[time-nuts] can of worms: time-of-day in a community radio station

[Fiorenzo Cattaneo]

Hello David,

>>> I'm surprised you don't trust "pool" servers.  My experience is that using
>>> the pool directive, and allowing NTP to expand its server list automatically
>>> to the maximum number of servers, gives good results usually with one of two
>>> servers at least being stratum-1 GPS-locked.

The main reason I do not trust "pool" servers is because there is no
guarantee of which server you will get. I might be paranoid, but I am
worried about rogue servers, and I much rather trust well known public
stratum-1 NTP servers. So mainly a matter of trust. As far as their
quality, I've actually never really seen public stratum-1 servers
being overloaded. A good number of them are actually a group of
servers and clients round-robin among all of them. For instance if I
lookup the DNS records of the stratum-1 NTP severe closer to me
(University of Washington, Seattle), I see it is actually 3 servers:

fcattane at linux-mint-64:~$ dig bigben.cac.washington.edu
;; ANSWER SECTION:
bigben.cac.washington.edu. 5 IN CNAME time.u.washington.edu.
time.u.washington.edu. 4 IN A 140.142.234.133
time.u.washington.edu. 4 IN A 140.142.1.8
time.u.washington.edu. 4 IN A 140.142.2.8


>>> If you have a PPS feed from that GPS board, you could easily add it to a
>>> Linux PC (Raspberry Pi, for example) or even a Windows box and use that
>>> locally as your own stratum-1 server.

Yes, all three stratum-1 servers I run at home all have a PPS output
from GPS (either vanilla GPS receiver or GPSDO) which I feed to NTPD
via serial port DCD input.

>>> I never had much success with
>>> peering - it seemed that when one server had a higher offset for whatever
>>> reason it dragged the other with it.  I use multiple independent stratum-1
>>> servers - one a Linux X86, one a Raspberry Pi, and one LeoNTP box.

That's interesting you had experience with peering. My own experience
with it was pretty good, but in my case I did the work to set up
internal time sync for the cloud computing megacorp I worked for, and
thus I had the luxury of choosing among several hundred servers and I
picked the ones which had the lowest PPM drift.

I wonder if NTPD's peering algorithm runs into trouble if the clock
jitter between machines is too high -- hard to say without doing more
experimenting. My experience with peering was good but only lasted
about 6 months, as few months later we actually bought some
Symmetricom stratum-1 boxes to avoid external dependencies and certify
NIST traceability.

The obvious advantage of peering is that if you don't have your own
stratum-1 sources you can continue to serve time even if the internet
connection goes down. Of course these days it's so incredibly cheap --
not to mention fun -- to build your own stratum-1 server with cheap
GPS receiver with PPS output using either a Rasperry PI, a cheap old
X86 PC, or (as I do for 2 of my servers) a pcengines box.
Luckily both of us went down this route for our own NTPD servers :-)

Cheers !

-- Fio Cattaneo

Universal AC, can Entropy be reversed? -- "THERE IS AS YET
INSUFFICIENT DATA FOR A MEANINGFUL ANSWER."

On Sat, Oct 19, 2019 at 9:04 AM David J Taylor via time-nuts
<time-nuts at lists.febo.com> wrote:
>
> Hi David, I don't particularly trust NTP servers from pool.ntp.org (I
> assume that is what you mean by "pool"), and I use public stratum-1
> servers chosen from a public list. Of course I make sure that my usage
> complies with the policies and terms of use for each server (some
> allow regional use only, some say do avoid using iburst keyword, some
> require prior permission and/or notification).
> I have found that using 3 to 5 public stratum-1 servers works very
> well, and gives a time synchronization which is within 3 to 5
> milliseconds when compared with a reference timing board with GPSDO.
> This time offset discrepancy is actually due to the fact that my ISP
> (comcast cable) has asymmetric send/receive delays. It disappears when
> I tried this setup at my office, which has symmetrical fiber optic
> connection to internet.
>
> Your point about avoiding having all the internal machine hit
> stratum-1 servers is a good one however. To avoid that in my setup I
> designate 3 machines which serve as an internal stratum-2 pool for
> internal distribution. Each of them has 3 to 5 external NTP stratum-1
> servers and they all peer with each other. Then every other internal
> machine uses these 3 machines. I have more details on my setup in my
> reply to Eric.
>
> Kind regards,
>
> -- Fio Cattaneo
> =====================================
>
> Fio,
>
> I'm surprised you don't trust "pool" servers.  My experience is that using
> the pool directive, and allowing NTP to expand its server list automatically
> to the maximum number of servers, gives good results usually with one of two
> servers at least being stratum-1 GPS-locked.  That's using UK and NL
> servers.  I suppose if you have a poor or overloaded internet connection
> server quality doesn't matter as much - well, almost.  My ISP is 200/20, and
> used to be 200/12.  Talk about asymmetric!
>
> If you have a PPS feed from that GPS board, you could easily add it to a
> Linux PC (Raspberry Pi, for example) or even a Windows box and use that
> locally as your own stratum-1 server.  I never had much success with
> peering - it seemed that when one server had a higher offset for whatever
> reason it dragged the other with it.  I use multiple independent stratum-1
> servers - one a Linux X86, one a Raspberry Pi, and one LeoNTP box.
>
> Cheers,
> David
> --
> SatSignal Software - Quality software for you
> Web: http://www.satsignal.eu
> Email: david-taylor at blueyonder.co.uk
> Twitter: @gm8arv
>
>
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