What about NTP?

A first try on a non-technical Mini-HOWTO and FAQ on NTP

1999-04-12

Edited by

Ulrich Windl

Ulrich Windl

...work in progress ... contributions welcome...
Revision History
Revision 0.31999-04-12
...still working on it...

Copyright

The material in this document is covered by the respective contributor's copyright. © Copyright 1999, by the contributors and editors of this document. For details see the list of editors and authors.

All editors and contributors agree that this manual can be redistributed under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

If you don't already have the GNU General Public License at hand, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.


Table of Contents
Time, what Time?
How do Computer Clocks work?
Bits and Registers
Making Time go on
Clock Quality
What is NTP?
How does it work?
Configuration of xntp
Reference Clocks
Troubleshooting
General Issues
Specific Products
Background and Related Information
Common Terms and Acronyms

Copyright

The material in this document is covered by the respective contributor's copyright. © Copyright 1999, by the contributors and editors of this document. For details see the list of editors and authors.

All editors and contributors agree that this manual can be redistributed under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

If you don't already have the GNU General Public License at hand, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

This is a first try to explain the most basic features of NTP, and to answer the most frequently asked questions.

Time, what Time?

This section discusses general topics related to time and time-keeping.

Note: Note from the editor: This part is under construction!

.1. What is Time?
.2. What is UTC?
.3. Is UTC the same as GMT?

.1. What is Time?

Note from the editor: Tough question, any volunteers?

.2. What is UTC?

UTC (Universal Time Coordinated, Temps Universel Coordonné) is an official standard for the current time. UTC evolved from the former GMT (Greenwich Mean Time) that once was used to set the clocks on ships before they left for a long journey. Later GMT had been adopted as as the world's standard time. One of the reasons that GMT; had been replaced as official standard time was the fact that it was based on the mean solar time. Newer methods of time measurement showed that the mean solar time varied a lot by itself.

The following list will explain the main components of UTC:

  • Universal means that the time can be used everywhere in the world, meaning that it is independent from time zones (i.e. it's not local time). To convert UTC to local time, one would have to add or subtract the local time zone.

  • Coordinated means that several institutions contribute their estimate of the current time, and UTC is built by combining these estimates.

The UTC second has been defined by the 13th General Conference of Weights and Measures in 1967 as "The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom."

.3. Is UTC the same as GMT?

The observatory in Greenwich derived GMT from astronomical events like the solar day. UTC is based on a quantum resonance of a cesium atom, being quite more accurate.

Unfortunately the earth's rotation is not very much impressed by the definition of the UTC second. Having 86400 UTC seconds per day on an earth that's slowing down would mean that midnight would eventually fall in the middle of the day. As this is probably unacceptable, some extra seconds can be added or removed inside the UTC time-scale to keep synchronization. That patch work is named leap seconds.

To make things worse, leap seconds can be predicted as much as the earths rotation, which is not at all. Therefore you can't easily make calculations for dates in the future using UTC; at least not with accuracy of a few seconds.