ntp questions: slewing versus jumping the time

Question

ntp doesn't want to jump your clock, because discontinuous time jumps are Bad. It wants to adjust your clock gradually -- very gradually. It's very conservative: by default, it won't slew your clock by more than xx parts per million (ppm).

But since ntp is so conservative, if it finds out that your clock is too far off, such that adjusting it gradually would take forever, it will fall back and jump your clock anyway (even though that's Bad). It does this, by default, if it would take longer than yy hours to adjust your clock gradually.

If you want, you can tell ntp to adjust your clock faster (i.e. less gradually), up to a maximum of zz ppm.

My question is, what are xx, yy, and zz? I know those thresholds exist, I'm pretty sure they're documented, I'm pretty sure they're configurable, but I can never remember the values and I can never find them.

If you know what they are, ideally I'd like to know (a) what the default values are and (b) where they're documented and (c) how they can be configured and (d) where the actual code is that makes the decision to slew or jump. Thanks.

Deprave

Thanks for asking this question. These days, in the age of distributed systems (most of which heavily rely on synchronized clocks across machines one way or another) this is indeed a highly relevant quesiton, and I am also indeed a little shocked that there are little concise resources about that. – Placement

By the way, this document is a very authoritative source: eecis.udel.edu/~mills/ntp/html/clock.html – Placement

Answer 1

xx: 128ms

The ntpd algorithms discard sample offsets exceeding 128 ms, unless the interval during which no sample offset is less than 128 ms exceeds 900s.

I don't know of a way to change xx.

yy: 600s

In practice, the need for a step has been extremely rare and almost always the result of a hardware failure or operator error. The step threshold and stepout threshold can be changed using the step and stepout options of the tinker command, respectively. If the step threshold is set to zero, the step function is entirely disabled and the clock is always slewed. The daemon sets the step threshold to 600 s using the -x option on the command line.

zz : 500ppm

The maximum slew rate possible is limited to 500 parts-per-million (PPM) as a consequence of the correctness principles on which the NTP protocol and algorithm design are based. As a result, the local clock can take a long time to converge to an acceptable offset, about 2,000 s for each second the clock is outside the acceptable range.

I don't believe there is a way to change zz.

As for actual code that shows how the decision is made to slew/step, you probably won't find it online. Only the algorithm. Here and here are great links on that algorithm works.

Yser