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A Kernel Model for Precision Timekeeping
Revised 3 April 1994
Note: This memorandum is a substantial revision of RFC-1589, "A Kernel
Model for Precision Timekeeping," March, 1994. It includes several
changes to the daemon and user interfaces, as well as a new feature
which disciplines the CPU clock oscillator in both time and frequency to
a source of precision time signals. This memorandum is included in the
distributions for the SunOS, Ultrix and OSF/1 kernels and in the NTP
Version 3 distribution (xntp3.v.tar.Z) as the file README.kern, where v
is the version identifier. Availability of the kernel distributions,
which involve licensed code, will be announced separately. The NTP
Version 3 distribution can be obtained via anonymous ftp from
louie.udel.edu in the directory pub/ntp. In order to utilize all
features of this distribution, the NTP version identifier should be 3q
or later.
Overview
This memorandum describes an engineering model which implements a
precision time-of-day function for a generic operating system. The model
is based on the principles of disciplined oscillators and phase-lock
loops (PLL) and frequency-lock loops (FLL) often found in the
engineering literature. It has been implemented in the Unix kernels for
several workstations, including those made by Sun Microsystems and
Digital Equipment. The model changes the way the system clock is
adjusted in time and frequency, as well as provides mechanisms to
discipline its frequency to an external precision timing source. The
model incorporates a generic system-call interface for use with the
Network Time Protocol (NTP) or similar time synchronization protocol.
The NTP Version 3 daemon xntpd operates with this model to provide
synchronization limited in principle only by the accuracy and stability
of the external timing source.
This memorandum does not obsolete or update any RFC. It does not propose
a standard protocol, specification or algorithm. It is intended to
provoke comment, refinement and implementations for kernels not
considered herein. While a working knowledge of NTP is not required for
an understanding of the design principles or implementation of the
model, it may be helpful in understanding how the model behaves in a
fully functional timekeeping system. The architecture and design of NTP
is described in [MIL91], while the current NTP Version 3 protocol
specification is given in RFC-1305 [MIL92a] and a subset of the
protocol, the Simple Network Time Protocol (SNTP), is given in RFC-1361
[MIL92c].
The model has been implemented in the Unix kernels for three Sun
Microsystems and Digital Equipment workstations. In addition, for the
Digital machines the model provides improved precision to one
microsecond (us). Since these specific implementations involve
modifications to licensed code, they cannot be provided directly.
Inquiries should be directed to the manufacturer's representatives.
However, the engineering model for these implementations, including a
simulator with code segments almost identical to the implementations,
but not involving licensed code, is available via anonymous FTP from
host louie.udel.edu in the directory pub/ntp and compressed tar archive
kernel.tar.Z. The NTP Version 3 distribution can be obtained via
anonymous ftp from the same host and directory in the compressed tar
archive xntp3.3q.tar.Z, where the version number shown as 3.3q may be
adjusted for new versions as they occur.
1. Introduction
This memorandum describes a model and programming interface for generic
operating system software that manages the system clock and timer
functions. The model provides improved accuracy and stability for most
computers using the Network Time Protocol (NTP) or similar time
synchronization protocol. This memorandum describes the design
principles and implementations of the model, while related technical
reports discuss the design approach, engineering analysis and
performance evaluation of the model as implemented in Unix kernels for
modern workstations. The NTP Version 3 daemon xntpd operates with these
implementations to provide improved accuracy and stability, together
with diminished overhead in the operating system and network. In
addition, the model supports the use of external timing sources, such as
precision pulse-per-second (PPS) signals and the industry standard IRIG
timing signals. The NTP daemon automatically detects the presence of the
new features and utilizes them when available.
There are three prototype implementations of the model presented in this
memorandum, one each for the Sun Microsystems SPARCstation with the
SunOS 4.1.x kernel, Digital Equipment DECstation 5000 with the Ultrix
4.x kernel and Digital Equipment 3000 AXP Alpha with the OSF/1 V1.x
kernel. In addition, for the DECstation 5000/240 and 3000 AXP Alpha
machines, a special feature provides improved precision to 1 us (stock
Sun kernels already do provide this precision). Other than improving the
system clock accuracy, stability and precision, these implementations do
not change the operation of existing Unix system calls which manage the
system clock, such as gettimeofday(), settimeofday() and adjtime();
however, if the new features are in use, the operations of
gettimeofday() and adjtime() can be controlled instead by new system
calls ntp_gettime() and ntp_adjtime() as described below.
A detailed description of the variables and algorithms that operate upon
them is given in the hope that similar functionality can be incorporated
in Unix kernels for other machines. The algorithms involve only minor
changes to the system clock and interval timer routines and include
interfaces for application programs to learn the system clock status and
certain statistics of the time synchronization process. Detailed
installation instructions are given in a specific README files included
in the kernel distributions.
In this memorandum, NTP Version 3 and the Unix implementation xntp3 are
used as an example application of the new system calls for use by a
synchronization daemon. In principle, these system calls can be used by
other protocols and implementations as well. Even in cases where the
local time is maintained by periodic exchanges of messages at relatively
long intervals, such as using the NIST Automated Computer Time Service
[LEV89], the ability to precisely adjust the system clock frequency
simplifies the synchronization procedures and allows the telephone call
frequency to be considerably reduced.
2. Design Approach
While not strictly necessary for an understanding or implementation of
the model, it may be helpful to briefly describe how NTP operates to
control the system clock in a client computer. As described in [MIL91],
the NTP protocol exchanges timestamps with one or more peers sharing a
synchronization subnet to calculate the time offsets between peer clocks
and the local clock. These offsets are processed by several algorithms
which refine and combine the offsets to produce an ensemble average,
which is then used to adjust the local clock time and frequency. The
manner in which the local clock is adjusted represents the main topic of
this memorandum. The goal in the enterprise is the most accurate and
stable system clock possible with the available computer hardware and
kernel software.
