iperf - Man Page

• Run on server: iperf -s
• Run on server using UDP mode and set server port to listen on 5001: iperf -u -s -p 5001
• Run on client: iperf -c server_address
• Run on client every 2 seconds: iperf -c server_address -i 2
• Run on client with 5 parallel threads: iperf -c server_address -P 5
• Run on client using UDP mode: iperf -u -c server_address -p 5001

tldr.sh

iperf -s [options]

iperf -c server [options]

iperf -u -s [options]

iperf -u -c server [options]

iperf 2 is a testing tool which performs network traffic measurements using network sockets. The performance metrics supported include throughput and latency (or link capacity and responsiveness.) Latency measurements include both one way delay (OWD) and round trip times (RTTs.) Iperf can use both TCP and UDP sockets (or protocols.) It supports unidirectional, full duplex (same socket) and bidirectional traffic, and supports multiple, simultaneous traffic streams. It supports multicast traffic including source specific multicast (SSM) joins. Its multi-threaded design allows for peak performance. Metrics displayed help to characterize host to host network performance. Setting the enhanced (-e) option provides all available metrics. Note: the metrics are at the socket level reads and writes. They do not include the overhead associated with lower level protocol layer headers.

The user must establish both a server (to receive traffic) and a client (to generate and send traffic) for a test to occur. The client and server typically are on different hosts or computers but need not be.

-b,  --bandwidth

set the target bandwidth and optional standard deviation per <mean>,[<stdev>] (See Notes for suffixes) Setting the target bitrate on the client to 0 will disable bitrate limits (particularly useful for UDP tests). Will limit the read rate on the server.

-e,  --enhanced

Display enhanced output in reports otherwise use legacy report (ver 2.0.5) formatting (see Notes)

-f, --format [abkmgBKMG]

format to report: adaptive, bits, Bytes, Kbits, Mbits, Gbits, KBytes, MBytes, GBytes (see Notes for more)

-h,  --help

print a help synopsis

   --hide-ips

obscure ip addresses in output (useful when wanting to publish results and not display the full ip addresses. v4 only)

-i,  --interval < t | f >

sample or display interval reports every t seconds (default) or every frame or burst, i.e. if f is used then the interval will be each frame or burst. The frame interval reporting is experimental.  Also suggest a compile with fast-sampling, i.e. ./configure --enable-fastsampling

-l,  --len n[kmKM]

set read/write buffer size (TCP) or length (UDP) to n (TCP default 128K, UDP default 1470)

   --l2checks

perform layer 2 length checks on received UDP packets (requires systems that support packet sockets, e.g. Linux)

-m,  --print_mss

print TCP maximum segment size

   --NUM_REPORT_STRUCTS <count>

Override the default shared memory size between the traffic thread(s) and reporter thread in order to mitigate mutex lock contentions. The default value of 5000 should be sufficient for 1Gb/s networks. Increase this upon seeing the Warning message of reporter thread too slow. If the Warning message isn't seen, then increasing this won't have any significant effect (other than to use some additional memory.)

-o,  --output filename

output the report or error message to this specified file

   --permit-key [=<value>]

Set a key value that must match for the server to accept traffic on a connection. If the option is given without a value on the server a key value will be autogenerated and displayed in its initial settings report. The lifetime of the key is set using --permit-key-timeout and defaults to twenty seconds. The value on clients required the use of '=', e.g. --permit-key=password (even though it's required command line option.) The server will auto-generate a value if '=password' is not given.  The value will also be used as part of the transfer id in reports. The option set on the client but not the server will also cause the server to reject the client's traffic. TCP only, no UDP support.

-p,  --port m[-n]

set client or server port(s) to send or listen on per m (default 5001) w/optional port range per m-n (e.g. -p 6002-6008) (see Notes)

   --sum-dstip

sum traffic threads based upon the destination IP address (default is source ip address)

   --sum-only

set the output to sum reports only. Useful for -P at large values

   --tcp-tx-delay n

Set TCP_TX_DELAY on the socket. Delay units are milliseconds. Value takes float. See Notes for qdisc requirements.

-t,  --time n

time in seconds to listen for new traffic connections, receive traffic or send traffic

-u,  --udp

use UDP rather than TCP

   --utc

use coordinated universal time (UTC) when outputting time (otherwise use local time)

-w,  --window n[kmKM]

TCP window size (socket buffer size)

-z,  --realtime

Request real-time scheduler, if supported.

-B,  --bind host[:port][%dev]

bind to host, ip address or multicast address, optional port or device (see Notes)

-C,  --compatibility

for use with older versions does not sent extra msgs

-M,  --mss n

set TCP maximum segment size using TCP_MAXSEG

-N,  --nodelay

set TCP no delay, disabling Nagle's Algorithm

-v,  --version

print version information and quit

-x,  --reportexclude [CDMSV]

exclude C(connection) D(data) M(multicast) S(settings) V(server) reports

-y,  --reportstyle C|c

if set to C or c report results as CSV (comma separated values)

   --tcp-cca

Set the congestion control algorithm to be used for TCP connections. See SPECIFIC OPTIONS for more

   --working-load-cca

Set the congestion control algorithm to be used for TCP working loads. See SPECIFIC OPTIONS for more

-Z,  --tcp-congestion

Set the default congestion control algorithm to be used for new connections. Platforms must support setsockopt's TCP_CONGESTION. (Notes: See sysctl and tcp_allowed_congestion_control for available options. May require root privileges.)

-1, --singleclient

set the server to process only one client at a time

-b, --bandwidth n[kmgKMG]

set target read rate to n bits/sec. TCP only for the server.

-s, --server

run in server mode

   --histograms[=binwidth[u],bincount,[lowerci],[upperci]]

enable latency histograms for udp packets (-u), for tcp writes (with --trip-times), or for either udp or tcp with --isochronous clients, or for --bounceback. The binning can be modified. Bin widths (default 1 millisecond, append u for microseconds, m for milliseconds) bincount is total bins (default 1000), ci is confidence interval between 0-100% (default lower 5%, upper 95%, 3 stdev 99.7%)

   --jitter-histograms[=<binwidth>]

enable jitter histograms for udp packets (-u). Optional value is the bin width where units are microseconds and defaults to 100 usecs

   --permit-key [=<value>]

Set a key value that must match for the server to accept traffic from a client (also set with --permit-key.) The server will auto-generate a globally unique key when the option is given without a value. This value will be displayed in the server's initial settings report. The lifetime of the key is set using --permit-key-timeout and defaults to twenty seconds. TCP only, no UDP support.

   --permit-key-timeout <value>

Set the lifetime of the permit key in seconds. Defaults to 20 seconds if not set. A value of zero will disable the timer.

   --skip-rx-copy

Set the server threads to use MSG_TRUNC on recv when possible. This flag causes the received bytes of data to be discarded and should offload the receiving CPU some.

   --tap-dev <dev>

Set the receive interface to the TAP device as specified.

   --tcp-rx-window-clamp n[kmKM]

Set the socket option of TCP_WINDOW_CLAMP, units is bytes.

