Copyright Notice

The NTPv4 implementation includes provisions for an external clock, where the system clock is implemented by some external hardware device. diff --git a/html/filter.html b/html/filter.html index ed5a17c96..0b6976321 100644 --- a/html/filter.html +++ b/html/filter.html @@ -8,9 +8,9 @@

Clock Filter Algorithm

The clock filter algorithm processes the offset and delay samples produced by the on-wire protocol for each peer process separately. It uses a sliding window of eight samples and picks out the sample with the least expected error. This page describes the algorithm design principles along with an example of typical performance.

diff --git a/html/hints.html b/html/hints.html index 73350a5f8..a4e575929 100644 --- a/html/hints.html +++ b/html/hints.html @@ -9,9 +9,9 @@

Hints and Kinks

from Alice's Adventures in Wonderland, Lewis Carroll

Mother in law has all the answers.

Last update: - 03-Sep-2010 21:56 - UTC

+ Modified: 11-Sep-2011 +

This is an index for a set of troubleshooting notes contained in individual text files in the ./hints directory. They were supplied by various volunteers in the form of mail messages, patches or just plain word of mouth. Each note applies to a specific computer and operating system and gives information found useful in setting up the NTP distribution or site configuration. The notes are very informal and subject to errors; no attempt has been made to verify the accuracy of the information contained in them.

diff --git a/html/history.html b/html/history.html index 506f1d3af..59eb7d120 100644 --- a/html/history.html +++ b/html/history.html @@ -8,9 +8,9 @@

Historical Notes

Last update: - 05-Aug-2011 20:22 - UTC

+ Modified: 03-Oct-2011 +

Historical Notes on NTP Upgrades

This is an interim report on recent upgrades to the NTPv4 reference implementation code base and documentation. This report documents the upgrade program, which began in June 2007 and continued until March 2008. It is very important to recognize that this historic document describes the upgrade status as of 2008. Additional upgrades have been implemented since then. As of mid 2011, the additional upgrades are documented on the NTP Version 4 Release Notes page.

diff --git a/html/howto.html b/html/howto.html index b9baae362..be23dc810 100644 --- a/html/howto.html +++ b/html/howto.html @@ -10,9 +10,9 @@

How to Write a Reference Clock Driver

from Pogo, Walt Kelly

You need a little magic.

Last update: - 03-Sep-2010 1:37 - UTC

+ Modified: 11-Sep-2010 +

The Huff-n'-Puff Filter

Last update: - 13-Oct-2011 19:21 - UTC

+ Modified: 15-Nov-2012 +

In scenarios where a considerable amount of data are downloaded or uploaded using DSL or telephone modem lines, timekeeping quality can be seriously degraded. This occurs because the traffic volume, and thus the queuing delays, on the upload and download directions of transmission can be very different. In many cases the apparent time errors are so large as to exceed the step threshold and a step correction can occur during and after the data transfer.

The huff-n'-puff filter is designed to correct the apparent time offset in these cases. It depends on knowledge of the propagation delay when no other traffic is present, such as during other than work hours. The filter remembers the minimum delay over the most recent interval measured usually in hours. Under conditions of large delay, the filter corrects the apparent offset using the sign of the offset and the difference between the apparent delay and minimum delay. The name of the filter reflects the negative (huff) and positive (puff) correction, which depends on the sign of the offset. The filter is activated by the tinker huffpuff command, as described in the Miscellaneous Options page.

diff --git a/html/index.html b/html/index.html index 18a7102f3..cf215494a 100644 --- a/html/index.html +++ b/html/index.html @@ -10,9 +10,9 @@

The Network Time Protocol (NTP) Distribution

P.T. Bridgeport Bear; from Pogo, Walt Kelly

Pleased to meet you.

Last update: - 11-Nov-2011 1:25 - UTC

+ Modified: 15-Nov-2011 +

Kernel Model for Precision Timekeeping

gif from Pogo, Walt Kelly

Alice finds the kernel a house of cards.

Last update: - 22-Sep-2010 21:25 - UTC

+ Modified: 15-Nov-2012 +

PPSAPI Interface for Precision Time Signals

from NBS Special Publication 432, 1979

Last update: - 03-Sep-2010 23:16 - UTC

+ Modified: 11-Sep-2010 +

`ntp-keygen` - generate public and private keys

gif from Alice's Adventures in Wonderland, Lewis Carroll

Alice holds the key.

Last update: - 03-Oct-2011 7:03 +

+ Modified: 29-Nov-2012

Leap Second Processing

Last update: - 29-Nov-2012 16:43 - UTC

+ Modified: 30-Nov-2012 +

About every eighteen months the International Earth Rotation Service (IERS) issues a bulletin announcing the insertion of a leap second in the Universal Coordinated Time (UTC) timescale. Ordinarily, this happens at the end of the last day of June or December; but, in principle, it could happen at the end of any month. While these bulletins are available on the Internet at www.iers.org, advance notice of leap seconds is also available in signals broadcast from national time and frequency stations, in GPS signals and in telephone modem services. Many, but not all, reference clocks recognize these signals and many, but not all, drivers for them can decode the signals and set the leap bits in the timecode accordingly. This means that many, but not all, primary servers can pass on these bits in the NTP packet heard to dependent secondary servers and clients. Secondary servers can pass these bits to their dependents and so on throughout the NTP subnet.

