When the iperf3 client specifies a value larger than MTU with the -l parameter in UDP mode, the server receives zero traffic.

[Mrlidie]

【Descriptions】When the iperf3 client specifies a value larger than MTU with the -l parameter in UDP mode, the server receives zero traffic.
【Setp】
1.start iperf3 server：

iperf3 -s -p 6001

2.start iperf3 client：

iperf3 -c xxx -u -b 0 -P 2 -p 6003 -t 0 -l 20k

3.server result:

Server listening on 6003 (test #1)
-----------------------------------------------------------
Accepted connection from 99.1.2.2, port 42342
[  5] local 99.1.2.1 port 6003 connected to 99.1.2.2 port 57943
[  6] local 99.1.2.1 port 6003 connected to 99.1.2.2 port 33399
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-1.00   sec  35.9 MBytes   301 Mbits/sec  0.013 ms  999/2839 (35%)
[  6]   0.00-1.00   sec  35.4 MBytes   297 Mbits/sec  0.011 ms  937/2749 (34%)
[SUM]   0.00-1.00   sec  71.3 MBytes   598 Mbits/sec  0.012 ms  1936/5588 (35%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec  0.013 ms  0/0 (0%)
[  6]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec  0.011 ms  0/0 (0%)
[SUM]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec  0.012 ms  0/0 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec  0.013 ms  0/0 (0%)
[  6]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec  0.011 ms  0/0 (0%)
[SUM]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec  0.012 ms  0/0 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec  0.013 ms  0/0 (0%)
[  6]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec  0.011 ms  0/0 (0%)
[SUM]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec  0.012 ms  0/0 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec  0.013 ms  0/0 (0%)
[  6]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec  0.011 ms  0/0 (0%)
[SUM]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec  0.012 ms  0/0 (0%)
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec  0.013 ms  0/0 (0%)
[  6]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec  0.011 ms  0/0 (0%)
[SUM]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec  0.012 ms  0/0 (0%)

4.client result:

Connecting to host 99.1.2.1, port 6003
[  5] local 99.1.2.2 port 57943 connected to 99.1.2.1 port 6003
[  7] local 99.1.2.2 port 33399 connected to 99.1.2.1 port 6003
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec  1.47 GBytes  12.7 Gbits/sec  77280
[  7]   0.00-1.00   sec  1.41 GBytes  12.1 Gbits/sec  74000
[SUM]   0.00-1.00   sec  2.89 GBytes  24.8 Gbits/sec  151280
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   1.00-2.00   sec  1.48 GBytes  12.7 Gbits/sec  77550
[  7]   1.00-2.00   sec  1.41 GBytes  12.1 Gbits/sec  73690
[SUM]   1.00-2.00   sec  2.88 GBytes  24.8 Gbits/sec  151240
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   2.00-3.00   sec  1.48 GBytes  12.7 Gbits/sec  77350
[  7]   2.00-3.00   sec  1.41 GBytes  12.1 Gbits/sec  73890
[SUM]   2.00-3.00   sec  2.88 GBytes  24.8 Gbits/sec  151240
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   3.00-4.00   sec  1.47 GBytes  12.6 Gbits/sec  77180
[  7]   3.00-4.00   sec  1.41 GBytes  12.1 Gbits/sec  74070
[SUM]   3.00-4.00   sec  2.88 GBytes  24.8 Gbits/sec  151250
- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   4.00-5.00   sec  1.47 GBytes  12.6 Gbits/sec  77040
[  7]   4.00-5.00   sec  1.42 GBytes  12.2 Gbits/sec  74200
[SUM]   4.00-5.00   sec  2.88 GBytes  24.8 Gbits/sec  151240
^C- - - - - - - - - - - - - - - - - - - - - - - - -
[  5]   5.00-5.95   sec  1.40 GBytes  12.7 Gbits/sec  73403
[  7]   5.00-5.95   sec  1.34 GBytes  12.1 Gbits/sec  70073
[SUM]   5.00-5.95   sec  2.74 GBytes  24.8 Gbits/sec  143476

When the -l parameter is set to 8000 (with MTU value of 9000 on both server and client), the server-side rate does not become zero.

OS info：
Linux 4.19.113-67.nfs4.x86_64
iperf3 -v
iperf 3.19.1 (cJSON 1.7.15)

[davidBar-On]

It is not clear that the problem is related to the MTU, or at least not directly, as initially the server does get something. Since you set -b 0, the client tries to send as much data is the CPU allows, which is probably in much higher rate than the network throughput. Therefore, many/post of the UDP segments are getting lost, and it may be that no UDP packet gets all its segments, and this is the reason for the 0 throughput. You may try running Wireshark or tcpdump to see if and what is received on the server machine.

The evaluate what happens, I suggest to remove the -P 2 and test with different increasing client sending rate. E.g., start with -b 1M, then 10M, 50M, 100M, 500M, 1G, etc. My guess it that the lower rates tests will succeed and in some point received throughput will start to decrease.

You may try running a TCP test without limiting the bandwidth, so see an estimated rate of the network bandwidth.

[bmah888]

@Mrlidie: I'm curious to know why you are using such a large value for the -l parameter because this is almost guaranteed to cause IP fragmentation. What scenario are you trying to simulate or what are you trying to measure here?