   --test-exchange-timeout <value>

Set the maximum wait time for a test excahnge in seconds. Defaults to 60 seconds if not set. A value of zero will disable the timeout.

-t, --time n

time in seconds to listen for new traffic connections and/or receive traffic (defaults to infinite)

   --tos-override <val>

set the socket's IP_TOS value for reverse or full duplex traffic. Supported in versions 2.1.5 or greater. Previous versions won't set IP_TOS on reverse traffic. See Notes for values.

-B, --bind ip | ip%device

bind src ip addr and optional src device for receiving

-D, --daemon

run the server as a daemon. On Windows this will run the specified command-line under the IPerfService, installing the service if necessary. Note the service is not configured to auto-start or restart - if you need a self-starting service you will need to create an init script or use Windows "sc" commands.

-H, --ssm-host host

Set the source host (ip addr) per SSM multicast, i.e. the S of the S,G

-R, --remove

remove the IPerfService (Windows only).

-U, --single_udp

run in single threaded UDP mode

-V, --ipv6_domain

Enable IPv6 reception by setting the domain and socket to AF_INET6 (Can receive on both IPv4 and IPv6)

   --tcp-cca

Set the congestion control algorithm to be used for TCP connections - will overide any client side settings (same as --tcp-congestion)

   --working-load

Enable support for TCP working loads on UDP traffic streams

   --working-load-cca

Set the congestion control algorithm to be used for TCP working loads - will overide any client side settings

-b, --bandwidth n[kmgKMG][,n[kmgKMG]] | n[kmgKMG]pps

set target bandwidth to n bits/sec (default 1 Mbit/sec) or n packets per sec. This may be used with TCP or UDP. Optionally, for variable loads, use format of  mean,standard deviation

   --bounceback[=n]

run a TCP bounceback or rps test with optional number writes in a burst per value of n. The default is ten writes every period and the default period is one second (Note: set size with --bounceback-request). See Notes on clock unsynchronized detections.

   --bounceback-hold n

request the server to insert a delay of n milliseconds between its read and write (default is no delay)

   --bounceback-no-quickack

request the server not set the TCP_QUICKACK socket option (disabling TCP ACK delays) during a bounceback test (see Notes)

   --bounceback-period[=n]

request the client schedule its send(s) every n seconds (default is one second, use zero value for immediate or continuous back to back)

   --bounceback-request n

set the bounceback request size in units bytes. Default value is 100 bytes.

   --bounceback-reply n

set the bounceback reply size in units bytes. This supports asymmetric message sizes between the request and the reply. Default value is zero, which uses the value of --bounceback-request.

   --bounceback-txdelay n

request the client to delay n seconds between the start of the working load and the bounceback traffic (default is no delay)

   --burst-period n

Set the burst period in seconds. Defaults to one second. (Note: assumed use case is low duty cycle traffic bursts)

   --burst-size n

Set the burst size in bytes. Defaults to 1M if no value is given.

-c, --client host | host%device

run in client mode, connecting to host  where the optional %dev will SO_BINDTODEVICE that output interface (requires root and see Notes)

   --connect-only[=n]

only perform a TCP connect (or 3WHS) without any data transfer, useful to measure TCP connect() times. Optional value of n is the total number of connects to do (zero is run forever.) Note that -i will rate limit the connects where -P will create bursts and -t will end the client and hence end its connect attempts.

   --connect-retry-time n

time value in seconds for application level retries of TCP connect(s). See --connect-retry-timer for the retry time interval. See operating system information for the details of system or kernel TCP connect related settings. This is an application level retry of the connect() call and not the system level connect.

   --connect-retry-timer n

The minimum time value in seconds to wait before retrying the connect. Note: This a minimum time to  wait between retries and can be longer dependent upon the system connect time taken. See operating system information for the details of system or kernel TCP connect related settings.

   --dscp

set the DSCP field (masking ECN bits) in the TOS byte (used by IP_TOS & setsockopt)

-d, --dualtest

Do a bidirectional test simultaneous test using two unidirectional sockets

   --fq-rate n[kmgKMG]

Set a rate to be used with fair-queuing based socket-level pacing, in bytes or bits per second. Only available on platforms supporting the SO_MAX_PACING_RATE socket option. (Note: Here the suffixes indicate bytes/sec or bits/sec per use of uppercase or lowercase, respectively)

   --fq-rate-step n[kmgKMG]

Set a step of rate to be used with fair-queuing based socket-level pacing, in bytes or bits per second. Step occurs every fq-rate-step-interval (defaults to one second)

   --fq-rate-step-interval n

Time in seconds before stepping the fq-rate

   --full-duplex

run a full duplex test, i.e. traffic in both transmit and receive directions using the same socket

   --histograms[=binwidth[u],bincount,[lowerci],[upperci]]

enable select()/write() histograms with --tcp-write-times or --bounceback (these options are mutually exclusive.) The binning can be modified. Bin widths (default 100 microseconds, append u for microseconds, m for milliseconds) bincount is total bins (default 10000), ci is confidence interval between 0-100% (default lower 5%, upper 95%, 3 stdev 99.7%)

   --ignore-shutdown

don't wait on the TCP shutdown or close (fin & finack) rather use the final write as the ending event

   --incr-dstip

increment the destination ip address when using the parallel (-P) or port range option

   --incr-dstport

increment the destination port when using the parallel (-P) or port range option

   --incr-srcip

increment the source ip address when using the parallel (-P) or port range option

   --incr-srcport

increment the source ip address when using the parallel (-P) or port range option, requires -B to set the src port

   --ipg n

set the inter-packet gap to n (units of seconds) for packets or within a frame/burst when --isochronous is set

   --isochronous[=fps:mean,stdev]

send isochronous traffic with frequency frames per second and load defined by mean and standard deviation using a log normal distribution, defaults to 60:20m,0. (Note: Here the suffixes indicate bytes/sec or bits/sec per use of uppercase or lowercase, respectively. Also the p suffix is supported to set the burst size in packets, e.g. isochronous=2:25p will send two 25 packet bursts every second, or one 25 packet burst every 0.5 seconds.)

   --local-only[=1|0]

Set 1 to limit traffic to the local network only (through the use of SO_DONTROUTE) set to zero otherwise with optional override of compile time default (see configure --default-localonly)

   --near-congestion[=n]

Enable TCP write rate limiting per the sampled RTT. The delay is applied after the -l number of bytes have completed. The optional value is the multiplier to the RTT and defines the time delay. This value defaults to 0.5 if it is not set. Values less than 1 are supported but the value cannot be negative. This is an experimental feature. It is not likely stable on live networks. Suggested use is over controlled test networks.

   --no-connect-sync

By default, parallel traffic threads (per -P greater than 1) will synchronize after their TCP connects and prior to each sending traffic, i.e. all the threads first complete (or error) the TCP 3WHS before any traffic thread will start sending. This option disables that synchronization such that each traffic thread will start sending immediately after completing its successful connect.

