Which Frame Relay flow control mechanism is used to signal routers that they should reduce the flow rate frames?

If there is a congestion in one direction, the FECN (forward explicit congestion notification) bit is set to inform routers in the forward direction (from the point of view of the traffic) about the congestion. Also, eventually some frames will travel back to the origin; in that case, the BECN (backWord explicit congestion notification) bit is also set to inform the origin about the congestion.