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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.
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Importance of OpenFlow Protocol

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

Types of Messages in OpenFlow

Computing

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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

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  • They ensure that message dependencies have been met.
  • They receive notifications for completed operations.
7. Role-Request
  • They set the role of their OpenFlow channel & also query that role
8. Asynchronous Configuration
Computing
  • 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

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  • 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

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  • 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
  • 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.

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