Flexibility in Softwarized Networks: Classifications and Research Challenges

The increase of flexibility is a common objective of softwarized networks based on concepts such as Software Defined Networking, Network Function Virtualization and Network Virtualization. Hence, in the state of the art, flexibility is used as an argument for a certain proposed architecture, solution mechanism or design choice in general. The meaning of flexibility behind such rather qualitative arguments is highly diversified in the literature, as a common understanding of flexibility is missing so far. In this article, we survey the state of the art in softwarized networks with a focus on the flexibility that is provided by each proposed concept, mechanism or system. In particular, we show that the flexibility provided by different network softwarization technologies can be classified into six different aspects within the three high-level flexibility categories, i.e., configuration adaptation, functions location and scalability. We analyze the state of the art in flexibility from several viewpoints including flexibility aspects, network technologies, domains and planes in order to derive a common understanding of how flexibility can be provided in softwarized wireline and wireless networks. Moreover, we reveal open issues, which are mostly related to the fact that flexibility is not clearly defined in the literature, and derive concrete research challenges accordingly. Our classification of flexibility and the derived research challenges aim at stimulating the discussion towards a more quantitative analysis of the design requirement of flexibility that has demonstrated increasing importance for softwarized networks and beyond.