iperf3 FAQ

What is the history of iperf3, and what is the difference between iperf2 and iperf3?

iperf2 was orphaned in the late 2000s at version 2.0.5, despite some known bugs and issues. After spending some time trying to fix iperf2’s problems, ESnet decided by 2010 that a new, simpler tool was needed, and began development of iperf3. The goal was make the tool as simple as possible, so others could contribute to the code base. For this reason, it was decided to make the tool single threaded, and not worry about backwards compatibility with iperf2. Many of the feature requests for iperf3 came from the perfSONAR project (http://www.perfsonar.net).

Then in 2014, Bob (Robert) McMahon from Broadcom restarted development of iperf2 (See https://sourceforge.net/projects/iperf2/). He fixed many of the problems with iperf2, and added a number of new features similar to iperf3. iperf2.0.8, released in 2015, made iperf2 a useful tool. iperf2’s current development is focused is on using UDP for latency testing, as well as broad platform support.

In 2023, iperf3 was modified and restructured to support multi-threading, so that it uses one thread per test stream. This allows it to use multiple CPU cores during tests, which in turn permit it to keep up with continually increasing network link and path bandwidths across the backbones of ESnet and other network providers.

As of this writing (2024), both iperf2 and iperf3 are being actively (although independently) developed. We recommend being familiar with both tools, and use whichever tool’s features best match your needs.

A feature comparison of iperf2, iperf3, and nuttcp is available at: https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/throughput-tool-comparision/

iperf3 parallel stream performance is much less than iperf2. Why?

Versions of iperf3 before version 3.16 were all single threaded, and iperf2 is multi-threaded. This could result in a performance gap because iperf3 was only able to use one CPU core on a host, which turned into a bottleneck when trying to do high bitrate tests (faster than about 25 Gbps).

Beginning with version 3.16, iperf3 is multi-threaded, which allows it to take advantage of multiple CPU cores during a test (one thread per stream). iperf3 has been observed to send and receive approximately 160Gbps on a 200Gbps path in a test involving multiple TCP flows, with little or no tuning.

Prior to multi-threading support in iperf3, one might need to use the method described here to achieve faster speeds.

I’m trying to use iperf3 on Windows, but having trouble. What should I do?

iperf3 is not officially supported on Windows, but iperf2 is. We recommend you use iperf2.

Some people are using Cygwin to run iperf3 in Windows, but not all options will work. Some community-provided binaries of iperf3 for Windows exist.

How can I build a statically-linked executable of iperf3?

There are a number of reasons for building an iperf3 executable with no dependencies on any shared libraries. Unfortunately this isn’t quite a straight-forward process.

The steps below have nominally been tested on CentOS 7.4, but can probably be adapted for use with other Linux distributions:

  1. If necessary, install the static C libraries; for CentOS this is the glibc-static package.

  2. If OpenSSL is installed, be sure that its static libraries are also installed, from the openssl-static package.

  3. Be sure that lksctp-* packages are not installed, because as of this writing, there do not appear to be any static libraries available for SCTP.

  4. Configure iperf3 thusly: configure "LDFLAGS=--static" --disable-shared These options are necessary to disable the generation of shared libraries and link the executable statically. For iperf-3.8 or later, configuring as configure --enable-static-bin is another, shorter way to accomplish this. If SCTP is installed on the system it might also be necessary to pass the --without-sctp flag at configure time.

  5. Compile as normal.

It appears that for FreeBSD (tested on FreeBSD 11.1-RELEASE), only the last two steps are needed to produce a static executable.

How can I build on a system that doesn’t support profiled executables?

This problem has been noted by users attempting to build iperf3 for Android systems, as well as some recent versions of macOS. There are several workarounds. In order from least effort to most effort:

  1. Beginning with iperf-3.8, profiled executables are actually not built by default, so this question becomes somewhat moot. Pass the --enable-profiling flag to configure to build profiled executables.

  2. In iperf-3.6 and iperf-3.7, the --disable-profiling flag can be passed to configure to disable the building of profiled object files and the profiled executable.

  3. At the time the linking of the iperf3 profiled executable fails, the “normal” iperf3 executable is probably already created. So if you are willing to accept the error exit from the make process (and a little bit of wasted work on the build host), you might not need to do anything.

  4. After the configure step, there will be a definition in src/Makefile that looks like this:

    noinst_PROGRAMS = t_timer$(EXEEXT) t_units$(EXEEXT) t_uuid$(EXEEXT) \

    If you edit it to look like this, it will disable the build of the profiled iperf3:

    noinst_PROGRAMS = t_timer$(EXEEXT) t_units$(EXEEXT) t_uuid$(EXEEXT)
  5. Similar to item 2 above, but more permanent…if you edit src/Makefile.am and change the line reading like this:

    noinst_PROGRAMS         = t_timer t_units t_uuid iperf3_profile

    To look like this:

    noinst_PROGRAMS         = t_timer t_units t_uuid

    And then run ./bootstrap.sh, that will regenerate the project Makefiles to make the exclusion of the profiled iperf3 executable permanent (within that source tree).

I’m seeing quite a bit of unexpected UDP loss. Why?

First, confirm you are using iperf 3.1.5 or higher. There was an issue with the default UDP send size that was fixed in 3.1.5. Second, try adding the flag -w2M to increase the socket buffer sizes. That seems to make a big difference on some hosts.

iperf3 UDP does not seem to work at bandwidths less than 100Kbps. Why?

