1: .. _faq:
2:
3: iperf3 FAQ
4: ==========
5:
6: What is the history of iperf3, and what is the difference between iperf2 and iperf3?
7: iperf2 was orphaned in the late 2000s at version 2.0.5, despite some
8: known bugs and issues. After spending some time trying to fix
9: iperf2's problems, ESnet decided by 2010 that a new, simpler tool
10: was needed, and began development of iperf3. The goal was make the
11: tool as simple as possible, so others could contribute to the code
12: base. For this reason, it was decided to make the tool single
13: threaded, and not worry about backwards compatibility with
14: iperf2. Many of the feature requests for iperf3 came from the
15: perfSONAR project (http://www.perfsonar.net).
16:
17: Then in 2014, Bob (Robert) McMahon from Broadcom restarted
18: development of iperf2 (See
19: https://sourceforge.net/projects/iperf2/). He fixed many of the
20: problems with iperf2, and added a number of new features similar to
21: iperf3. iperf2.0.8, released in 2015, made iperf2 a useful tool. iperf2's
22: current development is focused is on using UDP for latency testing, as well
23: as broad platform support.
24:
25: As of this writing (2017), both iperf2 and iperf3 are being actively
26: (although independently) developed. We recommend being familiar with
27: both tools, and use whichever tool’s features best match your needs.
28:
29: A feature comparison of iperf2, iperf3, and nuttcp is available at:
30: https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/throughput-tool-comparision/
31:
32: iperf3 parallel stream performance is much less than iperf2. Why?
33: iperf3 is single threaded, and iperf2 is multi-threaded. We
34: recommend using iperf2 for parallel streams.
35: If you want to use multiple iperf3 streams use the method described `here <https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/iperf/multi-stream-iperf3/>`_.
36:
37: I’m trying to use iperf3 on Windows, but having trouble. What should I do?
38: iperf3 is not officially supported on Windows, but iperf2 is. We
39: recommend you use iperf2.
40:
41: Some people are using Cygwin to run iperf3 in Windows, but not all
42: options will work. Some community-provided binaries of iperf3 for
43: Windows exist.
44:
45: How can I build a statically-linked executable of iperf3?
46: There are a number of reasons for building an iperf3 executable with
47: no dependencies on any shared libraries. Unfortunately this isn't
48: quite a straight-forward process.
49:
50: The steps below have nominally been tested on CentOS 7.4, but
51: can probably be adapted for use with other Linux distributions:
52:
53: #. If necessary, install the static C libraries; for CentOS this is
54: the ``glibc-static`` package.
55:
56: #. If OpenSSL is installed, be sure that its static libraries are
57: also installed, from the ``openssl-static`` package.
58:
59: #. Be sure that ``lksctp-*`` packages are not installed, because
60: as of this writing, there do not appear to be any static
61: libraries available for SCTP.
62:
63: #. Configure iperf3 thusly: ``configure "LDFLAGS=--static"
64: --disable-shared`` These options are necessary to disable the
65: generation of shared libraries and link the executable
66: statically. For iperf-3.8 or later, configuring as ``configure
67: --enable-static-bin`` is another, shorter way to accomplish
68: this. If SCTP is installed on the system it might also be
69: necessary to pass the ``--without-sctp`` flag at configure
70: time.
71:
72: #. Compile as normal.
73:
74: It appears that for FreeBSD (tested on FreeBSD 11.1-RELEASE), only
75: the last two steps are needed to produce a static executable.
76:
77: How can I build on a system that doesn't support profiled executables?
78: This problem has been noted by users attempting to build iperf3 for
79: Android systems, as well as some recent versions of macOS.
80: There are several workarounds. In order from least
81: effort to most effort:
82:
83: #. Beginning with iperf-3.8, profiled executables are actually not
84: built by default, so this question becomes somewhat moot. Pass
85: the ``--enable-profiling`` flag to ``configure`` to build
86: profiled executables.
87:
88: #. In iperf-3.6 and iperf-3.7, the ``--disable-profiling`` flag can be
89: passed to ``configure`` to disable the building of profiled
90: object files and the profiled executable.
