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Software-defined networking (SDN) separates network control logic from packet forwarding so software can program and coordinate how multiple devices handle traffic. Switches and routers still forward packets; controller software provides a broader, logically centralized way to direct their behavior. SDN describes an architectural approach, not one mandatory product, protocol, or universal network blueprint.
What does software-defined networking mean?
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 carry packets while software-based control logic programs network behavior across devices. The control can be logically centralized without relying on a single physical controller.
The IETF/IRTF’s RFC 7426 describes SDN more broadly as network programmability through open interfaces and abstraction between the control and forwarding planes. Published as an Informational RFC in January 2015, it notes that SDN terminology and layer structures vary; it is not an Internet Standard that mandates one architecture.
How is an SDN network organized?
A three-part model helps explain the roles. It is a teaching model, not a required SDN layer taxonomy.
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Applications and policy
Applications or policy systems express desired services and rules—for example, which traffic should receive priority or which resources a service may use.
Control software
Controller software translates those intentions into instructions for the network and maintains a logical view of it. ONF describes the controller as able to present the network to applications and policy engines as a single logical switch, abstracting some device-by-device detail.
Infrastructure and forwarding
Physical or virtual switches and routers forward packets according to installed behavior. They remain part of the data path; SDN does not mean that the devices stop forwarding traffic or have no local functions.
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Interfaces between these areas are often described directionally. “Northbound” commonly means application or orchestration interaction with control software; “southbound” commonly means controller interaction with forwarding devices. There is no single universal API or protocol for either direction across all SDN deployments.
What are SDN’s main features?
- Direct programmability: Control logic is separated from forwarding functions so software can change network behavior.
- Agility: Abstraction can make it possible to adjust traffic handling across a network as needs change.
- Logical centralization: Controller software can provide a broader view than configuring each device in isolation. This does not require one physical controller.
- Programmatic configuration: Software can be used to configure, manage, secure, and optimize network resources.
- Potential openness: Open standards may support vendor neutrality, but only when the standards and compatible implementations are actually used.
These are architectural capabilities, not guarantees that every product marketed as SDN will be easy to manage, interoperable, or secure.
Is OpenFlow the same as SDN?
No. SDN is the broader architectural and programmability approach; OpenFlow is one standardized interface associated with it. ONF describes OpenFlow as communication between a supported switch and controller. In this model, the switch retains the data path while high-level routing decisions move to a separate controller, often hosted on a standard server. OpenFlow messages can report packet events, send packets, modify forwarding tables, and retrieve statistics.
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Support varies by device and protocol version, so compatibility must be checked for the specific controller, switch, and deployment. OpenFlow is not required to define every SDN system, and its presence alone does not establish that a network is fully open or multi-vendor.
What can SDN improve, and what does it not guarantee?
SDN can support centralized management across devices, automation, abstraction from individual device configuration, and faster changes to policy or services. Cisco’s SDN overview frames the approach as a way to automate network lifecycle tasks and manage policy across physical and virtual environments; that is a vendor perspective rather than independent evidence of savings or outcomes.
SDN does not automatically lower costs, improve security, increase reliability, or boost performance. Results depend on device support, controller resilience, policy correctness, staffing, and operational practice. ONF’s historical SDN white paper presents possible benefits from centralized and automated management, but its qualitative claims are not a current independent benchmark. No current attributable impact statistic is established here.
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What does a concrete SDN implementation look like?
ONF’s NG-SDN reference-design material describes a programmable, hardware-independent platform built from open-source software and 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 one ONF example, not evidence that this stack is dominant or universally deployed.
For background on the components and design, see ONF’s NG-SDN material. A project name or reference design is not a substitute for checking current component status, versions, and device compatibility before deployment.
How should you evaluate an SDN approach?
Compare a proposed design against the network you need to operate, rather than relying on the SDN label. Check:
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- Compatibility: Which devices, protocols, and versions are supported? Confirm multi-vendor operation rather than assuming it.
- Controller design: How is the controller deployed, and what happens during failure? Consider availability and scale.
- Interfaces: Which application, orchestration, and device interfaces are available, and how programmable are they?
- Policy and verification: How are policy consistency, access control, monitoring, and correctness handled?
- Operations: What skills, migration work, troubleshooting methods, and lifecycle processes will the network require?
- Portability: Which open standards and data models are used, and can the control and data planes interoperate across implementations?
ONF’s technical library collects specifications and technical material for readers examining particular designs. A standards claim should be verified against the actual specifications and the implementations being considered.
Further reading
For a terminology-focused reference, consult RFC 7426. ONF also lists Software-Defined Networks: A Systems Approach as an open-source micro-book for deeper study; availability and format can vary.
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