In order to understand how the new model works, it is useful to review
how most Unix kernels maintain the system clock. In the Unix design a
hardware counter interrupts the kernel at a fixed rate: 100 Hz in the
SunOS kernel, 256 Hz in the Ultrix kernel and 1024 Hz in the OSF/1
kernel. Since the Ultrix timer interval (reciprocal of the rate) does
not evenly divide one second in microseconds, the kernel adds 64 us once
each second, so the timescale consists of 255 advances of 3906 us plus
one of 3970 us. Similarly, the OSF/1 kernel adds 576 us once each
second, so its timescale consists of 1023 advances of 976 us plus one of
1552 us.
2.1. Mechanisms to Adjust Time and Frequency
In most Unix kernels it is possible to slew the system clock to a new
offset relative to the current time by using the adjtime() system call.
To do this the clock frequency is changed by adding or subtracting a
fixed amount (tickadj) at each timer interrupt (tick) for a calculated
number of timer interrupts. Since this calculation involves dividing the
requested offset by tickadj, it is possible to slew to a new offset with
a precision only of tickadj, which is usually in the neighborhood of 5
us, but sometimes much larger. This results in a roundoff error which
can accumulate to an unacceptable degree, so that special provisions
must be made in the clock adjustment procedures of the synchronization
daemon.
In order to implement a frequency discipline function, it is necessary
to provide time offset adjustments to the kernel at regular adjustment
intervals using the adjtime() system call. In order to reduce the system
clock jitter to the regime consistent with the model, it is necessary
that the adjustment interval be relatively small, in the neighborhood of
1 s. However, the Unix adjtime() implementation requires each offset
adjustment to complete before another one can be begun, which means that
large adjustments must be amortized over possibly many adjustment
intervals. The requirement to implement the adjustment interval and
compensate for roundoff error considerably complicates the synchronizing
daemon implementation.
In the new model this scheme is replaced by another that represents the
system clock as a multiple-word, precision-time variable in order to
provide very precise clock adjustments. At each timer interrupt a
precisely calibrated quantity is added to the kernel time variable and
overflows propagated as required. The quantity is computed as in the NTP
local clock model described in [MIL92b], which operates as an adaptive-
parameter, first-order, type-II phase-lock loop (PLL). In principle,
this PLL design can provide precision control of the system clock
oscillator within 1 us and frequency to within parts in 10^11. While
precisions of this order are surely well beyond the capabilities of the
CPU clock oscillator used in typical workstations, they are appropriate
using precision external oscillators, as described below.
The PLL design is identical to the one originally implemented in NTP and
described in [MIL92b]. In the original design the software daemon
simulates the PLL using the adjtime() system call; however, the daemon
implementation is considerably complicated by the considerations
described above. The modified kernel routines implement the PLL in the
kernel using precision time and frequency representations, so that these
complications are avoided. A new system call ntp_adjtime() is called
only as each new time update is determined, which in NTP occurs at
intervals of from 16 s to 1024 s. In addition, doing frequency
compensation in the kernel means that the system clock runs true even if
the daemon were to cease operation or the network paths to the primary
synchronization source fail.
In the new model the new ntp_adjtime() operates in a way similar to the
original adjtime() system call, but does so independently of adjtime(),
which continues to operate in its traditional fashion. When used with
NTP, it is the design intent that settimeofday() or adjtime() be used
only for system clock adjustments greater than +-128 ms, although the
dynamic range of the new model is much larger at +-512 ms. It has been
the Internet experience that the need to change the system clock in
increments greater than +-128 ms is extremely rare and is usually
associated with a hardware or software malfunction or system reboot.
The easiest way to set the time is with the settimeofday() system call;
however, this can under some conditions cause the clock to jump
backwards. If this cannot be tolerated, adjtime() can be used to slew
the clock to the new value without running backward or affecting the
frequency discipline process. Once the system clock has been set within
+-128 ms, the ntp_adjtime() system call is used to provide periodic
updates including the time offset, maximum error, estimated error and
PLL time constant. With NTP the update interval and time constant depend
on the measured delay and dispersion; however, the scheme is quite
forgiving and neither moderate loss of updates nor variations in the
update interval are serious.
2.2 Daemon and Application Interface
Unix application programs can read the system clock using the
gettimeofday() system call, which returns only the system time and
timezone data. For some applications it is useful to know the maximum
error of the reported time due to all causes, including clock reading
errors, oscillator frequency errors and accumulated latencies on the
path to the primary synchronization source. However, in the new model
the PLL adjusts the system clock to compensate for its intrinsic
frequency error, so that the time error expected in normal operation
will usually be much less than the maximum error. The programming
interface includes a new system call ntp_gettime(), which returns the
system time, as well as the maximum error and estimated error. This
interface is intended to support applications that need such things,
including distributed file systems, multimedia teleconferencing and
other real-time applications. The programming interface also includes a
new system call ntp_adjtime(), which can be used to read and write
kernel variables for time and frequency adjustment, PLL time constant,
leap-second warning and related data.
In addition, the kernel adjusts the indicated maximum error to grow by
an amount equal to the maximum oscillator frequency tolerance times the
elapsed time since the last update. The default engineering parameters
have been optimized for update intervals in the order of 64 s. As shown
in [MIL93], this is near the optimum interval for NTP used with ordinary
room-temperature quartz oscillators. For other intervals the PLL time
constant can be adjusted to optimize the dynamic response over intervals
of 16-1024 s. Normally, this is automatically done by NTP. In any case,
if updates are suspended, the PLL coasts at the frequency last
determined, which usually results in errors increasing only to a few
tens of milliseconds over a day using typical modern workstations.
