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NTP is a protocol to acquire, keep and distribute highly precise system time.

It uses Port-123-udp since UDP has lower latency. TCP handshakes and retransmits would increase the round-trip time.

The RFC links above point to version 3 and version 4 of the protocol, NTPv4 is backward compatibe to NTPv3.
Earlier versions are obsolete.

NTP up to version 3 uses 64-bit binary Timestamps, 32-bit seconds since the 1900-01-01 Epoch and 32-bit fractional seconds.
This means it will rollover at 2036-02-07. NTPv4 fixes this using 128-bit Timestamps.

NTP can use multiple time servers and finds the best host by measuring round-trip delay, jitter, drift, etc.
It is best effort possible highly precise and acts like a PLL. It can even be used to derive a master clock.

A less complex and less precise implementation is the SNTP protocol.


In early FRITZ!OS only SNTP was supported, implemented in multid.
Since fw 4.63 NTP is supported by chronyd, which is controlled by multid using chronyc.
In fw 7.39 AVM added avmntpd, a SNTP client to quickly get a valid system time before chronyd takes over.


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