NF descriptor or uses a VM and the corresponding network and stor- age resources previously allocated to it. Following successful completion of this process, the VNF manager requests to start the VM [37]. 1.1.2.4 Legacy Support and Interworking Aspects In general, the behavior of a complete system can be characterized when the constituent functional blocks and their interconnections are specified. An inherent property of a functional block (in traditional sense) is that its operation is autonomous. The behavior of a functional block is characterized by the static transfer function of the functional block, the dynamic state of the functional block, and the inputs/outputs received/generated at the corresponding reference points. If a functional block is disconnected from an immediately preceding functional block, it will continue to function and generate outputs; however, it will process a null or invalid input. As we mentioned earlier, the objective of NFV is to sep- arate software that defines the NF (the VNF) from the hardware and the generic software that creates the hosting NFVI on which the VNF runs. Therefore, it is a requirement that the VNFs and the NFVI be separately specified. However, this is a requirement that is not immediately satisfied by traditional method of functional blocks and associated interfaces. Fig. 1.6 shows an example where a traditional network comprising three functional blocks is evolved into a hypothetical case where two of the three functional blocks have been vir- tualized. In each case, the functional block is implemented as a VNF that runs on a host function in the NFVI. However, in this process, there are two important differences with the standard functional block representation that must be noted. The VNF is not a functional block independent of its host function, because the VNF cannot exist autonomously in the way that a functional block can exist. The VNF depends on the host function for its Virtualized interface Virtualized interface Network Network Network Network Contain