While any synchronization daemon can in principle be modified to use the
new system calls, the most likely will be users of the NTP Version 3
daemon xntpd. The xntpd code determines whether the new system calls are
implemented and automatically reconfigures as required. When
implemented, the daemon reads the frequency offset from a system file
and provides it and the initial time constant via ntp_adjtime(). In
subsequent calls to ntp_adjtime(), only the time offset and time
constant are affected. The daemon reads the frequency from the kernel
using ntp_adjtime() at intervals of about one hour and writes it to a
system file. This information is recovered when the daemon is restarted
after reboot, for example, so the sometimes extensive training period to
learn the frequency separately for each oscillator can be avoided.
2.3. Precision Clocks for DECstation 5000/240 and 3000 AXP Alpha
The stock microtime() routine in the Ultrix kernel for Digital Equipment
MIPS-based workstations returns system time to the precision of the
timer interrupt interval, which is in the 1-4 ms range. However, in the
DECstation 5000/240 and possibly other machines of that family, there is
an undocumented IOASIC hardware register that counts system bus cycles
at a rate of 25 MHz. The new microtime() routine for the Ultrix kernel
uses this register to interpolate system time between timer interrupts.
This results in a precision of 1 us for all time values obtained via the
gettimeofday() and ntp_gettime() system calls. For the Digital Equipment
3000 AXP Alpha, the architecture provides a hardware Process Cycle
Counter and a machine instruction (rpcc) to read it. This counter
operates at the fundamental frequency of the CPU clock or some
submultiple of it, 133.333 MHz for the 3000/400 for example. The new
microtime() routine for the OSF/1 kernel uses this counter in the same
fashion as the Ultrix routine. Support for this feature is conditionally
compiled in the kernel only if the MICRO option is used in the kernel
configuration file.
In both the Ultrix and OSF/1 kernels the gettimeofday() and
ntp_gettime() system call use the new microtime() routine, which returns
the interpolated value to 1-us resolution, but does not change the
kernel time variable. Therefore, other routines that access the kernel
time variable directly and do not call either gettimeofday(),
ntp_gettime() or microtime() will continue their present behavior. The
microtime() feature is independent of other features described here and
is operative even if the kernel PLL or new system calls have not been
implemented.
The SunOS kernel already includes a system clock with 1-us resolution;
so, in principle, no microtime() routine is necessary. An existing
kernel routine uniqtime() implements this function, but it is coded in
the C language and is rather slow at 42-85 us per call on a SPARCstation
IPC. A replacement microtime() routine coded in assembler language is
available in the NTP Version 3 distribution and is much faster at about
3 us per call. Note that, as explained later, this routine should be
called at an interrupt priority level not greater than that of the timer
interrupt routine. Otherwise, it is possible to miss a tick increment,
with result the time returned can be late by one tick. This is always
true in the case of gettimeofday() and ntp_gettime(), but might not be
true in other cases, such as when using the PPS signal described later
in this memorandum.
2.4. External Time and Frequency Discipline
The overall accuracy of a time synchronization subnet with respect to
Coordinated Universal Time (UTC) depends on the accuracy and stability
of the primary synchronization source, usually a radio or satellite
receiver, and the CPU clock oscillator of the primary server. As
discussed in [MIL93], the traditional interface using a ASCII serial
timecode and RS232 port precludes the full accuracy of most radio
clocks. In addition, the poor frequency stability of typical CPU clock
oscillators limits the accuracy, whether or not precision time sources
are available. There are, however, several ways in which the system
clock accuracy and stability can be improved to the degree limited only
by the accuracy and stability of the synchronization source and the
jitter of the interface and operating system.
Many radio clocks produce special signals that can be used by external
equipment to precisely synchronize time and frequency. Most produce a
pulse-per-second (PPS) signal that can be read via a modem-control lead
of a serial port and some produce a special IRIG signal that can be read
directly by a bus peripheral, such as the KSI/Odetics TPRO IRIG SBus
interface, or indirectly via the audio codec of some workstations, as
described in [MIL93]. In the NTP Version 3 daemon xntpd, the PPS signal
can be used to augment the less precise ASCII serial timecode to improve
accuracy to the order of a few tens of microseconds. Support is also
included in the NTP distribution for the TPRO interface, as well as the
audio codec; however, the latter requires a modified kernel audio driver
contained in the compressed tar archive bsd_audio.tar.Z in the same host
and directory as the NTP Version 3 distribution mentioned previously.
2.4.1. PPS Signal
The most convenient way to interface a PPS signal to a computer is
usually with a serial port and RS232-compatible signal; however, the PPS
signal produced by most radio clocks and laboratory instruments is
usually a TTL pulse signal. Therefore, some kind of level
converter/pulse generator is necessary to adapt the PPS signal to a
serial port. An example design, including schematic and printed-circuit
board artwork, is in the compressed tar archive gadget.tar.Z in the same
host and directory as the NTP Version 3 distribution mentioned
previously. There are several ways the PPS signal can be used in
conjunction with the NTP Version 3 daemon xntpd, as described in [MIL93]
and in the documentation included in the distribution.
