I just finished reading sections 1-6.2 of the OpenFlow specification here.
Section 6.1.2 says:
Packet-in events can be configured to buffer packets. For packet-in generated by an output action in a flow entries or group bucket, it can be specified individually in the output action itself (see 7.2.6.1), for other packet-in it can be configured in the switch configuration (see 7.3.2). If the packet-in event is configured to buffer packets and the switch has sufficient memory to buffer them, the packet-in event contains only some fraction of the packet header and a buffer ID to be used by a controller when it is ready for the switch to forward the packet. Switches that do not support internal buffering, are configured to not buffer packets for the packet-in event, or have run out of internal buffering, must send the full packet to controllers as part of the event. Buffered packets will usually be processed via a Packet-out or Flow-mod message from a controller, or automatically expired after some time
This makes it sound like for every packet that hits the OpenFlow switch, an asynchronous message must be sent to the controller to make a forwarding decision. However Chapter 5 makes it sound like a switch has a set of OpenFlow flows and at the end of that generates an action set which determines what should be done with a packet and the packet is only forwarded to the controller when there is a flow table miss.
Under what conditions is a packet sent to the controller for a decision? Is it always? Or is it only circumstantial?
Packets will be sent to the OpenFlow controller any time the out port is set to be the controller.
PACKET_IN events occur when a flow wasn't matched on the switch and are then sent to the controller. Otherwise no event is created - the switch simply forwards the packet according to the flow rules and the controller is none the wiser.