Software-defined networking (SDN) separates network control logic from the devices that forward packets, allowing software to program and coordinate behavior across multiple switches or routers. The devices still carry traffic; a controller provides a broader, logically centralized way to direct them. SDN is an architectural approach, not one product or a single required protocol.
What software-defined networking means
The Open Networking Foundation (ONF) defines SDN as “The physical separation of the network control plane from the forwarding plane, and where a control plane controls several devices.” In practical terms, forwarding devices handle packet movement while software-based control logic can coordinate network behavior across them. The controller’s view may be logically centralized without relying on one physical controller.
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The IETF/IRTF describes SDN more broadly as network programmability: the ability to initialize, control, change, and manage network behavior through software and open interfaces. RFC 7426, published in January 2015, is an Informational RFC—not an Internet Standard—and notes that SDN terminology and layer models are not uniform. Read RFC 7426.
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How the architecture fits together
A useful way to understand SDN is to follow a policy from the software that requests it to the devices that act on it. These are conceptual areas, not a mandatory layer taxonomy.
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- Applications and policy: Network applications or administrators express desired services, rules, or outcomes.
- Control: Controller software translates those intentions into instructions for network devices and maintains a logical view of the network.
- Infrastructure and forwarding: Physical or virtual switches and routers forward packets according to their installed behavior.
ONF says controller software can present the network to applications and policy engines as a single logical switch. In common usage, “northbound” refers to interaction between applications or orchestration systems and the control software; “southbound” refers to interaction between the controller and forwarding devices. Those labels describe directions, not one universal API or protocol.
Features SDN can provide
- Direct programmability: Control logic is separated from packet forwarding so software can change network behavior.
- Agility: An abstraction over individual devices can make it possible to adjust traffic behavior as requirements change.
- Logical centralization: Controllers can maintain a broader network view than device-by-device management alone.
- Programmatic configuration: Software can configure, manage, secure, and optimize network resources.
- Potential openness: Open standards can support vendor neutrality, but the SDN label alone does not guarantee compatible implementations.
These are architectural capabilities, not proof that a particular deployment will be simpler, more secure, or interoperable.
OpenFlow is one part of SDN
OpenFlow is a standard interface described by ONF for communication between a supported switch and controller. In that model, the switch retains the data path while high-level routing decisions can be handled by separate controller software. OpenFlow messages can report packet events, send packets, modify forwarding tables, and retrieve statistics. ONF’s technical library provides its specifications and technical material.
SDN is the broader architecture or programmability approach; OpenFlow is one interface associated with it, not a synonym for SDN. Device support and protocol versions vary, so compatibility must be checked for the particular controller and hardware rather than assumed.
Potential benefits—and what they depend on
Centralized management, automation, abstraction from individual device configuration, and faster policy or service changes are common reasons organizations consider SDN. Cisco’s enterprise overview also frames SDN as a way to automate network operations across physical and virtual environments; that is a vendor perspective, not independent evidence of savings or results. Cisco’s SDN overview.
Benefits are not automatic. Results depend on device support, controller resilience, policy correctness, staff expertise, migration choices, and day-to-day operations. ONF’s older white paper discusses possible reliability and security gains from centralized management and consistent policy, but it does not establish a current independent comparison or quantified outcome. ONF’s historical white paper.
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Moving control functions into software creates capabilities, but it also makes the controller architecture, compatibility, and operational process important design concerns. Before choosing an approach or platform, examine:
- Device and protocol compatibility: Which switches and routers, protocol versions, and functions are supported? Verify the exact combinations.
- Controller availability and failure behavior: How is control software deployed, and what happens to existing forwarding behavior if a controller or connection is unavailable?
- Interfaces and programmability: Which application, orchestration, and device interfaces are actually available, and can the needed workflows use them?
- Policy and verification: How are access controls, policy consistency, monitoring, and configuration correctness handled?
- Operational fit: What migration, troubleshooting, skills, and lifecycle-management work will the deployment require?
- Portability: Are the standards and implementations sufficiently compatible to avoid dependence on one vendor or controller?
These checks matter because logical centralization and open standards do not by themselves guarantee resilience or portability.
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An example: ONF’s NG-SDN reference design
ONF’s NG-SDN material describes a programmable, hardware-independent reference platform combining open-source software with white-box hardware. Named components include Stratum, P4Runtime, gNMI, gNOI, gRIBI, OpenConfig models, μONOS, and a verification engine. The design emphasizes programmability and ongoing network verification; it is an example, not evidence that this stack is dominant or universally deployed. See ONF’s NG-SDN material.
Where to learn more
For foundational terminology, read RFC 7426 alongside ONF’s SDN definition and technical library. ONF also recommends the open-source micro-book Software-Defined Networks: A Systems Approach for deeper study; availability and formats may vary.
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