External management systems or network applications (Net Apps) may wish to extract information about the underlying network or control an aspect of the network behavior or policy. Additionally, controllers may find it necessary to communicate with each other for a variety of reasons. For instance, an internal control application may need to reserve resources across multiple domains of control, or a primary controller may need to share policy information with a backup controller.

Unlike controller-switch communication (that is, the southbound interface), currently, there is no accepted standard for the northbound interface and they are more likely to be implemented on an ad-hoc basis for particular applications.

A potential reason is that the northbound interface is defined entirely in the software, while controller-switch interactions must enable the hardware implementation. If we consider the controller as a network operating system, then there should be a clearly defined interface by which applications can access the underlying hardware (switches), coexist, and interact with other applications and utilize system services (for example, topology discovery, forwarding, and so on), without requiring the application developer to know the implementation details of the controller (that is, the network operating system). While there are several controllers that exist, their application interfaces are still in the early stages and independent from each other and incompatible. Until a clear northbound interface standard emerges, SDN applications will continue to be developed in an ad-hoc fashion and the concept of flexible and portable Net Apps may have to wait for some time.