FECN BECN and the discard eligibility DE bit
 


IOS includes Traffic Shaping to deal with times in which a router can send more data than the VC allows.  Traffic Shaping allows the router to decrease the overall rate of sending bits to a speed slower than the access rate, and maybe even as low as the CIR of a VC.  With a T1 access link and a 128-kbps CIR, Traffic Shaping could be defined to send an average of only 256 kbps over that VC.  The idea is that the Frame Relay provider will probably discard a lot of traffic if the router averages sending data over that VC at close to T1 speed, which is 12 times the CIR in this case.  However, the Frame Relay provider may not discard traffic if the average is only 256 kbps – twice the CIR in this case.

You can set Traffic Shaping to use a single speed, or to adapt to a range between two speed settings.  When it’s configured to adapt between two speeds, if the network is not congested, the higher speed is used; when the network is congested, the router adapts so that it shapes using the lower rate.

To adapt the shaping rates, the routers need a way to know whether congestion is occurring – and that’s where Forward Explicit Congestion Notification (FECN) and Backward Explicit Congestion Notification (BECN) are used.

FECN and BECN are bits in the Frame Relay header.  At any point – either in a router or inside the Frame Relay cloud – a device can set the FECN bit, meaning that this frame itself has experienced congestion.  In other words, congestion exists in the forward direction of that frame.

The goal of this process is to get the sending router to slow down.  So, knowing that it set FECN in a frame, the Frame Relay switch can set the BECN bit in the next frame going back to the router on that VC.  The BECN tells the router that congestion occurred in the direction opposite, or backward, of the direction of the frame.  In other words, it says that congestion occurred for the frame sent by the first router to the second router.  The first router can then choose to slow down or not, depending on how Traffic Shaping is configured.

The Discard Eligibility (DE) Bit

Frame Relay protocols define a means to lessen the blow when the customer sends more than CIR bits per second over a VC, causing the provider to discard some frames.  The customer can set the DE bit in some frames.  If the provider’s switches need to discard frames because of congestion, the switches can discard the frames with the DE bit set.  If the customer sets the DE bit in the right frames, such as for less important traffic, the customer can ensure that the important traffic gets through the Frame Relay network, even when the provider has to discard traffic.  When the provider’s network is not so congested, the customer can send lots of extra data through the Frame Relay network without its being discarded.