NIST Automated Computer Time Service. NIST provides a civil time reference for computers equipped with analog modems that can dial in to its phone lines. Due to variability of delays in the analog phone network accuracy is not very good, around 4 milliseconds compared to around a microsecond for [PPS]. But the service is still occasionally used as a backup to more precise sources.


An association is the relationship that an NTP client and server have when the server is computing and shipping time updates to the client and the client is expecting them. There are several different kinds of associations including unicast, manycast, and broadcast modes; another variable is whether the association is authenticated or unauthenticated. A significant amount of NTP’s complexity is the means for discovering association partners, creating and maintaining associations, and preventing attackers from forming associations.

association ID

A numeric identifier designating an association, normally internal to ntpd but exposed in [mode 6] packets.


Old name for the PPS driver. Origin lost in the mists of history, but probably related in some way to cesium atomic clocks.


An [era]; code comments sometimes use this term to emphasize that modular arithmetic is going on.


In an NTP context, drift refers to the frequency offset of a clock crystal in an NTP host that causes the system time to slowly drift. It is usually expressed in a parts-per-million (PPM) offset from its nominal frequency. It changes, slowly, in response to environmental factors (mainly ambient temperature). ntpd measures drift by sampling the clock and performing clock recovery against a phase-locked loop. The drift measurement can be stored and updated locally to a drift file so that when ntpd is stopped and restarted it doesn’t have to go through the entire resampling and resynchronization process before providing reliable time.


[Mills-speak] for a timeserver identified as not reliable by statistical filtering. Usually this does not imply a problem with the timeserver itself but rather with highly variable and asymmetric network delays between server and client, but firmware bugs in GPS receivers have produced falsetickers.

  1. The zero date of an NTP era. The "prime epoch" (of era 0) was 1900-00-00T00:00:00 in proleptic UTC (leap second correction was not introduced until 1972). 2. Other calendar systems have other definitions; notably, the Unix epoch is 1970-00-00T00:00:00.


One complete revolution of an NTP 64-bit timestamp; approximately 136 years. Eras are numbered from 0, but this era number is not represented internally in NTP code because modular-arithmetic trickery is used to deduce the nearest time that could fit a given timestamp.


Can have one of two senses. Either (1) an offset configured for a [refclock] or server to correct its time, reversing a fixed or nearly-fixed propagation delay, or (2) in the old config syntax, a "fudge bit" was what is now called a driver option - set in a refclock declaration to change the refclock’s behavior in some driver-dependent way.


Historical. An operating system running on PDP-11s used for early time-service and Internet routing experiments. Metonymously, a PDP-11 running the suite. While some of these remained in service as late as 1988 they are now long gone, but have left a few traces in the NTP codebase.


Global Positioning System; in common parlance "a GPS" is a radio receiver designed to get position and time fixes from the system. GPS fixes are derived from spherical trigonometry using the precisely known positions of satellites in a geocentric coordinate system. GPS also provides time service; those that emit [PPS] are suitable as clock sources for Stratum 1 timeservers. In timekeeping, the term is used to refer not only to the original U.S. GPS system, but newer constellations that work on the same principles, such as ГЛОНАСС (the Russian GLONASS), 北斗 (the Chinese BeiDou-2), and the EU’s Galileo.


GPS Disciplined Oscillator. A good crystal is synchronized to time from a GPS receiver. The combination gets the short term stability of the crystal and the long term stability of GPS. With appropriate firmware, the crystal can be used when the GPS receiver is temporarily non-operational; see [holdover]. Most cell towers have GPSDOs. You can usually spot the conical GPS antenna.


The GPS Daemon, an open-source device manager for GPSes and other geodetic sensors. Frequently used as a clock source by Stratum 1 sites via the SHM interface.

GPS pivot date

The GPS pivot date is a specific fixed arbitrary time. It is stored in the firmware of a GPS receiver, and is probably the date that firmware was released. For this reason, a GPS receiver will start reporting incorrect time 512 weeks (9.8 years) or 1024 weeks (19.6 years) after it’s pivot date. There is generally no way to determine what a given GPS receiver’s pivot date is, or to determine that it has failed in this manner.

It is recommended that any critical Stratum 1 NTP server that uses
a GPS receiver as a refclock not use one that is more than 9 years
old, and to have a peer or nopeer relationship with other NTP
servers, so as to detect when the GPS time is no longer sane.

In connection with a [GPSDO] or [time radio] that may lose signal from its time source, holdover is its ability to continue delivering accurate time from an internal oscillator. Due to [drift] in the oscillator, accuracy drops as holdover time (time since last signal lock) increases.

in-band time

GPS-based time sources often deliver two channels of time information. The timestamp delivered as ASCII data over the serial link is called "in-band time" to contrast it with the out-of-band [PPS] signal. Abbreviated IBT. Seldom useful by itself as it tends to have a large random wander from top of second. However, it is useful as a count of seconds.

leap second

Because the earth’s rotation varies in irregular ways (gradually slowing due to tidal drag among other forces) and the second is now defined in absolute terms rather than as a fraction of day length, keeping time of day synchronized with mean solar time requires occasional, unpredictable insertions of a standard second in the calendar. Leap second notifications are issued as Bulletin C by the International Earth Rotation Service when required, and obeyed by national time authorities. The current leap-second offset is automatically propagated through the [GPS] system.


