## 2 Related Work
### State of the Art of Research Works  
Dynamic service chaining is considered a relevant challenge for the evolution of network deployments toward flexible, cost-effective, and on-demand service delivery models. Recently, several works have been focused on this topic and related issues. Based on the analysis of existing works, we distinguish related literature in two main areas: architectural approaches for NFV/SDN integrated management; algorithms and techniques for specific issues in dynamic service chaining. Within the first area, several works include an architectural model integrating NFV management with SDN control capabilities as a main goal of the work or as a reference framework within which more focused contributions can be positioned. Some authors limited their discussion to high-level guidelines and architectural views. Reference [27] is a position paper that analyses the case of multi-domain distributed deployment of NFV. Some reference use cases are analyzed and challenges and research directions are discussed. [PERSON] _et al._[28] argue the need of conceiving a Network Operating System (NOS) to cope with the lack of network-wide abstractions, thus favoring the foundation for true network programmability. However, this is a short paper that not handles in detail the integration of network control capabilities with NFV management functions. [PERSON] _et al._[29] present a three-layer architectural view for both network and cloud domains (infrastructure, control and application layers). The integration of these two architectures is discussed by introducing an orchestration functional box, which is used for services that require a combination of these resources. However, its interaction with cloud and network control functions is only briefly described. Reference [30] analyzes benefits and challenges related to the adoption of the NFV paradigm toward the 5G cellular framework. The work moves from the high-level architecture conceived within the T-NOVA European Project to support the dynamic provision of VNFs on-demand and as-a-service. However, the authors provide only an introductory and conceptual description of the architecture. Other related works provide deeper insights into architectural design and related implementation issues [31; 32; 33; 34]. Reference [31] presents a functional architecture supporting automated, dynamic service creation leveraging NFV, SDN, and cloud virtualization techniques, which has been conceived in the framework of UNIFY, a three year European Project that has just concluded. The proposed architecture comprisesa service layer, an orchestration layer and an infrastructure layer. They also briefly discuss how some functions/layers of the architecture can be mapped to ETSI NFV MANO and ONF SDN architectural elements. UNIFY do not explicitly refer to SOA principles, although some similar concepts exists, such as the description of services at different levels of abstractions (_i.e._, service graphs and network function forwarding graph). Nevertheless, UNIFY does not aim at supporting a multi provider marketplace of network services as this work proposes using SOA practices, which were, in fact, designed to ease the operation of multi-provider environments where services can be dynamically-advertised and discovered. [PERSON] _et al._[32] propose a modular VNF architecture providing policy-based management of VNF and service chains. Their main contribution consists in a basic ontology-based information model to describe network resources, network control functions, and VNFs capabilities with a uniform language. However, the role of network control functions (_i.e._, SDN controller) in the architecture and its interaction with the NFV orchestrator are not explained. [PERSON] _et al._[33] address the problem of SDN-based virtual connectivity over multi-technological domains.