[ntp:questions] Help verifying accuracy of NTP server

Weber wsdl at osengr.org
Wed Dec 2 00:06:55 UTC 2015

I could use some advice about verifying the performance of a home-built 
stratum-1 NTP server.

The server is based on an HP55300A reference clock. The PPS signal and 
time-of-day serial port are connected to an Arduino Mega with an 
Ethernet shield and some miscellaneous interface circuitry. The Mega 
tracks time from the PPS signal generally within +-500ns by using 
internal timers, and a software-based PLL controlling a software-based 
fractional-N divider. There are hardware timestamps on incoming NTP 
requests. Transmit timestamps are not hardware generated but as best I 
can figure, are accurate to 10us worst case and probably just a couple 
of us in reality.

At least I ***think*** that's how accurate this thing is. In an attempt 
at independent verification, I've been running ntpd 4.8.2 on a Win7 PC. 
It is configured to use the Arduino NTP server (which is on the same 
network hub as the PC). Also configured are three internet stratum-1 
servers over a DSL connection (one is ACTS, the other two are GPS ref 
clocks). Polling between 16 and 32 seconds on each. The PC's loop stats 
look fine -- it tracks within 500us typically and jitter is around 300us 
or so.

Offset data in peerstats shows the two stratum-1 GPS servers have a -4ms 
time offset relative to the Arduino server. Pretty much exactly the same 
offset in fact. Peerstats shows a very stable delay to these two servers 
-- 30ms on one and 65ms for the other. This data was taken over a 
12-hour period.

So, here's my question: If the Arduino NTP server is accurate to better 
than 100us, is it believable that these two internet servers would show 
almost the same exact -4ms offset? Is this something to do with DSL? Or 
should I be digging into the Arduino server looking for bugs? Does 
anyone have suggestions for other experiments?

I've left out lots of details to keep the message shorter. I'll be glad 
to provide more info.

Thanks in advance if anyone can help out.

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