TIMESLAVE(1M)TIMESLAVE(1M)NAMEtimeslave - 'slave' local clock to a better one
SYNOPSIStimeslave [ options ] -H netmaster
timeslave [ options ] -C /dev/ttydXX
where options can be any of:
[-mdBQY] [-w icmp-wait] [-W time-wait]
[-t min-trials] [-T max-trials] [-r rate]
[-p port] [-D max-drift] [-i init-drift]
[-y year] [-P param-file] [-c clock-type]
DESCRIPTION
Timeslave matches the clock in the local machine to a better clock. It
does this by speeding up or slowing down the local clock, or if the local
clock is particularly wrong, by changing the date. When the date is
changed, because the difference is too great to correct smoothly,
timeslave logs the event in the system log.
Timeslave understands several options:
-H netmaster
specifies the hostname or Internet address of another machine that
has a better clock or that has, in turn, slaved its clock to a
better one. Either a host must be specified with -H or a device
must be specified with -C, but not both.
-C /dev/ttyXX
specifies the filename of a tty port, such as /dev/ttyd2, with a
Precision Time Standards or Traconex/Multisonics WWV receiver.
-P param-file
specifies a file in which to save a computed value of the timetrim
parameter. The active value in the operating system is set to the
value found in the file when the daemon started. A good choice for
the file name is /var/adm/timetrim.
-c clock-type
specifies the kind of WWV or GPS receiver/clock connected to the
device specified with -C. Supported types of clocks are:
TRC_WWV Traconix
PST_WWV or Precision Standard Time WWV receiver
TT600 TrueTime model 600 GPS receiver
The default clock type is PST_WWV.
It is usually not necessary to use the following options to change the
default values compiled in timeslave:
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TIMESLAVE(1M)TIMESLAVE(1M)-m causes timeslave to measure the difference between the local and
remote clock (or WWV receiver), but to not change the local clock.
-d increases the 'debugging level,' causing more messages to appear in
the system log. Specifying this option several times increases the
amount of debugging information. The signals SIGUSR1 and SIGUSR2
can be used to increase and decrease, respectively, the current
amount of debugging information.
-B disables the normal 'backgrounding' of the daemon.
-q disables error messages caused by network links that make the
netmaster intermittently unreachable. This can be useful for
"demand dialing" SLIP links (see slip(1M)).
-w icmp-wait
is the number of seconds before an ICMP timestamp packet is assumed
to have been lost in the network. The default is 1 second.
-W time-wait
is the number of seconds before the remote machine is assumed to be
not answering time service requests. The first time the remote
machine fails to answer a time service request is logged in the
system log. Additional failures are also logged if debugging is
requested. The default is 4 seconds.
-t min-trials
specifies the minimum number of ICMP packets to use to determine the
time difference. The default value is calculated as follows: min-
trials=4 if the default value of max-trials is used, otherwise min-
trials=max-trials/4.
-T max-trials
specifies the maximum number of ICMP packets to use to determine the
time difference. If the remote machine does not respond to this
many requests, it is assumed to be temporarily dead. The default
value is calculated as follows: max-trials=16 if the default value
of min-trials is used, otherwise max-trials=min-trials*4.
-r rate
specifies the approximate number of seconds between measurements.
The separation between measurements must be at least 5 seconds.
Shorter periods do not allow enough time to correct the local clock.
The default is 3 minutes and 30 seconds. A period of 90 seconds is
always used when first starting to start the filters. Very long
periods do not allow provide enough data to avoid over correction.
The time between one measurement and the next is changed slightly
each time to avoid systematic errors caused by other activity in the
machines or the network in general. For example, cron starts a
request in the first few seconds of a minute.
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TIMESLAVE(1M)TIMESLAVE(1M)-p port
is the port number on which the other machine provides Internet time
service, as describe in RFC-868. The default port comes from the
"time" udp service defined in /etc/services.
-D max-drift
specifies the maximum rate at which the local and remote clocks can
be expected to drift, in nanoseconds per second. This is used to
discard bad measurements. Its default value is 1% or 10,000,000
nanoseconds per second.
-i init-drift
specifies an initial value of the drift of the local clock in
nanoseconds per second. This value can be used to make timeslave
better behaved when it is first started. The default is 0 drift.
-y year
specifies a year, overriding the value obtained from the WWV
receiver or the remote system. This parameter is handy when -C is
used and the switch settings to change the year in the WWV receiver
are forgotten, or when -H is used and the netmaster is confused
about the year. The year is automatically adjusted at midnight,
January 1.