   --no-udp-fin

Don't perform the UDP final server to client exchange which means there won't be a final server report displayed on the client. All packets per the test will be from the client to the server and no packets should be sent in the other direction. It's highly suggested that -t be set on the server if this option is being used. This is because there will be only one trigger ending packet sent from client to server and if it's lost then the server will continue to run. (Requires ver 2.0.14 or better)

-n, --num n[kmKM]

number of bytes to transmit (instead of -t)

   --permit-key [=<value>]

Set a key value that must match the server's value (also set with --permit-key) in order for the server to accept traffic from the client. TCP only, no UDP support.

   --sync-transfer-id

Pass the clients' transfer id(s) to the server so both will use the same id in their respective outputs

-r, --tradeoff

Do a bidirectional test individually - client-to-server, followed by a reversed test, server-to-client

   --tcp-cca

Set the congestion control algorithm to be used for TCP connections & exchange with the server (same as --tcp-congestion)

   --tcp-quickack

Set TCP_QUICKACK on the socket

   --tcp-write-prefetch n[kmKM]

Set TCP_NOTSENT_LOWAT on the socket and use event based writes per select() on the socket.

   --tcp-write-times

Measure the socket write times

-t, --time n|0

time in seconds to transmit traffic, use zero for infinite (default is 10 secs)

   --trip-times

enable the measurement of end to end write to read latencies (client and server clocks must be synchronized.) See notes about tcp-write-prefetch being enabled.

   --txdelay-time

time in seconds to hold back or delay after the TCP connect and prior to the socket writes. For UDP it's the delay between the traffic thread starting and the first write.

   --txstart-time n.n

set the txstart-time to n.n using unix or epoch time format (supports microsecond resolution, e.g 1536014418.123456) An example to delay one second using command substitution is iperf -c 192.168.1.10 --txstart-time $(expr $(date +%s) + 1).$(date +%N)

   --udp-l4s

run an l4s traffic load (requires a iperf server that supports l4s)

-B, --bind ip | ip:port | ipv6 -V | [ipv6]:port -V

bind src ip addr and optional port as the source of traffic (see Notes)

-F, --fileinput name

input the data to be transmitted from a file

-I, --stdin

input the data to be transmitted from stdin

-L, --listenport n

port to receive bidirectional tests back on

-P, --parallel n

number of parallel client threads to run

-R, --reverse

reverse the traffic flow (useful for testing through firewalls, see Notes)

-S, --tos <val>

set the socket's IP_TOS value. Versions 2.1.5 or greater will reflect this tos setting back with --reverse or --full-duplex option. (Previous versions won't set tos on the reverse traffic.) Note: use server side --tos-override to override. See Notes for values.

-T, --ttl n

time-to-live, for multicast (default 1)

   --working-load[=up|down|bidir][,n]

request a concurrent working load, currently TCP stream(s), defaults to full duplex (or bidir) unless the up or down option is provided. The number of TCP streams defaults to 1 and can be changed via the n value, e.g. --working-load=down,4 will use four TCP streams from server to the client as the working load. The IP ToS will be BE (0x0) for working load traffic.

   --working-load-cca

Set the congestion control algorithm to be used for TCP working loads, exchange with the server

-V, --ipv6_domain

Set the domain to IPv6 (send packets over IPv6)

-X, --peerdetect

run peer version detection prior to traffic.

-Z, --linux-congestion algo

set TCP congestion control algorithm (Linux only)

TCP tests (client) iperf -c <host> -e -i 1
------------------------------------------------------------
Client connecting to 192.168.1.35, TCP port 5001 with pid 256370 (1/0 flows/load)
Write buffer size: 131072 Byte
TCP congestion control using cubic
TOS set to 0x0 (dscp=0,ecn=0) (Nagle on)
TCP window size:  100 MByte (default)
------------------------------------------------------------
[  1] local 192.168.1.103%enp4s0 port 41024 connected with 192.168.1.35 port 5001 (sock=3) (icwnd/mss/irtt=14/1448/158) (ct=0.21 ms) on 2024-03-26 10:48:47.867 (PDT)
[ ID] Interval        Transfer    Bandwidth       Write/Err  Rtry     InF(pkts)/Cwnd(pkts)/RTT(var)        NetPwr
[  1] 0.00-1.00 sec   201 MBytes  1.68 Gbits/sec  1605/0        73     1531K(1083)/1566K(1108)/13336(112) us  15775
[  1] 1.00-2.00 sec   101 MBytes   846 Mbits/sec  807/0         0     1670K(1181)/1689K(1195)/14429(83) us  7331
[  1] 2.00-3.00 sec   101 MBytes   847 Mbits/sec  808/0         0     1790K(1266)/1790K(1266)/15325(97) us  6911
[  1] 3.00-4.00 sec   134 MBytes  1.13 Gbits/sec  1075/0         0     1858K(1314)/1892K(1338)/16188(99) us  8704
[  1] 4.00-5.00 sec   101 MBytes   846 Mbits/sec  807/0         1     1350K(955)/1370K(969)/11620(98) us  9103
[  1] 5.00-6.00 sec   121 MBytes  1.01 Gbits/sec  966/0         0     1422K(1006)/1453K(1028)/12405(118) us  10207
[  1] 6.00-7.00 sec   115 MBytes   962 Mbits/sec  917/0         0     1534K(1085)/1537K(1087)/13135(105) us  9151
[  1] 7.00-8.00 sec   101 MBytes   844 Mbits/sec  805/0         0     1532K(1084)/1580K(1118)/13582(136) us  7769
[  1] 8.00-9.00 sec   134 MBytes  1.13 Gbits/sec  1076/0         0     1603K(1134)/1619K(1145)/13858(105) us  10177
[  1] 9.00-10.00 sec   101 MBytes   846 Mbits/sec  807/0         0     1602K(1133)/1650K(1167)/14113(105) us  7495
[  1] 10.00-10.78 sec   128 KBytes  1.34 Mbits/sec  1/0         0        0K(0)/1681K(1189)/14424(111) us  11.64
[  1] 0.00-10.78 sec  1.18 GBytes   941 Mbits/sec  9674/0        74        0K(0)/1681K(1189)/14424(111) us  8154

where (per -e,)

ct= TCP connect time (or three way handshake time 3WHS)
Write/Err Total number of successful socket writes. Total number of non-fatal socket write errors
Rtry Total number of TCP retries
Inf(pkts)/Cwnd(pkts)/RTT(var) (*nix only) TCP bytes and packets inflight, congestion window bytes and packets, and round trip time and variance (sampled where NA indicates no value). Inflight is in units of Kbytes and packets where packets_in_flight = (tcp_info_buf.tcpi_unacked - tcp_info_buf.tcpi_sacked - tcp_info_buf.tcpi_lost + tcp_info_buf.tcpi_retrans) RTT (var) is the sampled RTT variance. All stats taken from tcp_info or tcp_connection_info and sampled per the -i reportting interval
NetPwr (*nix only) Network power defined as (throughput / RTT)

