We propose two methods to passively measure and monitor changes in round-trip times
(RTTs) throughout the lifetime of a TCP connection. Our first method associates data
segments with the acknowledgments (ACKs) that trigger them by leveraging the TCP
timestamp option. Our second method infers TCP RTT by observing the repeating patterns of
segment clusters where the pattern is caused by TCP self-clocking. We evaluate the two
methods using both emulated and real Internet tests.

New Methods for Passive Estimation of TCP Round-Trip Times | CiNii Research … New
Methods for Passive Estimation of TCP Round-Trip Times …