# A Secure Link-Layer Connectivity Platform for Multi-Site NFV Services
## 2 Background on Multi-Site VNF Communications  
### ETSI Network Functions Virtualization  
The genesis of Network Functions Virtualization (NFV) took place at the end of 2012, when several leading network operators and service providers published a whitepaper introducing this innovative technology, outlining its potential benefits and identifying its main challenges [10]. As previously mentioned, NFV aims at supporting the creation of more dynamic and service-aware networks, to flexibly entail the increasing demands foreseen for upcoming services in the new generation of communication networks (i.e., the 5G). To this purpose, NFV intends to mitigate the heavy reliance of current network functions on highly specialized hardware, abstracting those functionalities by means of virtualization techniques. Based on this introductory collaborative work, the European Telecommunications Standards Institute (ETSI) founded the Network Functions Virtualization Industry Specification Group (NFV ISG). This group was chartered to lead the work of addressing the technical challenges for NFV realization. Since its inception to the present day, the ETSI NFV ISG has incorporated a wide variety of network stakeholders, from operators to manufacturers, including information technology and cloud players, with the objective of developing and consolidating normative specifications and informative studies that allow the implementation and evolution of NFV. In this context, within the baseline specifications, the ETSI NFV ISG defined an architectural framework for NFV [11], which provides a blueprint for NFV implementation by delineating the different constituents of the architecture, and outlining the interfaces that underpin interoperability. This architectural framework encompasses three main components that are described next. The first component comprises the virtual network functions (VNFs), which represent a software implementation of traditional network functions. VNFs leverage virtualization techniques, so network functionality can be provided without depending on dedicated hardware appliances. In NFV, an end-to-end network service is composed by a set of interconnected VNFs. The set of resources in terms of computing, storage, and networking that builds up the environment in which VNFs run, leads to the second main component of the architectural framework, which is referred to as the NFV infrastructure (NFVI). The NFVI not only encompasses commodity hardware that supports the execution of VNFs (i.e., non-proprietary, industry standard server computers and other heterogeneous appliances with different capacities), but also a virtualization layer, which abstracts and logically partitions the physical resources into virtual resources. These can be allocated to VNFs, ensuring that the VNF lifecycle is strictly independent of the underlying hardware platforms. In addition, this pool of resources offered by the NFVI to accommodate the instantiation of VNFs can be distributed into several locations, or NFVI points of presence (NFVI-PoPs). The third component, the NFV Management and Orchestration (MANO), addresses the virtualization-specific management operations required in the NFV framework to support the decoupling of network functions from the hardware layer, enabling the execution of VNFs. To that end, the MANO component comprises three interacting entities: (1) the Virtualized Infrastructure Manager (VIM), which is in charge of handling the hardware and virtualized resources provided by the NFVI to accommodate VNFs; (2) the VNF Manager (VNFM) covers the lifecycle management of VNFs, which implies operations related to their instantiation, configuration, increase and/or decrease of the allocated resources, and termination; and (3) the NFV Orchestrator (NFVO), responsible for coordinating the resource allocation, and for managing the interconnection of the VNFs to compose end-to-end network services, in collaboration with the VIM and VNFM entities.