The NTP Version 3 distribution includes a special ppsclock module for
the SunOS 4.1.x kernel that captures the PPS signal presented via a
modem-control lead of a serial port. Normally, the ppsclock module
produces a timestamp at each transition of the PPS signal and provides
it to the synchronization daemon for integration with the serial ASCII
timecode, also produced by the radio clock. With the conventional PLL
implementation in either the daemon or the kernel as described in
[MIL93], the accuracy of this scheme is limited by the intrinsic
stability of the CPU clock oscillator to a millisecond or two, depending
on environmental temperature variations.
The ppsclock module has been modified to in addition call a new kernel
routine hardpps() once each second. In addition, the Ultrix 4.3 kernel
has been modified to provide a similar functionality. The hardpps()
routine compares the timestamp with a sample of the CPU clock oscillator
in order to discipline the oscillator to the time and frequency of the
PPS signal. Using this method, the time accuracy is improved to
typically 20 us or less and frequency stability a few parts in 10^8,
which is about two orders of magnitude better than the undisciplined
oscillator. The new feature is conditionally compiled in the code
described below only if the PPS_SYNC option is used in the kernel
configuration file.
When using the PPS signal to adjust the time, there is a problem with
some kernels which is very difficult to fix. The serial port interrupt
routine often operates at an interrupt priority level above the timer
interrupt routine. Thus, as explained below, it is possible that a tick
increment can be missed and the time returned late by one tick. It may
happen that, if the CPU clock oscillator frequency is close to the PPS
oscillator frequency (less than a few ppm), this condition can persist
for two or more successive PPS interrupts. A useful workaround in the
code is to use a glitch detector and median filter to process the PPS
sample offsets. The glitch detector suppresses offset bursts greater
than half the tick interval and which last less than 30 successive PPS
interrupts. The median filter ranks the offsets in a moving window of
three samples and uses the median as the output and the difference
between the other two as a dispersion measure.
2.4.2. External Clocks
It is possible to replace the system clock function with an external bus
peripheral. The TPRO device mentioned previously can be used to provide
IRIG-synchronized time with a precision of 1 us. A driver for this
device tprotime.c and header file tpro.h are included in the
kernel.tar.Z distribution mentioned previously. Using this device, the
system clock is read directly from the interface; however, the device
does not record the year, so special provisions have been made to obtain
the year from the kernel time variable and initialize the driver
accordingly. Support for this feature is conditionally compiled in the
kernel only if the EXT_CLOCK and TPRO options are used in the kernel
configuration file.
While the system clock function is provided directly by the microtime()
routine in the driver, the kernel time variable must be disciplined as
well, since not all system timing functions use the microtime() routine.
This is done by measuring the time difference between the microtime()
clock and kernel time variable and using it to adjust the kernel PLL as
if the adjustment were provided by an external peer and NTP.
A good deal of error checking is done in the TPRO driver, since the
system clock is vulnerable to a misbehaving radio clock, IRIG signal
source, interface cables and TPRO device itself. Unfortunately, there is
no practical way to utilize the extensive diversity and redundancy
capabilities available in the NTP synchronization daemon. In order to
avoid disruptions that might occur if the TPRO time is far different
from the kernel time variable, the latter is used instead of the former
if the difference between the two exceeds 1000 s; presumably in that
case operator intervention is required.
2.4.2. External Oscillators
Even if a source of PPS or IRIG signals is not available, it is still
possible to improve the stability of the system clock through the use of
a specialized bus peripheral. In order to explore the benefits of such
an approach, a special SBus peripheral called HIGHBALL has been
constructed. The device includes a pair of 32-bit hardware counters in
Unix timeval format, together with a precision, oven-controlled quartz
oscillator with a stability of a few parts in 10^9. A driver for this
device hightime.c and header file high.h are included in the
kernel.tar.Z distribution mentioned previously. Support for this feature
is conditionally compiled in the kernel only if the EXT_CLOCK and
HIGHBALL options are used in the kernel configuration file.
Unlike the external clock case, where the system clock function is
provided directly by the microtime() routine in the driver, the HIGHBALL
counter offsets with respect to UTC must be provided first. This is done
using the ordinary kernel PLL, but controlling the counter offsets
directly, rather than the kernel time variable. At first, this might
seem to defeat the purpose of the design, since the jitter and wander of
the synchronization source will affect the counter offsets and thus the
accuracy of the time. However, the jitter is much reduced by the PLL and
the wander is small, especially if using a radio clock or another
primary server disciplined in the same way. In practice, the scheme
works to reduce the incidental wander to a few parts in 10^8, or about
the same as using the PPS signal.
As in the previous case, the kernel time variable must be disciplined as
well, since not all system timing functions use the microtime() routine.
However, the kernel PLL cannot be used for this, since it is already in
use providing offsets for the HIGHBALL counters. Therefore, a special
correction is calculated from the difference between the microtime()
clock and the kernel time variable and used to adjust the kernel time
variable at the next timer interrupt. This somewhat roundabout approach
is necessary in order that the adjustment does not cause the kernel time
variable to jump backwards and possibly lose or duplicate a timer event.
2.5 Other Features
It is a design feature of the NTP architecture that the system clocks in
a synchronization subnet are to read the same or nearly the same values
before during and after a leap-second event, as declared by national
standards bodies. The new model is designed to implement the leap event
upon command by an ntp_adjtime() argument. The intricate and sometimes
arcane details of the model and implementation are discussed in [MIL92b]
and [MIL93]. Further details are given in the technical summary later in
this memorandum.
