OpenFlow is a communications protocol that enables the interaction between the control and data planes of a software-defined network (SDN). OpenFlow messages play a crucial role in facilitating communication between the SDN controller and the network devices, such as switches and routers. These messages are used to convey instructions, queries, and updates to dynamically manage and control network behavior.
Key OpenFlow Messages:
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Packet-In:
- Purpose: Generated by the switch to inform the controller about a packet that does not match any existing flow entry.
- Usage: Enables the controller to make decisions about how to handle the packet, such as installing a new flow entry.
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Flow-Removed:
- Purpose: Sent by the switch to inform the controller about the removal of a flow entry from the flow table.
- Usage: Provides information about the duration and reason for removing the flow entry.
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Features-Request/Reply:
- Purpose: The controller requests information about the switch's capabilities, and the switch responds with its features.
- Usage: Helps the controller understand the capabilities of the switch, including the number of ports and supported features.
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Flow-Mod:
- Purpose: Sent by the controller to add, modify, or delete flow entries in the switch's flow table.
- Usage: Enables the controller to dynamically control how the switch processes packets based on various criteria.
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Packet-Out:
- Purpose: Used by the controller to instruct the switch to send a packet out of a specific port.
- Usage: Allows the controller to manipulate the flow of individual packets through the network.
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Port-Status:
- Purpose: Notifies the controller about changes in the status of ports on the switch, such as port addition or removal.
- Usage: Helps the controller adapt to changes in the network topology.
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Barrier-Request/Reply:
- Purpose: The controller can use this message to ensure that certain operations have completed before proceeding.
- Usage: Ensures the synchronization of actions in the network.
