Loop-Free Route Updates for Software-Defined Networks

Abstract

We consider the fundamental problem of updating arbitrary routes in a software-defined network in a (transiently) loop-free manner. Our objective is to compute fast network update schedules which minimize the number of interactions (i.e., rounds) between the controller and the network nodes. We first prove that this problem is difficult in general: The problem of deciding whether a k-round update schedule exists is NPcomplete already for k = 3, and there are problem instances requiring Ω(n) rounds, where n is the network size. Given these negative results, we introduce an attractive, relaxed notion of loop-freedom. We show that relaxed loop-freedom admits for much shorter update schedules (up to a factor Ω(n) in the best case), and present a scheduling algorithm which requires at most Θ(log n) rounds.

Distributed Consistent Network Updates in SDNs: Local Verification for Global Guarantees

“…The task is to update from the old (solid) to the new (dashed) forwarding rules for the destination d in a loop-free fashion. In a centralized setting, we need Ω(n) rounds to complete the migration, as only one rule (once: two) can be updated per round [12]. Else, asynchrony could lead to transient loops in the forwarding graph.…”

Survey of Consistent Software-Defined Network Updates

“…9. The authors also observe that loop-freedom can come in two flavors, strong and relaxed loop-freedom [64].…”

“…One first line of research focuses on providing additional guarantees, e.g., congestion-freedom (from [53] to [54], [55], [56], [57], [58], [59], [60]). In a second line of research, several algorithms [32], [61], [62], [63], [64] to compute a set of ordered rule replacements have been proposed to deal with specific SDN update cases (e.g., where only forwarding consistency is needed), avoiding the need for additional rules and hence enabling more resource-efficient approaches (e.g., TCAM memory slots are expensive and precious).…”

“…Moreover, one can distinguish between two different definitions for loop-free network updates: Strong Loop-Freedom (SLF) and Relaxed Loop-Freedom (RLF) [64]. SLF requires that at any point in time, the forwarding rules stored at the switches should be loop-free.…”

“…Ludwig et al in [74], extended in [64], and [67] initiated the study of arbitrary route updates: routes which are not necessarily destination-based. The authors show that the update problem in this case boils down to an optimization problem on a very simple directed graph: initially, before the first update round, the graph simply consists of two connected paths, the old and the new route.…”