The FECN, BECN and DE bits are used for network congestion control.
The Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) bits can be used to notify an end station, such as a router, of network congestion.
FECN bits notifies devices downstream that congestion is occurring. Downstream means destination.
BECN bits indicates to routers that could be contributing to congestion that they should slow down transmission.
Discard Eligible (DE) bits indicate that the frame is eligible for discard if congestion occurs.
Most routers set frames to be discards eligible if the traffic exceeds the PVC's Commited Information Rate (CIR).
The CIR is the transmission rate guaranteed by the service provider.