Software-Defined Networking 2

5Adaptive and Continuous Consistency for Distributed SDN Controllers: Quorum-Based Replication

5.1. Introduction

Existing SDN controller platforms have been created according to different SDN control plane designs with the aim of meeting specific requirements in terms of scalability, high availability and performance. Consistency has also been regarded as an essential design principle for the distributed SDN controller platforms. The latter use conventional consistency models to manage the distributed state among the controllers in the cluster. As explained in section 4.3.2, the consistency models used in SDN can be categorized into strong, eventual and weak (OpenDayLight; ONOS). These static and standard consistency models (OpenDayLight; ONOS) have both advantages and drawbacks.

In large-scale SDNs, the strong consistency control model might be extremely expensive and costly to maintain for certain applications. Indeed, it requires important synchronization efforts among the controller replicas at the cost of causing serious network scalability and performance issues. In contrast, the eventual consistency control model implies less inter-controller communication overhead as it sacrifices the strict consistency guarantees for higher availability and improved performance. In practice, many scalable control applications running in modern distributed storage systems such as Apache’s Cassandra (Lakshman and Malik 2010) and Amazon’s DynamoDB (Sivasubramanian 2012) opt for eventual ...