IRTT(1)

NAME
irtt - Isochronous Round-Trip Time

SYNOPSIS
irtt command [args]

irtt help command

DESCRIPTION
IRTT measures round-trip time and other latency related metrics using UDP packets sent on a fixed period, and produces both text and JSON output.

COMMANDS

client
    runs the client

server
    runs the server

report
    emits results from a JSON file

bench
    runs HMAC and fill benchmarks

timer
    runs timer resolution test

clock
    runs wall vs monotonic clock test

sleep
    runs sleep accuracy test

version
    shows the version

EXAMPLES
After installing IRTT, start a server:

        $ irtt server
        IRTT server starting...
        [ListenerStart] starting IPv6 listener on [::]:2112
        [ListenerStart] starting IPv4 listener on 0.0.0.0:2112

While that’s running, run a client. If no options are supplied, it will send a request once per second, like ping. Here we simulate a one minute G.711 VoIP conversation by using an interval of 20ms and randomly filled payloads of 172 bytes:

        $ irtt client -i 20ms -l 172 -d 1m --fill=rand --sfill=rand -q 192.168.100.10
        [Connecting] connecting to 192.168.100.10
        [Connected] connected to 192.168.100.10:2112
                                 Min     Mean   Median      Max  Stddev
                                 ---     ----   ------      ---  ------
                        RTT  11.93ms  20.88ms   19.2ms  80.49ms  7.02ms
                 send delay   4.99ms  12.21ms  10.83ms  50.45ms  5.73ms
              receive delay   6.38ms   8.66ms   7.86ms  69.11ms  2.89ms

              IPDV (jitter)    782ns   4.53ms   3.39ms  64.66ms   4.2ms
                  send IPDV    256ns   3.99ms   2.98ms  35.28ms  3.69ms
               receive IPDV    896ns   1.78ms    966µs  62.28ms  2.86ms

             send call time   56.5µs   82.8µs           18.99ms   348µs
                timer error       0s   21.7µs           19.05ms   356µs
          server proc. time   23.9µs   26.9µs             141µs  11.2µs
                        duration: 1m0s (wait 241.5ms)
           packets sent/received: 2996/2979 (0.57% loss)
         server packets received: 2980/2996 (0.53%/0.03% loss up/down)
             bytes sent/received: 515312/512388
               send/receive rate: 68.7 Kbps / 68.4 Kbps
                   packet length: 172 bytes
                     timer stats: 4/3000 (0.13%) missed, 0.11% error

In the results above, the client and server are located at two different sites, around 50km from one another, each of which connects to the Internet via point-to-point WiFi. The client is 3km NLOS through trees located near its transmitter, which is likely the reason for the higher upstream packet loss, mean send delay and IPDV.

BUGS

•   Windows is unable to set DSCP values for IPv6.

•   Windows is unable to set the source IP address, so --set-src-ip may not be used on the server.

LIMITATIONS

    “It is the limitations of software that give it life.”

            -Me, justifying my limitations

  Isochronous (fixed period) send schedule
Currently, IRTT only sends packets on a fixed period, foregoing the ability to simulate arbitrary traffic. Accepting this limitation offers some benefits:

•   It’s easy to implement

•   It’s easy to calculate how many packets and how much data will be sent in a given time

•   It simplifies timer error compensation

Also, isochronous packets are commonly seen in VoIP, games and some streaming media, so it already simulates an array of common types of traffic.

  Fixed packet lengths for a given test
Packet lengths are fixed for the duration of the test. While this may not be an accurate simulation of some types of traffic, it means that IPDV measurements are accurate, where they wouldn’t be in any other case.

  Stateful protocol
There are numerous benefits to stateless protocols, particularly for developers and data centers, including simplified server design, horizontal scalabity, and easily implemented zero-downtime restarts. However, in this case, a stateful protocol provides important benefits to the user, including:

•   Smaller packet sizes (a design goal) as context does not need to be included in every request

•   More accurate measurement of upstream vs downstream packet loss (this gets worse in a stateless protocol as RTT approaches the test duration, complicating interplanetary tests!)

