Cloud Definable Networking: Software-Defined Mobile Network: SDN Architecture

Various SDN architectures have been proposed for addressing the different demands. The first SDN architecture is proposed by ONF, which is widely accepted in the academia and industry. In addition, the NFV architecture is proposed by ETSI for carrier networks, which is supported by the industry. OpenDaylight is jointly proposed by the major equipment manufacturers and software companies to concrete SDN architecture in the actual deployment.

The SDN architecture is originally mentioned by ONF, and Fig. 4.5 illustrates its version released in 2013. From bottom to top (or from south to north), the SDN architecture consists of data plane, control plane, and application plane. The communication between the data plane and control planes is supported by the SDN control-data-plane interface (CDPI), which is a unified communication standard mainly based on the OpenFlow protocol. The communication between the control plane and application plane is supported by the SDN northbound interfaces (NBIs), which is available to be customized according to the actual demands. The data plane consists of the switches and other NEs, while the connections between the NEs follow the different rules. The control plane containing the SDN control logic is responsible for running the control logic, maintaining the network view. Furthermore, the controller abstracts the network view into network services, accesses CDPI proxy to invoke the appropriate network data path, and provides the operators, researchers, and other third parties with the convenient NBIs for customized private applications and logic network management. The application plane includes various SDN-based network applications. The users do not care about the technical details of the underlying device, and the new applications can be quickly deployed through simply programming. CDPI is responsible for forwarding the rules sent from the network operating system to the network devices, which needs to match the equipment with different manufacturers and models and does not affect the control layer and the upper logic. NBIs enable the third party to 2 https://datatracker.ietf.org/wg/forces/documents/. https://irtf.org/sdnrg. 4 http://www.itu.int/en/ITU-T/Pages/default.aspx. 34.3 Software-Defined Mobile Network 39 Fig. 4.5 SDN architecture (Source: SDN Architecture Overview, ONF, 2013) develop the individual network management software and applications, providing more options for managers. This network abstraction feature allows the user to select a different network operating system based on demand, and does not affect the normal operation of the physical device. The interface in SDN is open, and the controller is logic center. The southbound is responsible for communications with data plane, while the northbound is responsible for the communications with application plane. Furthermore, because the single control mechanism is likely to cause control node failure, seriously affecting the performance, multi-mode controller is adopted. In the researches on the open interfaces, the southbound interface of the controller is the core technology and research hotspot to separate control and data. Due to the decoupling between control layer and data layer, the improvements for these two layers are relatively independent that only the standard southbound interfaces are essential between the layers, which is a key component of the SDN layered architecture. Logically, it has to support the normal communication between the control layer and data layer, and the independent update of them. Physically, the manufacturers need to develop the devices supporting the standard southbound interfaces, because the traditional network equipment is not running among the SDN network. Therefore, the development of standard southbound interfaces is an important part of basic research on SDN. Many organizations have developed40 4 Definable Networking some standards for the southbound interfaces, and the CDPI proposed by ONF become mainstream southbound interfaces, which follows the OpenFlow protocol. OpenFlow is the first widely accepted data interface protocol for SDN control layer, which makes the single integrated and closed network equipment to be more flexible and controllable communication equipment. OpenFlow protocol matches rules based on the concept of flow, therefore, the switch needs to maintain a flow table for data forwarding, while the creation, maintained, and distribution of the flow table are completed by the controller. Except the southbound interface related researches, the NBIs, eastbound and westbound interfaces are also the hotspots. The NBIs support the communications between the control layer and various applications. The applications are able to request resources programmatically for learning the overall network information, simplifying the network configuration and accelerating the business deployment. However, due to the diversity of business applications, it is difficult to develop the NBI corresponding the various demands. Initially, SDN supports customized NBIs for different scenarios custom fit North, but the unified NBIs are considered to directly affect the smooth development of the application service. Thus, a lot organizations begin to develop the NBI standards, such as the NBI standards by ONF and REST API standards by OpenDaylight. However, these standards only describe the function without the detailed implementation. Therefore, how to achieve a unified NBI standard is the next major task in industry. Different from the north-southbound interfaces, the east-westbound interfaces provide the communications between the controllers. Because the limited performance of the single controller cannot meet the demand for deploying the large-scale SDN, the east-westbound interfaces are expected to improve the salability of the controller and provide technical support for load balancing and performance improvements.