If device A is sending data to device B accross a Frame Relay intrastructure and one of the intermediate Frame Relay switches encounters congestion, congestion being full buffers, over subscribed port, overloaded resources, etc, it will set the BECN bit on packets being returned to the sending device and the FECN bit on the packets being sent to the receiving device. This has the effect of telling the sending router to Back off and apply flow control like traffic Shaping and informs the receiving device that the flow is congested and that it should inform upper layer protocols, if possible, that it should close down windowing etc to inform the sending application to slow down.
A FECN tells the receiving device that the path is congested so that the upper layer protocols should expect some delay. The BECN tells the transmitting device that the Frame Relay network is congested and that it should "back off" to allow better throughput.
FECN (Forward Error Congestion Notification)
BECN (Backward Error Congestion Notification)
