Unruh wrote: > Harlan Stenn <[EMAIL PROTECTED]> writes: > >>>>> In article <[EMAIL PROTECTED]>, [EMAIL PROTECTED] (maxime louvel) writes: > >> maxime> Hi, I would like to know if NTP (and particularly ntpd) is able to >> maxime> synchronise a clock, only playing with the frequency. I want to >> maxime> avoid any step in the clock that would probably screw up my network >> maxime> appli. > >> It's *possible* and it can be a really bad idea. > >> For most folks, only steps *backward* are a problem. > > Actually, ntp does synchronize the clock by "playing with the frequency > only" unless the step size is so large (128ms by default but up to 600sec > by option) that it will step.
There are two supported settings for this limit, 128ms and 600,000ms, but you can set other values, including infinite, with an option that comes with severe health warnings. If you set a value of more than 500ms on systems that implement part of the clock discipline in the kernel, you will force them not to use that mechanism, and therefore get lower quality time. There are also options, unfortunately also with health warnings, that can effectively eliminate the main legitimate cause of this, which is high asymmetric traffic loads on medium speed internet connections which have not been traffic shaped for NTP. If you suffer large steps for other reasons, you should fix the underlying cause. Fixing asymmetric load can have commercial implications, as only high value ISP accounts are likely to support the necessary traffic shaping. (Another source of steps is lost clock ticks, but that generally only gives the, relatively benign, positive steps.) Systems which suffer large steps and aren't allowed to step have been reported to hunt (in the control theory sense) rather badly. Note, the subject is a little confusing as it could be read as meaning that steps are allowed to remain uncorrected. That would be unacceptable for a protocol where every node is both client and server. That would be like if you had to drive 30 miles and insisted that you drove at 30 mph, but you were delayed by a traffic jam, and still declared the journey complete after exactly one hour. _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
