It's surprising ntp doesn't take into account network delays in some way...
NTP does try to measure and account for network delay; otherwise our clocks would be 70ms or more off! But my understanding is it assumes that network delay is symmetric: ie delay from A to B is the same as delay from B to A. That hasn't been true on the Internet for 10+ years, but it's really not true if your NTP server is on the end of an ADSL link. Particularly one where the slow upstream pipe gets full.
What I don't know is how much error we get from the reality of asymmetric network routing. Has there been anything written about that? I was a little surprised ADSL servers are in the pool at all; I'd naively assumed their time would be bad enough to not be worth it. But better to have some uncertainty than no server at all.
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