## 3 SDN, NFV, and SFC: An Overview
### Network Function Virtualization  
The use of proprietary network hardware is expensive for service providers in terms of procurement, security, configuration, scalability, and maintenance costs. The European Telecommunications Standards Institute (ETSI) [54] introduced a high-level NFV architectural framework, envisaging the deployment of network functions as software, running on the network function virtual infrastructure (NFVI), which could be a general-purpose server. This proposal was introduced to take advantage of hardware virtualisation [55, 56]. The deployment of network services has been greatly simplified by NFV, because the cost of acquiring new hardware middle-boxes is reduced, and several middle boxes can be virtualized and deployed on single or multiple general-purpose servers.  
The ETSI architectural framework for NFV depicted in Figure 6 shows all the important components that are necessary for deploying NFV. The operations support system (OSS) and business support system (BSS) directly interact with the vNFs. The vNF component is the network functionality, for example, a traffic load balancer, a WAN optimizer, and a firewall, etc. (Table 2). The hardware infrastructure consists of a virtual infrastructure with virtual computing, storage, and network components. This infrastructure is managed by the virtual infrastructure manager (VIM), which is responsible for resource allocation and embedding of virtual network functions on the virtual infrastructure. NFV Orchestrator (NFVO), which is an integral part of the ETSI NFV framework [54], is responsible for service orchestration and management [29]. One of the functions of the orchestration layer is the mapping of virtual network functions in a service chain to available physical resources. The management and network orchestration (MANO) component is responsible for the orchestration of vNFs and the chaining of services in a scenario where service function chaining is used. As shown in the SFC scenario in Figure 7, general-purpose hardware could be a typical high-performance commercial off-the-shelf (COTS) hardware [54].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**vNF** & **Functionality** \\ \hline Application gateway & Layer 7 traffic management based on application profile \\ Layer 2 forwarder & Packet forwarding based on layer 2 information \\ Protocol Analyzer & Packet classification, based on protocol in use \\ Flow tracker & Storing, displaying and forwarding ingress flows \\ Layer 3 Switch/Router & Traffic routing and switching using IP addresses \\ Application Firewall & Layer 3 and layer 7 packet filtering \\ Bridge & Bridging between two networks or host devices \\ Carrier Grade NAT & IP address translation for WAN connectivity \\ IDS/IPS & Stateful or stateless intrusion detection and prevention \\ Protocol converter & Protocol translation between IPv4 to IPv6 \\ Encryption gateway & Packet encapsulation and packet encryption/decryption \\ ACL & User and application level access control \\ Protocol Accelerators & Performance improvements by ISPs \\ VLAN manager & VLAN encapsulation and decapsulation \\ \hline \hline \end{tabular}
\end{table}
Table 2: Commonly used vNFs in NFV.