[ntp:questions] Re: NTP seems unsuitable for this application... what do you think?

Brad Knowles brad at stop.mail-abuse.org
Thu Dec 2 15:10:00 UTC 2004


At 8:59 AM -0500 2004-12-02, John Seal wrote:

>  But page 4 of Sun's "Using NTP to Control and Synchronize System Clocks -
>  Part I: Introduction to NTP" says:
>
>  "if two NTP servers are synchronized to each other as peers, what
>  actually happens is the clocks decide among themselves which is the
>  better source of time, and both clocks attempt to synchronize to that".

	I'm not convinced that this is correct.  Perhaps the book you 
mention is not the best reference for this sort of thing.

>  It's true that we don't care so much about the time being absolutely
>  correct; we care more about the hosts all having the SAME time.
>  Synchronization is more important that accuracy, but we also have to
>  guard against things like time going backwards, because it messes up
>  Sybase and certain alarm-based scheduling algorithms we use.

	See the NTP FAQ, especially 
<http://www.ntp.org/ntpfaq/NTP-s-config.htm#AEN2660>.

>  Page 3 of the Sun NTP document says:
>
>  "Other schemes, such as DTS, are designed primarily for local area
>  networks, while NTP is designed specifically for Internet environments."
>
>  This is another reason we think NTP might not be appropriate (since our
>  system is not on the Internet).  Would DTS be appropriate?

	I don't know anything about DTS.  I know that, with a proper 
configuration, you should be able to get NTP working correctly on 
your network, even if you don't have any upstream time servers (e.g., 
you can't even use GPS).  I know that, with a proper configuration, 
if you can use upstream time servers such as GPS, then you should not 
only be able to have a configuration that will keep the clocks in 
reasonably close sync, but they will also be reasonably close to 
"correct".

	NTP wasn't designed to work this way, but if you provide enough 
time servers to which the clients can connect, and if you make sure 
that the configuration between the servers is such that they have 
enough input sources to be able to make the algorithms work 
correctly, you should be fine.  You'll have your own private little 
"Internet", where NTP works pretty much as intended.

>  What we really want is to take a system that boots with absolutely no
>  idea what time it is, and somehow get all hosts to agree on a time,
>  even if it's just "wristwatch time" entered by an operator, no matter
>  when or in what order the hosts are booted.

	That's a problem.  You will have to ensure that the "master" time 
server(s) is/are booted first, and whether you enter the time from 
wristwatch or you let it get the time from GPS, that will have to be 
turned on and settle down before you can turn on any of the other 
machines.

	NTP has a strict stratum hierarchy, and that has to be 
maintained.  You can't have everyone peer with everyone else, enter 
the time on one machine, and then hope that everyone else will 
consider that one to be the "master".

-- 
Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.



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