the NFVI, making the runtime environment available to one or more container virtualization. In turn, the CIS is exposed by one or more CIS clusters. CNFs are deployed and managed on CIS instances and make use of container cluster networks implemented in the CIS clusters [30].