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The OpenFlow protocol supports the following message types: controller-to-switch, asynchronous,
and symmetric. Each message type has its own subtypes.
Controller-to-Switch messages
IMPORTANT:
Support for Controller-to-Switch messages varies by device mode.
Controller-to-Switch messages are initiated by the controller and used to directly manage or inspect
the state of the switch. Controller-to-Switch messages might or might not require a response from
the switch.
The Controller-to-Switch messages include the following subtypes:
• Features—The controller requests the basic capabilities of a switch by sending a features
request. The switch must respond with a features reply that specifies the basic capabilities of
the switch.
• Configuration—The controller sets and queries configuration parameters in the switch. The
switch only responds to a query from the controller.
• Modify-State—The controller sends Modify-State messages to manage state on the switches.
Their primary purpose is to add, delete, and modify flow or group entries in the OpenFlow tables
and to set switch port properties.
• Read-State—The controller sends Read-State messages to collect various information from
the switch, such as current configuration and statistics.
• Packet-out—These are used by the controller to send packets out of the specified port on the
switch, or to forward packets received through Packet-in messages. Packet-out messages
must contain a full packet or a buffer ID referencing a packet stored in the switch. The message
must also contain a list of actions to be applied in the order they are specified. An empty action
list drops the packet.
• Barrier—Barrier messages are used to confirm the completion of the previous operations. The
controller send s Barrier request. The switch must send a Barrier reply when all the previous
operations are complete.
• Role-Request—Role-Request messages are used by the controller to set the role of its
OpenFlow channel, or query that role. It is typically used when the switch connects to multiple
controllers.
• Asynchronous-Configuration—These are used by the controller to set an additional filter on
the asynchronous messages that it wants to receive, or to query that filter. It is typically used
when the switch connects to multiple controllers.
Asynchronous messages
IMPORTANT:
Support for asynchronous messages varies by device mode.
Switches send asynchronous messages to controllers to inform a packet arrival or switch state
change. For example, when a flow entry is removed due to timeout, the switch sends a flow removed
message to inform the controller.
The asynchronous messages include the following subtypes:
• Packet-In—Transfer the control of a packet to the controller. For all packets forwarded to the
Controller reserved port using a flow entry or the table-miss flow entry, a packet-in event is
always sent to controllers. Other processing, such as TTL checking, can also generate
packet-in events to send packets to the controller. The Packet-in events can include the full
packet or can be configured to buffer packets in the switch. If the packet-in event is configured
to buffer packets, the packet-in events contain only some fraction of the packet header and a
buffer ID. The controller processes the full packet or the combination of the packet header and
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