You’ll need to reduce the default packet length to get UDP rates of less that 100Kbps. Try -l100.

TCP throughput drops to (almost) zero during a test, what’s going on?

A drop in throughput to almost zero, except maybe for the first reported interval(s), may be related to problems in NIC TCP Offload, which is used to offload TCP functionality to the NIC (see https://en.wikipedia.org/wiki/TCP_offload_engine). The goal of TCP Offload is to save main CPU performance, mainly in the areas of segmentation and reassembly of large packets and checksum computation.

When TCP packets are sent with the “Don’t Fragment” flag set, which is the recommended setting, segmentation is done by the TCP stack based on the reported next hop MSS in the ICMP Fragmentation Needed message. With TCP Offload, active segmentation is done by the NIC on the sending side, which is known as TCP Segmentation offload (TSO) or in Windows as Large Send Offload (LSO). It seems that there are TSO/LSO implementations which for some reason ignore the reported MSS and therefore don’t perform segmentation. In these cases, when large packets are sent, e.g. the default iperf3 128KB (131,072 bytes), iperf3 will show that data was sent in the first interval, but since the packets don’t get to the server, no ack is received and therefore no data is sent in the following intervals. It may happen that after certain timeout the main CPU will re-send the packet by re-segmenting it, and in these cases data will get to the server after a while. However, it seems that segmentation is not automatically continued with the next packet, so the data transfer rate be very low.

The recommended solution in such a case is to disable TSO/LSO, at least on the relevant port. See for example: https://atomicit.ca/kb/articles/slow-network-speed-windows-10/. If that doesn’t help then “Don’t Fragment” TCP flag may be disabled. See for example: https://support.microsoft.com/en-us/help/900926/recommended-tcp-ip-settings-for-wan-links-with-a-mtu-size-of-less-than. However, note that disabling the “Don’t Fragment” flag may cause other issues.

To test whether TSO/LSO may be the problem, do the following:

  • If different machine configurations are used for the client and server, try the iperf3 reverse mode (-R). If TSO/LSO is only enabled on the client machine, this test should succeed.

  • Reduce the sending length to a small value that should not require segmentation, using the iperf3 -l option, e.g. -l 512. It may also help to reduce the MTU by using the iperf3 -M option, e.g. -M 1460.

  • Using tools like Wireshark, identify the required MSS in the ICMP Fragmentation Needed messages (if reported). Run tests with the -l value set to 2 times the MSS and then 4 times, 6 times, etc. With TSO/LSO issue in each test the throughput should be reduced more. It may help to increase the testing time beyond the default 10 seconds to better see the behavior (iperf3 -t option).

What congestion control algorithms are supported?

On Linux, run this command to see the available congestion control algorithms (note that some algorithms are packaged as kernel modules, which must be loaded before they can be used):

/sbin/sysctl net.ipv4.tcp_available_congestion_control

On FreeBSD, the equivalent command is:

/sbin/sysctl net.inet.tcp.cc.available
I’m using the --logfile option. How do I see file output in real time?

Use the --forceflush flag.

I’m using the –fq-rate flag, but it does not seem to be working. Why?

You need to add ‘net.core.default_qdisc = fq’ to /etc/sysctl.conf for that option to work.

I’m having trouble getting iperf3 to work on Windows, Android, etc. Where can I get help?

iperf3 only supports Linux, FreeBSD, and OSX. For other platforms we recommend using iperf2.

I managed to get a Windows executable built, but why do I get a BSOD on Windows 7?

There seems to be a bug in Windows 7 where running iperf3 from a network filesystem can cause a system crash (in other words Blue Screen of Death, or BSOD). This is a Windows bug addressed in kb2839149:


A hotfix is available under kb2732673:


Why can’t I run a UDP client with no server?

This is potentially dangerous, and an attacker could use this for a denial of service attack. We don’t want iperf3 to be an attack tool.

I’m trying to use iperf3 to test a 40G/100G link…What do I need to know?

See the following pages on fasterdata.es.net:

My receiver didn’t get all the bytes that got sent but there was no loss. Huh?

iperf3 uses a control connection between the client and server to manage the start and end of each test. Sometimes the commands on the control connection can be received and acted upon before all of the test data has been processed. Thus the test ends with data still in flight. This effect can be significant for short (a few seconds) tests, but is probably negligible for longer tests.

A file sent using the -F option got corrupted…what happened?

The -F option to iperf3 is not a file transfer utility. It’s a way of testing the end-to-end performance of a file transfer, including filesystem and disk overheads. So while the test will mimic an actual file transfer, the data stored to disk may not be the same as what was sent. In particular, the file size will be rounded up to the next larger multiple of the transfer block size, and for UDP tests, iperf’s metadata (containing timestamps and sequence numbers) will overwrite the start of every UDP packet payload.

I have a question regarding iperf3…what’s the best way to get help?

Searching on the Internet is a good first step. http://stackoverflow.com/ has a number of iperf3-related questions and answers, but a simple query into your favorite search engine can also yield some results.

There is a mailing list nominally used for iperf3 development, iperf-dev@googlegroups.com.

We discourage the use of the iperf3 issue tracker on GitHub for support questions. Actual bug reports, enhancement requests, or pull requests are encouraged, however.