Frequency tolerance PHI (unit: seconds per second)
NtpDuration is used to represent signed intervals between NtpTimestamps.
A negative duration interval is interpreted to mean that the first
timestamp used to define the interval represents a point in time after
the second timestamp.
NtpInstant is a monotonically increasing value modelling the uptime of the NTP service
NtpTimestamp represents an ntp timestamp without the era number.
Stores when we will next exchange packages with a remote server.
Used to determine whether the server is reachable and the data are fresh