SDN: From ideal to reality

SDN is more than 10 years old. When it first came out, its concept of separation of control plane and data plane attracted great attention and research in the network industry. Today, SDN is no longer popular. Some people say, "SDN is dead, so let's burn it." Some people say, "The spirit of SDN is immortal, and it remains the same no matter how it changes." What is SDN? What is the current situation of SDN? What is the gap between the ideal and reality of SDN?

The development of virtualization has led to significant changes in network traffic, especially data center traffic. The three-layer network architecture of Core/Distribution/Access is very effective in the traditional Client-Server network architecture. However, today, the north-south traffic from users to servers is much lower than the east-west traffic from servers to servers. When the east-west traffic increases sharply, the three-layer network architecture begins to become incapable.

At the same time, traditional network systems have also exposed many limitations - the system is too complex and cannot flexibly respond to changes according to the business needs of enterprises. Therefore, rapid deployment, flexible scalability, automation and easy integration are the key attributes that the network must meet.

Market demand has given rise to a new trend in the network industry: SDN (Software Defined Network)!

SDN: From Inspiration to Essence

According to ONF (Open Networking Foundation), SDN is a network architecture in which the control plane and data plane are separated and programmable.

What are the control plane and data plane? For example, we know the route from home to the company and can set off without navigation because we have obtained data and decisions in advance. However, if we want to go from home to Company A where a stranger works, how do we plan the route? We need to think, find and choose the right path. Calculating and deciding the path is equivalent to using the control plane in network language. However, walking from home to the company does not require calculation and decision-making, but just starting directly, which is equivalent to processing on the data plane.

The same is true for network systems. To transfer data from point A to point B, each network device must contain both computing functions (i.e., control plane) and data transmission functions (i.e., data plane). In an SDN network, the control plane is separated from the device and centralized in a place called an SDN controller (essentially software programmed to control the network). The network system has hundreds of devices, but only one controller is needed for computing and decision-making, and the devices only transmit data through the controller's instructions.

In terms of architecture, SDN is divided into three layers. The infrastructure layer includes all network devices with data forwarding functions (i.e., data plane execution). The SDN control layer contains one or more controllers responsible for managing and controlling the packet forwarding of the underlying network devices. The two layers communicate using southbound interfaces (such as OpenFlow). The top layer is the application layer, which includes applications and services (such as traffic control, security, QoS, etc.), which interact directly with the control layer through northbound interfaces.

With the above network architecture, SDN network brings many changes:

First, separating the control plane and the data plane helps create a "global view" for the entire network, improving visibility and policy consistency. In theory, if the device only performs data plane tasks, it will simplify the design and greatly reduce the cost of hardware production, because the device itself does not need to run too many protocols, but only executes commands from the controller. Most importantly, this separation can better abstract the network system.

"Abstraction" is a very important feature of SDN networks. Today's networks are becoming simpler and more automated, so network systems must be abstracted. Through a layered network architecture, network administrators can program network systems as needed and push down to the underlying implementation without having to interact with hundreds of devices, thousands of lines of code, and complex protocols.

From centralized control planes to create a "global view" of the entire network, to inter-layer communication via APIs, all network elements in SDN support custom programming. This combination helps network administrators increase the level of automation in their networks, and applications and network services can run in real time. Network systems can be fully customized and optimized for the desired features without relying on equipment manufacturers.

SDN: From ideal to reality

There are many differences between the ideal and reality of SDN. There are many differences between actual SDN products compared to SDN definitions and research. For actual SDN products, are the control plane and data plane really separated? Will these devices be independently manufactured without relying on vendors? Is Openflow a popular communication protocol?

SDN is constantly evolving, and network architectures are becoming more diverse:

The SDN network architecture mentioned above is the architecture (2a) in the figure above, where the control plane and the data plane are completely separated. Routing protocols such as OSPF and BGP or MAC learning mechanisms are all concentrated in the controller, and the device is only the physical part that implements data forwarding. This is considered to be the original SDN architecture.

However, this architecture may exist more in research labs and some experimental products. It is different when it comes to commercial SDN products. For example, SD-WAN uses architecture (2b), where the components that control the entire network are concentrated on the controller, but the control plane is retained on the device so that the device can operate relatively independently. Even some solutions are called SDN, but in fact the control plane is completely on the device, and the controller only tends to centrally manage the entire network to support communication with the application layer and users.

The architecture (3) in the figure shows an SDN-based Overlay architecture, which is also part of many software-defined products on the market today. Overlay networks are designed to use network virtualization to establish connected logical networks on top of physical infrastructure.

From the perspective of SDN, the solution products provided in the market are very diverse, which are called SDX (Software Defined xx). Because SDN is not a product, it is an architecture and a concept. The final product depends on the purpose of construction and the specific solution.

For example, SDN can be used for network traffic scheduling. Usually, the path selection of traditional networks is based on the "optimal" path calculated by the routing protocol, but the result may cause traffic congestion on the "optimal" path, and other non-"optimal" paths may be idle. When the SDN network architecture is adopted, SDN with a "global vision" can intelligently adjust the network traffic path according to the network traffic status and improve network utilization. SD-WAN is a good example.

For the virtualization of network functions, including the virtualization of physical network infrastructure in a multi-tenant environment, SDN helps create virtual networks in the data center, providing rapid provisioning for both the infrastructure layer and applications.

From this we can see that from the perspective of the SDN network, it can be developed into different products and solutions to overcome many problems that are impossible or difficult to solve with traditional networks.

SDN (or more precisely, the idea of ​​SDN) will continue to be applied and developed, bringing more diverse and improved products in the future. I hope that this article will help interested readers better grasp the market trends and understand the essence of SDN networks and their applications in practice.