iperf -c host.domain.com -i 1 --bounceback --permit-key=mytest --hide-ips

------------------------------------------------------------
Client connecting to (**hidden**), TCP port 5001
Bursting:  100 Byte writes 10 times every 1.00 second(s)
Bounce-back test (size= 100 Byte) (server hold req=0 usecs)
TCP window size: 16.0 KByte (default)
------------------------------------------------------------
[mytest(1)] local *.*.*.96 port 38044 connected with *.*.*.123 port 5001 (bb len/hold=100/0) (icwnd/mss/irtt=14/1448/10605)
[ ID] Interval        Transfer    Bandwidth         BB cnt=avg/min/max/stdev         Rtry  Cwnd/RTT    RPS
[mytest(1)] 0.00-1.00 sec  1.95 KBytes  16.0 Kbits/sec    10=11.949/9.662/19.597/3.127 ms    0   14K/10930 us    83 rps
[mytest(1)] 1.00-2.00 sec  1.95 KBytes  16.0 Kbits/sec    10=10.004/9.651/10.322/0.232 ms    0   14K/10244 us    99 rps
[mytest(1)] 2.00-3.00 sec  1.95 KBytes  16.0 Kbits/sec    10=10.582/9.720/14.831/1.573 ms    0   14K/10352 us    94 rps
[mytest(1)] 3.00-4.00 sec  1.95 KBytes  16.0 Kbits/sec    10=11.303/9.940/15.114/2.026 ms    0   14K/10832 us    88 rps
[mytest(1)] 4.00-5.00 sec  1.95 KBytes  16.0 Kbits/sec    10=11.148/9.671/14.803/1.837 ms    0   14K/10858 us    89 rps
[mytest(1)] 5.00-6.00 sec  1.95 KBytes  16.0 Kbits/sec    10=10.207/9.695/10.729/0.356 ms    0   14K/10390 us    97 rps
[mytest(1)] 6.00-7.00 sec  1.95 KBytes  16.0 Kbits/sec    10=10.871/9.770/14.387/1.547 ms    0   14K/10660 us    91 rps
[mytest(1)] 7.00-8.00 sec  1.95 KBytes  16.0 Kbits/sec    10=11.224/9.760/14.993/1.837 ms    0   14K/11027 us    89 rps
[mytest(1)] 8.00-9.00 sec  1.95 KBytes  16.0 Kbits/sec    10=10.719/9.887/14.553/1.455 ms    0   14K/10620 us    93 rps
[mytest(1)] 9.00-10.00 sec  1.95 KBytes  16.0 Kbits/sec    10=10.775/9.689/14.746/1.562 ms    0   14K/10596 us    92 rps
[mytest(1)] 0.00-10.02 sec  19.5 KBytes  16.0 Kbits/sec    100=10.878/9.651/19.597/1.743 ms    0   14K/11676 us    91 rps
[  1] 0.00-10.02 sec BB8(f)-PDF: bin(w=100us):cnt(100)=97:5,98:8,99:10,100:8,101:12,102:10,103:6,104:7,105:2,106:2,107:3,108:3,109:2,110:1,114:1,115:1,118:1,120:2,121:1,124:1,125:1,128:1,140:1,143:1,144:1,146:2,148:1,149:2,150:1,151:1,152:1,196:1 (5.00/95.00/99.7%=97/149/196,Outliers=0,obl/obu=0/0)

where

BB cnt=avg/min/max/stdev Count of bouncebacks, average time, minimum time, maximum time, standard deviation units of ms
Rtry Total number of TCP retries
Cwnd/RTT (*nix only) TCP congestion window and round trip time (sampled where NA indicates no value)
RPS Responses per second

TCP tests (server)

iperf -s -e -i 1 -l 8K
------------------------------------------------------------
Server listening on TCP port 5001 with pid 13430
Read buffer size: 8.00 KByte
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
[  4] local 45.33.58.123 port 5001 connected with 45.56.85.133 port 49960
[ ID] Interval        Transfer    Bandwidth       Reads   Dist(bin=1.0K)
[  4] 0.00-1.00 sec   124 MBytes  1.04 Gbits/sec  22249    798:2637:2061:767:2165:1563:589:11669
[  4] 1.00-2.00 sec   136 MBytes  1.14 Gbits/sec  24780    946:3227:2227:790:2427:1888:641:12634
[  4] 2.00-3.00 sec   137 MBytes  1.15 Gbits/sec  24484    1047:2686:2218:810:2195:1819:728:12981
[  4] 3.00-4.00 sec   126 MBytes  1.06 Gbits/sec  20812    863:1353:1546:614:1712:1298:547:12879
[  4] 4.00-5.00 sec   117 MBytes   984 Mbits/sec  20266    769:1886:1828:589:1866:1350:476:11502
[  4] 5.00-6.00 sec   143 MBytes  1.20 Gbits/sec  24603    1066:1925:2139:822:2237:1827:744:13843
[  4] 6.00-7.00 sec   126 MBytes  1.06 Gbits/sec  22635    834:2464:2249:724:2269:1646:608:11841
[  4] 7.00-8.00 sec   110 MBytes   921 Mbits/sec  21107    842:2437:2747:592:2871:1903:496:9219
[  4] 8.00-9.00 sec   126 MBytes  1.06 Gbits/sec  22804    1038:1784:2639:656:2738:1927:573:11449
[  4] 9.00-10.00 sec   133 MBytes  1.11 Gbits/sec  23091    1088:1654:2105:710:2333:1928:723:12550
[  4] 0.00-10.02 sec  1.25 GBytes  1.07 Gbits/sec  227306    9316:22088:21792:7096:22893:17193:6138:120790

where (per -e,)

Reads Total number of socket reads
Dist(bin=size) Eight bin histogram of the socket reads returned byte count. Bin width is set per size. Bins are separated by a colon. In the example, the bins are 0-1K, 1K-2K, .., 7K-8K.

