Flent is a wrapper around netperf and similar tools to run predefined tests and aggregate and plot the results. It defines several tests that can be run against one or more hosts, primarily targeted at testing for the presence of bufferbloat under various conditions.

The aggregated data is saved in (gzipped) JSON format for later processing and/or import into other tools. The JSON format is documented below.

Apart from the JSON format, the data can be output as csv values, emacs org mode tables or plots. Each test can specify several different plots, including time-series plots of the values against each other, as well as CDF plots of (e.g.) ping times.

Plots can be output to the formats supported by matplotlib by specifying the output filename with -o output.{png,ps,pdf,svg}. If no output file is specified, the plot is displayed using matplotlib’s interactive plot browser, which also allows saving of the output (in .png format).


Flent runs on Python, versions 2.7+ and 3.3+. Plotting requires a functional matplotlib installation (but everything else can run without matplotlib). For the interactive plot viewer, a graphical display (and suitably configured matplotlib) is required.

Most tests employ the netperf benchmarking tool to run the tests. Version 2.6 or higher is required, and netperf must be compiled with the --enable-demo option passed to ./configure. Some tests use iperf in addition to, or instead of netperf. Both tools must be available in the PATH.

For ICMP ping measurements, the version of ping employed must support output timestamping (the -D parameter to GNU ping). This is not supported by the BSD and OSX versions of ping. As an alternative to the regular ping command, the fping utility (see can be employed. In that case fping must be version 3.5 or greater. Flent will attempt to detect the presence of fping in the PATH and check for support for the -D parameter. If this check is successful, fping will be employed for ping data, otherwise the system ping will be used.

The irtt tool is highly recommended for UDP measurements. See Flent will automatically detect if irtt is available in the PATH and use it if it is detected. Note that the server component of irtt needs to be running on the target host(s) as well.