  Business models of network service providers are undergoing an evolving
transformation fueled by vertical customer demands and technological advances
such as 5G, Software Defined Networking~(SDN), and Network Function
Virtualization~(NFV). Emerging scenarios call for agile network services
consuming network, storage, and compute resources across heterogeneous
infrastructures and administrative domains. Coordinating resource control and
service creation across interconnected domains and diverse technologies becomes
a grand challenge. Research and development efforts are being devoted to
enabling orchestration processes to automate, coordinate, and manage the
deployment and operation of network services. In this survey, we delve into the
topic of Network Service Orchestration~(NSO) by reviewing the historical
background, relevant research projects, enabling technologies, and
standardization activities. We define key concepts and propose a taxonomy of
NSO approaches and solutions to pave the way towards a common understanding of
the various ongoing efforts around the realization of diverse NSO application
scenarios. Based on the analysis of the state of affairs, we present a series
of open challenges and research opportunities, altogether contributing to a
timely and comprehensive survey on the vibrant and strategic topic of network
service orchestration.