A leap second is inserted following second 59 of the last minute of the day and becomes second 60 of that day. A leap second is deleted by omitting second 59 of the last minute of the day, although this has never happened and is highly unlikely to happen in future. So far as is known, there are no provisions in the Unix or Windows libraries to account for this occasion other than to affect the conversion of an NTP datestamp or timestamp to conventional civil time.

`ntpd` System Log Messages

from Alice's Adventures in Wonderland, Lewis Carroll

The log can be shrill at times.

Last update: - 04-Sep-2010 0:54 - UTC

+ Modified: 11-Sep-2010 +

Poll Process

Last update: - 20-Apr-2012 19:21 - UTC

+ Modified: 15-Nov-2012 +

The poll process sends NTP packets at intervals determined by the clock discipline algorithm. The process is designed to provide a sufficient update rate to maximize accuracy while minimizing network overhead. The process is designed to operate over a poll exponent range between 3 (8 s) and 17 (36 hr). The minimum and maximum poll exponent within this range can be set using the minpoll and maxpoll options of the server command, with default 6 (64 s) and 10 (1024 s), respectively.

The poll interval is managed by a heuristic algorithm developed over several years of experimentation. It depends on an exponentially weighted average of clock offset differences, called clock jitter, and a jiggle counter, which is initially set to zero. When a clock update is received and the offset exceeds the clock jitter by a factor of 4, the jiggle counter is increased by the poll exponent; otherwise, it is decreased by twice the poll exponent. If the jiggle counter is greater than an arbitrary threshold of 30, it is reset to 0 and the the poll exponent is increased by 1. If the jiggle counter is less than -30, it is set to 0 and the poll exponent decreased by 1. In effect, the algorithm has a relatively slow reaction to good news, but a relatively fast reaction to bad news.

Pulse-Per-Second (PPS) Signal Interfacing

from Alice's Adventures in Wonderland, Lewis Carroll

Alice is trying to find the PPS signal connector.

Last update: - 02-Nov-2010 22:57 - UTC

+ Modified: 15-Nov-2012 +

Mitigation Rules and the `prefer` Keyword

from Alice's Adventures in Wonderland, Lewis Carroll

Listen carefully to what I say; it is very complicated.

Last update: - 18-Jul-2012 16:41 - UTC

+ Modified: 15-Nov-2012 +

Rate Management and the Kiss-o'-Death Packet

from Pogo, Walt Kelly

Our junior managers and the administrators.

Last update: - 21-Apr-2012 16:27 - UTC

+ Modified: 15-Nov-2012 +

Debugging Reference Clock Drivers

from The Wizard of Oz, L. Frank Baum

Call the girls and the'll sweep your bugs.

Last update: - - 03-Sep-2010 1:35 - - UTC

+ Modified: 11-Sep-2010 +

Reference Clock Support

UDel Internet Research Laboratory

- Modified: 31-Jan-2014 + Modified: 01-Mar-2014

Clock Select Algorithm

Last update: - 13-Apr-2012 14:58 - UTC

+ Modified: 15-Nov-2012 +

The clock select algorithm determines from a set of sources , which are correct (truechimers) and which are not (falsetickers) according to a set of formal correctness assertions. The principles are based on the observation that the maximum error in determining the offset of a candidate cannot exceed one-half the roundtrip delay to the primary reference clock at the time of measurement. This must be increased by the maximum error that can accumulate since then. The selection metric, called the root distance,, is one-half the roundtrip root delay plus the root dispersion plus minor error contributions not considered here.

First, a number of sanity checks is performed to sift the selectable candidate from among the source population. The sanity checks are sumarized as follows:.

Performance Metrics

Last update: - 20-Jun-2012 18:33 - UTC

+ Modified: 15-Nov-2012 +

`tickadj` - set time-related kernel variables

Last update: - 04-Sep-2010 17:59 - UTC

+ Modified: 11-Jul-2011 +

Synopsis

tickadj [ -Aqs ] [ -a tickadj ] [ -t tick ]

How NTP Works

Last update: - 18-Aug-2012 19:04 - UTC

+ Modified: 15-Nov-2012 +

NTP Interleaved Modes

from Pogo, Walt Kelly

You need a little magic.

Last update: - 06-Jun-2012 14:59 - UTC

+ Modified: 15-Nov-2012 +

In the protocol described in the NTP specification and reference implementation up to now, the transmit timestamp, which is captured before the message digest is computed and the packet queued for output, is properly called as a softstamp The receive timestamp, which is captured after the input driver interrupt routine and before the packet is queued for input, is properly called a drivestamp. For enhanced accuracy it is desirable to capture the transmit timestamp as close to the wire as possible; for example, after the output driver interrupt routine.

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