[ntp:questions] Re: NTP Pool rules vs. four-server advice

Adrian von Bidder grazdan at fortytwo.ch
Tue Dec 20 10:43:58 UTC 2005

Hash: SHA1

Clinging to sanity, Michael Deutschmann mumbled in his beard:

> On Mon, 19 Dec 2005, Richard B. Gilbert wrote:
>> Michael Deutschmann wrote:
>> >I've noticed a conflict between the instructions on the NTP server
>> >Pool project page and other NTP advice.
>> >
>> > [... NTP guideline is to use 1, 4 or more servers.  Pool ettiquette
>> >  says only 3 servers total ...]
>> [... Stuff about how hard it is for the pool ...]
> That's not really relevant to the argument.
> If by bringing this up you are trying to say that the pool would
> collapse with a "ration" of 4 servers per user, then the "ration"
> should be set at 1, since those two extra servers for people ignoring
> the 1-or-4 rule
> aren't doing them any good.  (Assuming that the 1-or-4 rule itself is
> actually technically justified, of course).

Well, I'm not the authority on the ntp error correction mathematics, but
I know a bit about the pool :-P

The most frequent failure you'll see from pool.ntp.org is that they are
unreachable temporarily or permanently.  So even if you configure two
pool.ntp.org servers (which is, as far I understand, a very bad
configuration from the error correction/statistics point of view) you
can afford to lose one of the pool.ntp.org timeservers.  Remember that
the pool.ntp.org servers are monitored, so there is a big probability
that even if you have only one remaining server in your configuration,
you will still get some relatively sane time.

Btw, the [012].pool DNS names are far from useless, because if you add
server pool.ntp.org
server 0.pool.ntp.org
server 1.pool.ntp.org
server 2.pool.ntp.org
you get four servers and you're guaranteed that they'll not overlap (if
Bjorn still uses my script to generate the DNS zone, that is.)

The reason why the pool.ntp.org web pages suggest using three servers is
IIRC purely historical - this may be subject to discussion.  What hurts
the pool, traffic-wise, is not regular well-configured and up-to-date
ntp clients, but misconfigured clients polling every second or every 6
seconds or so (read in the timekeepers mailing list archives about some
numbers - I think it was something like less than 10 clients making
more than 50% of one servers traffic...)

- -- vbi

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