For udp traffic, if send is ubuntu and receive is windows, -P, --parallel will lead iperf error

[llh8991]

Send: linux(22.04.4)+iperf3 3.21
Receive: windows(win11 24H2)+iperf3 3.21

without -P, iperf3 pump udp traffic can success
with -P 4, iperf3 pump udp traffic failed with error "unable to read from stream socket: Resource temporarily unavailable"

root@WXCAB1-EPC1:~# iperf3 -c 192.168.1.14 -i 1 -u -b 40M -t 10
Connecting to host 192.168.1.14, port 5201
[  5] local 192.168.1.11 port 40798 connected to 192.168.1.14 port 5201
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  5]   0.00-1.00   sec  4.77 MBytes  40.0 Mbits/sec  3457
[  5]   1.00-2.00   sec  4.77 MBytes  40.0 Mbits/sec  3453
[  5]   2.00-3.00   sec  4.77 MBytes  40.0 Mbits/sec  3453
[  5]   3.00-4.00   sec  4.77 MBytes  40.0 Mbits/sec  3453
[  5]   4.00-5.00   sec  4.77 MBytes  40.0 Mbits/sec  3453
[  5]   5.00-6.00   sec  4.77 MBytes  40.0 Mbits/sec  3451
[  5]   6.00-7.00   sec  4.77 MBytes  40.0 Mbits/sec  3455
[  5]   7.00-8.00   sec  4.77 MBytes  40.0 Mbits/sec  3453
[  5]   8.00-9.00   sec  4.77 MBytes  40.0 Mbits/sec  3453
[  5]   9.00-10.00  sec  4.77 MBytes  40.0 Mbits/sec  3453
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  5]   0.00-10.00  sec  47.7 MBytes  40.0 Mbits/sec  0.000 ms  0/34534 (0%)  sender
[  5]   0.00-10.01  sec  45.1 MBytes  37.8 Mbits/sec  0.530 ms  1898/34534 (5.5%)  receiver

iperf Done.
root@WXCAB1-EPC1:~# iperf3 -c 192.168.1.14 -i 1 -u -b 40M -t 10 -P 4
Connecting to host 192.168.1.14, port 5201
[  5] local 192.168.1.11 port 35980 connected to 192.168.1.14 port 5201
iperf3: error - unable to read from stream socket: Resource temporarily unavailable
root@WXCAB1-EPC1:~# iperf3 -v
iperf 3.21 (cJSON 1.7.15)
Linux WXCAB1-EPC1 6.8.0-52-generic #53~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Jan 15 19:18:46 UTC 2 x86_64
Optional features available: CPU affinity setting, IPv6 flow label, TCP congestion algorithm setting, sendfile / zerocopy, socket pacing, bind to device, support IPv4 don't fragment, POSIX threads                    , GSO/GRO support

[davidBar-On]

This is a known issue. See PR #1163 that includes a suggested fix. As Windows is not formally supported by iperf3 it is not clear whether this PR will ever be merged into the main branch (it is from 2021). In any case, I re-based PR #1163 to the latest iperf3 master branch (3.21+).