3. Technical Summary
In order to more fully understand the workings of the model, a stand-
alone simulator kern.c and header file timex.h are included in the
kernel.tar.Z distribution mentioned previously. In addition, an example
kernel module kern_ntptime.c which implements the ntp_gettime() and
ntp_adjtime() system calls is included. Neither of these programs
incorporate licensed code. Since the distribution is somewhat large, due
to copious comments and ornamentation, it is impractical to include a
listing of these programs in this memorandum. In any case, implementors
may choose to snip portions of the simulator for use in new kernel
designs; but, due to formatting conventions, this would be difficult if
included in this memorandum.
The kern.c program is an implementation of an adaptive-parameter, first-
order, type-II phase-lock loop. The system clock is implemented using a
set of variables and algorithms defined in the simulator and driven by
explicit offsets generated by the main() routine in the program. The
algorithms include code fragments almost identical to those in the
machine-specific kernel implementations and operate in the same way, but
the operations can be understood separately from any licensed source
code into which these fragments may be integrated. The code fragments
themselves are not derived from any licensed code. The following
discussion assumes that the simulator code is available for inspection.
3.1. PLL Simulation
The simulator operates in conformance with the analytical model
described in [MIL92b]. The main() program operates as a driver for the
fragments hardupdate(), hardclock(), second_overflow(), hardpps() and
microtime(), although not all functions implemented in these fragments
are simulated. The program simulates the PLL at each timer interrupt and
prints a summary of critical program variables at each time update.
There are three defined options in the kernel configuration file
specific to each implementation. The PPS_SYNC option provides support
for a pulse-per-second (PPS) signal, which is used to discipline the
frequency of the CPU clock oscillator. The EXT_CLOCK option provides
support for an external kernel-readable clock, such as the KSI/Odetics
TPRO IRIG interface or HIGHBALL precision oscillator, both for the SBus.
The TPRO option provides support for the former, while the HIGHBALL
option provides support for the latter. External clocks are implemented
as the microtime() clock driver, with the specific source code selected
by the kernel configuration file.
The PPS signal is carefully monitored for error conditions which can
affect accuracy, stability and reliability. The time_status kernel
variable contains bits that both control the use of the PPS signal and
reveal its operational status. The function of each bit is described in
a later section of this memo.
3.1.1. The hardupdate() Fragment
The hardupdate() fragment is called by ntp_adjtime() as each update is
computed to adjust the system clock phase and frequency. Note that the
time constant is in units of powers of two, so that multiplies can be
done by simple shifts. The phase variable is computed as the offset
divided by the time constant, but clamped to a maximum (for robustness).
Then, the time since the last update is computed and clamped to a
maximum and to zero if initializing. The offset is multiplied (sorry
about the ugly multiply) by the result and divided by the square of the
time constant and then added to the frequency variable. Note that all
shifts are assumed to be positive and that a shift of a signed quantity
to the right requires a little dance.
The STA_PLL and STA_PPSTIME status bits, which are set by the
ntp_adjtime() system call, serve to enable or inhibit the kernel PLL and
PPS time-discipline functions. The STA_PPSSIGNAL status bit is set by
the hardpps() code fragment when the PPS signal is present and operating
within nominal bounds. Time discipline from the PPS signal operates only
if both the STA_PPSTIME and STA_PPSSIGNAL bits are set; otherwise, the
discipline operates from the offset given in the ntp_adjtime() system
call. In the intended mode of operation, the synchronization daemon sets
STA_PLL to enable the PLL when first initialized, then sets STA_PPSTIME
when reliable synchronization to within +-128 ms has been achieved with
either a radio clock or external peer. The daemon can detect and
indicate this condition for monitoring purposes by noting that both
STA_PPSTIME and STA_PPSSIGNAL are set.
With the defines given in the program and header files, the maximum time
offset is determined by the size in bits of the long type (32 or 64)
less the SHIFT_UPDATE scale factor (12) or at least 20 bits (signed).
The scale factor is chosen so that there is no loss of significance in
later steps, which may involve a right shift up to SHIFT_UPDATE bits.
This results in a time adjustment range over +-512 ms. Since
time_constant must be greater than or equal to zero, the maximum
frequency offset is determined by the SHIFT_USEC scale factor (16) or at
least 16 bits (signed). This results in a frequency adjustment range
over +-31,500 ppm.
In the addition step, the value of offset * mtemp is not greater than
MAXPHASE * MAXSEC = 31 bits (signed), which will not overflow a long add
on a 32-bit machine. There could be a loss of precision due to the right
shift of up to 12 bits, since time_constant is bounded at 6. This
results in a net worst-case frequency resolution of about .063 ppm,
which is not significant for most quartz oscillators. The worst case
could be realized only if the NTP peer misbehaves according to the
protocol specification.
The time_offset value is clamped upon entry. The time_phase variable is
an accumulator, so is clamped to the tolerance on every call. This helps
to damp transients before the oscillator frequency has been stabilized,
as well as to satisfy the correctness assertions if the time
synchronization protocol or implementation misbehaves.
3.1.2. The hardclock() Fragment
The hardclock() fragment is inserted in the hardware timer interrupt
routine at the point the system clock is to be incremented by the value
of tick. Previous to this fragment the time_update variable has been
initialized to the tick increment plus the value computed by the
adjtime() system call in the stock Unix kernel, normally plus/minus the
tickadj value, which is usually in the order of 5 us. The time_phase
variable, which represents the instantaneous phase of the system clock,
is advanced by time_adj, which is calculated in the second_overflow()
fragment described below. If the value of time_phase exceeds 1 us in
scaled units, time_update is increased by the (signed) excess and
time_phase retains the residue.
In those cases where a PPS signal is connected by a serial port
operating at an interrupt priority level greater than the timer
interrupt, special consideration should be given the location of the
hardclock() fragment in the timer interrupt routine. The system clock
should be advanced as early in the routine as possible, preferably
before the hardware timer interrupt flag is cleared. This reduces or
eliminates the possibility that the microtime() routine may latch the
time after the flag is cleared, but before the system clock is advanced,
which results in a returned time late by one tick.
