[Bloat] [Starlink] [Rpm] Researchers Seeking Probe Volunteers in USA

Dick Roy dickroy at alum.mit.edu
Thu Jan 12 16:57:32 EST 2023





Nothing earth-shaking :-)




From: Starlink [mailto:starlink-bounces at lists.bufferbloat.net] On Behalf Of
Robert McMahon via Starlink
Sent: Thursday, January 12, 2023 9:50 AM
To: Sebastian Moeller
Cc: Dave Taht via Starlink; mike.reynolds at netforecast.com; libreqos; David
P. Reed; Rpm; bloat
Subject: Re: [Starlink] [Rpm] Researchers Seeking Probe Volunteers in USA


Hi Sebastien,

You make a good point. What I did was issue a warning if the tool found it
was being CPU limited vs i/o limited. This indicates the i/o test likely is
inaccurate from an i/o perspective, and the results are suspect. It does
this crudely by comparing the cpu thread doing stats against the traffic
threads doing i/o, which thread is waiting on the others. There is no
attempt to assess the cpu load itself. So it's designed with a singular
purpose of making sure i/o threads only block on syscalls of write and read.

I probably should revisit this both in design and implementation. Thanks for
bringing it up and all input is truly appreciated. 


On Jan 12, 2023, at 12:14 AM, Sebastian Moeller <moeller0 at gmx.de> wrote:

Hi Bob,

 On Jan 11, 2023, at 21:09, rjmcmahon <rjmcmahon at rjmcmahon.com> wrote:


 Iperf 2 is designed to measure network i/o. Note: It doesn't have to move
large amounts of data. It can support data profiles that don't drive TCP's
CCA as an example.


 Two things I've been asked for and avoided:


 1) Integrate clock sync into iperf's test traffic

 [SM] This I understand, measurement conditions can be unsuited for tight
time synchronization...

 2) Measure and output CPU usages

 [SM] This one puzzles me, as far as I understand the only way to properly
diagnose network issues is to rule out other things like CPU overload that
can have symptoms similar to network issues. As an example, the cake qdisc
will if CPU cycles become tight first increases its internal queueing and
jitter (not consciously, it is just an observation that once cake does not
get access to the CPU as timely as it wants, queuing latency and variability
increases) and then later also shows reduced throughput, so similar things
that can happen along an e2e network path for completely different reasons,
e.g. lower level retransmissions or a variable rate link. So i would think
that checking the CPU load at least coarse would be within the scope of
network testing tools, no?



 I think both of these are outside the scope of a tool designed to test
network i/o over sockets, rather these should be developed & validated
independently of a network i/o tool.


 Clock error really isn't about amount/frequency of traffic but rather
getting a periodic high-quality reference. I tend to use GPS pulse per
second to lock the local system oscillator to. As David says, most every
modern handheld computer has the GPS chips to do this already. So to me it
seems more of a policy choice between data center operators and device mfgs
and less of a technical issue.



  Yall can call me crazy if you want.. but... see below [RWG]
 Hi Bib,
 On Jan 9, 2023, at 20:13, rjmcmahon via Starlink
<starlink at lists.bufferbloat.net> wrote:

 My biggest barrier is the lack of clock sync by the devices, i.e. very
limited support for PTP in data centers and in end devices. This limits the
ability to measure one way delays (OWD) and most assume that OWD is 1/2 and
RTT which typically is a mistake. We know this intuitively with airplane
flight times or even car commute times where the one way time is not 1/2 a
round trip time. Google maps & directions provide a time estimate for the
one way link. It doesn't compute a round trip and divide by two.

 For those that can get clock sync working, the iperf 2 --trip-times options
is useful.
  [SM] +1; and yet even with unsynchronized clocks one can try to measure
how latency changes under load and that can be done per direction. Sure this
is far inferior to real reliably measured OWDs, but if life/the internet
deals you lemons....
 [RWG] iperf2/iperf3, etc are already moving large amounts of data

 back and forth, for that matter any rate test, why not abuse some of

 that data and add the fundemental NTP clock sync data and

 bidirectionally pass each others concept of "current time".  IIRC (its

 been 25 years since I worked on NTP at this level) you *should* be

 able to get a fairly accurate clock delta between each end, and then

 use that info and time stamps in the data stream to compute OWD's.

