Missing or incorrect rtt, rttvar, pmtu, and snd_cwnd values during reverse mode TCP tests

[ejv]

Context

When running TCP tests in reverse mode (--reverse), the following values are not reported at the specified report interval and also return a value of 0 on the very last report:

• pmtu
• rtt
• rttvar
• snd_cwnd

Note: Running in the default standard mode (not reverse mode) seems to work perfectly fine and is not affected.

Command:

iperf3 --client <host> --interval 5 --time 15 --parallel 5 --reverse --json

Version of iperf3:

3.6

Note: The listener (on the remote side) is also running an identical operating system and build of iperf3 3.6.

Hardware:

Virtual machine

Operating system (and distribution, if any):

Ubuntu 18.04.1 LTS

Expected Behavior

Note: Per the command provided above, the test runs for a total of 15 seconds, reporting at 5 second intervals, therefore yielding 3 distinct reports.

In the default standard mode ("forward") the rtt,rttvar,pmtu, and snd_cwnd values are correctly reported. In reverse mode these values are either missing or returning an incorrect value of 0.

This is demonstrated below.

// Default mode ("forward")
// Report 1 (5 seconds)
// Note: rtt, rttvar, pmtu, and snd_cwnd are correctly provided.
                                }, {
                                        "socket":       8,
                                        "start":        0,
                                        "end":  5.00017,
                                        "seconds":      5.0001702308654785,
                                        "bytes":        10109056,
                                        "bits_per_second":      16173938.939275235,
                                        "retransmits":  0,
                                        "snd_cwnd":     394240,
                                        "rtt":  93604,
                                        "rttvar":       78,
                                        "pmtu": 1460,
                                        "omitted":      false
                                }, {

// Default mode ("forward")
// Report 2 (10 seconds)
// Note: rtt, rttvar, pmtu, and snd_cwnd are correctly provided.
                                }, {
                                        "socket":       8,
                                        "start":        5.00017,
                                        "end":  10.000167,
                                        "seconds":      4.9999971389770508,
                                        "bytes":        9650432,
                                        "bits_per_second":      15440700.03523943,
                                        "retransmits":  0,
                                        "snd_cwnd":     394240,
                                        "rtt":  93594,
                                        "rttvar":       78,
                                        "pmtu": 1460,
                                        "omitted":      false
                                }, {

// Default mode ("forward")
// Report 3 (15 seconds, final report)
// Note: rtt, rttvar, pmtu, and snd_cwnd are correctly provided.
                                        "omitted":      false
                                }, {
                                        "socket":       8,
                                        "start":        10.000167,
                                        "end":  15.000311,
                                        "seconds":      5.0001440048217773,
                                        "bytes":        9520896,
                                        "bits_per_second":      15232994.875057576,
                                        "retransmits":  0,
                                        "snd_cwnd":     394240,
                                        "rtt":  93994,
                                        "rttvar":       683,
                                        "pmtu": 1460,
                                        "omitted":      false
                                }, {

Actual Behavior

// Reverse mode
// Report 1 (5 seconds)
// Note: rtt, rttvar, pmtu, and snd_cwnd are missing in reverse mode.
                                }, {
                                        "socket":       8,
                                        "start":        0,
                                        "end":  5.000139,
                                        "seconds":      5.0001392364501953,
                                        "bytes":        9054848,
                                        "bits_per_second":      14487353.366468906,
                                        "omitted":      false
                                }, {

// Reverse mode
// Report 2 (10 seconds)
// Note: rtt, rttvar, pmtu, and snd_cwnd are missing in reverse mode.
                                }, {
                                        "socket":       8,
                                        "start":        5.000139,
                                        "end":  10.000138,
                                        "seconds":      4.9999990463256836,
                                        "bytes":        9534976,
                                        "bits_per_second":      15255964.509844305,
                                        "omitted":      false
                                }, {

// Reverse mode
// Report 3 (15 seconds, final report)
// Note: rtt, rttvar, pmtu, and snd_cwnd are now visible, however, they return values of 0.
// Note: In reverse mode, the 0 values seem to always be provided on the very last report.
                                }, {
                                        "socket":       8,
                                        "start":        10.000138,
                                        "end":  15.000168,
                                        "seconds":      5.0000300407409668,
                                        "bytes":        9527936,
                                        "bits_per_second":      15244606.008147953,
                                        "retransmits":  0,
                                        "snd_cwnd":     0,
                                        "rtt":  0,
                                        "rttvar":       0,
                                        "pmtu": 0,
                                        "omitted":      false
                                }, {

Steps to Reproduce

• Clone iperf repo.
• Build (./configure; make; make install) on Ubuntu 18.04.1 LTS.
• Run: iperf3 --client <host> --interval 5 --time 15 --parallel 5 --reverse --json

I'm happy to provide further information if necessary. Thanks.

[ejv]

Bump. This remains unfixed in release version 3.7.

@bmah888 Anyway this can be prioritized? A great deal of important data (latency, variance, etc.) continues to be lost in reverse mode TCP tests.

Thank you for your efforts.

[bmah888]

What's happening here is that the retransmit, rtt, rtt_var, pmtu, and snd_cwnd are all gathered and reported on the sending side of the test, regardless of whether it's the client or the server that's doing the sending.

So if you run --reverse tests to a server that was invoked with the --json flag, you'll see that information in the server-side output. That's not necessarily workable.

So another approach is to run both the server and the client with --json, and also add --get-server-output to the client flags, you'll see the server data in a JSON structure on the client side called server_output_json. Admittedly this isn't the most convenient in terms of presenting the data, but that's probably the best we can do given that there is not an easy way to synchronize the interval reports on the client and server (so we could merge the data).