iperf3 client summary reports 0% UDP loss despite non-zero loss in server output

[kindacute]

Whenever I introduce 30% UDP packet drop on my router, iperf3 always report 0% losses. I don't understand why it happens. I run command with --debug and --verbose parameters and see that losses about 30%:

"server_output_text": "-----------------------------------------------------------\nServer listening on 5201\n-----------------------------------------------------------\nAccepted connection from 194.37.101.163, port 49961\n[129] local 51.158.1.21 port 5201 connected to 194.37.101.163 port 50509\n[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams\n[129] 0.00-1.00 sec 86.3 KBytes 707 Kbits/sec 0.236 ms 22/83 (27%) \n[129] 1.00-2.00 sec 89.1 KBytes 730 Kbits/sec 0.212 ms 23/86 (27%) \n[129] 2.00-3.00 sec 89.1 KBytes 730 Kbits/sec 0.213 ms 24/87 (28%) \n[129] 3.00-3.05 sec 1.41 KBytes 256 Kbits/sec 0.200 ms 0/1 (0%) \n- - - - - - - - - - - - - - - - - - - - - - - - -\n[ ID] Interval Transfer Bitrate Jitter Lost/Total Datagrams\n[129] 0.00-3.05 sec 266 KBytes 715 Kbits/sec 0.200 ms 69/257 (27%) receiver\n",

So why in the end (and without --verbose, --debug) I see 0 packet losses:

Reading new State from the Server - current state is 13-EXCHANGE_RESULTS
State change: client received and changed State to 14-DISPLAY_RESULTS
interval_len 0.997376 bytes_transferred 124528
interval forces keep
[  7]   2.01-3.00   sec   122 KBytes   999 Kbits/sec  86
- - - - - - - - - - - - - - - - - - - - - - - - -
Test Complete. Summary Results:
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  7]   0.00-3.00   sec   366 KBytes   999 Kbits/sec  0.000 ms  0/259 (0%)  sender
[  7]   0.00-3.05   sec   266 KBytes   715 Kbits/sec  0.200 ms  0/257 (0%)  receiver
CPU Utilization: local/sender 0.9% (0.1%u/0.8%s), remote/receiver 0.1% (0.0%u/0.1%s)
State change: State set to 16-IPERF_DONE (from 14-DISPLAY_RESULTS)
Receiver threads stopped
iperf Done.

[davidBar-On]

... I run command with --debug and --verbose parameters and see that losses about 30%:

I assume these settings are for the server, as per the output "without --verbose, --debug" these are still set for the client. If not please clarify.

What iperf3 version are you using (iperf3 -v)?
Also, can you try running with --debug=3 (instead of just --debug) and see if this makes any change?

[kindacute]

Iperf version:

iperf3 -v
iperf 3.20 (cJSON 1.7.15)
Darwin %username% 25.1.0 Darwin Kernel Version 25.1.0: Mon Oct 20 19:33:36 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6030 arm64
Optional features available: sendfile / zerocopy, authentication, bind to device, support IPv4 don't fragment, POSIX threads

When I said --verbose and --debug I meant for client, here is exact command line that I run:
iperf3 -c ping.online.net -u -b 1M -t 3 --verbose --debug

here is output with --debug=3:

iperf3 -c ping.online.net -u -b 1M -t 3 --verbose --debug=3
iperf 3.20
Darwin %username% 25.1.0 Darwin Kernel Version 25.1.0: Mon Oct 20 19:33:36 PDT 2025; root:xnu-12377.41.6~2/RELEASE_ARM64_T6030 arm64
Control connection MSS 1448
Setting UDP block size to 1448
Reading new State from the Server - current state is 0-Test reset
State change: client received and changed State to 9-PARAM_EXCHANGE - Client to Server Parameters Exchange
send_parameters:
{
	"udp":	true,
	"omit":	0,
	"time":	3,
	"num":	0,
	"blockcount":	0,
	"parallel":	1,
	"len":	1448,
	"bandwidth":	1000000,
	"pacing_timer":	1000,
	"client_version":	"3.20"
}
Time: Tue, 16 Dec 2025 20:31:39 UTC
Connecting to host ping.online.net, port 5201
      Cookie: jwcahhcobn2bgdhbc6h6ufx7rjit6uqeqvqx
      Target Bitrate: 1000000
Reading new State from the Server - current state is 9-PARAM_EXCHANGE - Client to Server Parameters Exchange
State change: client received and changed State to 10-CREATE_STREAMS
SNDBUF is 9216, expecting 0
RCVBUF is 786896, expecting 0
Setting application pacing to 125000
Sending Connect message to Socket 7
Connect received for Socket 7, sz=4, buf=3ade68b1, i=0, max_len_wait_for_reply=4
[  7] local 192.168.88.236 port 64510 connected to 51.158.1.21 port 5201
Reading new State from the Server - current state is 10-CREATE_STREAMS
State change: client received and changed State to 1-TEST_START - starting a new test
Starting Test: protocol: UDP, 1 streams, 1448 byte blocks, omitting 0 seconds, 3 second test, tos 0
Reading new State from the Server - current state is 1-TEST_START - starting a new test
State change: client received and changed State to 2-TEST_RUNNING
Thread FD 7 created
All threads created
interval_len 1.005044 bytes_transferred 125976
interval forces keep
[ ID] Interval           Transfer     Bitrate         Total Datagrams
[  7]   0.00-1.01   sec   123 KBytes  1.00 Mbits/sec  87
interval_len 0.999373 bytes_transferred 124528
interval forces keep
[  7]   1.01-2.00   sec   122 KBytes   997 Kbits/sec  87
Thread FD 7 stopped
Sender threads stopped
State change: State set to 4-TEST_END (from 2-TEST_RUNNING)
Reading new State from the Server - current state is 4-TEST_END
State change: client received and changed State to 13-EXCHANGE_RESULTS
send_results
{
	"cpu_util_total":	1.0661168602445217,
	"cpu_util_user":	0.072310479371249847,
	"cpu_util_system":	0.99383829194095563,
	"sender_has_retransmits":	0,
	"streams":	[{
			"id":	1,
			"bytes":	376480,
			"retransmits":	-1,
			"jitter":	0,
			"errors":	0,
			"omitted_errors":	0,
			"packets":	260,
			"omitted_packets":	0,
			"start_time":	0,
			"end_time":	3.005025
		}]
}
get_results
{
	"cpu_util_total":	0.11669088267344714,
	"cpu_util_user":	0,
	"cpu_util_system":	0.11669088267344714,
	"sender_has_retransmits":	-1,
	"server_output_text":	"-----------------------------------------------------------\nServer listening on 5201\n-----------------------------------------------------------\nAccepted connection from 194.37.101.163, port 55902\n[ 88] local 51.158.1.21 port 5201 connected to 194.37.101.163 port 64510\n[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams\n[ 88]   0.00-1.00   sec  86.3 KBytes   707 Kbits/sec  0.252 ms  22/83 (27%)  \n[ 88]   1.00-2.00   sec  89.1 KBytes   730 Kbits/sec  0.084 ms  23/86 (27%)  \n[ 88]   2.00-3.00   sec  89.1 KBytes   730 Kbits/sec  0.188 ms  24/87 (28%)  \n[ 88]   3.00-3.05   sec  4.24 KBytes   745 Kbits/sec  0.200 ms  1/4 (25%)  \n- - - - - - - - - - - - - - - - - - - - - - - - -\n[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams\n[ 88]   0.00-3.05   sec   269 KBytes   722 Kbits/sec  0.200 ms  70/260 (27%)  receiver\n",
	"streams":	[{
			"id":	1,
			"bytes":	275120,
			"retransmits":	-1,
			"jitter":	0.00019994949078789008,
			"errors":	70,
			"packets":	260,
			"start_time":	0,
			"end_time":	3.046705
		}]
}
Reading new State from the Server - current state is 13-EXCHANGE_RESULTS
State change: client received and changed State to 14-DISPLAY_RESULTS
interval_len 1.000608 bytes_transferred 125976
interval forces keep
[  7]   2.00-3.01   sec   123 KBytes  1.01 Mbits/sec  86
- - - - - - - - - - - - - - - - - - - - - - - - -
Test Complete. Summary Results:
[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
[  7]   0.00-3.01   sec   368 KBytes  1.00 Mbits/sec  0.000 ms  0/260 (0%)  sender
[  7]   0.00-3.05   sec   269 KBytes   722 Kbits/sec  0.200 ms  0/260 (0%)  receiver
CPU Utilization: local/sender 1.1% (0.1%u/1.0%s), remote/receiver 0.1% (0.0%u/0.1%s)
State change: State set to 16-IPERF_DONE (from 14-DISPLAY_RESULTS)
Receiver threads stopped

iperf Done.

