On Tue, Jun 8, 2010 at 5:43 AM, Shoppa, Tim <[email protected]> wrote: > MC0FRED writes: >> 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.
Or it could just be "the Internet". Asymmetric routes are the norm when crossing ISP boundaries, not the exception. Everyone does "hot potato" routing, and dumps packets onto their peer and upstream networks at the nearest exit point from their own network. For example, when a packet travels from my hosting provider in Dallas, it jumps from the hosting provider to Level 3 to XO in the Dallas metro area, and then XO transports it to Chicago over their network. But a packet travelling from my Chicago office starts on XO, and is dumped to Level 3's chicago peering point, and then travels to Dallas via Level 3's network. -- RPM _______________________________________________ pool mailing list [email protected] http://lists.ntp.org/listinfo/pool
