## 2 Related Work
### Standardization Activities  
The NFV ISG (Industry Specification Group) of the European Telecommunications Standard Institute (ETSI) is the most noteworthy standardization initiative so far regarding the NFV domain aiming at specifying general, open, and scalable architectural and operation solutions to meet challenges placed by NFV. More specifically, current standardization efforts in ETSI [5] are paving the way toward the realization of a multi-provider NFV ecosystem through the definition of a comprehensive architectural framework composed of: (i) the NFV Infrastructure (NFVI), which includes the hardware and software resources that support a cloud of virtualized network, computational and storage resources; (ii) the VNFs, which refer to the software implementations of network functions capable of running over the NFVI; and (iii) NFV Management and Orchestration (MANO), which covers the lifecycle management of both physical and software resources as well as management and orchestration of VNF instances [12]. An important step for NFV MANO has been to include software-defined networking solutions into the NFV architectural framework. Such solutions consider SDN controllers acting as Network Controllers for the network part of the NFVI (_i.e._, network infrastructure domains) to deliver connectivity services involving physical or virtual resources (_i.e._, OpenFlow switches) while providing an abstract view of such resources for orchestration purposes [13]. Moreover, in [14] ETSI identified the most common design patterns for using SDN in an NFV architectural framework along with recommendations to be fulfilled by the entities that perform the integration. Current standardization efforts in the Internet Engineering Task Force (IETF) around SDN and NFV include, firstly, the Service Function Chaining (SFC) Working Group [15] efforts aiming at addressing the dynamic specification and instantiation of an ordered list of instances of service functions (such as firewalls, load balancers, _etc._) while subsequently steering of data traffic flows through those service functions, accordingly. Secondly, another related initiative carried out in IRTF is the Network Function Virtualization Research Group (NFVRG) [16] aiming at developing new architectures, systems, and software, and to explore trade-offs and possibilities for leveraging virtualized infrastructure to provide support for network functions. Thirdly, efforts in IETF are also devoted to the design of a comprehensive SDN controller, namely Application-Based Network Operations (ABNO) architecture [17], including instrumental guidelines and operational workflow specifications to coordinate network control functions to compute paths, enforce policies and manage topology while providing a full network automation and programmability for the benefit of the applications that use the network. To the best of our knowledge, these efforts are still poorly integrated. IEEE with the Next Generation Service Overlay Network (NGSON) standard [18] specifies a framework for the control and delivery of composite services over diverse IP-based networks (e.g., Internet, P2P overlay, IMS, PSTN, Mobile) with context-aware, dynamically-adaptive, and self-organizing capabilities. Dynamic context of users, devices, services, and networks are considered to adapt composite service delivery while optimizing network and computing resources consumption. In particular, network awareness allows for adapting service provisioning to the current status of the network (e.g., avoiding hot spots or congestions) while satisfying the ever-charging requirements of users. In this regard, recent emerging paradigms such as SDN and initiatives such as NFV can significantly contribute to achieve greater elasticity in network service deployments and accelerate a prominent innovation in NGSON service and data delivery functions. In this regard, a liaison has been established between the IEEE SDN Initiative and the NGSON WG to identify potential new standards in SDN/NFV areas to be developed in IEEE [19].