[ale] semi [OT] NTP questions - and NTP Windows - was: possibility of running an NTP server

[Ron Frazier]

On 1/14/2012 1:38 AM, mike at trausch.us wrote:
> On 01/13/2012 04:54 PM, Ron Frazier wrote:
>    
>> NTP is polling the NIST service every 4 minutes, and, as you can see
>> from the red graph, my clock offset is all over the place, varying from
>> -5 ms to +110 ms.  I also notice that the frequency is drifting up from
>> -13 to -9.  Can someone explain to me in a few words what the frequency
>> means?  Does this graph mean that the system is trimming my clock?
>>      
> It is more than sufficient to set your clock once a day.  Much more
> frequently than that and it is considered impolite.
>
> Even the most horrible of PC clocks I have encountered don't drift more
> than 10 seconds per day.  The average, IME, is 0.5 to 4 seconds per day.
>   If you need to get more accurate than that, then purchase or build a
> WWVB receiver that can feed your computer the current time directly from
> the radio signal.  Then you can have your computer constantly up-to-date
> when the signal is coming in, and you shouldn't drift horribly when it
> isn't.
>
> 	--- Mike
>
>    
Hi Mike,

I understand what you're saying.  I think my worst clock is about 15 sec 
/ day.  Based on my recent experiments, it seems to vary widely with the 
usage of the machine and drift worse when the machine is busy.  That 
correlates with the information I found that said sometimes the 
interrupts get disabled when the computer is busy and the OS misses RTC 
pulses.  I know some Linux systems have "kernel discipline" functions 
which help NTP fine tune the kernel timing.  I don't know if Ubuntu 
11.04 has it, and I don't know if Windows has anything similar.

I thoroughly checked the NIST site's server list, and the only thing 
they mentioned about polling interval is not to do it more than every 4 
seconds.  Also, their posted access policy on the public lists at 
NTP.ORG says they're open access for up to 20 queries / hour, which 
would be once every 3 minutes.  Also, the default settings for the NTPD 
daemon are to poll starting every 64 seconds and expanding out to every 
~ 16 minutes.  So, it certainly seems that the systems are designed to 
poll more often.  On my Linux machines, I have set the poll settings set 
for 4 minutes minimum to 32 minutes maximum.  The minimum complies with 
the NIST lower limit.  Usually, after a while, the NTPD will settle down 
into a polling cycle of 32 minutes.  On the Windows side, I've got it 
set to 8 minutes to 32 minutes.  It usually settles down to either 
polling every 8 minutes or every 16 minutes and sometimes goes to 32 
minutes.  So, just by running the algorithm with it's normal default 
behavior, it polls much more than once / day.  Not only that, as I 
understand it, the NTPD will terminate if it senses an extremely large 
time offset from the server.  So, it would probably just bomb out if it 
detected 5 - 10 sec (5000 - 10000 ms) of offset.

Having said all that, I'm fine with the idea of maintaining accuracy 
while reducing internet polling, if possible.  I don't think it's 
unreasonable to ask a $1000 computer to maintain the same time as a $20 
wall clock.  That is, always within +/- .5 sec of true time if it's 
being synced to a trusted source.  I have literally searched hundreds of 
Google listings with keywords like "ntp reference clock", "ntp wwvb", 
"ntp lan clock", "ntp local clock", etc.  I keep getting references to 
rack mounted devices that cost hundreds or thousands of dollars.  I 
don't have that kind of budget.  It seems to me, that, if I can buy a 
wall mounted "atomic" clock for $20, I should be able to buy a "LAN 
mounted" atomic clock for under $100.  How much does it cost to hang a 
LAN port on the back.  I know I could dedicate an old computer to do 
this, but that's just another machine to patch and maintain.  I'd rather 
have some sort of time appliance that I just plug into the LAN and 
forget it.

Here's what I'd like to have:

An inexpensive local time source
    a) that sits on the LAN as an NTP server
    b) that autoboots and autosets and autoreboots and autoresets
    c) that's accurate to within +/- .5 sec / 24 to 48 hr without syncing
    d) that discipline's itself from WWVB or GPS or cell towers, etc.  
(By the way, it occurs to me that caller ID also provides the time when 
phone calls come in.)
    e) that autoselects internet time servers for backup sync services 
but also allows manual input through ntp.conf
    f) that automatically uses the internet servers for sync if it's 
normal source fails for a period of time
    g) that automatically "recuses" itself by reporting a higher stratum 
/ error message, if it cannot provide accurate time
    h) that provides status and error notifications
    i) that provides access to statistics

Then, what I want my computer to do is:

    a) poll the time appliance as frequently as necessary to maintain 
+/- .5 seconds offset from true time
    b) periodically, maybe daily, poll a NIST or other external server 
and cross check the local source
    c) use the external source if the local source is off by more than 
+/- .5 seconds
    d) continue to use the external source for as long as needed
    e) produce an error message / notification that the local source has 
failed
    f) periodically check the local source and resume using it when possible

That's all I want ... I think.  8-)

Sincerely,

Ron

-- 

(PS - If you email me and don't get a quick response, you might want to
call on the phone.  I get about 300 emails per day from alternate energy
mailing lists and such.  I don't always see new messages very quickly.)

Ron Frazier

770-205-9422 (O)   Leave a message.
linuxdude AT c3energy.com