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Open Networking Foundation (ONF) defines OpenFlow as the first standard communications interface defined between the control and forwarding layers of an SDN architecture.
OpenFlow allows direct access to and manipulation of the forwarding plane of network devices such as switches and routers.

Importance of OpenFlow Protocol

The OpenFlow protocol is a standard bearer& enabler in Software-defined Networking.
It allows the servers to tell the network: where to send the packets.
It enables the network controller to determine the path of network packets.
It allows for sophisticated traffic management & is feasible using ACL, routing protocols.
It allows switches from different vendors to be managed remotely using a single, open protocol.

Types of Messages in OpenFlow

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1. Controller-to-switch Messages

They are initiated by the controller.
They are used to directly manage or inspect the state of the switch. They are further categorized as following messages:

1. Features

Controller may request a capabilities of switch by sending a feature request message to the switch.
The switch must reply with a features reply message specifying the features and capabilities of the switch.

2. Configuration

The controller is able to set and query configuration parameters in the switch.

3. Modify-State

These messages are sent by the controller to manage the state of the switches.
They are used to add/delete or modify flow table entries or to set switch port priorities.

4. Read-State

They collect statistics from the switch flow tables, ports, and the individual flow entries.

5. Send-Packet

They send packets out of a specified port on the switch.
They forward the packets received via packet-in messages.

6. Barrier

They ensure that message dependencies have been met.
They receive notifications for completed operations.

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7. Role-Request

They set the role of their OpenFlow channel & also query that role

8. Asynchronous Configuration

These messages set an additional filter on asynchronous messages that it wants to receive/query.
It is used when the switch connects to multiple controller.

2. Asynchronous messages

Asynchronous messages are sent without the controller even soliciting them from the switch.
Switches send asynchronous messages to the controller to denote: a packet arrival, switch state change, or an error.
The four types of asynchronous messages:

1. Packet-in

For all packets that do not have a matching flow entry or if a packet matches an entry with a send to controller action, a packet-in message is sent to the controller.
It transfers the control of the packet to the controller.

2. Flow-Removal

This message informs controller about removal of a flow entry from a flow table.
The flow modify message also specifies whether the switch should send a flow removal message to the controller when the flow expires.

3. Port-status

It informs the controller of a change on the port.
These events include changes in port status (for example, disabled by the user) or a change in the port status as specified by 802.1D (Spanning Tree).
Switch is expected to send port status to controller via the port-update messages.

4. Error

The switch is able to notify the controller of problems using error message.

3. Symmetric messages

Symmetric messages are sent without solicitation, in any direction i.e initiated by switch or a controller.
The three types of symmetric message are:

1. Hello

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Hello messages are exchanged between the switch and controller upon connection setup.

2. Echo

These messages can be used to indicate the latency, bandwidth, and/or liveliness of a controller-switch connection (that is a heartbeat).

3. Vendor

These messages offer additional functionality within the OpenFlow message type space for future revisions of OpenFlow.