width is seldom justified.
Since NTP timestamps are cherished data and, in fact, represent the main
product of the protocol, a special timestamp format has been
established. NTP timestamps are represented as a 64-bit unsigned fixed-
point number, in seconds relative to 0h on 1 January 1900. The integer
part is in the first 32 bits and the fraction part in the last 32 bits.
This format allows convenient multiple-precision arithmetic and
conversion to Time Protocol representation (seconds), but does
complicate the conversion to ICMP Timestamp message representation
(milliseconds). The precision of this representation is about 200
picoseconds, which should be adequate for even the most exotic
requirements.
Timestamps are determined by copying the current value of the local
clock to a timestamp when some significant event, such as the arrival of
a message, occurs. In order to maintain the highest accuracy, it is
important that this be done as close to the hardware or software driver
associated with the event as possible. In particular, departure
timestamps should be redetermined for each link-level retransmission. In
some cases a particular timestamp may not be available, such as when the
host is rebooted or the protocol first starts up. In these cases the 64-
bit field is set to zero, indicating the value is invalid or undefined.
Note that since some time in 1968 the most significant bit (bit 0 of the
integer part) has been set and that the 64-bit field will overflow some
time in 2036. Should NTP be in use in 2036, some external means will be
necessary to qualify time relative to 1900 and time relative to 2036
(and other multiples of 136 years). Timestamped data requiring such
qualification will be so precious that appropriate means should be
readily available. There will exist an 200-picosecond interval,
henceforth ignored, every 136 years when the 64-bit field will be zero
and thus considered invalid.
State Variables and Parameters
Following is a summary of the various state variables and parameters
used by the protocol. They are separated into classes of system
variables, which relate to the operating system environment and local-
clock mechanism; peer variables, which represent the state of the
protocol machine specific to each peer; packet variables, which
represent the contents of the NTP message; and parameters, which
represent fixed configuration constants for all implementations of the
current version. For each class the description of the variable is
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