A Network Function (NF) is the implementation of a function block deployed in a network\break infrastructure with well-defined interfaces and behaviour. Nowadays a Physical Network Function (PNF) runs on a network appliance~\cite{ETSI-NFV}.

Virtual Network Function (VNF) is the software implementation of a Network Function which is deployed on virtual resources. A single VNF could be made up of more than one component that could be run on a single or multiple Virtual Machines (VMs). 
VMs could be implemented not in a single place, but across different Data Center of the same TSP and the Network point of Presence (N-PoP) provide the information of where these VMs are located~\cite{NFV-survey}.

VNF features are agrees with the NFV characteristics such as:
\begin{itemize}
	\item
	energy efficiency, this technology demonstrates its potential to reduce the energy\break consumption of network equipment, due to the possibility of turn off physical servers or devices during off-peak hours in which the workload is low;
	\item
	reliability, it represents the system capability not to be affected by unexpected or hostile situations. The challenge here is to make VNFs more or equal reliable then the traditional hardware-based functions; 
	\item
	migration, it is referred to change the location of VNF instance depending on the actual necessity and this must be done without data loss or discontinuity service;
	\item
	performance, it is a crucial point of virtualized environment because traditional network functions run on specialized hardware-devices that provide a guaranteed efficiency. The software developed must follows all the feature of micro-service idea to provide the same level of quality, measured in terms of latency and throughput using efficient algorithms;
	\item
	security, since the virtualization technologies used in cloud environments are also applied in NFV to provide a virtualized system, all the challenges caused by these virtualization technologies are also faced by NFV~\cite{NFV-survey}.
	The Security Expert Group (SEG) created by ETSI lists all possible problems and threads that VFN technology could be affected as multi-tenant isolation, secure boot or crash, user authentication, authorization and accounting.
	The list of potential security issues investigated by SEG could be found in the ETSI deliver ``Network Functions Virtualization (NFV) Security''~\cite{ETSI-NFV-security}.
\end{itemize}
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The notion of Service Function Chain (SFC) came out with the spread of VNF technology, which means more than one network function can be interconnected. This is easily accessible due to the standard interfaces of VNF instances.
SFCs are used by the network operators and the TSPs to offer custom services to users and, thanks to Could Computing techniques, they could re-designed applications and provide redundancy of the services.
