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Top Forums UNIX for Dummies Questions & Answers Timekeeping in Linux question ... Post 302516691 by Perderabo on Sunday 24th of April 2011 10:23:52 PM
Old 04-24-2011
For better or worse it is very easy to be sure that the tires are spinning at the same rate but much harder to know what the rate is. In the clock example, you need a piece of code to read the clock, convert to a format that you understand, and display it to you. Sadly, all of that takes time. How much depends on how busy the system is. This is where the .2 second come from.

Ideally, you should pick 10 servers or so that seem more or less close to you and tell NTP about all of them. NTP will compare them and separate the false-tickers from the true-chimers (and that is what the NTP docs call them). Network delay is not a big problem. A NTP reply includes the time the request was recieved and the time the reply was sent. The client calculates and compensates for in-transit time.

These days for a few thousand dollars you can get a GPS clock with an ovenized oscillator. You do need to put the antenna on the roof so it gets a good view of the satellites. The satellites know the time to within 30 nanoseconds or so. A decent GPS clock will be accurate to a microsecond or two all the time. We have a few of these and they work pretty well. Just goggle for GPS clocks.
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ntptrace(1M)						  System Administration Commands					      ntptrace(1M)

NAME
ntptrace - trace a chain of NTP hosts back to their master time source SYNOPSIS
/usr/sbin/ntptrace [-vdn] [-r retries] [-t timeout] [server] DESCRIPTION
ntptrace determines where a given Network Time Protocol (NTP) server gets its time from, and follows the chain of NTP servers back to their master time source. If given no arguments, it starts with localhost. OPTIONS
The following options are supported: -d Turns on some debugging output. -n Turns off the printing of host names; instead, host IP addresses are given. This may be necessary if a nameserver is down. -r retries Sets the number of retransmission attempts for each host. -t timeout Sets the retransmission timeout (in seconds); default = 2. -v Prints verbose information about the NTP servers. EXAMPLES
Example 1 Sample Output From the ntptrace Command The following example shows the output from the ntptrace command: % ntptrace localhost: stratum 4, offset 0.0019529, synch distance 0.144135 server2.bozo.com: stratum 2, offset 0.0124263, synch distance 0.115784 usndh.edu: stratum 1, offset 0.0019298, synch distance 0.011993, refid 'WWVB' On each line, the fields are (left to right): o The server's host name o The server's stratum o The time offset between that server and the local host (as measured by ntptrace; this is why it is not always zero for local- host) o The host's synchronization distance o The reference clock ID (only for stratum-1 servers) All times are given in seconds. Synchronization distance is a measure of the goodness of the clock's time. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Availability |SUNWntpu | +-----------------------------+-----------------------------+ SEE ALSO
xntpd(1M), attributes(5) BUGS
This program makes no attempt to improve accuracy by doing multiple samples. SunOS 5.11 19 Mar 1998 ntptrace(1M)
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