[ntp:questions] NTP 128ms

brian utterback brian.utterback at oracle.com
Thu Oct 6 23:35:39 UTC 2016

The maximum adjustment allowed by default is 17 minutes. If you control
enough of the system's upstream servers you can set the clock to
anything you want eventually. The only question is how fast you can do it.

On 10/6/2016 2:16 PM, Brian Inglis wrote:
> On 2016-10-06 06:49, MORRIS, Joe J wrote:
>> Probably a simple answer, but I don't seem to be able to find it. I
>> understand that any difference between the time source and expected
>> time on the client below 128ms will result in the local time being
>> changed to reflect the new time. What safeguards are there in place
>> to stop the time source (either through fault or malicious attack)
>> changing the time by say 100ms every time its polled? Assuming a poll
>> every minute, over 10 minutes, the time could be changed by a
>> second.
> With an offset below the step threshold (default 128ms), system time
> will be slewed to discipline it towards UTC; above the step threshold,
> system time will be stepped to set it to UTC: this is normally done only
> once at startup; if any offset exceeds the panic threshold (default
> 1000s), ntpd will exit: this may be bypassed once only at startup with
> option -g (panicgate); see:
> https://www.eecis.udel.edu/~mills/ntp/html/discipline.html
> https://www.eecis.udel.edu/~mills/ntp/html/clock.html
> Much of the ntpd code is sanity checking to ensure that bad data is
> ignored, and only truechimers are selected to estimate UTC; see:
> https://www.eecis.udel.edu/~mills/ntp/html/warp.html
> https://www.eecis.udel.edu/~mills/ntp/html/poll.html
> https://www.eecis.udel.edu/~mills/ntp/html/filter.html
> https://www.eecis.udel.edu/~mills/ntp/html/select.html
> https://www.eecis.udel.edu/~mills/ntp/html/cluster.html
> https://www.eecis.udel.edu/~mills/ntp/html/prefer.html
> If the number of sources deemed truechimers are not greater than the
> number of sources deemed falsetickers, no majority clique exists, and
> the system time adjustment is not changed: see select above Figure 2.
> Many default operating parameters may be changed by the configuration
> tos statement; see:
> https://www.eecis.udel.edu/~mills/ntp/html/miscopt.html#tos
> For greatest accuracy and consistency, a timing quality GPS with PPS
> can be connected to a low latency serial port or other IO connector
> on a system running with kernel PPS and NTP API support (most Unix).
> With a good sky view this keeps time within us of UTC; use a good
> GPSDO (with OCXO) supporting NTP and you can stay within ns of UTC.

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