The architecture of Software Defined Networking (SDN) is typically visualized as a set of layers that interact through well-defined interfaces. This layered approach is key to its flexibility and programmability. The primary layers are the Application Plane, the Control Plane, and the Data Plane (also known as the Infrastructure Plane).
This separation allows for independent evolution and development of each plane, fostering innovation and specialization. The central idea is the decoupling of the decision-making part of networking from the traffic-forwarding part.
The Control Plane: The Network's Brain
The Control Plane is often referred to as the "brain" of the SDN network. It contains the logic that governs network behavior.
Centralized Intelligence: Typically, a centralized SDN controller (or a distributed cluster acting as one) resides in the control plane. This controller has a global view of the network topology and status.
Decision Making: Based on policies defined by network administrators and information from the application plane, the controller makes decisions about how traffic should be routed, what Quality of Service (QoS) to apply, and how to implement security policies.
Communication with Data Plane: The controller communicates its decisions to the data plane devices using a southbound interface, with OpenFlow being a common protocol for this communication.
This centralization of control logic simplifies network management and enables sophisticated automation and orchestration of network services.
The Data Plane: The Network's Muscle
The Data Plane (or Infrastructure Plane) consists of the physical and virtual network devices (switches, routers, access points) that actually forward network traffic.
Packet Forwarding: The primary function of the data plane is to process and forward packets based on the instructions received from the control plane. These devices become simple forwarding elements.
Execution of Instructions: Data plane devices expose their capabilities to the control plane and execute the forwarding rules (e.g., flow rules in OpenFlow) pushed down by the controller.
Simplified Hardware: Because the complex control logic is moved to the controller, data plane devices can potentially be simpler and less expensive.
The data plane elements are responsible for the actual handling of data packets, ensuring they reach their destination efficiently as dictated by the control plane.
The Application Plane: Leveraging Network Programmability
The Application Plane consists of network applications and services that leverage the programmability offered by SDN. These applications communicate their network requirements to the SDN controller via northbound interfaces (NBIs).
Network Services: Examples include load balancers, firewalls, intrusion detection systems, network monitoring tools, and custom business applications.
Automation and Orchestration: Applications can dynamically request network resources, modify network policies, and adapt to changing conditions automatically.
Innovation: By exposing network capabilities through APIs, the application plane allows for the development of innovative services that were difficult or impossible to implement in traditional networks. Much like how algorithmic market analysis platforms innovate in financial services through deep data processing, SDN applications drive innovation in network services.
Key Interfaces in SDN Architecture
Southbound Interfaces (SBIs): These interfaces enable communication between the SDN controller (control plane) and the network forwarding devices (data plane). OpenFlow is a well-known example of an SBI protocol.
Northbound Interfaces (NBIs): These interfaces allow applications and services in the application plane to communicate their requirements and desired network behavior to the SDN controller. REST APIs are commonly used for NBIs.
East-West Interfaces: These interfaces facilitate communication and coordination between multiple SDN controllers in a distributed control plane setup, ensuring consistency and scalability.
Understanding these interfaces is crucial for comprehending how the different layers of the SDN architecture interact to create a cohesive and programmable network.