Hi RR,
On Jan 11, 2023, at 22:46, Dick Roy <[email protected]> wrote:
-----Original Message-----
From: Starlink [mailto:[email protected]] On
Behalf Of Sebastian Moeller via Starlink
Sent: Wednesday, January 11, 2023 12:01 PM
To: Rodney W. Grimes
Cc: Dave Taht via Starlink; [email protected]; libreqos;
David P. Reed; Rpm; rjmcmahon; bloat
Subject: Re: [Starlink] [Rpm] Researchers Seeking Probe Volunteers in
USA
Hi Rodney,
> On Jan 11, 2023, at 19:32, Rodney W. Grimes <[email protected]>
wrote:
>
> Hello,
>
> Yall can call me crazy if you want.. but... see below [RWG]
>> Hi Bib,
>>
>>
>>> On Jan 9, 2023, at 20:13, rjmcmahon via Starlink
<[email protected]> 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".
[RR] For this to work at a reasonable level of accuracy, the
timestamping circuits on both ends need to be deterministic and
repeatable as I recall. Any uncertainty in that process adds to
synchronization errors/uncertainties.
[SM] Nice idea. I would guess that all timeslot based access
technologies (so starlink, docsis, GPON, LTE?) all distribute "high
quality time" carefully to the "modems", so maybe all that would be
needed is to expose that high quality time to the LAN side of those
modems, dressed up as NTP server?
[RR] It’s not that simple! Distributing “high-quality time”, i.e.
“synchronizing all clocks” does not solve the communication problem in
synchronous slotted MAC/PHYs!
[SM] I happily believe you, but the same idea of "time slot" needs to
be shared by all nodes, no? So the clockss need to be reasonably
similar rate, aka synchronized (see below).
All the technologies you mentioned above are essentially P2P, not
intended for broadcast. Point is, there is a point controller (aka
PoC) often called a base station (eNodeB, gNodeB, …) that actually
“controls everything that is necessary to control” at the UE including
time, frequency and sampling time offsets, and these are critical to
get right if you want to communicate, and they are ALL subject to the
laws of physics (cf. the speed of light)! Turns out that what is
necessary for the system to function anywhere near capacity, is for
all the clocks governing transmissions from the UEs to be
“unsynchronized” such that all the UE transmissions arrive at the PoC
at the same (prescribed) time!
[SM] Fair enough. I would call clocks that are "in sync" albeit with
individual offsets as synchronized, but I am a layman and that might
sound offensively wrong to experts in the field. But even without the
naming my point is that all systems that depend on some idea of shared
time-base are halfway there of exposing that time to end users, by
"translating it into an NTP time source at the modem.
For some technologies, in particular 5G!, these considerations are
ESSENTIAL. Feel free to scour the 3GPP LTE 5G RLC and PHY specs if you
don’t believe me! J
[SM Far be it from me not to believe you, so thanks for the pointers.
Yet, I still think that unless different nodes of a shared segment
move at significantly different speeds, that there should be a common
"tick-duration" for all clocks even if each clock runs at an offset...
(I naively would try to implement something like that by trying to
fully synchronize clocks and maintain a local offset value to convert
from "absolute" time to "network" time, but likely because coming from
the outside I am blissfully unaware of the detail challenges that need
to be solved).
Regards & Thanks
Sebastian
>
>>
>>
>>>
>>> --trip-times
>>> 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
receiver
>
>>
>> [SM] Sweet!
>>
>> Regards
>> Sebastian
>>
>>>
>>> Bob
>>>> 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
>>>> seconds.
>>>> 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:
>>>> https://blog.cerowrt.org/post/found_in_flent/
>>>> 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
>>>> crusader.
>>>> 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
>>>> up)
>>>> 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:
>>>> flent
>>>> netperf
>>>> iperf2
>>>> iperf3
>>>> speedtest-cli
>>>> crusader
>>>> the broadband forum udp based test:
>>>> https://github.com/BroadbandForum/obudpst
>>>> trexx
>>>> 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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