How Do FECNs and BECNs Work?
Forward explicit congestion notification (FECN) and backwards explicit
congestion notification (BECN) are used in networks by intermediary
nodes to inform other nodes of congestion that was experienced as a
packet traveled across the network. In Frame Relay, setting a specific bit
in a normal Frame Relay packet indicates a FECN or BECN. Here’s how
it works.
If device A is sending data to device B across a Frame Relay infrastructure,
and one of the intermediary Frame Relay switches encounters
congestion (congestion being full buffers), an over-subscribed port,
overloaded resources, and so forth, it will set the BECN bit on packets
being returned to the sending device (A), and the FECN bit on the
packets being sent to the receiving device (B). This has the effect of
informing the sending router to slow down and apply flow control, such
as traffic shaping, and informing the receiving device that the flow is
congested and that upper layer protocols should expect some delays.