[Pool] Pool server monitoring 12ms offset
tshoppa at wmata.com
Tue Jun 8 10:43:42 UTC 2010
> with reference to the other NTP servers I am usually within +/- 2ms of them.
> Any ideas why the pool server thinks I am ~12ms off?
All your NTP times come from network connections. Network connections have latencies.
Typical latencies on the global network range from microseconds (local) to milliseconds (not far away) to 100ms+ (farther away or skinnier pipe).
NTP makes a simple assumption that the latency is symmetric, that the transmit path and receive path both have the same latencies.
>From the consistent 12ms offset that the NTP pool server sees, there is some kind of asymmetric latency going on.
Asymmetric latency can be related to assymetric signaling (e.g. ADSL) but you don't expect traffic to be so flat to give such a consistent 12ms delay.
You might check locally, if you have any very complex routing or firewall rules that may in fact be adding this latency in one direction. It is unusual for the latency to be so consistently 12ms.
NTP local clocks can also have intentional delays added (e.g. fudge flags in ntpd.conf) but this seems unlikely.
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