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In "Computer Networks : A Top-Down Approach" by Kurose and Ross they say:
Instead of measuring a SampleRTT for every transmitted segment, most TCP implementations take only one SampleRTT measurement at a time.
(SampleRTT for a segment is the amount of time between when the segment is sent (that is, passed to IP) and when an acknowledgment for the segment is received.)
Why is that? What's wrong with measuring for, say, at most half of the segments sent?
As in most systems engineering topics, there's a balance between performance and accuracy. In the case of TCP, it could take a SampleRTT measurement every other segment, but that would imply a higher processing delay, and thus a higher queueing delay, etc. The single SampleRTT measurement helps TCP get an idea of the RTT while staying lean and fast.
Each node wants to spend the minimum time possible processing each segment so that it can get forwarded onto the network and move on to the next segment.
[EDIT] Additionally, each subsequent measurement is less valuable than the one before. The first measurement lets you know the neighborhood of RTT, the second helps you fine-tune that, etc. So there's a diminishing payoff the more RTTs you measure, especially since after each measurement you have fewer segments remaining to send. What's the point of measuring the RTT if there's only 5 segments left to send? Thus the first RTT measurement is the most important and really the only one needed.
