OSF DCE Administration Guide--Core Components

Chapter 23. Introduction to DCE Distributed
Time Service
This chapter gives a conceptual overview of the DCE Distributed Time Service (DTS).
Some basic time and clock concepts, DTS time representation, and basic DTS operation
are also presented.
DTS is a software-based service that provides precise, fault-tolerant clock
synchronization for systems in local area networks (LANs) and wide area networks
(WANs). The clock synchronization that is provided by DTS enables distributed
computing applications to determine event sequencing, duration, and scheduling.
DTS consists of software components on a group of cooperating systems; it conforms to
the client/server model that is used in DCE. In the DTS implementation, each server
supplies the time to many client systems and applications through intermediaries called
clerks. Clerks reside on their client systems. (Note that, throughout this part of this
guide, the term entity is used to refer to either the server process or the clerk process
when they have the same functions.)
Most DCE nodes have a DTS clerk that adjusts the clock on its client system; clerks use
remote procedure calls (RPCs) to obtain time values from one or several servers in the
network. The nodes that do not have DTS clerks have DTS servers; in addition to
providing time values to clerks, servers also adjust the system clocks on their host
systems. Servers are also able to obtain reference time values from sources of
standardized time that are outside of the network.
Because no device can measure the exact time at a particular instant, DTS expresses the
time as an interval that contains the correct time. In the DTS model, clerks obtain time
intervals from several servers and compute the intersection where the intervals overlap.
Clerks then adjust the system clocks of their client systems to the midpoint of the
computed intersection. When clerks receive a time interval that does not intersect with
the majority, the clerks declare the nonintersecting value to be faulty. Clerks ignore
faulty values when computing new times, thereby ensuring that defective server clocks
do not affect clients.
DTS also permits the importation of time values from outside sources, such as the U.S.
National Institute for Standards and Technology (NIST). DTS uses the UTC
(Coordinated Universal Time) standard that has largely replaced Greenwich Mean Time
(GMT) as a reference. Many standards bodies disseminate UTC by radio, telephone, and
satellite; commercial devices (time-providers) are available to receive and interpret these
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