iperf3 Development

The iperf3 project is hosted on GitHub at:


This site includes the source code repository, issue tracker, and wiki.

Mailing Lists

The developer list for iperf3 is: iperf-dev@googlegroups.com. Information on joining the mailing list can be found at:


There is, at the moment, no mailing list for user questions, although a low volume of inquiries on the developer list is probably acceptable. If necessary, a user-oriented mailing list might be created in the future.

Bug Reports

Before submitting a bug report, try checking out the latest version of the code, and confirm that it’s not already fixed. Then submit to the iperf3 issue tracker on GitHub:


Note: Issues submitted to the old iperf3 issue tracker on Google Code (or comments to existing issues on the Google Code issue tracker) will be ignored.

Changes from iperf 2.x

New options (not necessarily complete, please refer to the manual page for a complete list of iperf3 options):

-V, --verbose             more detailed output than before
-J, --json                output in JSON format
-Z, --zerocopy            use a 'zero copy' sendfile() method of sending data
-O, --omit N              omit the first n seconds (to ignore slowstart)
-T, --title str           prefix every output line with this string
-F, --file name           xmit/recv the specified file
-A, --affinity n/n,m      set CPU affinity (Linux and FreeBSD only)
-k, --blockcount #[KMG]   number of blocks (packets) to transmit (instead
                          of -t or -n)
-L, --flowlabel           set IPv6 flow label (Linux only)

Changed flags:

-C, --linux-congestion    set congestion control algorithm (Linux only)
                          (-Z in iperf2)

Deprecated flags (currently no plans to support):

-d, --dualtest           Do a bidirectional test simultaneously
-r, --tradeoff           Do a bidirectional test individually
-T, --ttl                time-to-live, for multicast (default 1)
-x, --reportexclude [CDMSV]   exclude C(connection) D(data) M(multicast)
                              S(settings) V(server) reports
-y, --reportstyle C      report as a Comma-Separated Values

Also deprecated is the ability to set the options via environment variables.

Known Issues

The following problems are notable known issues, which are probably of interest to a large fraction of users or have high impact for some users, and for which issues have already been filed in the issue tracker. These issues are either open (indicating no solution currently exists) or closed with the notation that no further attempts to solve the problem are currently being made:

  • UDP performance: Some problems have been noticed with iperf3 on the ESnet 100G testbed at high UDP rates (above 10Gbps, sometimes at speeds above 1Gbps). The symptom is that on any particular run of iperf3 the receiver reports a loss rate of about 20%, regardless of the -b option used on the client side. This problem appears not to be iperf3-specific, and may be due to the placement of the iperf3 process on a CPU and its relation to the inbound NIC. In some cases this problem can be mitigated by increasing the socket buffer sizes with the -w option, or by an appropriate use of the CPU affinity (-A) option.
  • The -Z flag sometimes causes the iperf3 client to hang on OSX. (Issue #129)
  • When specifying the TCP buffer size using the -w flag on Linux, the Linux kernel automatically doubles the value passed in to compensate for overheads. (This can be observed by using iperf3’s --debug flag.) However, CWND does not actually ramp up to the doubled value, but only to about 75% of the doubled value. Some part of this behavior is documented in the tcp(7) manual page.
  • Although the -w flag is documented as setting the (TCP) window size, it is also used to set the socket buffer size. This has been shown to be helpful with high-bitrate UDP tests.
  • On some platforms (observed on at least one version of Ubuntu Linux), it might be necessary to invoke ldconfig manually after doing a make install before the iperf3 executable can find its shared library. (Issue #153)
  • The results printed on the server side at the end of a test do not correctly reflect the client-side measurements. This is due to the ordering of computing and transferring results between the client and server. (Issue #293)
  • The server could have a very short measurement reporting interval at the end of a test (particularly a UDP test), containing few or no packets. This issue is due to an artifact of timing between the client and server. (Issue #278)

There are, of course, many other open and closed issues in the issue tracker.


iperf3 version numbers use (roughly) a Semantic Versioning scheme, in which version numbers consist of three parts: MAJOR.MINOR.PATCH

The developers increment the:

  • MAJOR version when making incompatible API changes,
  • MINOR version when adding functionality in a backwards-compatible manner, and
  • PATCH version when making backwards-compatible bug fixes.

