[ntp:questions] NTP.log interpretation
jason at extremeoverclocking.com
Sat Apr 19 02:43:29 UTC 2014
As others have suggested, any client running NTP should point to *at least* 3 time sources (usually ~5 is preferred)... The reason
being if one server goes wacko, but the other two agree, then the client knows to X out the bad one and keep the two others. With
only two you are essentially just flipping a coin...
I do not know where you are located, but if you are serving time to 100+ clients, you should probably consider the "pool" servers as
backup sources and look more into finding local public stratum 1 & 2 servers:
NTP uses very very very little bandwidth, it's one small UDP packet (less than 128 bytes) that (assuming default configuration)
works its way up to once every 17 minute... There's no reason to be stingy with selecting a handful of external internet time
servers (unless company policy prohibits it).
If your company has the funds and you have the ability to mount a GPS antenna, then going with a commercial GPS based NTP server
might be the way to go. You can choose from various oscillator options so that they will flywheel if they lose GPS lock but still
keep decent time for long hold-over periods. Likewise those same companies also offer CDMA based time servers if you have no
If you want the DIY route, any old PC running Linux or FreeBSD that has a serial port + a GPS module that will output a PPS will
yield you far better results than you could sync with over a network.
Finally, it would also be worthwhile to have a layer of your time servers "peer" with each other. For instance, I have several
Stratum-1 servers that get time via GPS. Then I have three Stratum-2 servers that use the "server" line for the S1 servers, but in
addition they use the "peer" line with each other S2 server. When you combine that with "orphan" mode if all my S1 servers went
down, the S2's would work with each other to figure out their best guess at the right time. Finally all my clients point to the S2
servers... Just because it's only my local LAN, I do not have any external NTP servers listed, but if I did then those would end up
being used as fallback sources for the S2 servers.
My S2 servers are also not dedicated time servers, but they are servers that don't go down, rebooted, or even tinkered with often.
For instance one is a NAS that is the primary network storage for all clients running. Another is a database server.
A dedicated NTP server doesn't have to be a huge powerful machine. Many commercial products if you open them up you would be
surprised to see 486-class PC104 SBCs.... The extra cost comes in their proprietary hardware that usually will discipline a TCXO,
OCXO, or Rubidium oscillator to GPS or CDMA (giving the flywheel ability)...
I have built probably half a dozen GPS based Stratum-1 NTP servers using Soekris SBCs and Motorola Oncore GPS receivers, all off
ebay... Maybe spending $50 total in hardware and an hour or less mounting everything in the little chassis and soldering wires. The
end result is a nice time server that consumes maybe 5-10 watts... I also purchased a handful of old commercial time servers that
also pop up on eBay from time to time at decent "hobbyist" prices... But to be honest they provide no better time than my homemade
ones, and most are running outdated OSes and NTP distros that I would not trust in a commercial environment because of the potential
for exploiting (which is probably why they ended up on eBay). Not to mention most are using old GPS receivers from the 90's (some
aren't even timing receivers).
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