•   More accurate rate and test duration limiting on the server

  In-memory results storage
Results for each round-trip are stored in memory as the test is being run. Each result takes 77 bytes in memory (maybe 80 with alignment). When in streaming mode, this isn’t a problem as a circular buffer is used for round-trip data. But when in standard mode (needed for statistics and json output), this limits the effective duration of the test, especially at very low send intervals. However, the advantages are:

•   It’s easier to perform statistical analysis (like calculation of the median) on fixed arrays than on running data values

•   We don’t need to either send client timestamps to the server, or maintain a local running window of sent packet info, because they’re all in memory, no matter when server replies come back

•   Not accessing the disk during the test to write test output prevents inadvertently affecting the results

•   It simplifies the API

Before streaming mode was implemented, a decision was made to use 32-bit unsigned ints for sequence numbers, because in-memory results storage meant it would be impractical to use up the sequence number space. With streaming mode, it’s more likely that the sequence number space could wrap around. At a 10ms interval, the counter will wrap around in 497 days with undefined results. Keep that in mind if doing very long, low interval tests.

  64-bit received window
In order to determine per-packet differentiation between upstream and downstream loss, a 64-bit “received window” may be returned with each packet that contains the receipt status of the previous 64 packets. This can be enabled using --stats=window/both with the irtt client. Its limited width and simple bitmap format lead to some caveats:

•   Per-packet differentiation is not available (for any intervening packets) if greater than 64 packets are lost in succession. These packets will be marked with the generic Lost.

•   While any packet marked LostDown is guaranteed to be marked properly, there is no confirmation of receipt of the receive window from the client to the server, so packets may sometimes be erroneously marked LostUp, for example, if they arrive late to the server and slide out of the received window before they can be confirmed to the client, or if the received window is lost on its way to the client and not amended by a later packet’s received window.

There are many ways that this simple approach could be improved, such as by:

•   Allowing a wider window

•   Encoding receipt seqnos in a more intelligent way to allow a wider seqno range

•   Sending confirmation of window receipt from the client to the server and re-sending unreceived windows

However, the current strategy means that a good approximation of per-packet loss results can be obtained with only 8 additional bytes in each packet. It also requires very little computational time on the server, and almost all computation on the client occurs during results generation, after the test is complete. It isn’t as accurate with late (out-of-order) upstream packets or with long sequences of lost packets, but high loss or high numbers of late packets typically indicate more severe network conditions that should be corrected first anyway, perhaps before per-packet results matter. Note that in case of very high packet loss, the total number of packets received by the server but not returned to the client (which can be obtained using --stats=count) will still be correct, which will still provide an accurate average loss percentage in each direction over the course of the test.

  Use of Go
IRTT is written in Go. That carries with it:

•   Non-negligible system call overhead

•   A larger executable size than with C

•   Somewhat slower execution speed than C (although not that much slower (https://benchmarksgame.alioth.debian.org/u64q/compare.php?lang=go&lang2=gcc) )

However, Go also has characteristics that make it a good fit for this application:

•   Go’s target is network and server applications, with a focus on simplicity, reliability and efficiency, which is appropriate for IRTT

•   Memory footprint tends to be significantly lower than with some interpreted languages

•   It’s easy to support a broad array of hardware and OS combinations

SEE ALSO
irtt-client(1), irtt-server(1)

IRTT GitHub repository (https://github.com/heistp/irtt/)

AUTHOR
Pete Heist pete@heistp.net

Many thanks to both Toke Høiland-Jørgensen and Dave Täht from the Bufferbloat project (https://www.bufferbloat.net/) for their valuable advice. Any problems in design or implementation are entirely my own.

HISTORY
IRTT was originally written to improve the latency and packet loss measurements for the excellent Flent (https://flent.org) tool. Flent was developed by and for the Bufferbloat (https://www.bufferbloat.net/projects/) project, which aims to reduce “chaotic and laggy network performance,” making this project valuable to anyone who values their time and sanity while using the Internet.