Packets sent to the controller from the datapath always utilize the OFPT_PACKET_IN message. 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) set the buffer ID to OFP_NO_BUFFER and must send the full packet to the controller as part of the message.