same unclear situation: server reports 25-28% loss:

[ 88] 0.00-1.00 sec 86.3 KBytes 707 Kbits/sec 0.252 ms 22/83 (27%)
[ 88] 1.00-2.00 sec 89.1 KBytes 730 Kbits/sec 0.084 ms 23/86 (27%)
[ 88] 2.00-3.00 sec 89.1 KBytes 730 Kbits/sec 0.188 ms 24/87 (28%)
[ 88] 3.00-3.05 sec 4.24 KBytes 745 Kbits/sec 0.200 ms 1/4 (25%)

however 0% packet loss in summary:

[  7]   0.00-3.01   sec   368 KBytes  1.00 Mbits/sec  0.000 ms  0/260 (0%)  sender
[  7]   0.00-3.05   sec   269 KBytes   722 Kbits/sec  0.200 ms  0/260 (0%)  receiver

[kindacute]

just in case if this is important, here is my rules for mikrotik firewall, fasttrack is disabled:

[admin@MikroTik] > /ip firewall filter print 
Flags: X - disabled, I - invalid, D - dynamic 
 0 X  chain=forward action=drop protocol=udp dst-address=18.216.157.169 dst-port="" random=1 log=no log-prefix="" 

 1 X  chain=forward action=drop dst-address=192.168.88.203 random=1 log=no log-prefix="" 

 2 X  chain=forward action=drop dst-address=99.83.182.69 log=no log-prefix="" 

 3 X  chain=forward action=drop dst-address=75.2.89.164 log=no log-prefix="" 

 4 X  chain=forward action=drop src-address=192.168.88.253 dst-address=3.33.189.196 log=yes log-prefix="60PERDR" 

 5 X  chain=forward action=drop src-address=192.168.88.253 dst-address=108.138.51.0/24 log=no log-prefix="" 

 6 X  chain=forward action=drop src-address=192.168.88.253 dst-address=18.172.112.0/24 log=no log-prefix="" 

 7 X  chain=forward action=drop protocol=udp src-address=192.168.88.231 random=70 log=no log-prefix="" 

 8 X  chain=forward action=drop protocol=udp dst-address=192.168.88.231 random=70 log=no log-prefix="" 

 9    chain=forward action=drop protocol=udp dst-address=51.158.1.21 nth=10,3 log=no log-prefix="" 

10    chain=forward action=drop protocol=udp dst-address=51.158.1.21 nth=10,7 log=no log-prefix="" 

11    chain=forward action=drop protocol=udp dst-address=51.158.1.21 nth=10,9 log=no log-prefix="" 

12    ;;; defconf: accept established,related,untracked
      chain=input action=accept connection-state=established,related,untracked 

13    ;;; defconf: drop invalid
      chain=input action=drop connection-state=invalid 

14    ;;; defconf: accept ICMP
      chain=input action=accept protocol=icmp log=no log-prefix="" 

15    ;;; defconf: accept to local loopback (for CAPsMAN)
      chain=input action=accept dst-address=127.0.0.1 

16    ;;; defconf: drop all not coming from LAN
      chain=input action=drop in-interface-list=!LAN 

17    ;;; defconf: accept in ipsec policy
      chain=forward action=accept ipsec-policy=in,ipsec 

18    ;;; defconf: accept out ipsec policy
      chain=forward action=accept ipsec-policy=out,ipsec 

19 X  ;;; defconf: fasttrack
      chain=forward action=fasttrack-connection connection-state=established,related log=no log-prefix="" 

20    ;;; defconf: accept established,related, untracked
      chain=forward action=accept connection-state=established,related,untracked 

21    ;;; defconf: drop invalid
      chain=forward action=drop connection-state=invalid 

22    ;;; defconf: drop all from WAN not DSTNATed
      chain=forward action=drop connection-state=new connection-nat-state=!dstnat in-interface-list=WAN 
[admin@MikroTik] >  

[davidBar-On]

Thanks for the additional information. Found that this is an iperf3 backward compatibility issue with older servers that do not send the omitting statistics to the client. Submitted PR #1988 to fix this issue.