  Distributed software-defined networks (SDN), consisting of multiple
inter-connected network domains, each managed by one SDN controller, is an
emerging networking architecture that offers balanced centralized control and
distributed operations. Under such networking paradigm, most existing works
focus on designing sophisticated controller-synchronization strategies to
improve joint controller-decision-making for inter-domain routing. However,
there is still a lack of fundamental understanding of how the performance of
distributed SDN is related to network attributes, thus impossible to justify
the necessity of complicated strategies. In this regard, we analyze and
quantify the performance enhancement of distributed SDN architectures,
influenced by intra-/inter-domain synchronization levels and network structural
properties. Based on a generic weighted network model, we establish analytical
methods for performance estimation under four synchronization scenarios with
increasing synchronization cost. Moreover, two of these synchronization
scenarios correspond to extreme cases, i.e., minimum/maximum synchronization,
which are, therefore, capable of bounding the performance of distributed SDN
with any given synchronization levels. Our theoretical results reveal how
network performance is related to synchronization levels and inter-domain
connections, the accuracy of which are confirmed by simulations based on both
real and synthetic networks. To the best of our knowledge, this is the first
work quantifying the performance of distributed SDN analytically, which
provides fundamental guidance for future SDN protocol designs and performance
estimation.