91:
92: #. At the time the linking of the iperf3 profiled executable fails,
93: the "normal" iperf3 executable is probably already created. So if
94: you are willing to accept the error exit from the make process
95: (and a little bit of wasted work on the build host), you might
96: not need to do anything.
97:
98: #. After the configure step, there will be a definition in
99: ``src/Makefile`` that looks like this::
100:
101: noinst_PROGRAMS = t_timer$(EXEEXT) t_units$(EXEEXT) t_uuid$(EXEEXT) \
102: iperf3_profile$(EXEEXT)
103:
104: If you edit it to look like this, it will disable the build of the profiled iperf3::
105:
106: noinst_PROGRAMS = t_timer$(EXEEXT) t_units$(EXEEXT) t_uuid$(EXEEXT)
107:
108: #. Similar to item 2 above, but more permanent...if you edit
109: ``src/Makefile.am`` and change the line reading like this::
110:
111: noinst_PROGRAMS = t_timer t_units t_uuid iperf3_profile
112:
113: To look like this::
114:
115: noinst_PROGRAMS = t_timer t_units t_uuid
116:
117: And then run ``./bootstrap.sh``, that will regenerate the project
118: Makefiles to make the exclusion of the profiled iperf3 executable
119: permanant (within that source tree).
120:
121: I'm seeing quite a bit of unexpected UDP loss. Why?
122: First, confirm you are using iperf 3.1.5 or higher. There was an
123: issue with the default UDP send size that was fixed in
124: 3.1.5. Second, try adding the flag ``-w2M`` to increase the socket
125: buffer sizes. That seems to make a big difference on some hosts.
126:
127: iperf3 UDP does not seem to work at bandwidths less than 100Kbps. Why?
128: You'll need to reduce the default packet length to get UDP rates of less that 100Kbps. Try ``-l100``.
129:
130: TCP throughput drops to (almost) zero during a test, what's going on?
131: A drop in throughput to almost zero, except maybe for the first
132: reported interval(s), may be related to problems in NIC TCP Offload,
133: which is used to offload TCP functionality to the NIC (see
134: https://en.wikipedia.org/wiki/TCP_offload_engine). The goal of TCP
135: Offload is to save main CPU performance, mainly in the areas of
136: segmentation and reassembly of large packets and checksum
137: computation.
138:
139: When TCP packets are sent with the "Don't Fragment" flag set, which
140: is the recommended setting, segmentation is done by the TCP stack
141: based on the reported next hop MSS in the ICMP Fragmentation Needed
142: message. With TCP Offload, active segmentation is done by the NIC on
143: the sending side, which is known as TCP Segmentation offload (TSO)
144: or in Windows as Large Send Offload (LSO). It seems that there are
145: TSO/LSO implementations which for some reason ignore the reported
146: MSS and therefore don’t perform segmentation. In these cases, when
147: large packets are sent, e.g. the default iperf3 128KB (131,072
148: bytes), iperf3 will show that data was sent in the first interval,
149: but since the packets don’t get to the server, no ack is received
150: and therefore no data is sent in the following intervals. It may
151: happen that after certain timeout the main CPU will re-send the
152: packet by re-segmenting it, and in these cases data will get to the
153: server after a while. However, it seems that segmentation is not
154: automatically continued with the next packet, so the data transfer
155: rate be very low.
156:
157: The recommended solution in such a case is to disable TSO/LSO, at
158: least on the relevant port. See for example:
159: https://atomicit.ca/kb/articles/slow-network-speed-windows-10/. If
160: that doesn’t help then "Don't Fragment" TCP flag may be
161: disabled. See for example:
162: https://support.microsoft.com/en-us/help/900926/recommended-tcp-ip-settings-for-wan-links-with-a-mtu-size-of-less-than. However,
163: note that disabling the “Don’t Fragment” flag may cause other
164: issues.
165:
166: To test whether TSO/LSO may be the problem, do the following:
167:
168: * If different machine configurations are used for the client and
169: server, try the iperf3 reverse mode (``-R``). If TSO/LSO is only
170: enabled on the client machine, this test should succeed.
