OSF DCE Administration Guide--Core Components

Introduction to DCE Distributed Time Service
(Midnight)
Key:
= Coordinated Universal Time (UTC)
ZK−1996A−GE
30 35 40 45 50 55 5 10 15 20 25 30
−8+8
00:00
Boundary of
inaccuracy
Hardware
clock time
00:00:00.00
Boundary of
inaccuracy
23.2.3 Synchronizing System Clocks
To maintain uniform system times, DTS servers and clerks periodically synchronize the
clocks in all network systems. The DTS entity that is on each system performs these
synchronizations by requesting that servers send their combined clock and inaccuracy
values (time intervals) to the originating system. The entity then uses the values that are
sent by the servers to compute a new system time.
DTS servers and clerks have slightly different synchronization procedures. Before
attempting to synchronize with other systems, DTS servers always check that an external
time-provider is present on the server system. A given server requests times from other
servers if no time-provider is available. When no time-provider is available and a server
synchronizes with its peer servers, the server uses its own system time as one of the input
values when it computes a new system time.
Most network systems run the DTS clerk process. Clerks cannot have time-providers,
and they do not use the system time of their client systems to compute new times. When
a clerk is synchronizing its client system’s clock, the clerk uses only the time values that
it obtains from servers to compute a new system time.
When a DTS clerk requests time intervals from several servers, it uses them to calculate
a new time that is correct (that is, contains UTC) and that minimizes inaccuracy. When
the servers respond and the DTS clerk calculates network communications uncertainties
and drift for each of the time values, the clerk has a set of intervals (t1 through t4 in
Figure 23-2). Since each interval contains UTC, the intersection is the smallest interval
the clerk can choose that also contains UTC. This intersection is the computed time.
The DTS entity uses the computed time interval to adjust the clock on the system that
receives the server values.
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