From: Deepak Jain <deepak@ai.net> Date: Tue, 17 Mar 2009 15:54:28 -0400
As long as the end-user is made aware that the accuracy of said NTP clock is +/- 30.000 seconds (or whatever jitter might exist). Seems kind of ridiculous to use an NTP source that is, for many purposes, wildly inaccurate. For my purposes, wildly is more than +/- 0.1 seconds. Trying to troubleshoot a problem, network or server, where the timestamps on each server/router/device vary inconsistently, is like walking on broken fluorescent bulbs -- painful and dangerous to one's health.
Not being a time geek, since Cisco's were called out for being wild jitter-mongers... how much jitter are we talking about?
Clock is synchronized, stratum 2, xxxxxxxxxxxxxxxxxxxxxxxx nominal freq is 250.0000 Hz, actual freq is 249.9989 Hz, precision is 2**18 reference time is CD6A7CD4.45A9BB00 (19:47:32.272 UTC Tue Mar 17 2009) clock offset is 2.0581 msec, root delay is 29.62 msec root dispersion is 6.81 msec, peer dispersion is 3.30 msec
Are we talking about +/- 30 seconds, or a problem bounded by +/- 30 msec?
In my testing about 2 years ago (when we deployed dedicated NTP servers to our POPs), the jitter was highly variable. On a lightly loaded router we were seeing a reasonable 1 ms., but, if the router was fairly well loaded it increased to the 20-30 ms. range and we considered that unacceptable. Now days we use NTP for latency measurements and we really want better than 10 usec. and usually get better than 2 usec. on servers with attached reference clocks and about 150 usec. on systems syncing over the network. Since NTP and ICMP are treated about the same, try running a long term set or pings from a host to the router and see what you get. The PLL in ntpd will keep the local time much better than what you see, but it does not make for a very good time source. -- R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634 Key fingerprint:059B 2DDF 031C 9BA3 14A4 EADA 927D EBB3 987B 3751