A local file containing the current [leap second] offset, typically fetched from [USNO] or [NIST] and potentially needing updates near the end of each half year. Normally retrieved by a cron(1) job, but some varieties of [refclock] (notably GPSes) update the same information during normal operation.


Dr. David Mills, the original architect of NTP and its standards, wrote in a vivid and idiosyncratic style which is still preserved in much of NTP’s documentation. He coined many neologisms which connoisseurs refer to as "Mills-speak"; examples in this glossary include [falseticker], [proventic], and [truechimer]. The close-to-definitive reference is The NTP dictionary, though not all of those terms are still in use.

mode 6

Mode 6 is a control protocol used to get various kinds of status information from a running ntpd and configure it on the fly. So-called from the value 6 (0110) in the packet mode field. It is described in detail mode6.html[here].


National Institute of Standards and Technology. The civilian national time authority of the U.S.; runs [WWVB]. Responsible for keeping U.S. civil time coordinated with UTC. Civil [NIST] and military [USNO] time agree to within nanoseconds.

NTP Classic

The original reference implementation of NTP by Dave Mills, later maintained by the Network Time Foundation. It is available at http://www.ntp.org . NTPsec forked from it on June 6th, 2015.


An arbitrary number that may only be used once. A random or pseudo-random number issued in an authentication protocol (such as NTP’s) to ensure that old communications cannot be reused in replay attacks.

parse driver

A refclock, now known as generic, which handles multiple [time radio] protocols - two dozen mostly European precision time radios mostly using DCF, the broadcast time from the German national authority. Each clock type is internally represented as a few parse methods and table entries in a table-driven parser. Older documentation used the term "parse driver" interchangeably with "generic driver".

popcorn spike

A packet from a particular peer that has a computed theta more than three standard deviations from the previous sample. These are normally treated as random outliers and suppressed.


(Also as "1PPS") Pulse Per Second. A top-of-second pulse emitted by time sources, typically a GPS receiver or atomic clocks. It is often connected to a RS-232 modem control signal pin. A PPS signal doesn’t tell you which second a pulse corresponds to so it must be used with an ancillary signal, typically text over the same RS-232 connector (see [in-band time]). The advantage of a PPS signal is improved accuracy. Most OSes have provisions to grab a timestamp at interrupt time. The API is described in RFC 2783.


Precision Time Protocol, which does for hosts on a local area network what NTP does on the general Internet. By taking advantage of the (usually) more controlled conditions of a LAN, it is capable of much higher sustained accuracy than NTP, into the sub-microsecond range.


In an NTP context, "the pool" is usually the NTP Pool Project, a collection of thousands of NTP servers around the world that you can use to keep time. The DNS names are designed to keep the timeservers you select relatively "close" to you on the internet, with varying degrees of specificity, e.g. using north-america.pool.ntp.org may connect you to timeservers from Canada to Panama, while us.pool.ntp.org is more likely to connect you to timeservers within the continental U.S.. These pool timeservers are usually run by volunteers. See the Quick Start for Client Configurations for a more in-depth discussion of pools.


[Mills-speak] for "the transitive completion of the authentication relationship", defined in RFC 5906. Time is proventic if it is provided by a chain of time servers between which packets are authenticated and the chain reaches back to Stratum 1.


Shorthand for a "reference clock", a primary time source. A computer with a refclock is implicitly Stratum 1. Examples include [GPS] or [time radio] receivers, or even a master cesium clock.


A "stratum" is a layer in the hierarchy of time servers. A [refclock] is stratum 0; a computer directly attached to a refclock is stratum 1; and a client served by a stratum N is stratum N+1. Often capitalized, especially when referring to all members of a stratum. The larger the number, the further away from the source clock, thus the less accurate. Stratum 16 is the maximum, and indicates a device that is unreachable and unsynchronized. The majority of public timeservers are Stratum 1 and Stratum 2.

time radio

A radio receiver specialized for picking up accurate time reference signals broadcast over the air by a national time authority; notable ones include [WWVB] (U.S.), CHU (Canada), DCF (Germany), MSF (United Kingdom), and JJY (Japan). Usable as a Stratum 1 time source; may be qualified by "precision time radio" to distinguish from consumer-grade "atomic clocks", which are time radios that normally take radio synchronization just once a day and are not accurate enough to be used for Stratum 1. Precision time radios used to be important time sources, but (especially in the U.S.) have been largely obsolesced by [GPS] and [GPSDO]-based clocks.


[Mills-speak] for a timeserver that provides time believed good, that is with low jitter with respect to UTC. As with a [falseticker], this is usually less a property of the server itself than it is of favorable network topology.


The United States Naval Observatory, one of the two U.S. national time authorities and the source of the U.S. military time reference, now delivered primarily by GPS signals. U.S. civil [NIST] and military [USNO] time agree to within nanoseconds.


The U.S. national time radio station, run by [NIST] and broadcasting a highly accurate U.S. civil-time reference. Formerly an important primary time source, until NIST changed its modulation at 2012-10-29T15:00:00Z and obsolesced all then-existing U.S. precision time radios. No WWVB-compatible precision time radios have been manufactured since, though consumer-grade "atomic clocks" unaffected by the modulation change are still made.

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