[ntp:questions] Re: Sufficient # servers to sync to

John Sasso jsassojr at nycap.rr.com
Thu Mar 17 15:19:20 UTC 2005


Thanks for the response!  The data centers are roughly 20 mi apart, so
latency won't be an issue.

Question regarding 3 NTP servers and sanity-checking.  I can understand that
if one out of the 3 failed such that it could not longer be contacted then
the other two would pose a dilemma to the client, as you noted.  However,
suppose the one failed in such a manner that it could still be contacted and
give out time but it provided clearly erroneous time.  In this situation,
wouldn't the other two provide a "sanity check" against the falseticker?  My
logic here is that if this is the case, and since we will monitor the
health/uptime of the Stratum-1's (since we own them), then 3 would be
sufficient since if one of the Stratum-1's fail then its outage time would
be 1-3 days (depending on how fast we can get a replacement shipped to us).
The remaining two would still provide time.



"Richard B. Gilbert" <> wrote in message
news:88SdnTK4KtYVAaTfRVn-qg at comcast.com...
> John Sasso wrote:
> > I am working on a design for the NTP infrastructure for our company.  We
> >purchased 6 Stratum-1, GPS-sync'd NTP servers, three for each of our two
> >data centers located at remote sites.  We have a number of subnets at
> >of our secured sites, each secured by a firewall.
> >
> >According to
> > it suggests NTP clients should sync to a minimum of 4 NTP servers.
> >Specifically, it states:
> >
> >"While the general rule is for 2n+1 to protect against "n" falsetickers,
> >this actually isn't true for the case where n=1. It actually takes 2
> >to produce a "candidate" time, which is really an interval. The winner is
> >the shortest interval for which more than half (counting the two that
> >the interval) have an offset (+/- the dispersion) that lies on the
> >and that contains the point of greatest overlap."
> >
> >In the past, I've had NTP clients sync to up to 3 [out of 4] Stratum-2
> >servers.  The 4 NTP servers each sync'd to 4 off-site Stratum-1 NTP
> >as well as off one-another for additional sanity checking.
> >
> >For the design, is it overkill for me to require to NTP clients to sync
to 4
> >NTP servers?  How about just 3?  The NTP clients consist of Cisco routers
> >and firewalls, Windows, Sun, and Linux systems.  Part of the environment
> >uses Windows AD w/ Kerberos as well as SSL, which I think require
> >time.
> >
> >--john
> >
> >
> >
> >
> Many people would be satisfied with one "good" server.  If the
> consequences of that one "good" server being wrong someday are
> sufficiently serious to justify the expense, then four servers is the
> way to go.   Those four servers don't all have to be on-site and running
> GPS reference clocks, but you do need four.  The problem with three is
> that if one fails you have two left and no way to determine which, if
> either, is correct when they disagree.
> If your two data centers are not unreasonably far apart it might make
> sense to have each serve as a backup to the other.   Everybody
> configures six servers.   In each data center, one of the local servers
> will probably be selected but five others are available as a sanity
> check and "advisory committee".  For sites more than two or three
> hundred miles apart, the network delays may add enough uncertainty to
> make this choice undesirable.

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