 entity deployed in the form of a virtualization container. A VNF realized by a set of one or more VNFCs appears to the outside as a single, integrated system; however, the same VNF may be realized differently by each VNF provider. For example, one VNF developer may implement a VNF as a monolithic, vertically integrated VNFC, and another VNF developer may implement the same VNF using separate VNFCs, for example, one for the control plane, one for the user plane, and one for the EM. VNFCs of a VNF are con- nected in a graph. For a VNF with only a single VNFC, the internal connectivity graph is a null graph [37]. A VNF can assume a number of internal states to represent the status of the VNF. Transitions between these states provide architectural patterns for some expected VNF func- tionality. Before a VNF can start its life cycle, it is a prerequisite that the VNF was on- boarded (process of registering the VNF with the NFVO and uploading the VNF descriptor). Fig. 1.5 provides a graphical overview of the VNF states and state transitions. Each VNFC of a VNF is either parallelizable or nonparallelizable. If it is parallelizable, it may be instan- tiated multiple times per VNF instance, but there may be a constraint on the minimum and maximum number of parallel instances. If it is nonparallelizable, it is instantiated once per VNF instance. Each VNFC of a VNF may need to handle the state information, where it can be either stateful or stateless. A VNFC that does not have to handle state information is 5G Network Architecture Configure upgrade/update/ rollback/scale (in/out /up/down) Instantiated Reset configured Instantiated Inactive Start Active not configured Configure Upgrade/update/ rollback/scale Terminate Instantiate Terminate (in/out/up/down) Terminated Figure 1.5 VNF instance and state transitions [37]. a stateless VNFC. A VNFC that needs to handle state information may be implemented either as a stateful VNFC or as a stateless VNFC with an external state where the state information is held in a data reposito