TCP tests (server with --trip-times on client) iperf -s -i 1 -w 4M
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 8.00 MByte (WARNING: requested 4.00 MByte)
------------------------------------------------------------
[  4] local 192.168.1.4%eth0 port 5001 connected with 192.168.1.7 port 44798 (trip-times) (MSS=1448) (peer 2.0.14-alpha)
[ ID] Interval        Transfer    Bandwidth    Burst Latency avg/min/max/stdev (cnt/size) inP NetPwr  Reads=Dist
[  4] 0.00-1.00 sec  19.0 MBytes   159 Mbits/sec  52.314/10.238/117.155/19.779 ms (151/131717) 1.05 MByte 380.19  781=306:253:129:48:18:15:8:4
[  4] 1.00-2.00 sec  20.0 MBytes   168 Mbits/sec  53.863/21.264/79.252/12.277 ms (160/131080) 1.08 MByte 389.38  771=294:236:126:60:18:24:10:3
[  4] 2.00-3.00 sec  18.2 MBytes   153 Mbits/sec  58.718/22.000/137.944/20.397 ms (146/130964) 1.06 MByte 325.64  732=299:231:98:52:18:19:10:5
[  4] 3.00-4.00 sec  19.7 MBytes   165 Mbits/sec  50.448/ 8.921/82.728/14.627 ms (158/130588)  997 KByte 409.00  780=300:255:121:58:15:18:7:6
[  4] 4.00-5.00 sec  18.8 MBytes   158 Mbits/sec  53.826/11.169/115.316/15.541 ms (150/131420) 1.02 MByte 366.24  761=302:226:134:52:22:17:7:1
[  4] 5.00-6.00 sec  19.5 MBytes   164 Mbits/sec  50.943/11.922/76.134/14.053 ms (156/131276) 1.03 MByte 402.00  759=273:246:149:45:16:18:4:8
[  4] 6.00-7.00 sec  18.5 MBytes   155 Mbits/sec  57.643/10.039/127.850/18.950 ms (148/130926) 1.05 MByte 336.16  710=262:228:133:37:16:20:8:6
[  4] 7.00-8.00 sec  19.6 MBytes   165 Mbits/sec  52.498/12.900/77.045/12.979 ms (157/131003) 1.00 MByte 391.78  742=288:200:135:68:16:23:4:8
[  4] 8.00-9.00 sec  18.0 MBytes   151 Mbits/sec  58.370/ 8.026/150.243/21.445 ms (144/131255) 1.06 MByte 323.81  716=268:241:108:51:20:17:8:3
[  4] 9.00-10.00 sec  18.4 MBytes   154 Mbits/sec  56.112/12.419/79.790/13.668 ms (147/131194) 1.05 MByte 343.70  822=330:303:120:26:16:14:9:4
[  4] 10.00-10.06 sec  1.03 MBytes   146 Mbits/sec  69.880/45.175/78.754/10.823 ms (9/119632) 1.74 MByte 260.40  62=26:30:5:1:0:0:0:0
[  4] 0.00-10.06 sec   191 MBytes   159 Mbits/sec  54.183/ 8.026/150.243/16.781 ms (1526/131072) 1.03 MByte 366.98  7636=2948:2449:1258:498:175:185:75:48

where (per -e,)

Burst Latency One way TCP write() to read() latency in mean/minimum/maximum/standard deviation format (Note: requires the client's and server's system clocks to be synchronized to a common reference, e.g. using precision time protocol PTP. A GPS disciplined OCXO is a recommended reference.)
cnt Number of completed bursts received and used for the burst latency calculations
size Average burst size in bytes (computed average and estimate only)
inP inP, short for in progress, is the average number of bytes in progress or in flight. This is taken from the application level write to read perspective. Note this is a mean value. The parenthesis value is the standard deviation from the mean.  (Requires --trip-times on client. See Little's law in Notes.)
NetPwr Network power defined as (throughput / one way latency)

TCP tests (with one way delay sync check -X and --trip-times on the client)

iperf -c 192.168.1.4 -X -e --trip-times -i 1 -t 2
------------------------------------------------------------
Client connecting to 192.168.1.4, TCP port 5001 with pid 16762 (1 flows)
Write buffer size: 131072 Byte
TCP window size: 85.0 KByte (default)
------------------------------------------------------------
[ 1] Clock sync check (ms): RTT/Half=(3.361/1.680) OWD-send/ack/asym=(2.246/1.115/1.131)
[  1] local 192.168.1.1%ap0 port 47466 connected with 192.168.1.4 port 5001 (MSS=1448) (trip-times) (sock=3) (peer 2.1.4-master)
[ ID] Interval        Transfer    Bandwidth       Write/Err  Rtry     Cwnd/RTT        NetPwr
[  1] 0.00-1.00 sec  9.50 MBytes  79.7 Mbits/sec  77/0          0     2309K/113914 us  87
[  1] 1.00-2.00 sec  7.12 MBytes  59.8 Mbits/sec  57/0          0     2492K/126113 us  59
[  1] 2.00-2.42 sec   128 KBytes  2.47 Mbits/sec  2/0          0     2492K/126113 us  2
[  1] 0.00-2.42 sec  16.8 MBytes  58.0 Mbits/sec  136/0          0     2492K/126113 us  57

UDP tests (client)

iperf -c <host> -e -i 1 -u -b 10m
------------------------------------------------------------
Client connecting to <host>, UDP port 5001 with pid 5169
Sending 1470 byte datagrams, IPG target: 1176.00 us (kalman adjust)
UDP buffer size:  208 KByte (default)
------------------------------------------------------------
[  3] local 45.56.85.133 port 32943 connected with 45.33.58.123 port 5001
[ ID] Interval        Transfer     Bandwidth      Write/Err  PPS
[  3] 0.00-1.00 sec  1.19 MBytes  10.0 Mbits/sec  852/0      851 pps
[  3] 1.00-2.00 sec  1.19 MBytes  10.0 Mbits/sec  850/0      850 pps
[  3] 2.00-3.00 sec  1.19 MBytes  10.0 Mbits/sec  850/0      850 pps
[  3] 3.00-4.00 sec  1.19 MBytes  10.0 Mbits/sec  851/0      850 pps
[  3] 4.00-5.00 sec  1.19 MBytes  10.0 Mbits/sec  850/0      850 pps
[  3] 5.00-6.00 sec  1.19 MBytes  10.0 Mbits/sec  850/0      850 pps
[  3] 6.00-7.00 sec  1.19 MBytes  10.0 Mbits/sec  851/0      850 pps
[  3] 7.00-8.00 sec  1.19 MBytes  10.0 Mbits/sec  850/0      850 pps
[  3] 8.00-9.00 sec  1.19 MBytes  10.0 Mbits/sec  851/0      850 pps
[  3] 0.00-10.00 sec  11.9 MBytes  10.0 Mbits/sec  8504/0      850 pps
[  3] Sent 8504 datagrams
[  3] Server Report:
[  3] 0.00-10.00 sec  11.9 MBytes  10.0 Mbits/sec   0.047 ms    0/ 8504 (0%)  0.537/ 0.392/23.657/ 0.497 ms  850 pps  2329.37

where (per -e,)

Write/Err Total number of successful socket writes. Total number of non-fatal socket write errors
PPS Transmit packet rate in packets per second

UDP tests (server) iperf -s -i 1 -w 4M -u
------------------------------------------------------------
Server listening on UDP port 5001
Receiving 1470 byte datagrams
UDP buffer size: 8.00 MByte (WARNING: requested 4.00 MByte)
------------------------------------------------------------
[  3] local 192.168.1.4 port 5001 connected with 192.168.1.1 port 60027 (WARN: winsize=8.00 MByte req=4.00 MByte) (trip-times) (0.0) (peer 2.0.14-alpha)
[ ID] Interval        Transfer     Bandwidth        Jitter   Lost/Total  Latency avg/min/max/stdev PPS  inP NetPwr
[  3] 0.00-1.00 sec  44.5 MBytes   373 Mbits/sec   0.071 ms 52198/83938 (62%) 75.185/ 2.367/85.189/14.430 ms 31854 pps 3.64 MByte 620.58
[  3] 1.00-2.00 sec  44.8 MBytes   376 Mbits/sec   0.015 ms 59549/143701 (41%) 79.609/75.603/85.757/ 1.454 ms 31954 pps 3.56 MByte 590.04
[  3] 2.00-3.00 sec  44.5 MBytes   373 Mbits/sec   0.017 ms 59494/202975 (29%) 80.006/75.951/88.198/ 1.638 ms 31733 pps 3.56 MByte 583.07
[  3] 3.00-4.00 sec  44.5 MBytes   373 Mbits/sec   0.019 ms 59586/262562 (23%) 79.939/75.667/83.857/ 1.145 ms 31767 pps 3.56 MByte 583.57
[  3] 4.00-5.00 sec  44.5 MBytes   373 Mbits/sec   0.081 ms 59612/322196 (19%) 79.882/75.400/86.618/ 1.666 ms 31755 pps 3.55 MByte 584.40
[  3] 5.00-6.00 sec  44.7 MBytes   375 Mbits/sec   0.064 ms 59571/381918 (16%) 79.767/75.571/85.339/ 1.556 ms 31879 pps 3.56 MByte 588.02
[  3] 6.00-7.00 sec  44.6 MBytes   374 Mbits/sec   0.041 ms 58990/440820 (13%) 79.722/75.662/85.938/ 1.087 ms 31820 pps 3.58 MByte 586.73
[  3] 7.00-8.00 sec  44.7 MBytes   375 Mbits/sec   0.027 ms 59679/500548 (12%) 79.745/75.704/84.731/ 1.094 ms 31869 pps 3.55 MByte 587.46
[  3] 8.00-9.00 sec  44.3 MBytes   371 Mbits/sec   0.078 ms 59230/559499 (11%) 80.346/75.514/94.293/ 2.858 ms 31590 pps 3.58 MByte 577.97
[  3] 9.00-10.00 sec  44.4 MBytes   373 Mbits/sec   0.073 ms 58782/618394 (9.5%) 79.125/75.511/93.638/ 1.643 ms 31702 pps 3.55 MByte 588.99
[  3] 10.00-10.08 sec  3.53 MBytes   367 Mbits/sec   0.129 ms 6026/595236 (1%) 94.967/80.709/99.685/ 3.560 ms 31107 pps 3.58 MByte 483.12
[  3] 0.00-10.08 sec   449 MBytes   374 Mbits/sec   0.129 ms 592717/913046 (65%) 79.453/ 2.367/99.685/ 5.200 ms 31776 pps (null) 587.91

where (per -e,)

Latency End to end latency in mean/minimum/maximum/standard deviation format (Note: requires the client's and server's system clocks to be synchronized to a common reference, e.g. using precision time protocol PTP. A GPS disciplined OCXO is a recommended reference.)
PPS Received packet rate in packets per second
inP inP, short for in progress, is the average number of bytes in progress or in flight. This is taken from an application write to read perspective. (Requires --trip-times on client. See Little's law in Notes.)
NetPwr Network power defined as (throughput / latency)

Isochronous UDP tests (client)

iperf -c 192.168.100.33 -u -e -i 1 --isochronous=60:100m,10m --realtime
------------------------------------------------------------
Client connecting to 192.168.100.33, UDP port 5001 with pid 14971
UDP isochronous: 60 frames/sec mean= 100 Mbit/s, stddev=10.0 Mbit/s, Period/IPG=16.67/0.005 ms
UDP buffer size:  208 KByte (default)
------------------------------------------------------------
[  3] local 192.168.100.76 port 42928 connected with 192.168.100.33 port 5001
[ ID] Interval        Transfer     Bandwidth      Write/Err  PPS  frames:tx/missed/slips
[  3] 0.00-1.00 sec  12.0 MBytes   101 Mbits/sec  8615/0     8493 pps   62/0/0
[  3] 1.00-2.00 sec  12.0 MBytes   100 Mbits/sec  8556/0     8557 pps   60/0/0
[  3] 2.00-3.00 sec  12.0 MBytes   101 Mbits/sec  8586/0     8586 pps   60/0/0
[  3] 3.00-4.00 sec  12.1 MBytes   102 Mbits/sec  8687/0     8687 pps   60/0/0
[  3] 4.00-5.00 sec  11.8 MBytes  99.2 Mbits/sec  8468/0     8468 pps   60/0/0
[  3] 5.00-6.00 sec  11.9 MBytes  99.8 Mbits/sec  8519/0     8520 pps   60/0/0
[  3] 6.00-7.00 sec  12.1 MBytes   102 Mbits/sec  8694/0     8694 pps   60/0/0
[  3] 7.00-8.00 sec  12.1 MBytes   102 Mbits/sec  8692/0     8692 pps   60/0/0
[  3] 8.00-9.00 sec  11.9 MBytes   100 Mbits/sec  8537/0     8537 pps   60/0/0
[  3] 9.00-10.00 sec  11.8 MBytes  99.0 Mbits/sec  8450/0     8450 pps   60/0/0
[  3] 0.00-10.01 sec   120 MBytes   100 Mbits/sec  85867/0     8574 pps  602/0/0
[  3] Sent 85867 datagrams
[  3] Server Report:
[  3] 0.00-9.98 sec   120 MBytes   101 Mbits/sec   0.009 ms  196/85867 (0.23%)  0.665/ 0.083/ 1.318/ 0.174 ms 8605 pps  18903.85

where (per -e,)

frames:tx/missed/slips Total number of isochronous frames or bursts. Total number of frame ids not sent. Total number of frame slips

Isochronous UDP tests (server)

iperf -s -e -u --udp-histogram=100u,2000 --realtime
------------------------------------------------------------
Server listening on UDP port 5001 with pid 5175
Receiving 1470 byte datagrams
UDP buffer size:  208 KByte (default)
------------------------------------------------------------
[  3] local 192.168.100.33 port 5001 connected with 192.168.100.76 port 42928 isoch (peer 2.0.13-alpha)
[ ID] Interval        Transfer     Bandwidth        Jitter   Lost/Total  Latency avg/min/max/stdev PPS  NetPwr  Frames/Lost
[  3] 0.00-9.98 sec   120 MBytes   101 Mbits/sec   0.010 ms  196/85867 (0.23%)  0.665/ 0.083/ 1.318/ 0.284 ms 8585 pps  18903.85  601/1
[  3] 0.00-9.98 sec T8(f)-PDF: bin(w=100us):cnt(85671)=1:2,2:844,3:10034,4:8493,5:8967,6:8733,7:8823,8:9023,9:8901,10:8816,11:7730,12:4563,13:741,14:1 (5.00/95.00%=3/12,Outliers=0,obl/obu=0/0)
[  3] 0.00-9.98 sec F8(f)-PDF: bin(w=100us):cnt(598)=15:2,16:1,17:27,18:68,19:125,20:136,21:103,22:83,23:22,24:23,25:5,26:3 (5.00/95.00%=17/24,Outliers=0,obl/obu=0/0)

where,

Frames/lost Total number of frames (or bursts) received. Total number of bursts lost or error-ed
T8-PDF(f) Latency histogram for packets
F8-PDF(f) Latency histogram for frames

Note:

The environment variable option settings haven't been maintained well. See the source code if these are of interest.

Numeric options: Some numeric options support format characters per '<value>c' (e.g. 10M) where the c format characters are k,m,g,K,M,G. Lowercase format characters are 10^3 based and uppercase are 2^n based, e.g. 1k = 1000, 1K = 1024, 1m = 1,000,000 and 1M = 1,048,576

Rate limiting: The -b option supports read and write rate limiting at the application level.  The -b option on the client also supports variable offered loads through the <mean>,<standard deviation> format, e.g. -b 100m,10m. The distribution used is log normal. Similar for the isochronous option. The -b on the server rate limits the reads. Socket based pacing is also supported using the --fq-rate long option. This will work with the --reverse and --full-duplex options as well.

IP tos and dscp: Specifies the type-of-service or DSCP class for connections.  Accepted values are af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5, cs6, cs7, ef, le, nqb, nqb2, ac_be, ac_bk, ac_vi, ac_vo, lowdelay, throughput, reliability, a numeric value, or none to use the operating system default. The ac_xx values are the four access categories defined in WMM for Wi-Fi, and they are aliases for DSCP values that will be mapped to the corresponding ACs under the assumption that the device uses the DSCP-to-UP mapping table specified in IETF RFC 8325.

One can set the tos byte using:
--dscp on the client
--tos or -S on the client or server though the server side setting has caveats (per next item)
--tos-override for server side reversed traffic (use case is for bleaching the return path)

The --dscp value on the client is merely a convenience in that the user can set the dscp field's six bits vs set the tos byte. The --tos-override allows one to test the reflected TOS feature of APs. This is when the network bleaches the client side tos setting and the reverse traffic will then have no TOS setting. The AP is expected to apply the upstream traffic's TOS to the reverse traffic. The use case is that the --tos-overide would by zero simulating network bleaching of the tos byte. If there is no --tos setting then the setsockopt will never be called and the default values are used.

--trip-times The --trip-times option enables many one way delay (OWD) metrics. Also note that using --trip-times on a TCP client will cause --tcp-write-prefetch to be set to a small value if tcp-write-prefetch hasn't hasn't also been set. This is done to reduce send side bloat latency (which is unrelated to network induced latency.) Set --tcp-write-prefetch to zero to disable this (which will disable TCP_NOTSENT_LOWAT) and will allow for send side bloat.

Synchronized clocks: The --trip-times option indicates that the client's and server's clocks are synchronized to a common reference. Network Time Protocol (NTP) or Precision Time Protocol (PTP) are commonly used for this. The reference clock(s) error and the synchronization protocols will affect the accuracy of any end to end latency measurements. See bounceback NOTES section on clock unsynchronized detections

Histograms and non-parametric statistics: The --histograms option provides the raw data where nothing is averaged. This is useful for non-parametric distributions, e.g. latency. The standard output does use the central limit theorem to produce average, minimum, maximum and variation. This loses information when the underlining distribution is not Gaussian. Histograms are supported so this information is made available.

Histogram output interpretation: Below is an example bounceback histogram and how to interpret it

[  1] 0.00-5.10 sec BB8-PDF:
bin(w=100us):cnt(50)=35:1,37:1,39:1,40:3,41:4,42:1,43:1,52:1,57:1,65:1,68:1,69:1,70:1,72:2,74:1,75:5,78:1,79:2,80:4,81:3,82:1,83:1,88:2,90:2,92:1,94:1,117:1,126:1,369:1,1000:1,1922:1,3710:1 (5.00/95.00/99.7%=39/1000/3710,Outliers=4,obl/obu=0/0)

where,

[ 1] The traffic thread number
0.00-5.10 sec The time interval of the histogram
BB8-PDF BB8 is the histogram name and the PDF indicates a histogram raw output
bin(w=100us) provides the bin width. The bin width of this histogram is 100 microseconds
cnt(50) provides the total number of samples in the histogram. There are 50 samples in this histogram
35:1 provides the bin no then the number of samples in that bin. Bin 35 with bin width 100us is 3.4 ms - 3.5 ms and there was one sample that landed there
5.00/95.00/99.7%=39/1000/3710 provides the bin confidence intervals (per the integrated cumulative distribution function.) 5% landed in 3.9 ms or better (recall bin number multiplies by bin width.) 95% landed in 10 ms or better. 99.7% or 3 standards of deviation landed in 37.1 ms or better
Outliers=4 provides the outlier count, similar to 3IQR (3 times the inter quartile range) but uses 10% and 90% for inner & outer fence post, then 3 times that for outlier detection.
obl/obu=0/0 out of bounds lower and out of bands upper, provides the number of samples that could not be binned because the value landed outside of all possible bins

Binding is done at the logical level of port and ip address (or layer 3) using the -B option and a colon as the separator between port and the ip addr. Binding at the device (or layer 2) level requires the percent (%) as the delimiter (for both the client and the server.) An example for src port and ip address is -B 192.168.1.1:6001. To bind the src port only and let the operating system choose the source ip address use 0.0.0.0, e.g. -B 0.0.0.0:6001. On the client, the -B option affects the bind(2) system call, and will set the source ip address and the source port, e.g. iperf -c <host> -B 192.168.100.2:6002. This controls the packet's source values but not routing. These can be confusing in that a route or device lookup may not be that of the device with the configured source IP. So, for example, if the IP address of eth0 is used for -B and the routing table for the destination IP address resolves the output interface to be eth1, then the host will send the packet out device eth1 while using the source IP address of eth0 in the packet. To affect the physical output interface (e.g. dual homed systems) either use -c <host>%<dev> (requires root) which bypasses this host route table lookup, or configure policy routing per each -B source address and set the output interface appropriately in the policy routes. On the server or receive, only packets destined to -B IP address will be received. It's also useful for multicast. For example, iperf -s -B 224.0.0.1%eth0 will only accept ip multicast packets with dest ip 224.0.0.1 that are received on the eth0 interface, while iperf -s -B 224.0.0.1 will receive those packets on any interface, Finally, the device specifier is required for v6 link-local, e.g. -c [v6addr]%<dev> -V, to select the output interface.

Reverse, full-duplex, dualtest (-d) and tradeoff (-r): The --reverse (-R) and --full-duplex options can be confusing when compared to the older options of --dualtest (-d) and --tradeoff (-r). The newer options of --reverse and --full-duplex only open one socket and read and write to the same socket descriptor, i.e. use the socket in full duplex mode.  The older -d and -r open second sockets in the opposite direction and do not use a socket in full duplex mode. Note that full duplex applies to the socket and not to the network devices and that full duplex sockets are supported by the operating systems regardless if an underlying network supports full duplex transmission and reception. It's suggested to use --reverse if you want to test through a NAT firewall (or -R on non-windows systems). This applies role reversal of the test after opening the full duplex socket.  (Note: Firewall piercing may be required to use -d and -r if a NAT gateway is in the path.)

