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Unformatted text preview: TT that can be measured by the
sender is the time between the transmission of segment 4 (data bytes 1-1024) and the
reception of segment 7 (the ACK of bytes 1-2048), even though this ACK is for an additional
1024 bytes. We'll use M to denote the measured RTT.
The original TCP specification had TCP update a smoothed RTT estimator (called R) using
the low-pass filter R <- αR + (1-α)M
where α is a smoothing factor with a recommended value of 0.9. This smoothed RTT is
updated every time a new measurement is made. Ninety percent of each new estimate is from
the previous estimate and 10% is from the new measurement.
Given this smoothed estimator, which changes as the RTT changes, RFC 793 recommended
the retransmission timeout value (RTO) be set to RTO = Rβ
where β is a delay variance factor with a recommended value of 2.
[Jacobson 1988] details the problems with this approach, basically that it can't keep up with wide
fluctuations in the RTT, causing unnecessary retransmissions. As Jacobson notes, unnecessary
retransmissions add to the network load, when the network is already loaded. It is the network equivalent of
pouring gasoline on a fire. What's needed is to keep track of the variance in the RTT measurements, in addition to the
smoothed RTT estimator. Calculating the RTO based on both the mean and variance provides
much better response to wide fluctuations in the round-trip times, than just calculating the
RTO as a constant multiple of the mean. Figures 5 and 6 in [Jacobson 1988] show a
comparison of the RFC 793 RTO values for some actual round-trip times, versus the RTO
calculations we show below, which take into account the variance of the round-trip times.
As described by Jacobson, the mean deviation is a good approximation to the standard
deviation, but easier to compute. (Calculating the standard deviation requires a square root.)
This leads to the following equations that are applied to each RTT measurement M. file:///D|/Documents%20and%20Settings/bigini...
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- Spring '12