Discard eligibility (de) bit, Forward explicit congestion notification (fecn), Backward explicit congestion notification (becn) – Enterasys Networks Security Router X-PeditionTM User Manual

Page 213

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Controlling Congestion in Frame Relay Networks

XSR User’s Guide 9-5

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

FECN

BECN

Switch A

Switch C

Switch B

Source DTE

Destination DTE

Reduce Sent Traffic

Ignored

Congestion

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