con- nected by the supporting network infrastructure. These NFs can be implemented in a sin- gle operator network or inter-work between different operator networks. The underlying NF behavior contributes to the behavior of the higher level service. Therefore, the net- work service behavior is a combination of the behavior of its constituent functional blocks, which can include individual NFs, NF sets, NF forwarding graphs, and/or the infrastructure network. The end points and the NFs of the network service are represented as nodes and correspond to devices, applications, and/or physical server applications. An NF forwarding graph can have NF nodes connected by logical links that can be unidirec- tional, bidirectional, multicast, and/or broadcast. Fig. 1.4 shows the NFV architectural framework depicting the functional blocks and reference points in the NFV framework. The main reference points and execution reference points are shown by solid lines and are in the scope of European Telecommunications Standards Institute (ETSI) NFV specifica- tion [37-41]. The dotted reference points are available in present deployments but may need extensions for handling NFV. However, the dotted reference points are not the main focus of the NFV at present. The illustrated architectural framework focuses on the func- tionalities necessary for the virtualization and the resulting operation of the network. It does not specify which NFs should be virtualized, as that is solely a decision of the net- work operator. 1.1.2.2 Functional Aspects The NFV architectural framework, shown in Fig. 1.4, identifies functional blocks and the main reference points between the blocks. Some of these blocks are already present in cur- rent deployments, whereas others might be necessary additions in order to support the vir- tualization process and the subsequent operation. The functional blocks are as follows [37]: VNF is a virtualization of a network function in a legacy non-virtualized network. Element management (EM) performs the typical management 