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From: Neil Davies <neil.davies@pnsol.com>
To: Hal Murray <hmurray@megapathdsl.net>
Cc: bloat@lists.bufferbloat.net
Subject: Re: [Bloat] Steam In Home Streaming on ath9k wifi
Date: Wed, 22 Nov 2017 10:31:36 +0000	[thread overview]
Message-ID: <1CE2F13E-A918-4EC3-AAEF-3F92DA51C6E3@pnsol.com> (raw)
In-Reply-To: <20171121190826.5B80540605C@ip-64-139-1-69.sjc.megapath.net>

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Hal

We use this approach to automatically manage measurements.

There are a few more issues - the relative drift between the two clocks can be
as high as 200ppm, though typically 50-75ppm is what we observe, but this drift
is monotonic.

Also NTP can make changes at one (or both) ends - they show up as distinct
direction changes in the drift.

This gives a limit (or a measurement error function you have to work within).

Neil

> On 21 Nov 2017, at 19:08, Hal Murray <hmurray@megapathdsl.net> wrote:
> 
>> Right, no idea how Windows drivers behave. But odds are that the bottleneck
>> is at the client, since that often has worse antennas than the AP. If you're
>> in a position to take packet captures at both clients and AP you may be able
>> to figure it out; may require tightly synchronised clocks to do properly,
>> though.
> 
> It should be reasonable to synchronize the clocks at both ends well enough.
> 
> If that doesn't work and/or is inconvient, you could post process one trace
> to adjust the time stamps.  The idea is to scan both traces in parallel to
> find the minimum transit times in each direction, then adjust the time stamps
> on one end to allocate half the total time to each direction.  It would
> obviously be handy to have a few pings during a period of low traffic for
> calibration.
> 
> -------
> 
> There are two dimensions to clocks.  One is the current time.  The other is
> the frequency.  If the frequency is off, the clock will drift.  (ntpd's drift
> correction is usually stored in someplace like /var/lib/ntp/ntp.drift)
> 
> Unless you are interested in long runs, the clock will not drift far enough
> to be a serious problem, so all you have to do is get the time right before
> starting a run.
> 
> You might want to kill ntpd on the wifi end so it doesn't get confused and yank the clock around.
> 
> 
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> These are my opinions.  I hate spam.
> 
> 
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  reply	other threads:[~2017-11-22 10:31 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-21 19:08 Hal Murray
2017-11-22 10:31 ` Neil Davies [this message]
2017-11-23 17:48   ` Caleb Cushing
2017-11-24  9:20   ` Hal Murray
2017-11-24  9:34     ` Neil Davies
2017-11-26  7:25       ` Caleb Cushing
2017-11-26  7:29         ` Caleb Cushing
2017-11-26  7:55           ` Jan Ceuleers
2017-11-26 10:05             ` Jonathan Morton
2017-11-26 10:53               ` Jan Ceuleers
2017-11-26 10:55                 ` Jonathan Morton
2017-11-26 11:54                 ` Steinar H. Gunderson
2017-11-26 13:03                   ` Jan Ceuleers
2017-11-26 13:05                     ` Jan Ceuleers
2017-11-26 13:13                       ` Steinar H. Gunderson
2017-11-26 17:53               ` Dave Taht
2017-11-26 18:43                 ` Jan Ceuleers
2017-11-26 23:11                   ` Dave Taht
2017-12-04  7:24                     ` Caleb Cushing
  -- strict thread matches above, loose matches on Subject: below --
2017-11-18 22:14 Caleb Cushing
2017-11-19 16:28 ` Toke Høiland-Jørgensen
2017-11-19 21:18   ` Caleb Cushing
2017-11-19 21:27     ` Toke Høiland-Jørgensen
2017-11-19 22:03       ` Caleb Cushing
2017-11-19 22:13         ` Dave Taht

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