NTP Server

David Andersen dga at cs.cmu.edu
Sun Oct 24 13:18:18 CDT 2010


On Oct 24, 2010, at 1:09 PM, Randy Bush wrote:

>> 1) How necessary do you believe in local NTP servers? Do you really
>>   need the logs to be perfectly accurate?
> 
> what is "perfectly accurate?"  perfection is not very realistic.  to
> what use do you put these logs?  what precision and jitter are required
> for that use?
> 
> imiho, if you are just comparing router and server log files, run off
> public.  if you are trying to do fine-grained measurement, you are going
> to invest a lot in clock and propagation research.

As one of the aforementioned "time-nuts", I'd strongly second Randy's recommendation.  It's hard to find a middle ground in timing:  Most of the network-accessible stratum {1, 2} clocks are good enough for many uses.  If you find yourself needing really precise time with good guarantees, you're not just talking about buying one GPS unit -- you can easily go down a rathole of finding multiple units with good holdover.  (And if you don't need that, then ask yourself why public isn't good enough).

Possible very reasonable answers include needing to do one-way delay measurements;  others include wanting to depend on time for authentication protocols or other protocols and not have an external dependency (assuming you're not high-value enough for someone to try to spoof GPS at you).

The problem is that once you have a timing device or two, you've added to the set of crap you have to manage and monitor.  I use a lot of CDMA-based time receivers so that I can throw them in machine rooms with no sky access, and every year or two, I have to go upgrade a lot of firmware because some cellular company has changed their protocols.  I find a lot of cellular base stations that keep the wrong time (suggesting that their GPS-based time sync is fubared in some way).  Yadda, yadda.  Nothing is free.

  -Dave 



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