171: * Reduce the sending length to a small value that should not require
172: segmentation, using the iperf3 ``-l`` option, e.g. ``-l 512``. It
173: may also help to reduce the MTU by using the iperf3 ``-M`` option,
174: e.g. ``-M 1460``.
175: * Using tools like Wireshark, identify the required MSS in the ICMP
176: Fragmentation Needed messages (if reported). Run tests with the
177: ``-l`` value set to 2 times the MSS and then 4 times, 6 times,
178: etc. With TSO/LSO issue in each test the throughput should be
179: reduced more. It may help to increase the testing time beyond the
180: default 10 seconds to better see the behavior (iperf3 ``-t``
181: option).
182:
183: What congestion control algorithms are supported?
184: On Linux, run this command to see the available congestion control
185: algorithms (note that some algorithms are packaged as kernel
186: modules, which must be loaded before they can be used)::
187:
188: /sbin/sysctl net.ipv4.tcp_available_congestion_control
189:
190: On FreeBSD, the equivalent command is::
191:
192: /sbin/sysctl net.inet.tcp.cc.available
193:
194: I’m using the ``--logfile`` option. How do I see file output in real time?
195: Use the ``--forceflush`` flag.
196:
197: I'm using the --fq-rate flag, but it does not seem to be working. Why?
198: You need to add 'net.core.default_qdisc = fq' to /etc/sysctl.conf for that option to work.
199:
200: I'm having trouble getting iperf3 to work on Windows, Android, etc. Where can I get help?
201: iperf3 only supports Linux, FreeBSD, and OSX. For other platforms we recommend using iperf2.
202:
203: I managed to get a Windows executable built, but why do I get a BSOD on Windows 7?
204: There seems to be a bug in Windows 7 where running iperf3 from a
205: network filesystem can cause a system crash (in other words Blue
206: Screen of Death, or BSOD). This is a Windows bug addressed in kb2839149:
207:
208: https://support.microsoft.com/en-us/help/2839149/stop-error-0x00000027-in-the-rdbss-sys-process-in-windows-7-or-windows
209:
210: A hotfix is available under kb2732673:
211:
212: https://support.microsoft.com/en-us/help/2732673/-delayed-write-failed-error-message-when--pst-files-are-stored-on-a-ne
213:
214: Why can’t I run a UDP client with no server?
215: This is potentially dangerous, and an attacker could use this for a
216: denial of service attack. We don't want iperf3 to be an attack tool.
217:
218: I'm trying to use iperf3 to test a 40G/100G link...What do I need to know?
219: See the following pages on fasterdata.es.net:
220:
221: - https://fasterdata.es.net/host-tuning/100g-tuning/
222: - https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/iperf/multi-stream-iperf3/
223:
224: My receiver didn't get all the bytes that got sent but there was no loss. Huh?
225: iperf3 uses a control connection between the client and server to
226: manage the start and end of each test. Sometimes the commands on
227: the control connection can be received and acted upon before all of
228: the test data has been processed. Thus the test ends with data
229: still in flight. This effect can be significant for short (a few
230: seconds) tests, but is probably negligible for longer tests.
231:
232: A file sent using the ``-F`` option got corrupted...what happened?
233: The ``-F`` option to iperf3 is not a file transfer utility. It's a
234: way of testing the end-to-end performance of a file transfer,
235: including filesystem and disk overheads. So while the test will
236: mimic an actual file transfer, the data stored to disk may not be
237: the same as what was sent. In particular, the file size will be
238: rounded up to the next larger multiple of the transfer block size,
239: and for UDP tests, iperf's metadata (containing timestamps and
240: sequence numbers) will overwrite the start of every UDP packet
241: payload.
242:
243: I have a question regarding iperf3...what's the best way to get help?
244: Searching on the Internet is a good first step.
245: http://stackoverflow.com/ has a number of iperf3-related questions
246: and answers, but a simple query into your favorite search engine can
247: also yield some results.
248:
249: There is a mailing list nominally used for iperf3 development,
250: iperf-dev@googlegroups.com.
251:
252: We discourage the use of the iperf3 issue tracker on GitHub for
253: support questions. Actual bug reports, enhancement requests, or
254: pull requests are encouraged, however.
255:
256:
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