OSF DCE Administration Guide--Core Components

OSF DCE Administration Guide—Core Components
23.2.2 Inaccuracy Values
In order to synchronize system clocks to the most accurate settings, DTS needs a way to
determine the accuracy of time sources relative to each other and to UTC. This section
describes how DTS determines the relative accuracy of any time source that is available
in the network.
DTS uses an inaccuracy value, or tolerance, to determine the relative precision of time
values that it obtains from system clocks and external time-providers. This DTS feature
effectively transforms each time value into an interval, or range, rather than a point on a
continuum.
Inaccuracy values are determined by the following three factors:
• Drift—When reading a clock, DTS calculates the amount of time that the clock may
have drifted since DTS previously read the clock. Drift is the largest component of
most inaccuracy values.
• Communications Delay—The inaccuracy also contains the uncertain portions of the
communications delays between systems. Although DTS compensates for
processing delays, it cannot predict or directly measure the varying delays that occur
on network links. The inaccuracy values that a clerk or server obtains from
co-located systems on a LAN tend to be much lower than those obtained from servers
outside the LAN.
• Leap Seconds—UTC time is measured by atomic clocks, which are extremely stable.
The standard, however, keeps time based on the earth’s position. Due to the slowing
of the earth’s rotation, it occasionally becomes necessary to advance UTC time by 1
second. These events are known as leap seconds. Leap seconds may occur in the
final second of any month, and normally occur about once every 18 months. At the
end of each month, DTS accounts for leap seconds by increasing all inaccuracy
measurements by 1 second. DTS later adjusts the clocks to remove the extra second
of inaccuracy if an external time-provider determines that a leap second did not
actually occur.
Without DTS to correct it, a system clock’s inaccuracy is always increasing. For
example, suppose that a clock starts with a UTC time of 0:00:00.00 (midnight) and zero
inaccuracy. Due to drift, when the clock next shows a time of 0:00:00.00, the inaccuracy
is 8 seconds. UTC time may be 23:59:52.00 or 0:00:08.00, but is probably somewhere in
between. Therefore, the system time is an interval that contains UTC time and is
bounded by the inaccuracy, as shown in Figure 23-1. Using the DTS format for
displaying time, the combined time and inaccuracy interval is expressed as follows:
1993-08-03-00:00:00.000I08.000.
Figure 23-1. Time and Inaccuracy
23 − 6 Tandem Computers Incorporated 124243