Towards Causal Consistent Updates in Software-Defined Networks

Abstract

A network paradigm called the Software-Defined Network (SDN) has recently been introduced. The idea of SDN is to separate the control logic from forwarding devices to enable a centralized control platform. However, SDN is still a distributed and asynchronous system: events can be triggered by any network entity, while messages and packets are prone to arbitrary and unpredictable transmission delays. Moreover, the absence of a global temporal reference results in a broad combinatorial range space of event order. During network updates, an out-of-order execution of events may result in a deviation from desirable consistent network update properties, leading, for example, to forwarding loops and forwarding black holes, among others. In this paper, we introduce a study of the Transient Forwarding Loop (TFL) phenomenon during SDN updates; for this, we define a formal model of the TFL based on causal dependencies that capture the conditions under which it may occur. Based on this model, we introduce an algorithm that ensures the causal dependencies of the system oriented toward TFL-free SDN updating. We formally prove that it is sufficient to ensure the causal dependencies in order to guarantee TFL-free network updates. Finally, we analytically evaluate our algorithm and discuss how it outperforms the state-of-the-art in terms of updating overhead. transmission delays) interface to establish networking. Furthermore, an out-of-order execution of events may occur since no global temporal reference is shared between network entities and message delays are arbitrary. This leads to the following problem: when updating a network while packet flows are taking routes to their destination, an out-of-order execution could give rise to non-deterministic behavior that temporarily deviates from network properties, which in turn may result in an inconsistent network update. Moreover and as a result, the network-wide view from the controller can transitorily be inconsistent with the current data plane state, which could affect the consistency of future network updates. To ensure consistent updates, depending on the network application, the network should align with some properties, such as no Transient Forwarding Loop (TFL) and no forwarding black hole, among others. The no TFL is one of the essential network properties desired by several network applications, including traffic engineering, virtual machine migration, and planned maintenance [4]. Informally, it ensures that a packet is never forwarded along a loop back during an arbitrary time interval to a forwarding device in the network where it was previously processed. To the best of our knowledge, (i) no study formally specifies under which conditions TFLs may occur in the context of SDNs. Furthermore, (ii) no solution to this problem aligns with the distributed and asynchronous nature of the SDNs. Indeed, the proposed solutions are centralized or synchronized. In fact, a centralized-based solution is associated with memory overhead: to perform updates, it makes use of the centr...