Except in the case of an external oscillator such as the HIGHBALL
interface, the hardclock() fragment advances the system clock by the
value of tick plus time_update. However, in the case of an external
oscillator, the system clock is obtained directly from the interface and
time_update used to discipline that interface instead. However, the
system clock must still be disciplined as explained previously, so the
value of clock_cpu computed by the second_overflow() fragment is used
instead.
3.1.3. The second_overflow() Fragment
The second_overflow() fragment is inserted at the point where the
microseconds field of the system time variable is being checked for
overflow. Upon overflow the maximum error time_maxerror is increased by
time_tolerance to reflect the maximum time offset due to oscillator
frequency error. Then, the increment time_adj to advance the kernel time
variable is calculated from the (scaled) time_offset and time_freq
variables updated at the last call to the hardclock() fragment.
The phase adjustment is calculated as a (signed) fraction of the
time_offset remaining, where the fraction is added to time_adj, then
subtracted from time_offset. This technique provides a rapid convergence
when offsets are high, together with good resolution when offsets are
low. The frequency adjustment is the sum of the (scaled) time_freq
variable, an adjustment necessary when the tick interval does not evenly
divide one second fixtick and PPS frequency adjustment pps_freq (if
configured).
The scheme of approximating exact multiply/divide operations with shifts
produces good results, except when an exact calculation is required,
such as when the PPS signal is being used to discipline the CPU clock
oscillator frequency as described below. As long as the actual
oscillator frequency is a power of two in Hz, no correction is required.
However, in the SunOS kernel the clock frequency is 100 Hz, which
results in an error factor of 0.78. In this case the code increases
time_adj by a factor of 1.25, which results in an overall error less
than three percent.
On rollover of the day, the leap-second state machine described below
determines whether a second is to be inserted or deleted in the
timescale. The microtime() routine insures that the reported time is
always monotonically increasing.
3.1.4. The hardpps() Fragment
The hardpps() fragment is operative only if the PPS_SYNC option is
specified in the kernel configuration file. It is called from the serial
port driver or equivalent interface at the on-time transition of the PPS
signal. The code operates as a first-order, type-I, frequency-lock loop
(FLL) controlled by the difference between the frequency represented by
the pps_freq variable and the frequency of the hardware clock
oscillator. It also provides offsets to the hardupdate() fragment in
order to discipline the system clock time.
In order to avoid calling the microtime() routine more than once for
each PPS transition, the interface requires the calling program to
capture the system time and hardware counter contents at the on-time
transition of the PPS signal and provide a pointer to the timestamp
(Unix timeval) and counter contents as arguments to the hardpps() call.
The hardware counter contents are determined by saving the microseconds
field of the system time, calling the microtime() routine, and
subtracting the saved value. If a microseconds overflow has occurred
during the process, the resulting microseconds value will be negative,
in which case the caller adds 1000000 to normalize the microseconds
field.
In order to avoid large jitter when the PPS interrupt occurs during the
timer interrupt routine before the system clock is advanced, a glitch
detector is used. The detector latches when an offset exceeds a
threshold tick/2 and stays latched until either a subsequent offset is
less than the threshold or a specified interval MAXGLITCH (30 s) has
elapsed. As long as the detector remains latched, it outputs the offset
immediately preceding the latch, rather than the one received.
A three-stage median filter is used to suppress jitter less than the
glitch threshold. The median sample drives the PLL, while the difference
between the other two samples represents the time dispersion. Time
dispersion samples are averaged and used as a jitter estimate. If this
estimate exceeds a threshold MAXTIME/2 (100 us), an error bit
STA_PPSJITTER is raised in the status word.
The frequency of the hardware oscillator is determined from the
difference in hardware counter readings at the beginning and end of the
calibration interval divided by the duration of the interval. However,
the oscillator frequency tolerance, as much as 100 ppm, may cause the
difference to exceed the tick value, creating an ambiguity. In order to
avoid this ambiguity, the hardware counter value at the beginning of the
interval is increased by the current pps_freq value once each second,
but computed modulo the tick value. At the end of the interval, the
difference between this value and the value computed from the hardware
counter is the control signal for the FLL.
Control signal samples which exceed the frequency tolerance MAXFREQ (100
ppm) are discarded, as well as samples resulting from excessive interval
duration jitter. In these cases an error bit STA_PPSERROR is raised in
the status word. Surviving samples are then processed by a three-stage
median filter. The median sample drives the FLL, while the difference
between the other two samples represents the frequency dispersion.
Frequency dispersion samples are averaged and used as a stabiity
estimate. If this estimate is below a threshold MAXFREQ/4 (25 ppm), the
median sample is used to correct the oscillator frequency pps_freq with
a weight expressed as a shift PPS_AVG (2).
Initially, an approximate value for the oscillator frequency is not
known, so the duration of the calibration interval must be kept small to
avoid overflowing the tick. The time difference at the end of the
calibration interval is measured. If greater than tick/4, the interval
is reduced by half. If less than this fraction for four successive
calibration intervals, the interval is doubled. This design
automatically adapts to nominal jitter in the PPS signal, as well as the
value of tick. The duration of the calibration interval is set by the
pps_shift variable as a shift in powers of two. The minimum value
PPS_SHIFT (2) is chosen so that with the highest CPU oscillator
frequency 1024 Hz and frequency tolerance 100 ppm the tick will not
overflow. The maximum value PPS_SHIFTMAX (8) is chosen such that the
maximum averaging time is about 1000 s as determined by measurements of
Allan variance [MIL93].
Should the PPS signal fail, the current frequency estimate pps_freq
continues to be used, so the nominal frequency remains correct subject
only to the instability of the undisciplined oscillator. The procedure
to save and restore the frequency estimate works as follows. When
setting the frequency from a file, the time_freq value is set as the
file value minus the pps_freq value; when retrieving the frequency, the
two values are added before saving in the file. This scheme provides a
seamless interface should the PPS signal fail or the kernel
configuration change. Note that the frequency discipline is active
whether or not the synchronization daemon is active. Since all Unix
systems take some time after reboot to build a running system, usually
by that time the discipline process has already settled down and the
initial transients due to frequency discipline have damped out.
3.1.4. External Clock Interface
The external clock driver interface is implemented with two routines,
microtime(), which returns the current clock time, and clock_set(),
which furnishes the apparent system time derived from the kernel time
variable. The latter routine is called only when the clock is set using
the settimeofday() system call, but can be called from within the
driver, such as when the year rolls over, for example.
In the stock SunOS kernel and modified Ultrix and OSF/1 kernels, the
microtime() routine returns the kernel time variable plus an
interpolation between timer interrupts based on the contents of a
hardware counter. In the case of an external clock, such as described
above, the system clock is read directly from the hardware clock
registers. Examples of external clock drivers are in the tprotime.c and
hightime.c routines included in the kernel.tar.Z distribution.
The external clock routines return a status code which indicates whether
the clock is operating correctly and the nature of the problem, if not.
The return code is interpreted by the ntp_gettime() system call, which
transitions the status state machine to the TIME_ERR state if an error
code is returned. This is the only error checking implemented for the
external clock in the present version of the code.
The simulator has been used to check the PLL operation over the design
envelope of +-512 ms in time error and +-100 ppm in frequency error.
This confirms that no overflows occur and that the loop initially
converges in about 15 minutes for timer interrupt rates from 50 Hz to
1024 Hz. The loop has a normal overshoot of a few percent and a final
convergence time of several hours, depending on the initial time and
frequency error.
3.2. Leap Seconds
It does not seem generally useful in the user application interface to
provide additional details private to the kernel and synchronization
protocol, such as stratum, reference identifier, reference timestamp and
so forth. It would in principle be possible for the application to
independently evaluate the quality of time and project into the future
how long this time might be "valid." However, to do that properly would
duplicate the functionality of the synchronization protocol and require
knowledge of many mundane details of the platform architecture, such as
the subnet configuration, reachability status and related variables. For
the curious, the ntp_adjtime() system call can be used to reveal some of
these mysteries.
However, the user application may need to know whether a leap second is
scheduled, since this might affect interval calculations spanning the
event. A leap-warning condition is determined by the synchronization
protocol (if remotely synchronized), by the timecode receiver (if
available), or by the operator (if awake). This condition is set by the
synchronization daemon on the day the leap second is to occur (30 June
or 31 December, as announced) by specifying in a ntp_adjtime() system
call a status bit of either STA_DEL, if a second is to be deleted, or
STA_INS, if a second is to be inserted. Note that, on all occasions
since the inception of the leap-second scheme, there has never been a
deletion, nor is there likely to be one in future. If the bit is
STA_DEL, the kernel adds one second to the system time immediately
following second 23:59:58 and resets the clock state to TIME_WAIT. If
the bit is STA_INS, the kernel subtracts one second from the system time
immediately following second 23:59:59 and resets the clock stateto
TIME_OOP, in effect causing system time to repeat second 59. Immediately
following the repeated second, the kernel resets the clock status to
TIME_WAIT.
Following the leap operations, the clock remains in the TIME_WAIT state
until both the STA_DEL and STA_INS status bits are reset. This provides
both an unambiguous indication that a leap recently occured, as well as
time for the daemon or operator to clear the warning condition.
Depending upon the system call implementation, the reported time during
a leap second may repeat (with the TIME_OOP return code set to advertise
that fact) or be monotonically adjusted until system time "catches up"
to reported time. With the latter scheme the reported time will be
correct before and shortly after the leap second (depending on the
number of microtime() calls during the leap second), but freeze or
slowly advance during the leap second itself. However, Most programs
will probably use the ctime() library routine to convert from timeval
(seconds, microseconds) format to tm format (seconds, minutes,...). If
this routine is modified to use the ntp_gettime() system call and
inspect the return code, it could simply report the leap second as
second 60.
3.3. Clock Status State Machine
The various options possible with the system clock model described in
this memorandum require a careful examination of the state transitions,
status indications and recovery procedures should a crucial signal or
interface fail. In this section is presented a prototype state machine
designed to support leap second insertion and deletion, as well as
reveal various kinds of errors in the synchronization process. The
states of this machine are decoded as follows:
TIME_OK If a PPS signal or external clock is present, it is
working properly and the system clock is derived
from it. If not, the synchronization daemon is
working properly and the system clock is
synchronized to a radio clock or one or more peers.
TIME_INS An insertion of one second in the system clock has
been declared following the last second of the
current day, but has not yet been executed.
TIME_DEL A deletion of the last second of the current day has
been declared, but not yet executed.
TIME_OOP An insertion of one second in the system clock has
been declared following the last second of the
current day. The second is in progress, but not yet
completed. Library conversion routines should
interpret this second as 23:59:60.
TIME_WAIT The scheduled leap event has occurred, but the
STA_DEL and STA_INS status bits have not yet been
cleared.