-Y specifies that the year should be consistent with the modification
date of the file specified with -P.
Timeslave consumes less network bandwidth, fewer CPU cycles and less
memory on both the local and remote machines than timed(1M). Timed is
appropriate for a group of peers which average their equivalently
accurate clocks to find a better estimate of the correct time. Because
the master of the network time is elected by the timed participants, more
correct time can be maintained despite the failure of individual systems.
A small, homogeneous group of machines should use timed, rather than
timeslave. In a large network, one should use a small group of trusted,
well-administered machines running timed with the -F or -G option to
exclude other machines. The excluded machines should run timed with only
the -M flag. One of the trusted machines should, if possible, be slaved
with timeslave to a WWV receiver, or some remote machine with a very
accurate clock. This machine should be lightly loaded, and should use
the -F option with timed to make itself the most trusted time keeper in
the local network. This scheme, a ``king'' with a circle of trusted
substitutes surrounded by a larger number of machines should be
replicated on each subnetwork in an installation, building a hierarchy of
domains. The local king can use timeslave to synchronize its clock to
very remote machines over low-performance, wide-area networks such as the
DARPA Internet.
A subnetwork consisting of two or more sub-subnetworks, such as a group
of ethernets, can be synchronized in a similar fashion. The king of the
subnetwork would be synchronized with timeslave to a remote network or a
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TIMESLAVE(1M)TIMESLAVE(1M)
WWV receiver (see below). The circle of trusted substitutes would be the
gateways to the other sub-subnetworks.
Timeslave operates in one of two modes. In the first, it matches the
clock in the local machine to the clock in some other machine. It sends
UDP datagrams to the 'time' service on the other machine to determine the
current day. The time service on machines using 4.3BSD-style networking
is provided by the inetd(1M) daemon. This allows timeslave to determine
the correct day. Timeslave then uses ICMP timestamp packets to measure
difference between the local and remote clocks in milliseconds. It
assumes that the round trip delay for packets to and from the remote
machine is symmetric. This is usually a valid assumption. However,
large network data transfers can make transmission delays tens of
thousands of times larger or smaller than reception delays. Therefore,
timeslave elaborately averages its measurements. In addition, it lowers
its scheduling priority to minimize the effects of other programs on the
local machine.
The second mode in which timeslave can operate is with a Precision Time
Standards or Traconex/Multisonics WWV receiver. In this case, it still
must average its measurements to compensate for variable delays due to
the operating system. When using a clock, it is best to minimize any
latencies in the serial line connected to the clock, by adjusting the
relevant parameters for the driver. See cdsio(1M) and sduart(1M). The
switches in the receiver should be configured to use "eight bits without
parity," a "baud rate" of 9600, and daylight-saving time disabled.
Timeslave logs accumulated corrections in the system log (see syslogd(1M)
to ease adjusting the local clock. The clock can be adjusted by changing
the timetrim parameter using systune(1M). This parameter is used by the
operating system to compensate for variations among machines. It can be
used to improve the accuracy of the local clock.
Timeslave immediately replaces the kernel value of the timetrim parameter
with the contents of the file specified with the -P option, and after
measuring for several hours, chooses a better value. The value is
continually refined. Besides logging in the system the value, timeslave
periodically writes the value to the file specified with -P. An error
message is displayed when the daemon is first started if the file
contains nonsense. However, a corrected value will be placed in the file
during its normal updates.
Messages printed by the kernel on the system console occur with
interrupts disabled. This means that the clock stops while they are
printing. A machine with many disk or network hardware problems and
consequent messages cannot keep good time by itself. Each message
typically causes the clock to lose a dozen milliseconds. A time daemon
can correct the result, but will compute a bogus timetrim value.
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TIMESLAVE(1M)TIMESLAVE(1M)WARNING
Temporal chaos results if two or more time daemons, whether timed,
timeslave, or NTP, try to adjust the same clock.
FILES
/var/adm/SYSLOG system log
/etc/init.d/network start-up script
/etc/config/timeslave.options optional configuration flags
/dev/ttydXX tty port attached to clock.
SEE ALSOchkconfig(1M), inetd(1M), syslog(1M), timed(1M), timedc(1M), icmp(7P),
systune(1M)BUGS
The filtering is not good enough when the local clock is worse than one
part in 10**4 or the network is overloaded.
Timeslave does not communicate its confidence in the time to a timed
running on the local machine.
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