ntp_intro - Network Time Protocol (NTP) introductory
information
The Tru64 UNIX software supports two time services: Network
Time Protocol (NTP) Time Synchronization Protocol
(TSP)
Because it can be traced to clocks of high absolute accuracy,
NTP provides a more accurate time service than TSP.
By contrast, TSP synchronizes time to the average of the
network host times. TSP is an acceptable time service if
your system is not on the Internet and does not have
access to a highly accurate time server; otherwise, NTP is
recommended.
Network Time Protocol [Toc] [Back]
The Network Time Protocol (NTP) provides accurate, dependable,
and synchronized time for hosts on both wide area
networks (like the Internet) and local area networks. In
particular, NTP provides synchronization traceable to
clocks of high absolute accuracy, and avoids synchronization
to clocks keeping bad time.
Hosts running NTP periodically exchange datagrams querying
each other about their current estimate of the time.
Using the round-trip time of the packet, a host can estimate
the one-way delay to the other host. (The assumption
is that the delay is roughly equal in both directions.) By
measuring the one-way delay and examining the timestamps
that are returned with the NTP packet, a host computes the
difference between its clock time and that of the host it
queried.
A host queries a remote host several times over a period
and feeds the results from the multiple samples to a digital-filtering
algorithm. The algorithm provides a more
accurate estimate of the delay, clock offset, and clock
stability than could be obtained with a single sample.
NTP messages also contain information about the accuracy
and reliability of the time sources. An NTP host connected
directly to a highly accurate time source, such as
a radio receiver tuned to a time code signal broadcast by
a government agency, is called a stratum 1 server. Every
other NTP host adopts a stratum number that is one higher
than the host from which it sets its own time. For example,
a host synchronized to a stratum 1 server becomes a
stratum 2 host. Stratum determination is done automatically,
and the stratum of a host can vary as its connectivity
changes.
A host running NTP combines various information to decide
which of the hosts it queried provides the time it
believes to be the most accurate. This information
includes the output of the digital-filtering algorithm and
the stratum numbers of the hosts it queried. By communicating
with several other hosts, an NTP host can usually
detect those hosts that are keeping bad time, and is able
to stay synchronized even if some of the other hosts
become unavailable for long periods.
In practice, NTP is able to synchronize clocks to within a
few milliseconds even over wide area networks spanning
thousands of miles. To obtain even greater accuracy, use
the NTP_TIME kernel option. See System Administration for
more information.
You can optionally use a high-resolution clock, enabling
the time returned by the clock_gettime routine to be
extrapolated between the clock ticks. The granularity of
the time returned will be in microseconds. To use the
high-resolution clock, use the MICRO_TIME kernel option.
For detailed information on NTP, see Network Time Protocol
(Version 3) (RFC 1305).
Time Synchronization Protocol [Toc] [Back]
The Time Synchronization Protocol (TSP) is the protocol
used by the /usr/sbin/timed daemon. In its simplest
application, the TSP servers on a broadcast network (for
example, an Ethernet) periodically broadcast TSP packets.
The hosts on the network elect one of the hosts on the
network running TSP as a master. The master then controls
the further operation of the protocol until it fails and a
new master is elected. The master collects time values
from the other hosts and computes the average of all the
times reported. It then sets its own clock to this average,
and tells the other hosts to synchronize their clocks
with it.
TSP quickly synchronizes all participating hosts. However,
because TSP does not trace time back to sources of
known accuracy, it is unable to correct for systematic
errors. If a clock drifts significantly, or if a mistake
is made in setting the time on a participating host, the
average time calculated and distributed by the master can
be affected significantly.
For information on setting up the network time services,
refer to Network Administration: Services.
Commands:ntp(1), ntpdate(8), ntptrace(8), timedsetup(8),
xntpdc(8)
Functions: ctime(3)
Files: ntp.conf(4), ntp.drift(4), ntp.keys(4)
Daemons: timed(8), xntpd(8)
Network Time Protocol (Version 3) (RFC 1305)
Network Administration: Services System Administration
ntp_intro(7)
[ Back ] |
