wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

18
Clock Synchronization
in Distributed Systems
Georg Gaderer
Austrian Academy
of Sciences
Patrick Loschmidt
Austrian Academy
of Sciences
18.1 Introduction..................................................................................... 18-1
18.2 Precision Time Protocol................................................................. 18-1
18.3 IEEE 1588 System Model............................................................... 18-2
18.4 Service Access Points...................................................................... 18-3
18.5 Ordinary Clocks.............................................................................. 18-3
18.6 Boundary Clocks.............................................................................18-4
18.7 Precision Time Protocol, IEEE 1588–2008 (PTPv2).................18-5
18.8 Network Time Protocol..................................................................18-6
18.9 Network Time Protocol Strata...................................................... 18-7
18.10 Architecture, Protocol, and Algorithms......................................18-8
18.11 NTP Clock Synchronization Hardware Requirements.............18-8
18.12 Synchronization Algorithms of NTP...........................................18-8
References.................................................................................................. 18-10
18.1 Introduction
It is known that technologies to synchronize clocks are not an exclusive research topic of the computer
age. As a matter of fact, the construction of highly accurate clocks and the resulting requirements for
synchronization date back to the seventeenth century. However, the actual research for the usage in
distributed systems began with the introduction of networked computing. The issue was first discussed
in depth with the topic of event ordering by Lamport [Lamport1978]. A good, general overview on
available algorithms can be found in [Anceaume1997]. Later, especially with the upcoming real-time
networks [Kopetz1987,Palumbo1992,Liskov,Patt1994], the problem came again into scientific discussion.
This chapter gives an overview of the state of the art of clock synchronization protocols as well as
their application in real-time networks.
18.2 Precision Time Protocol
The precision time protocol (PTP or IEEE 1588) is a protocol designed for highly accurate clock synchronization
focused especially on test and measurement, as well as power engineering (e.g., substation
automation), and factory automation. The standard is IEEE approved since 2002 [IEEE1588v1].
In 2008, a positive ballot on version 2 of the protocol was submitted to the IEEE [IEEE1588v2].
While the test and measurement industry needs synchronized clocks for data collection
[Eidson2005,Owen2005, Pleasant2005], other classes of applications, like the factory automation community,
need synchronized clocks in order to coordinate the access to the network and thus guarantee
18-1
© 2011 by Taylor and Francis Group, LLC