OSF DCE Administration Guide--Core Components

Introduction to DCE Distributed Time Service
23.2.1 Time Measurement Factors
The following subsections describe the factors that affect time measurement and explain
how DTS handles them.
23.2.1.1 Clock Error
All system clocks have common properties that contribute to clock error and interfere
with the synchronization process. System clock error tends to increase over time; the
rate of change of error is known as drift. If each system clock in a network started at the
same time and ran at the same rate, the clocks would remain synchronized. Because
each system clock drifts at a different rate, however, the system clocks throughout a
network become desynchronized.
The difference between any two clock readings is known as the skew between the clocks.
The clocks that are used in many computer systems have a specified maximum drift of a
few seconds per day. If uncorrected for several days, the skew between networked
system clocks can inhibit the performance of distributed applications.
The DTS server or clerk on each node tracks the drift of its client’s system clock and
periodically synchronizes with other DTS nodes to reduce the skew between its client’s
time value and those of the other DTS nodes. The DTS server or clerk adjusts the system
clock on its client node as the final step in this repeating synchronization process.
23.2.1.2 Communications and Processing Uncertainties
Communications delays also inhibit the synchronization process, especially when two
systems communicate over a WAN or low-speed link. DTS can adjust for the known
processing delays that are required to send and receive messages between systems. Due
to the varying quality of communications links, however, the time that is required to
send, receive, and acknowledge messages varies from one message to the next. These
delays cannot be known exactly because transit over the network and the time required
to read an incoming timestamp both vary.
Rather than using estimates of communications and processing delays, DTS records all
known error factors that accompany a time measurement sent over the network. This
measurement enables DTS to determine the relative quality of a time source regardless of
its geographic location or changing conditions on communications links.
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