How to measure TCP latency?

OS is Debian on both servers. I found some examples showing how to measure TCP latency with nmap: nmap --packet-trace -p 22 192.168.0.10 But I'm getting some negative results for latency. ie, it will print some output like: Host is up (-0.0023s latency) at the end. What is nmap doing? Is it measuring the time difference between SYN and SYN-ACK? If so, how is it getting a negative result sometimes? Does it require clocks to be very precisely synced between the two servers? And in addition, can I use ssh as a good TCP test server or should I use a different service? Is there another tool I can use to measure TCP latency? I don't know if I trust nmap due to the negative results for the latency measurement.