Iperf3.18 - packet count for burst mode seems broken when sending UDP traffic (on Windows)

[fitchtravis]

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Context

• Version of iperf3:

Using a build from - https://github.com/ar51an/iperf3-win-builds

iperf 3.18 (cJSON 1.7.15)
CYGWIN_NT-10.0-20348 MELPRTG01PRD 3.5.4-1.x86_64 2024-08-25 16:52 UTC x86_64
Optional features available: CPU affinity setting, support IPv4 don't fragment, POSIX threads

• Hardware:
  System Manufacturer: VMware, Inc.
  System Model: VMware Virtual Platform
  Processor(s): 1 Processor(s) Installed.
  [01]: Intel64 Family 6 Model 85 Stepping 4 GenuineIntel ~2095 Mhz
  Total Physical Memory: 32,767 MB

• Operating system (and distribution, if any):

OS Name: Microsoft Windows Server 2022 Datacenter
OS Version: 10.0.20348 N/A Build 20348

Please note: iperf3 is supported on Linux, FreeBSD, and macOS.
Support may be provided on a best-effort basis to other UNIX-like
platforms. We cannot provide support for building and/or running
iperf3 on Windows, iOS, or Android.

• Other relevant information (for example, non-default compilers,
  libraries, cross-compiling, etc.):

Please fill out one of the "Bug Report" or "Enhancement Request"
sections, as appropriate. Note that submissions of bug fixes, new
features, etc. should be done as a pull request at
https://github.com/esnet/iperf/pulls

Bug Report

• Expected Behavior

When using -b 100M/100, we expect to see iperf3 sending at 10Mbps with a burst of 100 packets.

Identically issue to - #1192

Without the bust mode option, tests run normal.

With burst mode iperf seems to be ignoring the bandwidth throttle limit

• Actual Behavior

iPerf3 is sending as much data as possible.

PS C:\Temp\iperf-3.18-win64> .\iperf3.exe --get-server-output  -c 10.y.y.11-b 100M/100 -t 5 -u
Connecting to host 10.y.y.11, port 5201
[  5] local 10.x.x.242 port 65202 connected to 10.y.y.11 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.01   sec   165 MBytes  1.36 Gbits/sec  128257
[  5]   1.01-2.01   sec   191 MBytes  1.61 Gbits/sec  149129
[  5]   2.01-3.00   sec   207 MBytes  1.76 Gbits/sec  161664
[  5]   3.00-4.01   sec   217 MBytes  1.81 Gbits/sec  169560
[  5]   4.01-5.00   sec   219 MBytes  1.85 Gbits/sec  170887
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-5.00   sec  1000 MBytes  1.68 Gbits/sec  0.000 ms  0/779497 (0%)  sender
[  5]   0.00-5.56   sec   422 MBytes   637 Mbits/sec  0.111 ms  450227/779494 (58%)  receiver

Server output:

-----------------------------------------------------------
Server listening on 5201 (test #35)
-----------------------------------------------------------
Accepted connection from 10.x.x.242, port 54742
[  5] local 10.y.y.11 port 5201 connected to 10.x.c.242 port 65202
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-1.00   sec  50.6 MBytes   424 Mbits/sec  0.014 ms  72697/112137 (65%)
[  5]   1.00-2.01   sec  84.2 MBytes   702 Mbits/sec  0.008 ms  80335/145963 (55%)
[  5]   2.01-3.01   sec  87.7 MBytes   737 Mbits/sec  0.015 ms  76942/145311 (53%)
[  5]   3.01-4.00   sec  89.6 MBytes   755 Mbits/sec  0.007 ms  74864/144734 (52%)
[  5]   4.00-5.02   sec  88.9 MBytes   733 Mbits/sec  0.012 ms  101676/171019 (59%)
[  5]   5.02-5.56   sec  21.3 MBytes   329 Mbits/sec  0.111 ms  43713/60330 (72%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[SUM]  0.0- 5.6 sec  1 datagrams received out-of-order
[  5]   0.00-5.56   sec   422 MBytes   637 Mbits/sec  0.111 ms  450227/779494 (58%)  receiver


iperf Done.

• Steps to Reproduce

Just run iperf3 with these switches '-b 100M/100 -t 5 -u'

• Possible Solution

Enhancement Request

• Current behavior

• Desired behavior

• Implementation notes

[davidBar-On]

Submitter #1821 with a proposed fix to the issue.

@fitchtravis, just out of curiosity, what is the reason you are using burst? I am asking because my understanding is that the main use of burst was to improve performance, as before the multi-thread iperf3 version a select() was called after each sending cycle. However, with the multi-thread version select() is not called, so burst should not impact performance.

[fitchtravis]

@davidBar-On . We have an issue with our MS Teams Audio/Video traffic getting dropped across our WAN due to how QoS has been configured. I was hoping to use bust mode incrementality, to determine at which point triggers the QoS profiles to start dropping packets.

[davidBar-On]

@fitchtravis, thanks for the explanation. If you know how to build iperf3 for Windows you may use the PR code (it may be a while until it will be fixed in an official version).

[fitchtravis]

@davidBar-On .. Thanks for the update.