Enterasys Networks X-PeditionTM manual Discard Eligibility DE Bit

Controlling Congestion in Frame Relay Networks

Several other parameters work hand-in-hand with CIR in controlling traffic flow. Committed burst (Bc) is the peak number of bits that the network attempts to deliver during a given period.

Bc differs from CIR - it is a number, not a rate. CIR is equal to the committed burst divided by time interval Tc, expressed in the formula: CIR = Bc/Tc. The frame-relay bc command sets outgoing committed burst size.

Excess burst (Be) is the maximum number of bits that you may send in excess of Bc. Sent on a best- effort basis, these bits will likely be discarded during congestion. The frame-relay be command sets outgoing excess burst size.

Another method of traffic shaping is the use of queues to limit surges that can congest a network. Data is buffered and then sent to the network in regulated amounts to ensure that traffic will fit within the promised traffic envelope for the particular connection. Traffic shaping is also known as metering, shaping, and smoothing.

Discard Eligibility (DE) Bit

The Discard Eligibility (DE) bit is one of the many fields in a frame header. A FR DTE device sets this bit to one (1) to indicate that the current frame rate is above the committed burst and that the switch may discard this packet before any other packets with the DE bit set to 0 during congested periods. When discarding DE-eligible data, by itself, is insufficient to ease severe congestion, more incoming frames are discarded without regard to the DE bit setting.

Forward Explicit Congestion Notification (FECN)

Forward Explicit Congestion Notification (FECN) sets a bit to inform the DTE device getting the frame of congestion in the path from source to destination. A DTE device receiving frames with the FECN bit set can request higher-level protocols to take flow-control action as needed. The XSR counts frames with the FECN bit set, otherwise it does not act on frames with the bit set.

Backward Explicit Congestion Notification (BECN)

Backward Explicit Congestion Notification (BECN) sets a bit in frames traveling the opposite direction of frames encountering a congested path, as shown in Figure 9-2. A DTE device getting frames with the BECN bit set assuredly is sending frames into a congested network. This means all data sent at the excess data rate are being tagged for discard and possibly lost. It is helpful to the DTE to proactively throttle output to help the network exit from a congested state.

Figure 9-2 Congestion Notification

Congestion
Source DTE	Destination DTE
Switch B
BECN
	FECN
Reduce Sent Traffic	Ignored
Switch A	Switch C
	XSR User’s Guide	9-5