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