Software-Defined Networking (SDN) has been one of the major buzz words of the networking industry for the past couple of years. And yet, no clear definition of what SDN actually covers has been broadly admitted so far. This document aims at contributing to the clarification of the SDN landscape by providing a perspective on requirements, issues and other considerations about SDN, as seen from within a service provider environment. It is not meant to endlessly discuss what SDN truly means, but rather to suggest a functional taxonomy of the techniques that can be used under a SDN umbrella and to elaborate on the various pending issues the combined activation of such techniques inevitably raises. As such, a definition of SDN is only mentioned for the sake of clarification.