The Network Time Protocol (NTP) Distribution

From: Alice's Adventures in Wonderland, Lewis Carroll, illustrations by Sir John Tenniel


Note: The software contained in this distribution is available without charge under the conditions set forth in the Copyright Notice.

The Network Time Protocol (NTP) is used to synchronize the time of a computer client or server to another server or reference time source, such as a radio or satellite receiver or modem. It provides client accuracies typically within a millisecond on LANs and up to a few tens of milliseconds on WANs relative to a primary server synchronized to Coordinated Universal Time (UTC) via a Global Positioning Service (GPS) receiver, for example. Typical NTP configurations utilize multiple redundant servers and diverse network paths, in order to achieve high accuracy and reliability. Some configurations include cryptographic authentication to prevent accidental or malicious protocol attacks.

Background information on computer network time synchronization can be found on the Executive Summary - Computer Network Time Synchronization page. Discussion on protocol conformance issues and interoperability with previous NTP versions can be found in the Protocol Conformance Statement page. Discussion on year-2000 issues can be found in the Year 2000 Conformance Statement page. Background information, bibliography and briefing slides suitable for presentations can be found in the NTP Reference Library page.

Building and Installing NTP

The Building and Installing the Distribution page presents an overview of the procedures for compiling the distribution and installing it on a typical client or server. The build procedures inspect the system hardware and software environment and automatically select the appropriate options for that environment. While these procedures work with most computers and operating systems marketed today, exceptions requiring manual intervention do exist, as documented in the Configuration Options and Release Notes pages.

Bringing up a NTP primary server requires a radio or satellite receiver or modem. The distribution includes hardware drivers for over two dozen radio clocks and modem services. A list of the particular receivers and modem drivers supported in the distribution is given in the Reference Clock Drivers page. For most popular workstations marketed by DEC, Sun and Hewlett Packard, the automatic build procedures select all drivers that run on the target machine. While this increases the size of the executable binary somewhat, individual drivers can be included or excluded using the configure utility documented in the Configuration Options page..

Configuring Clients and Servers

In order to participate in the existing NTP synchronization subnet and obtain accurate, reliable time, it is necessary to construct an appropriate configuration file, usually called ntp.conf, which establishes the servers and/or external receivers or modems to be used by this particular machine. Directions for constructing this file are in the Notes on Configuring NTP and Setting up a NTP Subnet page.

The most important factor in providing accurate, reliable time is the selection of modes and servers to be used in the configuration file. NTP support for one or more computers is normally engineered as part of the existing NTP synchronization subnet. The existing NTP subnet consists of a multiply redundant hierarchy of servers and clients, with each level in the hierarchy identified by stratum number. Primary servers operate at stratum one and provide synchronization to secondary servers operating at stratum two and so on to higher strata. In this hierarchy, clients are simply servers that have no dependents.

The NTP subnet in early 1998 includes 70 public primary (stratum 1) servers synchronized directly to UTC by radio, satellite or modem and located in every continent of the globe, except Antarctica (soon). Normally, client workstations and servers with a relatively small number of clients do not synchronize to primary servers. There are 106 public secondary (stratum 2) servers synchronized to the primary servers and providing synchronization to a total in excess of 100,000 clients and servers in the Internet. The current lists are maintained in the List of Public NTP Servers page, which is updated frequently. There are numerous private primary and secondary servers not normally available to the public as well. You are strongly discouraged from using these servers, since they sometimes hide in little ghettos behind dinky links to the outside world and your traffic can bring up expensive ISDN lines, causing much grief and anger.

Resolving Problems

Like other things Internet, the NTP synchronization subnets tend to be large and devilishly intricate, with many opportunities for misconfiguration and network problems. The NTP engineering model is specifically designed to help isolate and repair such problems using an integrated management protocol, together with a suite of monitoring and debugging tools. There is an optional data recording facility which can be used to record normal and aberrant operation, log problems to the system log facility, and retain records of client access. The NTP Debugging Techniques and Hints and Kinks pages contain useful information for identifying problems and devising solutions.  ntp.htm

Users are requested to report bugs, offer suggestions and contribute additions to this distribution. The Patching Procedures page suggests procedures which greatly simplify distribution updates, while the Porting Hints suggest ways to make porting this code to new hardware and operating systems easier. Additional information on reference clock driver construction and debugging can be found in the Reference Clock Drivers page. Further information on NTP in the Internet can be found in the NTP web page.

Program Manual Pages

ntpd - Network Time Protocol (NTP) daemon
ntpq - standard NTP query program
ntpdc - special NTP query program
ntpdate - set the date and time via NTP
ntptrace - trace a chain of NTP servers back to the primary source
tickadj - set time-related kernel variables
ntptime - read kernel time variables
authspeed - determine authentication delays
authcert - DES validation program
md5 - generate message digest

Supporting Documentation

NTP Reference Library
Copyright Notice
Executive Summary - Computer Network Time Synchronization
Protocol Conformance Statement
Year 2000 Conformance Statement
Notes on Configuring NTP and Setting up a NTP Subnet
NTP Version 4 Release Notes
Building and Installing the Distribution
Configuration Options
NTP Debugging Techniques
Reference Clock Drivers
Patching Procedures
Hints and Kinks
Porting Hints

Application Notes

Mitigation Rules and the prefer Keyword
Association Management
Gadget Box PPS Level Converter and CHU Modem
Time and Time Interval Measurement with Application to Computer and Network Performance Evaluation
A Kernel Model for Precision Timekeeping
A Kernel Programming Interface for Precision Time Signals  
David L. Mills (