I'm playing with NTP measurements and I observe that RIPE Atlas probes can talk to some public NTP servers but not all. See for instance measurements #74488034 (it works) and #74487872 (it fails). Testing with other NTP clients succeed with these two servers (ntp.nic.fr and ntp.netnod.se). I do not understand the reason for this difference. The servers which do work: ntp.nic.fr time.cloudflare.com time.google.com 0.pool.ntp.org ntp-p1.obspm.fr The servers which fail with RIPE Atlas probes but work otherwise: ntp.netnod.se time.nist.gov