I was reading a textbook which says:
Let’s begin our study of TCP timer management by considering how TCP estimates the round-trip time between sender and receiver. This is accomplished as follows. The sample RTT, denoted 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. Instead of measuring a SampleRTT for every transmitted segment, most TCP implementations take only one SampleRTT measurement at a time. That is, at any point in time, the SampleRTT is being estimated for only one of the transmitted but currently unacknowledged segments, leading to a new value of SampleRTT approximately once every RTT.
I'm a little bit confused here, the text in black says it won't measure SampleRTT for every segement, then it says a new value of SampleRTT will be approximately once every RTT, which still sounds like TCP measure SampleRTT for every segement to the an average RTT?