Hi Sebastian (et. al.),
[I'll comment up here instead of inline.]
Let me start by saying that I have not been intimately involved with
the IEEE 1588 effort (PTP), however I was involved in the 802.11
efforts along a similar vein, just adding the wireless first hop
component and it's effects on PTP.
What was apparent from the outset was that there was a lack of
understanding what the terms "to synchronize" or "to be synchronized"
actually mean. It's not trivial … because we live in a
(approximately, that's another story!) 4-D space-time continuum where
the Lorentz metric plays a critical role. Therein, simultaneity (aka
"things happening at the same time") means the "distance" between two
such events is zero and that distance is given by sqrt(x^2 + y^2 + z^2
- (ct)^2) and the "thing happening" can be the tick of a clock
somewhere. Now since everything is relative (time with respect to
what? / location with respect to where?) it's pretty easy to see that
"if you don't know where you are, you can't know what time it is!"
(English sailors of the 18th century knew this well!) Add to this the
fact that if everything were stationary, nothing would happen (as
Einstein said "Nothing happens until something moves!"), special
relativity also pays a role. Clocks on GPS satellites run approx.
7usecs/day slower than those on earth due to their "speed" (8700 mph
roughly)! Then add the consequence that without mass we wouldn't exist
(in these forms at leastJ), and gravitational effects (aka General
Relativity) come into play. Those turn out to make clocks on GPS
satellites run 45usec/day faster than those on earth! The net effect
is that GPS clocks run about 38usec/day faster than clocks on earth.
So what does it mean to "synchronize to GPS"? Point is: it's a
non-trivial question with a very complicated answer. The reason it is
important to get all this right is that the "what that ties time and
space together" is the speed of light and that turns out to be a
"foot-per-nanosecond" in a vacuum (roughly 300m/usec). This means if
I am uncertain about my location to say 300 meters, then I also am not
sure what time it is to a usec AND vice-versa!
All that said, the simplest explanation of synchronization is
probably: Two clocks are synchronized if, when they are brought
(slowly) into physical proximity ("sat next to each other") in the
same (quasi-)inertial frame and the same gravitational potential (not
so obvious BTW … see the FYI below!), an observer of both would say
"they are keeping time identically". Since this experiment is rarely
possible, one can never be "sure" that his clock is synchronized to
any other clock elsewhere. And what does it mean to say they "were
synchronized" when brought together, but now they are not because they
are now in different gravitational potentials! (FYI, there are land
mine detectors being developed on this very principle! I know someone
who actually worked on such a project!)
This all gets even more complicated when dealing with large networks
of networks in which the "speed of information transmission" can vary
depending on the medium (cf. coaxial cables versus fiber versus
microwave links!) In fact, the atmosphere is one of those media and
variations therein result in the need for "GPS corrections" (cf. RTCM
GPS correction messages, RTK, etc.) in order to get to sub-nsec/cm
accuracy. Point is if you have a set of nodes distributed across the
country all with GPS and all "synchronized to GPS time", and a second
identical set of nodes (with no GPS) instead connected with a network
of cables and fiber links, all of different lengths and composition
using different carrier frequencies (dielectric constants vary with
frequency!) "synchronized" to some clock somewhere using NTP or PTP),
the synchronization of the two sets will be different unless a common
reference clock is used AND all the above effects are taken into
account, and good luck with that! J
In conclusion, if anyone tells you that clock synchronization in
communication networks is simple ("Just use GPS!"), you should feel
free to chuckle (under your breath if necessaryJ)
Cheers,
RR
-----Original Message-----
From: Sebastian Moeller [mailto:moell...@gmx.de]
Sent: Thursday, January 12, 2023 12:23 AM
To: Dick Roy
Cc: Rodney W. Grimes; mike.reyno...@netforecast.com; libreqos; David
P. Reed; Rpm; rjmcmahon; bloat
Subject: Re: [Starlink] [Rpm] Researchers Seeking Probe Volunteers in
USA
Hi RR,
On Jan 11, 2023, at 22:46, Dick Roy <dick...@alum.mit.edu> wrote:
-----Original Message-----
From: Starlink [mailto:starlink-boun...@lists.bufferbloat.net] On
Behalf Of Sebastian Moeller via Starlink
Sent: Wednesday, January 11, 2023 12:01 PM
To: Rodney W. Grimes
Cc: Dave Taht via Starlink; mike.reyno...@netforecast.com; 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
<starl...@gndrsh.dnsmgr.net> 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
<starl...@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".
[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
awhile
>>>> test other web services.
>>>> _______________________________________________
>>>> Rpm mailing list
>>>> r...@lists.bufferbloat.net
>>>> https://lists.bufferbloat.net/listinfo/rpm
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