HP 600 Series, Services And Applications

Page: 286 / 316

Congestion Control The most interesting aspects of the Q.922
address field are the three bits used for congestion control—FECN,
BECN, and DE.
Figure 3. Congestion on a Frame Relay PVC
Figure 3 shows a Frame Relay network with two attached routers. Router 5
(R5) is sending data to R6 on a particular PVC. Assume that node B in the
Frame Relay network detects that the network is congested or about to
become congested on the route from R5 to R6. Node B responds to this
network congestion condition by setting the Forward explicit congestion
notification (FECN) bit in frames going from R5 to R6. The setting of this bit
tells R6 that incoming data on the PVC from R5 is experiencing congestion.
Alternatively, node B may discard the frame from R5 if the discard eligibility
(DE) bit in the frame is set. The discard eligibility bit would typically have
been set by node A in frames that exceeded the CIR of the PVC from R5 to
R6. HP routers never set the DE bit in frames transmitted to the Frame Relay
network.
To notify R5 that a congestion condition exists on the route to R6, node B
sets the backward explicit congestion notification (BECN) bit in frames
going from R6 to R5 (same PVC as before). This signal is intended to signal
R5 to reduce its transmission rate on that PVC.
Frame Relay standards do not require attached devices (routers) to respond
to congestion notifications. In fact, HP routers ignore these indications, as
do most other router implementations, although the number of BECNs and
FECNs received is maintained in the MIB.
A
B
C
R6
R5
BECN
FECN
Linking Up with Frame Relay
Frame Relay Data-Link Interface
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