Open Network Operating System, which represents ONOS, provides the control plane for a SDN, managing network components, such as switches and links, and running software programs or modules to provide communication services to end hosts and neighboring networks. It is the first open source network OS designed for service providers and mission-critical networks. Its aim is to provide high availability and optimum performance to these networks.

In server operating systems, ONOS provides analogous functionalities, some of which are APIs and abstractions, resource allocation, and permissions, as well as user-facing software such as a CLI, a GUI, and system applications. ONOS is used to manage the entire network rather than a single device in traditional switching operating systems, which, in turn. simplifies the management, configuration, and deployment of new software, hardware, and services. In SDN controllers, ONOS acts as an extensible, modular, and distributed SDN controller for the whole network.

ONOS kernel, core services, and ONOS applications are written in Java as bundles that are loaded into the karaf OSGi container. OSGi is a component system for Java that allows modules to be installed and run dynamically in a single JVM. Since ONOS runs in the JVM, it can run on several underlying OS platforms.

ONOS comprises four major features that make the architecture. They are as follows:

• Distributed core: This provides the service provider features (scalability and high availability) to the control plane of SDN. The same ONOS software is generally deployed on the cluster of servers present on the network. As a result of this, the network applications and network devices do not know whether they are working on a single instance or multiple instances of ONOS, thus providing network scalability for the network administrator. The distributed core is the main architectural feature that presents the service-provider-grade features to the SDN control plane.
• Northbound abstraction/API: This includes the network graph and applications whose purpose is to ease the development of control, management, and configuration services. It comprises two abstractions, that is, internet framework and global network view:
  • The internet framework permits an application to request a certain service from the network without prior knowledge of how the service will be performed. Network administrators and application developers use this to their advantage in high-level network programming. A common example of intent would be setting up an optical path between switch A and switch B and restricting the bandwidth assigned to the path.
  • The global network view is used to give the application a view of the network. This view gives the application the capability to program the network via the API. Each API allows the application look at the view as a network graph. Network graphs can be used in traffic engineering, practically by maximizing the network utilization by monitoring the network view and programming changes to the path to adjust the load.
• Southbound abstraction/API: The southbound is a key component for migration from traditional switches to open flow white boxes. It insulates the core of ONOS from the details of diverse protocols and devices by representing each network element as an object in a generic form. This abstraction permits the distributed core to maintain the state of the network node without knowing the specifics of the element being represented. The benefits of the southbound include the ability to manage various devices using multiple protocols with no effect on the distributed core, adding new devices to the system easily, as well as a smooth migration of traditional devices and protocols to white boxes that support full OpenFlow standards.
• Software modularity: This is what accommodates the easy development, debugging, and maintaining of ONOS as a software system by the community of users and developers. ONOS is an open source project backed by an expanding community of developers and users.