OpenFlow Overview

OpenFlow defines an interface between and SDN controller and a switch. Within the larger context of SDN, it is the "southbound" interface of the controller.

OpenFlow is not alone in that role. There are alternative messaging protocols and alternative means of communication between the controller and the switch. 

The following picture shows a diagram of the basic OpenFlow components.They are 

• OpenFlow Controller
• OpenFlow Logical Switch, which contains
• Ports
• Secure channel
• Flow Table containing a set of flow instructions - flow entries - that tell the switch how to handle a given packet

Controller 

The controller is responsible for communicating instructions to the switch across the secure channel, but the OpenFlow protocol has no say in how the controller determines what instructions to send. The instructions might be determined by automated orchestration component, by direct operator intervention, or by traditional path determination protocol such as OSPF or BGP. 

Switch

Packet processing happens through a set of tables: flow tables, a group table, and a meter table. The tables are populated with instructions from the controller through the OpenFlow channel. Packet enter and exit processing through ports, just as they would in normal switch.

Ports 

Ports on an OpenFlow switch serve the same input/output purpose that they do on any switch. A port can be added, changed, or removed in the switch configuration using, for example from the controller. Port changes must be communicated to the controller so that the controller can clean up any flow entries related to the port. 

Secure Channel

The OpenFlow channel is the communications interface between the switch and the controller. Anything OpenFlow related happening between the switch and controller - instructions and configuration from the controller to the switch, notifications from the switch to the controller, and packets passing from processing to the controller or from the controller into processing- goes over this channel. 

Flow Table

The flow table consists of a set of flow entries. Based on flow entries, the switch forwards the packet.