OpenFlow is a southbound protocol for controlling a switch’s match-action tables.
SDN defines the architecture. OpenFlow provides 1 possible controller-to-switch interface.
Flow-Based Forwarding
A flow is a set of packets identified by shared packet-header fields or ingress information.
- Matching can combine addresses, transport ports, protocol, VLAN, and ingress port.
- A rule can represent an individual connection or an aggregate of connections.
- Forwarding can depend on more than the destination IP prefix.
- Available match fields and actions depend on the device and interface.
Flow Entries
- Match fields select packets belonging to a flow.
- Priority resolves overlapping matches.
- Instructions and actions determine packet processing.
- Counters record traffic handled by an entry.
- Timeouts can remove stale entries.
Core Messages
FLOW-MOD
Adds, modifies, or removes flow entries.PACKET-IN
Reports a packet to the controller when configured processing requires it.PACKET-OUT
Requests the switch to process a packet using specified actions.
Reactive and Proactive Installation
| Property | Reactive Installation | Proactive Installation |
|---|---|---|
| Trigger | Traffic arrival causes rule installation. | Rules are installed before traffic arrives. |
| Initial packet delay | Controller interaction adds delay. | Existing rules avoid a setup round trip. |
| Controller load | New flows generate control work. | Routine traffic requires less controller interaction. |
| Table use | Rules are allocated as traffic appears. | Rules may occupy space before they are used. |
| Main concern | Bursts of new flows can overload the controller. | Rule coverage and table capacity require planning. |
A hybrid design installs base connectivity proactively and handles exceptions reactively.
Programmable Data Planes
P4 is a language for describing packet parsing, match-action tables, and supported actions in a programmable data plane.
- OpenFlow operates within the match fields and actions exposed by its specification and implementation.
- P4 allows the pipeline itself to be described programmatically.
- P4Runtime controls entries in the resulting pipeline.
- Target hardware still limits supported operations and resources.