 You need to put 4 time stamps in the packet, and with that you can

 compute "offset".


  enable the measurement of end to end write to read latencies (client and
server clocks must be synchronized)

 [RWG] --clock-skew

  enable the measurement of the wall clock difference between sender and
  [SM] Sweet!



 I have many kvetches about the new latency under load tests being

 designed and distributed over the past year. I am delighted! that they

 are happening, but most really need third party evaluation, and

 calibration, and a solid explanation of what network pathologies they

 do and don't cover. Also a RED team attitude towards them, as well as

 thinking hard about what you are not measuring (operations research).

 I actually rather love the new cloudflare speedtest, because it tests

 a single TCP connection, rather than dozens, and at the same time folk

 are complaining that it doesn't find the actual "speed!". yet... the

 test itself more closely emulates a user experience than speedtest.net

 does. I am personally pretty convinced that the fewer numbers of flows

 that a web page opens improves the likelihood of a good user

 experience, but lack data on it.

 To try to tackle the evaluation and calibration part, I've reached out

 to all the new test designers in the hope that we could get together

 and produce a report of what each new test is actually doing. I've

 tweeted, linked in, emailed, and spammed every measurement list I know

 of, and only to some response, please reach out to other test designer

 folks and have them join the rpm email list?

 My principal kvetches in the new tests so far are:

 0) None of the tests last long enough.

 Ideally there should be a mode where they at least run to "time of

 first loss", or periodically, just run longer than the

 industry-stupid^H^H^H^H^H^Hstandard 20 seconds. There be dragons

 there! It's really bad science to optimize the internet for 20

 seconds. It's like optimizing a car, to handle well, for just 20


 1) Not testing up + down + ping at the same time

 None of the new tests actually test the same thing that the infamous

 rrul test does - all the others still test up, then down, and ping. It

 was/remains my hope that the simpler parts of the flent test suite -

 such as the tcp_up_squarewave tests, the rrul test, and the rtt_fair

 tests would provide calibration to the test designers.

 we've got zillions of flent results in the archive published here:


 ps. Misinformation about iperf 2 impacts my ability to do this.

 The new tests have all added up + ping and down + ping, but not up +

 down + ping. Why??

 The behaviors of what happens in that case are really non-intuitive, I

 know, but... it's just one more phase to add to any one of those new

 tests. I'd be deliriously happy if someone(s) new to the field

 started doing that, even optionally, and boggled at how it defeated

 their assumptions.

 Among other things that would show...

 It's the home router industry's dirty secret than darn few "gigabit"

 home routers can actually forward in both directions at a gigabit. I'd

 like to smash that perception thoroughly, but given our starting point

 is a gigabit router was a "gigabit switch" - and historically been

 something that couldn't even forward at 200Mbit - we have a long way

 to go there.

 Only in the past year have non-x86 home routers appeared that could

 actually do a gbit in both directions.

 2) Few are actually testing within-stream latency

 Apple's rpm project is making a stab in that direction. It looks

 highly likely, that with a little more work, crusader and

 go-responsiveness can finally start sampling the tcp RTT, loss and

 markings, more directly. As for the rest... sampling TCP_INFO on

 windows, and Linux, at least, always appeared simple to me, but I'm

 discovering how hard it is by delving deep into the rust behind


 the goresponsiveness thing is also IMHO running WAY too many streams

 at the same time, I guess motivated by an attempt to have the test

 complete quickly?

 B) To try and tackle the validation problem:ps. Misinformation about iperf
2 impacts my ability to do this.

 In the libreqos.io project we've established a testbed where tests can

 be plunked through various ISP plan network emulations. It's here:

 https://payne.taht.net (run bandwidth test for what's currently hooked


 We could rather use an AS number and at least a ipv4/24 and ipv6/48 to

 leverage with that, so I don't have to nat the various emulations.

 (and funding, anyone got funding?) Or, as the code is GPLv2 licensed,

 to see more test designers setup a testbed like this to calibrate

 their own stuff.

 Presently we're able to test:







 the broadband forum udp based test:



 There's also a virtual machine setup that we can remotely drive a web

 browser from (but I didn't want to nat the results to the world) to

 test other web services.


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