iperf3

iperf3 is a command-line tool used to measure the bandwidth between two network hosts. It is a rewrite of the original iperf project, designed to provide a clean, well-documented protocol and codebase. iperf3 can test TCP and UDP throughput, supports IPv4 and IPv6, and operates in a client-server model: one host runs in server mode and another runs as a client to perform the test. The default port is 5201, though it can be configured.

In operation, start a server with iperf3 -s and run a test from a client with iperf3 -c host. For UDP tests, include -u and use -b to specify the target bandwidth. The -P option enables multiple parallel streams; -t sets the total test duration; -i sets the reporting interval; -n transmits a fixed amount of data; -l sets the UDP datagram length; -w adjusts the socket buffer size. The -d option enables a bidirectional test and -R enables a reverse test direction. The -J option outputs results in JSON for automation, and -S can set DSCP for QoS testing.

Output typically reports throughput; for UDP tests it also includes jitter and packet loss. By default, results are human-readable, but the -J flag provides machine-readable data. iperf3 is cross-platform, running on Linux, Windows, macOS, and other Unix-like systems. It is available as open-source software, commonly distributed under a permissive BSD-style license, and is widely used in labs and networks to assess link capacity and performance.