Re: NTP Server

[Sean Donelan <sean () donelan com>]

On Sun, 24 Oct 2010, Brandon Kim wrote:
> 1) How necessary do you believe in local NTP servers? Do you really 
> need the logs to be perfectly accurate?
> 2) If you do have a local NTP server, is it only for local internal 
> use, or do you provide this NTP server to your clients as an added 
> service?
> 3) If you do have a local NTP server, do you have a standby local NTP 
> server or do you use the internet as your standby server?

First terminology.  What do you mean by a local NTP server?

Almost any Cisco/Juniper router, Unix server and some recent Windows 
servers have NTP server software and can synchronize clocks in your 
network.  So you may already have a NTP server capable device.  You just 
need to configure it, and give it a good source of time.  It would be a 
Stratum 2 or greater NTP server because the good source of time is 
another NTP server.  Left to itself, NTP is pretty good at keeping clocks 
in arbitrary networks synchronized with each other. But most people are 
also interested in synchronizing clocks with some official time source.

The Network Time Protocol doesn't really have the notion of a "standby" 
server.  It uses multiple time sources together, and works best with about 
four time sources.  But for many end-systems, the Simple Network Time 
Protocol with a single time source may be sufficient.

If you are in a regulated industry (stock broker, electric utility, 9-1-1 
answering point, etc) there are specific time and frequency standards you 
must follow.

On the other hand, are you are asking about a local clock receiver (radio, 
satellite, etc) for a stratum 1 NTP server?  Clock receivers are getting 
cheaper, the problem is usually the antenna location.

Or on the third hand, are you asking about local primary reference clock 
(caesium, rubium, etc) for a stratum 1 NTP server?  These are still 
relatively expensive up to extremely expensive.

Or on the fourth hand, are you a time scientist working to improve 
international time standards.  If you are one of these folks, you already
know.


Most major ISPs use NTP across their router backbone, and incidently 
provide it to their customers. The local ISP router connected to your 
circuit probably has NTP enabled.

Required accuracy is in the eye of the beholder. NASDAQ requires brokers 
to have their clocks synchronized within 3 seconds of UTC(NIST).  9-1-1 
centers are required to have their clocks synchronized within 0.5 seconds 
of UTC.  Kerberos/Active Directory requires clocks to be synchronized 
within 5 minutes of each other.

If your log files have a resolution of 1 second, you probably won't see 
much benefit of sub-second clock precision or accuracy.  If you are 
conducting distributed measurements with sub-microsecond resolution, you
probably will want something more.