e., ringlet1).
Figure 12.4 Congestion control in a ring
S0 S1 S2 S3 S4 S6 S7 S8 S5
b1 b2 b3 b4 b5 b6
Figure 12.5 The fairRate advertisement
S0 S1 S2 S3 S4 S6 S7 S8 S5
Ringlet0 congestion feedback
Ringlet0 data
FairRate = 5
5 5 5 5 5
Advertised fairrate
Resilient Packet Ring (RPR) 337
The advertisement allows each contributing station to limit its rate, to the current
weight-adjusted fairRate, resulting in changes to rate statistics measured on the downstream
link of the congested station. The rate statistics are used to ensure that the condition
b1+??¦+b6 ?‰¤ LINK_RATE(1-fa) is met when adjusting the fairRate. The adjusted
fairRate is then advertised upstream. The feedback between the congested station and
the contributing stations allows continuous adjustment of rates to meet the fairness
objectives.
The fairness procedure is able to indicate a single congestion point, known as a choke
point, to each station. Figure 12.6 provides an example in which multiple stations on
the ringlet are congested. Each station independently computes a fairRate. Congested
station S4 computes a fairRate of 10 units and receives an advertised fairRate of 5 units
from downstream neighbor S5.
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