View source for 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
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https://datatracker.ietf.org/wg/forces/documents/.
https://irtf.org/sdnrg.
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http://www.itu.int/en/ITU-T/Pages/default.aspx.
34.3 Software-Defined Mobile Network
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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.