Re: NTP question

[Rubens Kuhl <rubensk () gmail com>]

On Wed, May 1, 2019 at 9:56 PM William Herrin <bill () herrin us> wrote:

> On Wed, May 1, 2019 at 5:48 PM Keith Medcalf <kmedcalf () dessus com> wrote:
>
> > If you have one such installation, then you really do not care about the
> > "accuracy" of the time.  However if you have multiple such installations
> > then you want them all to have the same time (if you will be comparing logs
> > between them, for example).  At some point it becomes "cheaper" to spend
> > thousands of dollars per site to have a single Stratum 0 timesource (for
> > example, the GPS system) at each site (and thus comparable time stamps)
> > than it is to pay someone to go though the rigamarole of computing offsets
> > and slew rates between sites to be able to do accurate comparison.  And if
> > you communicate any of that info to outsiders then being able to say "my
> > log timestamps are accurate to +/- 10 nanoseconds so it must be you who is
> > farked up" (and be able to prove it) has immense value.
>
> If your network is air gapped from the Internet then sure. If it's not,
> you can run NTP against a reasonably reliable set of time sources (not
> random picks from Pool) and be able to say, "my log timestamps are accurate
> to +/- 10 milliseconds so it must be you who is farked up." While my
> milliseconds loses the pecking order contest, it's just as good for
> practical purposes and a whole lot less expensive.
And while time source stability is a good criteria, the most important NTP
criteria is path latency symmetry between directions. It's better to have a
path that is 100 ms of 1-way latency both ways than a path that is 1 ms one
way, 100 ms the other way.


Rubens