Commit 9795a14c authored by Eric S. Raymond's avatar Eric S. Raymond

Add a glossary of NTP-speak.

parent 4afc2e27
== Miscellaneous Pages ==
* link:copyright.html[Copyright Notice]
* link:ntpspeak.html[A Glossary of NTP-speak]
* link:decode.html[Event Messages and Status Words]
* link:kern.html[Kernel Model for Precision Timekeeping]
* link:msyslog.html[+{ntpd}+ System Log Messages]
......@@ -175,9 +175,11 @@ The link:sitemap.html[Site Map] page contains a list of document
collections arranged by topic. The Program Manual Pages collection may
be the best place to start. The link:comdex.html[Command Index]
collection contains a list of all configuration file commands together
with a short function description. A great wealth of additional
information is available via the External Links collection, including a
book and numerous background papers and briefing presentations.
with a short function description. There is a
link:ntpsleak.html[glossary of NTP-speak]. A great wealth of
additional information is available via the External Links collection,
including a book and numerous background papers and briefing
Background information on computer network time synchronization is on
the {millshome}exec.html[Executive Summary -
= A Glossary of NTP-speak =
{millshome}pictures.html[from 'Pogo', Walt Kelly]
In an NTP context, refers to the frequency drift of a clock crystal
in an NTP host, expressed in a parts-per-million offset from its
nominal frequency. Changes, slowly, in response to environmental
factors (mainly ambient temperature). {ntpd} measures this by
sampling the clock and performing clock recovery against a
phase-locked loop. The drift measurement is occasionally stored
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 any
problem with the timeserver itself but rather with highly variable
and asymmetric network delays between server and client/
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) a "fudge bit" set in
a configuration file 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.
GPS-constrained Oscillator. A very high-precision atomic clock,
usually a rubidium or cesium crystal oscillator, periodically
resynchronized to <<USNO>> time via <<PPS>>. The most accurate time
source generally available; the only better ones are the
custom-built atomic clocks maintained by national time authorities.
Only cost-effective for time service when intrinsic variability in
network delays is well under GPSDO accuracy; thus, unsuitable for
use on a WAN, for which plain GPSes with <<PPS>> are cheaper and
generally good enough.
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
link:driver28.html[(Type 28)] interface.
A local file containing the current leap-second offset, typically
fetched from <<USNO>> or <<NIST>> and potentially needing updates
near the very beginning or end of each quarter. Normally retrieved
by a cron(1) job, but some varieties of <<refclock>> (notably
GPses) update it themselves suring 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>>.
NIST::[National Institute of Standards and
Technology]. The civilian national time authority of the USA;
runs <<WWVB>>. Responsible for keeping U.S. civil time
coordinated with international UTC time. NIST time tracks <<USNO>>
time to within a few nanoseconds.
NTP Classic::
The original reference implementation of NTP by Dave Mills, later
maintained by the Network Time Foundation. 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
parse driver::
A refclock link:driver8.html[(type 8)] which, uniquely, 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.
(Also as "1PPS") Pulse Per Second. A top-of-second pulse emitted
over RS232 handshake lines by some GPSes that, along with in-band
time delivered to 1-second accuracy, represents U.S. military time
(a very close approximation of UTC) to 50-nanosecond accuracy.
Nowadays the single most important time reference for most NTP
servers en outside the U.S.A.
<<Mills-speak>> for "the transitive completion of the
authentication relstionship", defined in RFC5906. 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.
Sun Audio Driver API, a software interface to audio devices
directly supported by NTP and used by the Irig Audio Decoder (type
6) and IRIG (type 7) refclocks. Originating in SunOS, it was later
implemented in Solaris and OpenSolaris, supported in the Linux OSS
sound layer, and is still used by the *BSD family of open-source
operating systems.
A "stratum" is a layer in the hieratchy of time servers. a
<<refclock>> is considered 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. While strata up to 15 are defined, it is
unusual to see a public timeserver with stratum > 3, and thus
almost all NTP clients are at Stratum 4 or lower.
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), and MSF
(United Kingdom). 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
link#falseticker[falseticker], this is usually less a property of
the server itself than it is of favorable network topology.
USNO::[The United States Naval Observatory],
one of tthe 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 and military 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
in 2013 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.
// end
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