SDN controller and Northbound Interface Technology

The control layer is the SDN brain. It is responsible for centralized and unified control of underlying forwarding devices and provides interfaces for calling network capabilities for upstream services. It plays an important role in the SDN architecture, SDN controller is also the focus of SDN. In terms of technical implementation, in addition to the southbound network control and northbound business support, the Controller also needs to pay attention to the expansion of things to avoid performance and Security bottlenecks caused by centralized SDN control, SDN controllers also introduce core technologies in the South, North, and East directions to effectively solve the problems of communication with various layers and horizontal scaling of control clusters.

Currently, there are many open-source controller implementations based on OpenFlow control protocols, such as NOX, Onix, and Floodlight. They all have their own distinctive designs, supports basic operations that support SDN network operations, such as link discovery, topology management, policy formulation, and table item delivery. Although the functions and performance of different controllers are still different, we can summarize the technical features that SDN controllers should possess, the experiences and lessons learned from the R & D and practice of these open-source systems will help promote the standardized development of SDN controllers.

In addition, the Controller Used for centralized network control is the core of the SDN network, and its performance and security are very important, the possible problems such as excessive load and single point of failure have always been urgent issues in the SDN field. At present, the industry has also made a lot of discussions on this, from the deployment architecture, technical measures and other aspects of the proposed many innovative methods.

SDN controllers control networks through the southbound interface protocol, including link discovery, topology management, policy formulation, and table item delivery, link discovery and topology management mainly control the use of the uplink channel of the southbound interface to centrally monitor and collect statistics on the information reported by the underlying switch device; policy formulation and table item issuance are the unified control of network devices using the downlink channel of the southbound interface.

SDN northbound interfaces are interfaces opened to upstream business applications through controllers. Their goal is to enable business applications to conveniently call underlying network resources and capabilities. Through the Northbound Interface, network service developers can call various network resources in the form of software programming; at the same time, the upper-layer network resource management system can use the Controller's Northbound Interface to globally control the resource status of the entire network and perform unified resource scheduling. Because the Northbound Interface directly serves business applications, its design needs to closely relate to business application requirements and has diversified features. At the same time, whether the design of the Northbound Interface is reasonable and convenient, so that it can be widely called by business applications will directly affect the market prospects of SDN controller manufacturers.

Unlike OpenFlow and other international standards, the Northbound Interface lacks industry-recognized standards. Therefore, the Protocol formulation of northbound interfaces has become the focus of competition in the SDN field, different participants, from the user perspective, from the Operation perspective, or from the product capability perspective, have proposed many solutions. It is reported that there are at least 20 controllers, each of which provides an external Northbound Interface for upper-layer application development and resource orchestration. Although it is still difficult to reach consensus on northbound interface standards, full openness, convenience, and flexibility are important criteria for measuring the advantages and disadvantages of interfaces, for example, REST APIs are preferred interfaces for upper-layer business application developers. Some traditional network equipment vendors provide programming interfaces for business applications to directly call on their existing devices. They can also be considered as one of the northbound interfaces, the purpose is to improve the configuration management flexibility without changing the existing device architecture and cope with open protocol competition.

The Controller is responsible for centralized control of the entire SDN network and plays an important role in grasping the resource view of the entire network and improving network resource delivery. However, the centralization of control capabilities also means that the security and performance of the controller board has become a bottleneck for the whole network. In addition, a single controller cannot cope with SND network problems across multiple regions, A Distributed cluster composed of multiple SDN controllers is required to avoid the reliability, scalability, and performance problems of a single controller node. Currently, no standards have been defined for East-West interfaces used for communication and connection between multiple controllers, but experts say some very mature cluster technologies can be applied to SDN networks to solve the above problems.