S4 advertises its
own fairRate rather than propagating the advertised fairRate of S6. By advertising the
more restrictive fairRate of S4 between S4 and S1, and the less restrictive fairRate of
S6 between stations S6 and S4, it is ensured that the condition bi/wi ?‰¤ fairRate is met
for both sets of contributing stations (i.e., S1??“S4 and S5??“S6), while not unnecessarily
restricting the rates of stations S5 and S6.
Figure 12.6 Received fairRate more restrictive than local fairRate
S0 S1 S2 S3 S4 S6 S7 S8 S5
Fairrate = 5 Fairrate = 10
5 5 5 5 5
Advertised fairRate
338 Chapter 12
As illustrated in Figure 12.8, an exception to the rule of propagating the more restrictive
fairRate is made when it is determined that no station upstream of S4 is adding
traffic that passes through both S4 and S6 at a rate greater than the fairRate advertised
by S6. In the example, the stations S1, S2, and S3 add fairness eligible traffic at rates of
1 unit, 2 units, and 1 unit, respectively. While S4 does not have knowledge of these individual
rates, it does measure the rate of transiting traffic bound for destinations beyond
S6; in this case, 4 units.
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