In most cases, the conservative method requires that the
fairRate not be adjusted until sufficient time has passed to ensure that the effect of any
previous adjustment has been observed. This waiting period is known as the fairness
round trip time (FRTT).
The FRTT is computed only by a station that performs conservative rate computation
and is the head of a congestion domain (the station generating the advertisement).
The FRTT value is recomputed when a valid pair of fairness differential delay (FDD)
frames from the tail station (the last station advertising the fairRate generated by the
head station) has been received by the head station. The FDD pair is sent by the tail
at regular intervals.
The head station estimates the FRTT by referencing the FDD and the loop round trip
time (LRTT). FRTT is the sum of FDD and LRTT.
FRTT = FDD + LRTT
FDD is a measure of the difference in delay between the classA and classC paths from
the tail station to the head station, as shown in Figure 12.9. Stations are requested
to generate the FDD frames only if they are a tail of a congestion domain whose head
station is deploying the Conservative method.
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