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I'm trying to do some throughput measurements for a azure VM ubuntu server using iperf3.(version iperf 3.11).

In the server side I run the following command

iperf3 -s -V -p xxxx

On the client side when I give the following command for 5 second test and 0.5 seconds report interval

iperf3 -c hostname -p xxxx -t 5  -i 0.5

I sometimes get 0.0 on the Transfer and Bitrate fields like the following

Connecting to host xxxxxxxxx, port xxxx
[  5] local 10.0.2.15 port 36280 connected to xxxxxxxxx port xxxxxxx
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-0.50   sec   656 KBytes  10.7 Mbits/sec    0   65.6 KBytes       
[  5]   0.50-1.01   sec  0.00 Bytes  0.00 bits/sec    0   65.6 KBytes       
[  5]   1.01-1.52   sec   188 KBytes  3.04 Mbits/sec    0   65.6 KBytes       
[  5]   1.52-2.00   sec   188 KBytes  3.18 Mbits/sec    0   65.6 KBytes       
[  5]   2.00-2.50   sec  0.00 Bytes  0.00 bits/sec    0   65.6 KBytes       
[  5]   2.50-3.00   sec   157 KBytes  2.57 Mbits/sec    0   62.7 KBytes       
[  5]   3.00-3.50   sec   314 KBytes  5.14 Mbits/sec    0   65.6 KBytes       
[  5]   3.50-4.00   sec   188 KBytes  3.09 Mbits/sec    0   65.6 KBytes       
[  5]   4.00-4.52   sec   188 KBytes  2.98 Mbits/sec    0   65.6 KBytes       
[  5]   4.52-5.13   sec   188 KBytes  2.53 Mbits/sec    0   65.6 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-5.13   sec  2.02 MBytes  3.30 Mbits/sec    0             sender
[  5]   0.00-5.13   sec  1.68 MBytes  2.74 Mbits/sec

Even though on the server side I get non zero values

[  5] local 10.0.1.4 port yyyyyy connected to xxxxxxx port xxxxxxx
[ ID] Interval           Transfer     Bandwidth
[  5]   0.00-1.00   sec   199 KBytes  1.63 Mbits/sec
[  5]   1.00-2.00   sec   315 KBytes  2.58 Mbits/sec
[  5]   2.00-3.00   sec   302 KBytes  2.47 Mbits/sec
[  5]   3.00-4.00   sec   417 KBytes  3.42 Mbits/sec
[  5]   4.00-5.00   sec   391 KBytes  3.20 Mbits/sec
[  5]   5.00-5.22   sec  93.9 KBytes  3.57 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth
[  5]   0.00-5.22   sec  0.00 Bytes  0.00 bits/sec                  sender
[  5]   0.00-5.22   sec  1.68 MBytes  2.70 Mbits/sec                  receiver
----------------------------------------------------------

The above happens even if I start the server with the following command

iperf -s -V -p xxxx -i 0.5

Which means that there is no way for the uplink to be at that moment 0.

E.x

client side

[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-0.50   sec   730 KBytes  11.9 Mbits/sec    0   92.7 KBytes       
[  5]   0.50-1.00   sec  0.00 Bytes  0.00 bits/sec    0   88.4 KBytes       
[  5]   1.00-1.51   sec   220 KBytes  3.52 Mbits/sec    0   88.4 KBytes       
[  5]   1.51-2.01   sec  0.00 Bytes  0.00 bits/sec    0   91.2 KBytes       
[  5]   2.01-2.52   sec   251 KBytes  4.06 Mbits/sec    0   91.2 KBytes       
[  5]   2.52-3.00   sec   220 KBytes  3.73 Mbits/sec    0   91.2 KBytes       
[  5]   3.00-3.50   sec   220 KBytes  3.59 Mbits/sec    0   91.2 KBytes       
[  5]   3.50-4.00   sec   220 KBytes  3.60 Mbits/sec    0   91.2 KBytes       
[  5]   4.00-4.50   sec  0.00 Bytes  0.00 bits/sec    0   91.2 KBytes       
[  5]   4.50-5.00   sec   188 KBytes  3.10 Mbits/sec    0   91.2 KBytes       
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-5.00   sec  2.00 MBytes  3.35 Mbits/sec    0             sender
[  5]   0.00-5.00   sec  1.59 MBytes  2.66 Mbits/sec                  receiver

iperf Done.

server side

[ ID] Interval           Transfer     Bandwidth
[  5]   0.00-0.50   sec  63.3 KBytes  1.04 Mbits/sec
[  5]   0.50-1.00   sec   101 KBytes  1.66 Mbits/sec
[  5]   1.00-1.50   sec   116 KBytes  1.91 Mbits/sec
[  5]   1.50-2.00   sec   145 KBytes  2.37 Mbits/sec
[  5]   2.00-2.50   sec   194 KBytes  3.18 Mbits/sec
[  5]   2.50-3.00   sec   194 KBytes  3.17 Mbits/sec
[  5]   3.00-3.50   sec   197 KBytes  3.22 Mbits/sec
[  5]   3.50-4.00   sec   201 KBytes  3.29 Mbits/sec
[  5]   4.00-4.50   sec   199 KBytes  3.27 Mbits/sec
[  5]   4.50-5.00   sec   199 KBytes  3.27 Mbits/sec
[  5]   5.00-5.07   sec  14.3 KBytes  1.59 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth
[  5]   0.00-5.07   sec  0.00 Bytes  0.00 bits/sec                  sender
[  5]   0.00-5.07   sec  1.59 MBytes  2.62 Mbits/sec                  receiver

The above does not happen when I use the default interval 1 sec and also when I use the -R parameter to measure download.

Can anyone help me to locate where the problem is ?

dk13
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1 Answers1

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The problem is probably because iperf3 feels the network buffers fater then the transmit rate, so the buffers become full. For example, in the second test, the client sent 730KB in the first second, but the server received only 63KB.

Also, in both tests the client sent about 300-400KB more than the server received. This is the amount of data that was still in the network buffers when the client completed sending (in iperf3 the client is responsible to set the end of the test).

See the related discussion in iperf3 issue #1308.

David Bar-On
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