Release Engineering Checklist

  1. Update the README and RELEASE_NOTES files to be accurate. Make sure that the “Known Issues” section of the README file is up to date.

  2. Compose a release announcement. Most of the release announcement can be written before tagging. Usually the previous version’s announcement can be used as a starting point.

  3. Preferably starting from a clean source tree (be sure that git status emits no output), make the changes necessary to produce the new version, such as bumping version numbers:

    vi RELEASE_NOTES   # update version number and release date
    vi configure.ac    # update version parameter in AC_INIT
    vi src/iperf3.1    # update manpage revision date if needed
    vi src/libiperf.3  # update manpage revision date if needed
    git commit -a      # commit changes to the local repository only
    ./bootstrap.sh     # regenerate configure script, etc.
    git commit -a      # commit changes to the local repository only
    # Assuming that $VERSION is the version number to be released...
    ./make_release tag $VERSION # this creates a tag in the local repo
    ./make_release tar $VERSION # create tarball and compute SHA256 hash

    These steps should be done on a platform with a relatively recent version of autotools / libtools. Examples are MacOS / MacPorts or FreeBSD. The versions of these tools in CentOS 6 are somewhat older and probably should be avoided.

    The result will be a release artifact that should be used for pre-testing.

  4. Stage the tarball (and a file containing the SHA256 hash) to the download site. Currently this is located on downloads.es.net.

  5. From another host, test the link in the release announcement by downloading a fresh copy of the file and verifying the SHA256 checksum. Checking all other links in the release announcement is strongly recommended as well.

  6. Also verify (with file(1)) that the tarball is actually a gzipped tarball.

  7. For extra points, actually try downloading, compiling, and smoke-testing the results of the tarball on all supported platforms.

  8. Plug the SHA256 checksum into the release announcement.

  9. PGP-sign the release announcement text using gpg --clearsign. The signed announcement will be sent out in a subsequent emails, but could also be archived. Decoupling the signing from emailing allows a signed release announcement to be resent via email or sent by other, non-email means.

  10. At this point, the release can and should be considered finalized. To commit the release-engineering-related changes to GitHub and make them public, push them out thusly:

    git push            # Push version changes
    git push --tags     # Push the new tag to the GitHub repo
  11. Send the PGP-signed release announcement to the following addresses. Remember to turn off signing in the MUA, if applicable. Remember to check the source address when posting to lists, as “closed” list will reject posting from all from registered email addresses.

    Note: Thunderbird sometimes mangles the PGP-signed release announcement so that it does not verify correctly. This could be due to Thunderbird trying to wrap the length of extremely long lines (such as the SHA256 hash). Apple Mail and mutt seem to handle this situation correctly. Testing the release announcement sending process by sending a copy to oneself first and attempting to verify the signature is highly encouraged.

  12. Update the iperf3 Project News section of the documentation site to announce the new release (see docs/news.rst and docs/conf.py in the source tree) and deploy a new build of the documentation to GitHub Pages.

  13. If an update to the on-line manual page is needed, it can be generated with this sequence of commands (tested on CentOS 7) and import the result into invoking.rst:

    export TERM
    nroff -Tascii -c -man src/iperf3.1 | ul | sed 's/^/   /' > iperf3.txt

Code Authors

The main authors of iperf3 are (in alphabetical order): Jon Dugan, Seth Elliott, Bruce A. Mah, Jeff Poskanzer, Kaustubh Prabhu. Additional code contributions have come from (also in alphabetical order): Mark Ashley, Aaron Brown, Aeneas Jaißle, Susant Sahani, Bruce Simpson, Brian Tierney.

iperf3 contains some original code from iperf2. The authors of iperf2 are (in alphabetical order): Jon Dugan, John Estabrook, Jim Ferbuson, Andrew Gallatin, Mark Gates, Kevin Gibbs, Stephen Hemminger, Nathan Jones, Feng Qin, Gerrit Renker, Ajay Tirumala, Alex Warshavsky.