NTP question

Rubens Kuhl rubensk at gmail.com
Thu May 2 01:04:27 UTC 2019

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

> On Wed, May 1, 2019 at 5:48 PM Keith Medcalf <kmedcalf at 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.

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