ntp.drift(4) Kernel Interfaces Manual ntp.drift(4)NAME
ntp.drift - Network Time Protocol (NTP) drift file
DESCRIPTION
When the NTP daemon (xntpd) is first started, it computes the error in the intrinsic frequency of the clock on the computer it is running
on. This process usually takes about a day or two after the daemon is started to compute a good estimate of this (and it needs a good
estimate to synchronize closely to its server). Once the initial value is computed, it will change only by relatively small amounts during
the course of continued operation.
The driftfile declaration should always be included in the ntp.conf file. This provides xntpd with complete path name to a file in which
it can store the current value of the frequency error. That way, if the daemon is stopped and restarted, it can reinitialize itself to the
previous estimate without spending time recomputing the frequency estimate.
The ntp.drift file contains one line with the following format: freq_offset flag Specifies the nominal frequency offset. DO NOT modify
this field. Specifies the method used to improve system clock accuracy. Specifies that xntpd runs a Phase-Locked Loop (PLL) algorithm to
keep synchronized with other hosts. This is the default. Specifies the use of the PLL algorithm in the kernel. This requires the
NTP_TIME kernel option. After the option is configured in the kernel, edit the ntp.drift file and change the flag value to 1.
See System Administration for information on the NTP_TIME kernel option.
FILES
Conventional name of the drift file
RELATED INFORMATION
Commands: ntpdate(8), ntpq(8), xntpd(8), xntpdc(8)
Files: ntp.conf(4)
Network Administration, System Administration delim off
ntp.drift(4)
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ntp_intro(7) Miscellaneous Information Manual ntp_intro(7)NAME
ntp_intro - Network Time Protocol (NTP) introductory information
DESCRIPTION
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 syn-
chronizes 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
The Network Time Protocol (NTP) provides accurate, dependable, and synchronized time for hosts on both wide area networks (like the Inter-
net) and local area networks. In particular, NTP provides synchronization traceable to clocks of high absolute accuracy, and avoids syn-
chronization 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 direc-
tions.) 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 sam-
ple.
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 syn-
chronized 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 accu-
rate. This information includes the output of the digital-filtering algorithm and the stratum numbers of the hosts it queried. By commu-
nicating 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
The Time Synchronization Protocol (TSP) is the protocol used by the 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 run-
ning TSP as a master. The master then controls the further operation of the protocol until it fails and a new master is elected. The mas-
ter collects time values from the other hosts and computes the average of all the times reported. It then sets its own clock to this aver-
age, 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.
RELATED INFORMATION
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
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