## 7 Conclusions  
In this paper we discussed how service-oriented principles can be applied to effectively orchestrate virtualized network resources, _i.e._, VNFs, to provide dynamically-established VNF chains while taking  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Redirection Option** & **Average Number of Transmitted Bytes** & **Standard Deviation** \\ \hline With redirection feature & 23 MBytes & 5.56 \\ Without redirection feature & 43 MBytes & 34.43 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Switch load with and without path redirection.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Number of VNFs in the Chain** & **Average Number of Flow Entries** & **Network Set-up Time** \\ \hline
3 & 153 & 36.32 ms \\
5 & 253 & 50.14 ms \\ \hline \hline \end{tabular}
\end{table}
Table 2: Orchestrator performance with different number of VNFs in the chain and with five VNF clouds.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference Option** & **Number of Exchanged Messages** & **Redirection Time** \\ \hline With redirection feature & 11,195 & 30 ms \\ Without redirection feature & 3449 & 0 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Redirection overhead.  
full advantage of programmable data forwarding capabilities provided by SDN to adaptively deliver data throughout VNFs. The level of abstraction introduced by SOA principles provides a generalized mechanism for composing heterogeneous resources (in the computing, as well as in the networking domain) across different providers to provide users with an enhanced service experience. Moreover, the exposure of VNFs as independent services advertised through a service contract description promotes the delivery of complex network services implemented as a composition of dynamically-selected and bound VNFs. In addition, composition adaptation and service selection and substitution mechanisms can be put in place to assure the provision of QoS-aware dynamic adaptive services. We also present current results of our ongoing activities in the implementation of a prototype to validate the proposed approach. In the near future we plan to extend the prototype with VNF and virtual infrastructure management capabilities.  
This approach can give several benefits to NSPs: a faster innovation speed of their network infrastructure without the need of radically changing hardware systems, reduction in the time and the cost for rolling out new services, adaptive network service provisioning and, finally, new business opportunities fostered by a dynamic environment of multi-provider cooperation.  
The proposed approach poses a number of challenges for NFV and SDN research areas. Firstly, efficient mechanisms for the automated orchestration of VNF instances and their placement are needed for adaptively addressing service requirements. Indeed, optimization algorithms would be required to provide the best set of VNF instances (in case of orchestration) and the set of locations where to put VNFs (in case of placement) while minimizing a cost function, e.g., overall latency along the VNF path. Secondly, in the SDN area the main envisioned challenge is how to provide dynamic and granular traffic steering capabilities while scaling at the tenant or application flow level. In both areas, high-scale monitoring functions are required for tracing the actual service availability and properly trigger service recovery operations. Moreover, such data need to be exposed to the orchestration functions with the proper level of abstraction and granularity to keep the problem scalable [2].  
We acknowledge [PERSON] and [PERSON] for their technical assistance.  
The authors equally contributed to this work.  
The authors declare no conflict of interest.