SDN controllers

One of the key fundamentals of SDN is disaggregation. Disaggregation of software and hardware in a network and also disaggregation of control and forwarding planes.

The SDN controller is the main brain and controller of an SDN environment. It's a strategic control point within the network and it is responsible for communicating information to:

• Routers and switches and other network devices behind them. SDN controllers uses APIs or protocols (such as OpenFlow or NETCONF) to communicate with these devices. This communication is known as southbound.
• Upstream switches, routers or applications, and the aforementioned business logic (via APIs or protocols). This communication is known as northbound. An example of a northbound communication is a BGP session between a legacy router and SDN controller.

If you are familiar with chassis based switches such as Cisco Catalyst 6500 or Nexus 7k chassis, you can imagine an SDN network as a chassis, with switches and routers as its I/O line cards and SDN controller as its supervisor or management module. In fact SDN is similar to a very scalable chassis where you don't have any limitation on the number of physical slots.

SDN controller is similar to the role of the management module of a chassis-based switch and it controls all switches via its southbound protocols and APIs.

The following table compares the SDN controller and a chassis based switch:

The first protocol that popularized the concept behind SDN was OpenFlow. When conceptualized by networking researchers at Stanford back in 2008, it was meant to manipulate the data plane to optimize traffic flows and make adjustments, so the network could quickly adapt to changing requirements. Version 1.0 of the OpenFlow specification was released in December of 2009; it continues to be enhanced under the management of the Open Networking Foundation, which is a user-led organization focused on advancing the development of open standards and the adoption of SDN technologies.

OpenFlow protocol was the first protocol that helped in popularizing SDN. OpenFlow is a protocol designed to update the flow tables in a switch. Allowing the SDN controller to access the forwarding table of each member switch or in other words to connect control plane and data plane in the SDN world. Back in 2008, OpenFlow was conceptualized by networking researchers at Stanford University, the initial use of OpenFlow was to alter the switch forwarding tables to optimize traffic flows and make adjustments, so the network could quickly adapt to changing requirements.

After the introduction of OpenFlow, NOX was introduced as an original OpenFlow controller (there still wasn't a concept of the SDN controller). NOX was providing a high-level API capable of managing and also developing network control applications. Separate applications were required to run on top of NOX to manage the network. NOX was initially developed by Nicira networks (which was acquired by VMware, and finally became part of VMware NSX). NOX was introduced along with OpenFlow in 2009. NOX was a closed source product, but ultimately it was donated to the SDN community, which led to multiple forks and sub projects out of original NOX. For example, POX is a sub project of NOX, which provides Python support. Both NOX and POX were early controllers. NOX appears an inactive development, however POX is still in use by the research community as it is a Python-based project and can be easily deployed.

NOX apart from being the first OpenFlow or SDN controller also established a programming model that is inherited by other subsequent controllers. The model was based on processing of OpenFlow messages, with each incoming OpenFlow message triggering an event that had to be processed individually. This model was simple to implement, but not efficient and robust and couldn't scale.

Nicira along with NTT and Google started developing ONIX, which was meant to be more abstract and scalable for large deployments. ONIX became the base for Nicira (the core of VMware NSX or network virtualization platform) and there are rumors that it is also the base for Google WAN controller. ONIX was planned to become open source and donated to the community, but for some reasons the main contributors decided to not do it, which forced the SDN community to focus on developing other platforms.

Starting in 2010, a new controller was introduced. It was called the Beacon controller and it became one of the most popular controllers. It was born due to the contribution of developers from Stanford University. Beacon is a Java-based open source OpenFlow controller created in 2010. It has been widely used for teaching, research, and as the basis of Floodlight. Beacon had the first built-in web user-interface, which was a huge step forward in the market of SDN controllers. Also it provided an easier method to deploy and run compared to NOX. Beacon was an influence for design of later controllers after it; however, it was only supporting star topologies, which was one of the limitations of this controller.

Floodlight was a successful SDN controller that was built as a fork of Beacon. Big Switch networks are developing Floodlight along with other developers. In 2013, Beacon popularity started to shrink and Floodlight started to gain popularity. Floodlight had fixed many issues of Beacon and added lots of additional features, which made it one of the most feature rich controllers available. It also had a web interface, a Java-based GUI, and it also could get integrated with OpenStack using the quantum plugin. Integration with OpenStack was a big step forward as it could be used to provide networking to a large pool of virtual machines, compute, and storage. Floodlight adoption increased by evolution of OpenStack and OpenStack adopters. This gave Floodlight greater popularity and applicability than other controllers that came before. Most of the controllers that came after Floodlight also supported OpenStack integration.

Floodlight is still supported and developed by community and Big Switch networks, and it is a base for Big Cloud Fabric (the Big Switch's commercial SDN controller).

There are other open source SDN controllers that are introduced such as Trema (ruby-based from NEC), Ryu (supported by NTT), FlowER, LOOM, and the recent OpenMUL.

The following table shows the current open source SDN controllers: