Asynchronous messages are initiated by the switch and used to update the controller of network events and changes to the switch state. Switches send asynchronous messages to the controller to denote a packet arrival, switch state change, or an error. There are four main asynchronous messages as follows:

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. If the switch has sufficient memory to buffer packets that are sent to the controller, the packet-in message contains some fraction of the packet header (by default, 128 bytes) and a buffer ID to be used by the controller when it is ready for the switch to forward the packet. Switches that do not support internal buffering (or have run out of internal buffer space) must send the full packet to the controller as part of the message.

Flow-Removal

When a flow entry is added to the switch by a flow modify message (the Modify State section), an idle timeout value indicates when the entry should be removed due to the lack of activity as well as a hard timeout value. The hard timeout value indicates when the entry should be removed, regardless of activity. The flow modify message also specifies whether the switch should send a flow removal message to the controller when the flow expires. Flow modify messages, which delete flow entries may also cause flow removal messages.

Port-status

The switch is expected to send port-status messages to the controller as the port configuration state changes. 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). OpenFlow switches may optionally support 802.1D Spanning Tree Protocol (STP).These switches are expected to process all 802.1D packets locally before performing flow lookup. Ports status as specified by the STP is then used to limit packets forwarded to the OFP_FLOOD port to only those ports along the spanning tree. Port changes as a result of the spanning tree are sent to the controller via the port-update messages. Note that forward actions that specify the outgoing port of OFP_ALL ignore the port status set by the STP. Packets received on the ports that are disabled by the STP must follow the normal flow table processing path.

Northbound interface

External management systems or network applications (Net Apps) may wish to extract information about the underlying network or control an aspect of the network behavior or policy. Additionally, controllers may find it necessary to communicate with each other for a variety of reasons. For instance, an internal control application may need to reserve resources across multiple domains of control, or a primary controller may need to share policy information with a backup controller.

Unlike controller-switch communication (that is the southbound interface), there is no currently accepted standard for northbound interface and they are more likely to be implemented on an ad-hoc basis for particular applications. One potential reason is that the northbound interface is defined entirely in software, while controller-switch interactions must enable the hardware implementation.

If we consider the controller as a network operating system, then there should be a clearly defined interface by which applications can access the underlying hardware (switches), coexist and interact with other applications, and utilize system services (for example, topology discovery, forwarding, and so on), without requiring the application developer to know the implementation details of the controller (that is the network operating system).

While there are several controllers that exist, their application interfaces are still in the early stages and independent from each other and incompatible. Until a clear northbound interface standard emerges, SDN applications will continue to be developed in an ad-hoc fashion and the concept of flexible and portable network apps may have to wait for some time.