An essential part of Software-Defined Networking (SDN) are southbound APIs (Application Programming Interfaces), which allow communication between the data plane (underlying network devices) and the control plane (the SDN controller). Since the applications are at the top (“North”) and the physical hardware is at the bottom (“South”) of a typical architecture diagram, these interfaces are referred to as “southbound” since they lead down to the switches, routers, and firewalls, whether they are virtual or physical.
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Core Functions and Responsibilities
A Southbound API’s main function is to translate high-level network policies or “business intent” into precise, low-level instructions that hardware may follow. Their primary duties consist of:
- Forwarding and Flow Control: The controller programs forwarding tables and sends “flow entries” to switches using these APIs, specifying the precise path packets should take throughout the network.
- Device configuration and management: They allow remote network element setting installation, change, and removal.
- Telemetry and Monitoring: Devices send these interfaces real-time hardware health, traffic, and status data.
- Topology Discovery: To discover the network’s topology, the controller employs southbound APIs to determine device port connections.
How they function:
- Controller: Applications are used by a central brain (like the Cisco DNA Center) to make choices.
- Southbound API: These APIs are used by the controller to convert general directives (such as “permit App X”) into more detailed instructions (such as flow rules and ACLs).
- Network Devices: These commands are sent to switches and routers through the southbound API, where they are enforced in the data plane to regulate traffic flow.
Common Southbound Protocols
Southbound interfaces use a range of specialized protocols based on the vendor and the particular task, in contrast to Northbound APIs, which are usually REST-based:
- OpenFlow: OpenFlow, an industry standard, lets controllers directly control flow tables.
- NETCONF / RESTCONF: NETCONF and RESTCONF manage device configurations using YANG and XML/JSON data formats.
- OpFlex: Cisco-exclusive declarative protocol OpFlex. It transmits abstract “summary policies” to devices, in contrast to OpenFlow, letting the hardware determine how to apply the modifications locally.
- gNMI / gRPC / P4 Runtime: Modern, high-performance interfaces for programming next-generation data planes and streaming telemetry are known as gNMI, gRPC, and P4 runtimes.
- Traditional Methods: Controllers such as Cisco DNA Center may continue to use SNMP and CLI (SSH/Telnet) as southbound communication channels in order to maintain backward compatibility with legacy devices.
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Difference Between Northbound and Southbound API
While both are essential to SDN, they serve different directions and users:
| Aspect | Southbound APIs | Northbound APIs |
|---|---|---|
| Direction of Communication | Controller → Network Devices | Controller → Applications |
| Primary Purpose | Control and configure network hardware | Expose network capabilities to applications |
| Network Layer | Data Plane interaction | Application / Service Layer interaction |
| Users | SDN controllers, network engineers | Application developers, orchestration tools |
| Level of Abstraction | Low-level, device-specific | High-level, abstracted |
| Functionality | Flow rules, device configuration, telemetry | Policy definition, automation, analytics |
| Typical Technologies | OpenFlow, NETCONF, RESTCONF, SNMP, gNMI, P4 Runtime | REST APIs, Intent-based APIs, YANG models |
| Focus | How the network operates | What the network should achieve |
| Hardware Dependency | Closely tied to device capabilities | Largely hardware-agnostic |
| Examples of Platforms | OpenFlow-enabled switches, routers | Cisco DNA Center, ONOS, OpenDaylight |
| Change Frequency | Frequent, real-time updates | Event-driven or on-demand |
| Security Scope | Device access control | Application authorization and policy control |
The advantages of Southbound APIs
Administrators may manage numerous devices from a single location thanks to Southbound APIs’ centralized control, which separates network management from the actual hardware. This architecture facilitates automation by allowing for instantaneous configuration changes and hardware abstraction, which gives the controller the ability to control a variety of equipment via a single interface.
Consider an orchestra conductor to have a better idea of this. The conductor’s gestures and baton for interacting with the musicians (the switches and routers) are the Southbound API. Instead of playing the instruments, the conductor uses this communication to make sure each musician is aware of exactly the notes to play, allowing the various parts to perform in unison.
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