The SDN switch (for instance, an OpenFlow switch), the SDN controller, and the interfaces are present in the controller for communication with forwarding devices, generally southbound interface (OpenFlow), and northbound interface (the network application interface), which are the fundamental building blocks of an SDN deployment. Switches in an SDN are often represented as basic forwarding hardware accessible via an open interface, as the control logic and algorithms are offloaded to a controller. OpenFlow switches come in two varieties: pure (OpenFlow-only) and hybrid (OpenFlow-enabled).

Pure OpenFlow switches have no legacy features or onboard control and completely rely on a controller for forwarding decisions. Hybrid switches support OpenFlow in addition to traditional operation and protocols. Most commercial switches available today are hybrids. An OpenFlow switch consists of a flow table, which performs packet lookup and forwarding. Each flow table in the switch holds a set of flow entries that consists of the following:

• Header fields or match fields, with information found in the packet header, ingress port, and metadata used to match incoming packets.
• Counters used to collect statistics for the particular flow, such as the number of received packets, the number of bytes, and the duration of the flow.
• Actions, which are sets of instructions or actions to be applied after a match, show how to handle matching packets. For instance, the action might be to forward a packet to a specified port.
• Priority, with information showing the precedence of the flow entry.
• Timeouts used by the switch to determine the maximum amount of time or idle time in which the flow should be discarded.
• Cookies, which are transparent data used by the controller to filter flow entries affected by flow statistics, flow modification, and flow deletion requests. It is not used in packet processing.
• Flags are used to alter the way flows get managed. For example, the CHECK_OVERLAP flag is used to inform the switch to verify that there is no conflicting flow existing on the switch with the same priority.