How to measure one-way latency?

[choiyhking]

Hello, I want to measure one-way latency using iperf3.
Specifically, I want to measure the latency from A to B, not the round-trip time (A -> B -> A).

I found the following line in src/iperf_udp.c

iperf_time_diff(&arrival_time, &sent_time, &temp_time);
transit = iperf_time_in_secs(&temp_time);

Is transit the one-way latency?

Additionally, how can I do this for TCP?

[davidBar-On]

Hi,

Is transit the one-way latency?

Yes, but only if the timers of the client and the server are fully synchronized. Note that the code you found is for estimating the jitter (see the code that follows these lines), where the variance of the latency is important, not the latency itself.

Additionally, how can I do this for TCP?

tcp_info may help. You can get it using getsockopt(..., IPPROTO_TCP, TCP_INFO, ...) (see for example the iperf3 tcp_info.c file). However, it seems that it only calculates RTT, but maybe by dividing it by 2 you can get an approximate one-way latency.

[jkasten2]

Getting one-way latency is a hard problem, server and client clocks have to be in-sync within 1ms to get any level of useful accuracy.

iperf doesn't have this feature, however owamp / twamp has been designed to get this specifically.

[bmah888]

This is the way.