iperf3

Run iperf3 -s on one device and iperf3 -c ADDRESS on the other. It sends as fast as it can for a few seconds and reports the bitrate, the retransmissions (Retr) and TCP's congestion window (Cwnd). -R measures the other direction, -u -b 5M sends UDP at a fixed rate and reports loss and jitter instead. Ping while it runs to see bufferbloat: the delay grows as the queue fills.

iperf3 command usage

iperf3 -s [-p PORT] [-1] [-D]
iperf3 -c ADDRESS [-p PORT] [-t SECONDS] [-i SECONDS] [-f m] [-T TITLE] [-R] [-P STREAMS] [-u -b RATE] [-w WINDOW]

Options

-s

be the server

-c 10.0.2.10

be the client, testing to that server

-t 5

run for 5 seconds

-i 0.1

a report line every tenth of a second (default: every second)

-f m

every rate in Mbit/s, so lines compare

-T first

start every line with a title: tells two runs' lines apart

-p 5202

another port: a second server, since one serves a single test at a time

-1

server: serve one test, then exit

-R

reverse: the server sends, you receive

-P 2

two connections side by side

-u -b 5M

UDP at 5 Mbit/s: reports loss and jitter

-w 16K

limit the TCP window to 16 KB

iperf3 examples

iperf3 -s -D

Starts an iperf3 server in the background, on port 5201.

iperf3 -c 10.0.2.10 -t 5

Measures TCP throughput to 10.0.2.10 for five seconds.

iperf3 -c 10.0.2.10 -R

Measures the other direction: the server sends to you.

iperf3 -c 10.0.2.10 -u -b 5M -t 5

Sends UDP at 5 Mbit/s and reports how much was lost and the jitter.

iperf3 -c 10.0.2.10 -t 4 -w 16K

Measures throughput with the TCP window capped at 16 KB.

iperf3 -c 10.0.2.10 -t 3 -i 0.1

Prints a line every tenth of a second: the congestion window growing in slow start.

iperf3 -s -p 5202 -1 -D

Starts a second server, on port 5202, for one test, in the background.

iperf3 -c 10.0.2.10 -P 4

Runs four streams at once: one stream may not fill a fast or long link.

iperf3 -c 10.0.2.10 --bidir

Measures both directions at the same time (iperf3 3.7 and later).

iperf3 -c 10.0.2.10 -t 10 -O 2

Leaves the first two seconds out of the result, so slow start does not pull the average down.

iperf3 -c 10.0.2.10 -n 100M

Sends 100 MB and stops, instead of running for a time.

iperf3 -c 10.0.2.10 -C bbr

Uses the BBR congestion control for this test, to compare it with the default cubic (Linux).

iperf3 -c 10.0.2.10 -u -b 1M -l 200

Sends small 200-byte UDP packets at 1 Mbit/s, close to a voice call.

iperf3 -c 10.0.2.10 -J > result.json

Writes the result as JSON, for a script or a graph.

iperf3 -c 10.0.2.10 -B 10.0.1.50

Sends from that local address, to test one uplink.

Reading the output

$ iperf3 -c 10.0.2.10 -t 2
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  7.63 GBytes  65.4 Gbits/sec    0   1.53 MBytes
[  5]   1.00-2.01   sec  7.24 GBytes  61.9 Gbits/sec    0   1.79 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   0.00-2.01   sec  14.9 GBytes  63.7 Gbits/sec    0             sender
[  5]   0.00-2.01   sec  14.9 GBytes  63.7 Gbits/sec                  receiver

• Bitrate: the throughput each second, and in the sender/receiver summary.
• Retr: segments sent again. Many retransmissions: loss on the path.
• Cwnd: how much TCP dared to have in flight. Small and stuck: a small window, or loss keeps cutting it.
• An unshaped lab link is this fast: numbers in Mbit/s mean a tc limit is doing its job.

Pitfalls

• iperf3: error - unable to connect to server: no iperf3 -s on the other side, or port 5201 is blocked.

Run it on real machines