Also, the --reverse -b <rate> setting behaves differently for TCP and UDP. For TCP it will rate limit the read side, i.e. the iperf client (role reversed to act as a server) reading from the full duplex socket. This will in turn flow control the reverse traffic per standard TCP congestion control. The --reverse -b <rate> will be applied on transmit (i.e. the server role reversed to act as a client) for UDP since there is no flow control with UDP. There is no option to directly rate limit the writes with TCP testing when using --reverse.

Bounceback The bounceback test allows one to measure network responsiveness (which, in this test, is an inverse of latency.) The units are responses per second or rps. Latency is merely delay in units of time. Latency metrics require one to know the delay of what's being measured. For bounceback it's a client write to a server read followed by a server write and then the client read. The original write is bounce backed. Iperf 2 sets up the socket with TCP_NODELAY and possibly TCP_QUICKACK (unless disabled). The client sends a small write (which defaults to 100 bytes unless -l is set) and issues a read waiting for the "bounceback" from the server. The server waits for a read and then optionally delays before sending the payload back. This repeats until the traffic ends. Results are shown in units of rps and time delays.

The TCP_QUICKACK socket option will be enabled during bounceback tests when the bounceback-hold is set to a non-zero value. The socket option is applied after every read() on the server and before the hold delay call. It's also applied on the client. Use --bounceback-no-quickack to have TCP run in default mode per the socket (which is most likely TCP_QUICKACK being off.)

Unsynchronized clock detections with --bounceback and --trip-times (as of March 19, 2023): Iperf 2 can detect when the clocks have synchronization errors larger than the bounceback RTT. This is done via the client's send timestamp (clock A), the server's recieve timestamp (clock B) and the client's final receive timestamp (clock A.) The check, done on each bounceback, is write(A) < read(B) < read(A). This is supported in bounceback tests  with a slight adjustment: clock write(A) < clock read(B) < clock read(A) - (clock write(B) - clock read(B)). All the timestamps are sampled on the initial write or read (not the completion of.) Error output looks as shown below and there is no output for a zero value.

[  1] 0.00-10.00 sec  Clock sync error count = 100

TCP Connect times: The TCP connect time (or three way handshake) can be seen on the iperf client when the -e (--enhanced) option is set. Look for the ct=<value> in the connected message, e.g.in '[ 3] local 192.168.1.4 port 48736 connected with 192.168.1.1 port 5001 (ct=1.84 ms)' shows the 3WHS took 1.84 milliseconds.

Port-range Port ranges are supported using the hyphen notation, e.g. 6001-6009. This will cause multiple threads, one per port, on either the listener/server or the client. The user needs to take care that the ports in the port range are available and not already in use per the operating system. The -P is supported on the client and will apply to each destination port within the port range. Finally, this can be used for a workaround for Windows UDP and -P > 1 as Windows doesn't dispatch UDP per a server's connect and the quintuple.

Packet per second (pps) calculation The packets per second calculation is done as a derivative, i.e. number of packets divided by time. The time is taken from the previous last packet to the current last packet. It is not the sample interval time. The last packet can land at different times within an interval. This means that pps does not have to match rx bytes divided by the sample interval. Also, with --trip-times set, the packet time on receive is set by the sender's write time so pps indicates the end to end pps with --trip-times. The RX pps calculation is receive side only when -e is set and --trip-times is not set.

Little's Law in queuing theory is a theorem that determines the average number of items (L) in a stationary queuing system based on the average waiting time (W) of an item within a system and the average number of items arriving at the system per unit of time (lambda). Mathematically, it's L = lambda * W. As used here, the units are bytes. The arrival rate is taken from the writes.

Network power: The network power (NetPwr) metric is experimental. It's a convenience function defined as throughput/delay. For TCP transmits, the delay is the sampled RTT times. For TCP receives, the delay is the write to read latency. For UDP the delay is the end/end latency. Don't confuse this with the physics definition of power (delta energy/delta time) but more of a measure of a desirable property divided by an undesirable property. Also note, one must use -i interval with TCP to get this as that's what sets the RTT sampling rate. The metric is scaled to assist with human readability.

Multicast: Iperf 2 supports multicast with a couple of caveats. First, multicast streams cannot take advantage of the -P option. The server will serialize multicast streams. Also, it's highly encouraged to use a -t on a server that will be used for multicast clients. That is because the single end of traffic packet sent from client to server may get lost and there are no redundant end of traffic packets.  Setting -t on the server will kill the server thread in the event this packet is indeed lost.

TCP_QUICKACK: The TCP_QUICKACK socket option will be applied after every read() on the server such that TCP acks are sent immediately, rather than possibly delayed.

TCP_TX_DELAY (--tcp-tx-delay): Iperf 2 flows can set different delays, simulating real world conditions. Units is microseconds. This requires FQ packet scheduler or a EDT-enabled NIC. Note that FQ packet scheduler limits might need some tweaking
 man tc-fq
   PARAMETERS
   limit
       Hard  limit  on  the  real  queue  size. When this limit is
       reached, new packets are dropped. If the value is  lowered,
       packets  are  dropped so that the new limit is met. Default
       is 10000 packets.

    flow_limit
       Hard limit on the maximum  number  of  packets  queued  per
       flow.  Default value is 100.

Use of TCP_TX_DELAY option will increase number of skbs in FQ qdisc, so packets would be dropped if any of the previous limit is hit. Using big delays might very well trigger old bugs in TSO auto defer logic and/or sndbuf limited detection.

Fast Sampling: Use ./configure --enable-fastsampling and then compile from source to enable four digit (e.g. 1.0000) precision in reports' timestamps. Useful for sub-millisecond sampling.

Use ./configure --enable-thread-debug and then compile from source to enable both asserts and advanced debugging of the tool itself.

See https://sourceforge.net/p/iperf2/tickets/

Iperf2, based from iperf (originally written by Mark Gates and Alex Warshavsky), has a goal of maintenance with some feature enhancement. Other contributions from Ajay Tirumala, Jim Ferguson, Jon Dugan <jdugan at x1024 dot net>, Feng Qin, Kevin Gibbs, John Estabrook <jestabro at ncsa.uiuc.edu>, Andrew Gallatin <gallatin at gmail.com>, Stephen Hemminger <shemminger at linux-foundation.org>, Tim Auckland <tim.auckland at gmail.com>, Robert J. McMahon <rjmcmahon at rjmcmahon.com>

accept(2),bind(2),close(2),connect(2),fcntl(2),getpeername(2),getsockname(2),getsockopt(2),listen(2),read(2),recv(2),select(2),send(2),setsockopt(2),shutdown(2),write(2),ip(7),socket(7),tcp(7),udp(7)

Source code at http://sourceforge.net/projects/iperf2/

"Unix Network Programming, Volume 1: The Sockets Networking API (3rd Edition) 3rd Edition" by W. Richard Stevens (Author), Bill Fenner (Author), Andrew M. Rudoff (Author)

smcrouted(8).