Central Control over Distributed Asynchronous Systems: A Tutorial on Software-Defined Networks and Consistent Network Updates

This tutorial will give an introduction to a topic that lies at the intersection of distributed computing and networking, and combines asynchronous distributed systems with central control, namely consistent updates in Software-Defined Networks (SDNs). We will give an overview on current models and algorithms, but also selected related topics, in particular those of potential interest to the PODC community, showcasing avenues for further research. In more detail, SDNs have been an intensively studied topic in networking over the last years, but much of its focus has been on (logical) central control, abstracting away most of its underlying foundation, namely that a network is still a distributed asynchronous system at its core. Summarized in a simplified way, SDNs come with the promise that the network state can be optimized and updated from a global point of view. However, such a simplification becomes especially problematic when consistency guarantees have to maintained. In asynchronous distributed systems, it is not possible to simultaneously change the state of all nodes, such a naive approach will lead to an inconsistent mix of old and new states, introducing e.g. forwarding loops. Notwithstanding, most approaches tackle these issues from the viewpoints of the networking/systems communities, and we believe could henceforth greatly benefit from connections to and ideas from the PODC community.