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Windows Server® 2008 TCP/IP Protocols and Services by Joseph Davies

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Calculating the RTO

The determination of the RTO is an important function of TCP. The RTO must be adjusted to the internetwork’s changing conditions. If the determined RTO is less than the RTT, segments are unnecessarily retransmitted.

In RFC 793, the suggested method of computing the RTO—known as the smoothed round-trip time (SRTT)—is based on the following formulas:

SRTT = (α*SRTT) + ((1-α)*RTT)
RTO = min[UpperBound, max[LowerBound,(β*SRTT)]]

Thus, the new RTO is based on the determination of the current RTT, the previous SRTT, a smoothing factor (α), and a variance factor (β) . In practice, this formula was found to be inadequate in determining the RTO in an environment in which the RTT changed suddenly. Instead, RFC 1122 states that TCP must ...

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