How NTP selects the best time source

• Stratum: how many steps away a source is from a primary reference (lower is generally better).
• Reachability: whether the server is consistently responding.
• Delay: network round-trip time to the server.
• Offset: the measured time difference between client and server.
• Jitter: variation in offset measurements over time.
• Stability: whether the source behaves consistently and predictably.

NTP collects samples from available sources and rejects outliers. It prefers sources that are reachable, stable, and low-noise. If a previously good source becomes unstable or unreachable, NTP can switch to another source automatically.

If a client appears to pick an unexpected server, the cause is often network quality or reachability rather than stratum alone. Ensure your preferred source is reachable and stable, and avoid mixing very distant public servers with local sources on the same client when accuracy matters.