A Preliminary Study on Software Defined Network (SDN)

【51CTO.com Quick Translation】Before 2008, the entire network world was composed and controlled by hardware devices. With the emergence of the OpenFlow protocol, people first attracted attention to software-defined networking (SDN). As an outstanding network control mechanism and technology category, SDN greatly simplifies the automation of network resources and enterprise policy-based network management. Currently, SDN has developed into one of the most reliable and stable network technologies supported by many technology service providers (such as Cisco).

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What is Software Defined Networking?

In short, software-defined networking (SDN) is an architecture that promotes network agility and flexibility. The main technical purpose of SDN is to achieve a centralized controlled network through software applications. By digging deeper, you will find that the purpose behind software-defined networking is to meet the ever-changing business needs through intelligent programming services, thereby organizing and building a network that can achieve consistency and comprehensive management.

With software applications based on application programming interfaces (APIs), SDN allows developers to program networks in a centralized control environment. Therefore, enterprises can open up a new network platform by implementing common SDN controls and manage the entire network without increasing technical complexity.

As the content carried in our network becomes more and more diverse, the dependence on cloud computing services becomes deeper and deeper, and the interaction and linkage between different mobile devices become more and more frequent, we need to use SDN to timely integrate various services according to the dynamic changes of consumer service providers, so as to fundamentally change the way of handling traditional services. It can be said that in order to achieve the predetermined control effect, many enterprises are actively using SDN technical services to improve their original operating models and performance.

As we all know, network design is crucial to the digitalization of business models, and digitalization often benefits from the implementation and use of SDN technology in its related aspects. Enterprises that mainly provide SDN services separate the control plane and data plane of the network through the revolutionary implementation of network virtualization, thereby achieving flexible control of network traffic and providing a good environment platform for innovation of core networks and applications in a way similar to intelligent pipelines.

The popularity of Cisco SDN is largely due to the centralized applications based on controllers and programmable network applications. They can provide an overall view of the network in which the enterprise is located. Currently, SDN is also changing the existing architecture of major data centers and the network models to which they are connected. Based on this, the existing network of the enterprise can also be automatically adjusted according to actual application needs.

What is an SDN controller?

As the "brain" in the Cisco network, the SDN controller is actually a software that can provide a centralized view and comprehensive control of the enterprise's entire network. IT operations personnel, managers, and even network administrators inside and outside the enterprise can regulate the data flow through the controller by managing the forwarding layer of the infrastructure. Through a policy-based manager, SDN can effectively organize various resources and overall behaviors in the network. Network administrators can also apply the policies originally applied to devices to multiple nodes in the network in a consistent manner.

In essence, these policies are rules for incoming network traffic. They can classify the access level of the target network and then assign corresponding priorities to the resources allowed to be accessed. Through a centralized network status view, we can achieve simple and unified network management based on various preset policies.

Software Defined Network Architecture

The SDN architecture includes the following three levels:

• application
• control
• Infrastructure

The application layer includes many services running on the network, the control layer is managed by the SDN controller, which is regarded as the "brain" of the network, and the infrastructure layer is composed of various network switches, routers, and other supporting hardware. These layers communicate with each other to play various roles. To facilitate interaction, SDN usually comes with the following two independent application programming interfaces:

• Southbound API
• Northbound API

The northbound API is used for communication between the control layer and the infrastructure layer, while the southbound API is used for communication between the control layer and the application layer.

What is the Northbound API?

In order to obtain available resources, various SDN applications rely on controllers to determine the state of the network infrastructure. The network policies pre-established by the administrator can coordinate the routing of the SDN controller according to the needs of application traffic. The relationship between them is explained in detail in Cisco's Software Defined Network (SDN) and Network Function Virtualization (NFV) documents.

The application layer can learn the content and location of its resource requirements by actively communicating with the control layer. The control layer then organizes resource "orders" and delivers them based on network availability. The control layer then uses intelligence to generate an ideal application path with synchronized latency and security features. It is worth noting that the organization process of the northbound API is not manually configured, but is pre-programmed to be automatic.

What is a Southbound API?

In the southbound API, the SDN controller (control layer) communicates with the infrastructure layer in the network through various routers and switches. It can guide the network infrastructure to follow the application data path set by the controller. Although the main function of the controller is to improve the way routers and switches are used to move data, the destination of data in the network infrastructure does not depend on physical devices or routing channels. In terms of intelligent intelligence, the controller has the role of making the best decisions and adjusting the data movement path.

The role of SDN

In addition to providing a programmable centralized view of the network and consistent network resources to meet changing business needs, SDN also provides many practical technical and business services, including:

(1) Centralized management and programmability

The control and forwarding functions of SDN are separated from each other. You can configure the target network programmatically through open source code or proprietary automation tools. The network's intelligence is mainly concentrated in the controller (ie: brain) software, which can treat the target network as a single switch to provide a centralized view.

(2) Minimized operating expenses

Most of the applications of SDN (including the updates of various network software) are automated. Since it avoids the need to purchase network infrastructure for each business or network requirement, SDN can minimize operating expenses. It reduces the possibility of human error by unifying policies and disseminating them across the network. It can be said that by automating the routine tasks of network administrators or operators, SDN greatly reduces the time spent on network management.

(3) Agility and flexibility

SDN aims to help enterprises achieve their business goals by deploying the latest applications and services in existing infrastructure with better agility and flexibility. When services are actually updated and created, we only need to make simple modifications to integrate and publish them to the entire network.

Original title: Detailed Look into the Software-Defined Networking (SDN) , author: Sitwat Maroof

[Translated by 51CTO. Please indicate the original translator and source as 51CTO.com when reprinting on partner sites]