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An SDN controller is an application in a software-defined networking (SDN) architecture that manages flow control for improved network management and application performance. The SDN controller platform typically runs on a server and uses protocols to tell switches where to send packets.
SDN controllers direct traffic according to forwarding policies that a network operator puts in place, thereby minimizing manual configurations for individual network devices. By taking the control plane off of the network hardware and running it instead as software, the centralized controller facilitates automated network management and makes it easier to integrate and administer business applications. In effect, the SDN controller serves as a sort of operating system (OS) for the network.
The controller is the core of a software-defined network. It resides between network devices at one end of the network and applications at the other end. Any communication between applications and network devices must go through the controller.
The controller communicates with applications -- such as firewalls or load balancers -- via northbound interfaces. The Open Networking Foundation (ONF) created a working group in 2013 focused specifically on northbound APIs and their development. The industry never settled on a standardized set, however, largely because application requirements vary so widely.
The controller talks with individual network devices using a southbound interface, traditionally one like the OpenFlow protocol. These southbound protocols allow the controller to configure network devices and choose the optimal network path for application traffic. OpenFlow was created by ONF in 2011.
Pros and cons of SDN controllers
One major benefit of SDN controllers is that the centralized controller is aware of all the available network paths and can direct packets based on traffic requirements. Because of the controller's visibility into the network, it can automatically modify traffic flows and notify network operators about congested links.
Companies can -- and should -- use more than one controller, adding a backup for redundancy. Three seems to be a common number among both commercial and open source SDN options. This redundancy will enable the network to continue running in the event of lost connectivity or controller susceptibility.
The controller acts as a single point of failure, so securing it is pivotal to any software-defined network. Whoever owns the controller has access to the entire network. This means network operators should create security and authentication policies to ensure only the right people have access.
SDN controller vendors
Vendors that offer SDN controllers include the following:
- Big Switch Networks
- Cumulus Networks
- Hewlett Packard Enterprise
- Juniper Networks
- Nuage Networks
- Pluribus Networks
Open source SDN controllers
SDN controllers are available in a variety of open source options, including:
- Open Network Operating System
Traditionally, SDN controllers are used in data center networks. As SDN technology evolved, however, the WAN became a compelling use case, driving the growth of software-defined WAN (SD-WAN) technology. An SD-WAN controller performs many of the same duties as an SDN controller, following policy configurations to direct WAN traffic over the most efficient route. The SD-WAN market has fewer notable open source options than SDN, as most SD-WAN controllers typically come tied together with the vendor's proprietary SD-WAN platform.