Update Consistency in Software-defined Networking based Multicast Networks

When applying updates on distributed network elements with SDN, intra-update states may violate desired network properties, such as drop-and loop-freeness. Current stateless approaches cannot guarantee the constancy of arbitrary network invariants (correctness) in general, yet update procedures guaranteeing certain invariants do exist. In this paper, we investigate on update consistency for the case of multicast routing and show that there is no correct update procedure w.r.t. both drop-and duplicate-freeness. We show that certain updates of multicast routes inherently raise a concurrency issue, which necessarily results in the occurrence of either drops or duplicates. Furthermore, we present a generic update procedure for multicast routing updates that identifies concurrency-relevant update steps. This procedure allows for the selection of an update strategy, such that either drops or duplicates are avoided. These effects can severely degrade network performance or quality of experience. To investigate the implications of drops and duplicates, we evaluate their frequencies and impact for wide-area network scenarios both, analytically and empirically, through direct measurement in the data plane under update.