PacketIn

PacketIn is a message in the OpenFlow protocol used by software-defined networking (SDN). It is sent by an OpenFlow switch to the controller to report a data-plane packet that could not be matched by any existing flow entry or that is configured to be sent to the controller for decision. The PacketIn message allows the controller to learn about the network state, make a forwarding decision, and install new flow rules as needed.

A PacketIn typically includes information such as the port where the packet entered the switch (in_port), a reference to a buffered copy on the switch (buffer_id), the total length of the original packet, and a reason code indicating why the packet was sent to the controller (for example, no matching flow). Depending on the OpenFlow version, the message may also carry a switch-specific cookie, a table identifier, a match structure describing the packet’s fields, and the actual packet data if it was not buffered. Variants exist across OpenFlow versions (for example, OFPT_PACKET_IN in early versions and corresponding fields in newer ones).

Handling a PacketIn involves the controller inspecting the packet, determining an appropriate action, and replying with a flow-modification message to install a rule or with a packet-out action to forward the packet immediately. This mechanism enables dynamic, centralized control of the data plane and supports use cases such as learning switches, topology discovery, and policy enforcement.

Security and performance considerations include the potential for controller overload from excessive PacketIn messages and the need for appropriate buffering, rate limiting, and thoughtful flow-install strategies to minimize unnecessary controller interaction.

See also: OpenFlow, FlowMod, PacketOut, SDN.