> Unless the asymmetry is on a `critical' link that *all* my NTP packets > travel along (such as my link to my ISP) I don't have a problem, > right? Furthermore, X would have to be different to Y by a > non-negligible amount, no? So even if the link *was* asymmetric, a > very fast link would still be very accurate.
There are (at least) two sources of asymmetry. One is link speed - examples are ADSL and cable modems. The other is queuing delays. You can get some neat graphs if you plot offset vs delay. The typical pattern is a wedge pointing left. The point of the wedge is the no-delay case. To the right of that, above the horizontal is more delay in one direction. Below is more delay in the other direction. There are examples in various papers. I don't have URLs handy. Or you can make your own. > I know I'm pushing the issue, but I always thought that one of the > goals of NTP was to make timekeeping as accurate as possible *without* > requiring the kind of devices you're talking about, and that it does > this by using servers `democratically'. What sort of accuracy were you expecting for "as accurate as possible"? Why? Over the net, 1 second is easy. 1 ms is hard. I'd expect generally within a few 10s of ms. Depends on your network and which servers you are using. It's generally easy to get better than a ms with a local GPS clock. I'd expect 10s of microseconds. In general, ntpd is a good definition for "as accurate as possible". It's a hard problem. You could probably do a bit better by tweaking various parameters and/or heuristics if you know something about your setup that makes it different from the average/typical setup. -- The suespammers.org mail server is located in California. So are all my other mailboxes. Please do not send unsolicited bulk e-mail or unsolicited commercial e-mail to my suespammers.org address or any of my other addresses. These are my opinions, not necessarily my employer's. I hate spam. _______________________________________________ timekeepers mailing list [email protected] https://fortytwo.ch/mailman/cgi-bin/listinfo/timekeepers
