Frame Relay

Frame Relay notes on FECN, BECN, DE and related concepts.

Frame Relay Forum (frforum.com)

PVCs and CIR

• CIR = Committed Information Rate
• PVC = Permanent Virtual Circuit

The difference between PVC and CIR is that a PVC is a connection and CIR is a property of that connection. Two nodes connect to a Frame Relay service provider and to transfer data between them they use a PVC. The speed of this PVC is defined as a CIR, which the service provider commits to provide as a minimum bandwidth between the two nodes.

FECN vs BECN

During periods of congestion, a Frame Relay network will change the 3 bits in the 2-byte DLCI to indicate FECN (Forward Error Congestion Notification) or BECN (Backward Error Congestion Notification) state.

• FECN is issued to the destination device along the path from source to destination, indicating congestion in the Frame Relay network along that path. It tells the receiving device that the path is congested so that upper layer protocols should expect some delay.
• BECN is issued to frames returning along the path to indicate to the originating device that congestion occurred. It tells the transmitting device that the Frame Relay network is congested and that it should "back off" to allow better throughput.

If device A is sending data to device B across a Frame Relay infrastructure and one of the intermediate switches encounters congestion (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 packets being sent to the receiving device. This tells the sending router to back off and apply flow control like traffic shaping, and informs the receiving device that the flow is congested so it should inform upper layer protocols to close down windowing and slow down.

DE – Discard Eligibility

The DE bit is set to control which packets are discarded during congestion. The carrier network will generally set the DE bit on any packets which exceed the CIR during a given time period. The sending device can also manipulate traffic and select traffic that it would like to be marked as DE, thus "protecting" other more important traffic from being discarded first.