In this talk, I will present some recent and old results 
on measurements of round-trip time of TCP connections over a 
large scale sampleof the Internet. In particular I will 
address two questions: 1) "Is the Round-trip Time Correlated
with the Number of Packets in Flight?" (a paper by Saad Biaz,
and Nitin H. Vaidya) and 2) What is the "Variability in TCP
Round-trip Times"? (a paper by Jay Aikat, Jasleen Kaur, F.
Donelson Smith, and Kevin Jeffay). Both papers appeared in
IMC'03. 
Then I will discuss the consequences of these answers to the
problem of distiguishing congestion losses from wireless 
transmission losses in wire/wireless hybrid networks.
Finally, I'm going to present a related problem that I'm
working on in collaboration with Mark to use extra RTT 
information to detect packet losses for the cases when TCP
has to rely on timeouts.