## 1 Introduction  
Today's users continuously demand interactive, content-rich, and immersive networked experiences while imposing increasingly stringent requirements on the delivery capability of the network infrastructure. With a number of mobile digital services, _i.e._, apps, which are racing toward the two million mark, the over-the-top (OTT) service providers have a consistent margin for innovating their infrastructure to optimize service provisioning and to enrich their service offerings to users at a rapid pace. On the other hand, network service providers (NSPs) have many difficulties to roll out innovative network-based services and to benefit from increasing revenues of the new digital economy [1]. A relevant obstacle is the inflexibility of the network infrastructure, which is static and costly to change. In fact, the deployment of network services and functions (e.g., routers, middleboxes) traditionally require the acquisition and operation of specialized hardware devices and their interconnections. This results in static chains of network services that cannot flexibly cope with dynamic user and service requirements (e.g., delay constraints) [2]. Moreover, the inflexibility in the service delivery is even more prominent across different NSPs and geographical domains where the level of resource accessibility and exploitation in the service delivery chain is still limited. The lack of a multi-provider infrastructure service coordination and deployments prevents NSPs to benefit from agile service customization, service enhancement, and reaching new markets [3].  
Recent research efforts on promising network technologies, _i.e._, software defined networking (SDN) [4] and network function virtualization (NFV) [5], go in the direction of conceiving and developing novel network virtualization models and programmable network service delivery functions to address the aforementioned flexibility and scalability requirements. Indeed, in accordance with recent service virtualization paradigms adopted in the computing domain (_i.e._, cloud computing), they are expected to promote a service-centric vision where the data delivery function is decoupled from the underlying network physical infrastructure and is conceived as a chain of ready-to-use functional capabilities, dynamically deployed where more appropriate in the physical network as virtual resources and provisioned as a service in potentially multi-provider environments [6; 7]. Challenges related to dynamic service composition and provisioning in a multi-provider environment have been faced in the last two decades in the IT domain, especially with the efforts related to service-oriented architecture (SOA) [8]. We, thus, deem it important to take into account principles and best practices of SOA in order to foster the deployment of SDN and NFV solutions for the provision of network services that can also be dynamically discovered, negotiated, and composed from different providers to meet specific user and service requirements.  
Below, we introduce the main concepts of NFV and SDN in order to provide the background information for our work. We also introduce main concepts of SOA, since it provides a reference framework for service modeling and provisioning.