Measure Network Latency
Jul 22, 2020
Xing Lin
2 minute read

Quite often, we need to know the network round-trip time (RTT) between two nodes and a fairly well known tool is ping. However, I found out the proper way to use the ping tool only until today. Actually, I guess I should just use the netperf tool instead, to measure the network latency in the future.

By default, ping sends out a packet every second. The interval which ping uses to send out a packet can have a huge impact on the measured network latency. We can use the -i option, to set the interval. Below are the RTT values I collected between two servers connected directly with a cable.

Interval Latency (us)
default (1s) 161
200 ms 132
20 ms 130
2 ms 73
flood (-f) 22

As we can see, one can get very different RTTs when using different intervals. For the common cases, I guess we should use the flooding mode where a new packet is sent out immediately after the previous packet is received.

netperf is another tool used for network performance testing as well. By default, it sends out a request immediately after the previous one completes. One can use netperf to test TCP/UDP round-trip time. It is probably the preferred tool to do latency measurement, than ping. netperf shows the TCP RTT is 37 us for the same two servers where I used ping to test RTT.

To use netperf, we need to first start netserver at one server. Then, at the other server, start netperf to do the tests.

$netperf -p 12345 -H -l 100 -t TCP_RR -- -o min_latency,mean_latency,max_latency,stddev_latency,transaction_rate

One can download the netperf source code from git. However, they have removed the configure file in the source tree. To generate the configure file, we can run the script. We may also need to install texinfo package, to get the makeinfo tool.