TIME_ERROR Either (a) the synchronization daemon has declared
the protocol is not working properly, (b) all
sources of outside synchronization have been lost or
(c) a PPS signal or external clock is present, but
not working properly.
In all states the system clock is derived from either a PPS signal or
external clock, if present, or the kernel time variable, if not. If a
PPS error condition is recognized, the PPS signal is disabled and
ntp_adjtime() updates are used instead. If an external clock error
condition is recognized, the external clock is disabled and the kernel
time variable is used instead.
The state machine makes a transition once each second at an instant
where the microseconds field of the kernel time variable overflows and
one second is added to the seconds field. However, this condition is
checked when the timer overflows, which may not coincide with the actual
seconds increment. This may lead to some interesting anomalies, such as
a status indication of a leap second in progress (TIME_OOP) when the
leap second has already expired. This ambiguity is unavoidable, unless
the timer interrupt is made synchronous with the system clock.
The following state transitions are executed automatically by the kernel
at rollover of the microseconds field:
any state -> TIME_ERROR This transition occurs when an error
condition is recognized and continues as long
as the condition persists. The error indication
overrides the normal state indication, but does
not affect the actual clock state. Therefore,
when the condition is cleared, the normal state
indication resumes.
TIME_OK->TIME_DEL This transition occurs if the STA_DEL bit is
set in the status word.
TIME_OK->TIME_INS This transition occurs if the STA_INS bit is
set in the status word.
TIME_INS->TIME_OOP This transition occurs immediately following
second 86,400 of the current day when an
insert-second event has been declared.
TIME_OOP->TIME_WAIT This transition occurs immediately following
second 86,401 of the current day; that is, one
second after entry to the TIME_OOP state.
TIME_DEL->TIME_WAIT This transition occurs immediately following
second 86,399 of the current day when a delete-
second event has been declared.
TIME_WAIT->TIME_OK This transition occurs when the STA_DEL and
STA_INS bits are cleared by an ntp_adjtime()
call.
The following table summarizes the actions just before, during and just
after a leap-second event. Each line in the table shows the UTC and NTP
times at the beginning of the second. The left column shows the behavior
when no leap event is to occur. In the middle column the state machine
is in TIME_INS at the end of UTC second 23:59:59 and the NTP time has
just reached 400. The NTP time is set back one second to 399 and the
machine enters TIME_OOP. At the end of the repeated second the machine
enters TIME_OK and the UTC and NTP times are again in correspondence. In
the right column the state machine is in TIME_DEL at the end of UTC
second 23:59:58 and the NTP time has just reached 399. The NTP time is
incremented, the machine enters TIME_OK and both UTC and NTP times are
again in correspondence.
No Leap Leap Insert Leap Delete
UTC NTP UTC NTP UTC NTP
---------------------------------------------
23:59:58|398 23:59:58|398 23:59:58|398
| | |
23:59:59|399 23:59:59|399 00:00:00|400
| | |
00:00:00|400 23:59:60|399 00:00:01|401
| | |
00:00:01|401 00:00:00|400 00:00:02|402
| | |
00:00:02|402 00:00:01|401 00:00:03|403
| | |
To determine local midnight without fuss, the kernel code simply finds
the residue of the time.tv_sec (or time.tv_sec + 1) value mod 86,400,
but this requires a messy divide. Probably a better way to do this is to
initialize an auxiliary counter in the settimeofday() routine using an
ugly divide and increment the counter at the same time the time.tv_sec
is incremented in the timer interrupt routine. For future embellishment.
4. Programming Model and Interfaces
This section describes the programming model for the synchronization
daemon and user application programs. The ideas are based on suggestions
from Jeff Mogul and Philip Gladstone and a similar interface designed by
the latter. It is important to point out that the functionality of the
original Unix adjtime() system call is preserved, so that the modified
kernel will work as the unmodified one, should the new features not be
in use. In this case the ntp_adjtime() system call can still be used to
read and write kernel variables that might be used by a synchronization
daemon other than NTP, for example.
The kernel routines use the clock state variable time_state, which
records whether the clock is synchronized, waiting for a leap second,
etc. The value of this variable is returned as the result code by both
the ntp_gettime() and ntp_adjtime() system calls. It is set implicitly
by the STA_DEL and STA_INS status bits, as described previously. Values
presently defined in the timex.h header file are as follows:
TIME_OK 0 no leap second warning
TIME_INS 1 insert leap second warning
TIME_DEL 2 delete leap second warning
TIME_OOP 3 leap second in progress
TIME_WAIT 4 leap second has occured
TIME_ERROR 5 clock not synchronized
In case of a negative result code, the kernel has intercepted an invalid
address or (in case of the ntp_adjtime() system call), a superuser
violation.
4.1. The ntp_gettime() System Call
The syntax and semantics of the ntp_gettime() call are given in the
following fragment of the timex.h header file. This file is identical,
except for the SHIFT_HZ define, in the SunOS, Ultrix and OSF/1 kernel
distributions. (The SHIFT_HZ define represents the logarithm to the base
2 of the clock oscillator frequency specific to each system type.) Note
that the timex.h file calls the syscall.h system header file, which must
be modified to define the SYS_ntp_gettime system call specific to each
system type. The kernel distributions include directions on how to do
this.
/*
* This header file defines the Network Time Protocol (NTP)
* interfaces for user and daemon application programs. These are
* implemented using private system calls and data structures and
* require specific kernel support.
*
* NAME
* ntp_gettime - NTP user application interface
*
* SYNOPSIS
* #include <sys/timex.h>
*
* int system call(SYS_ntp_gettime, tptr)
*
* int SYS_ntp_gettime defined in syscall.h header file
* struct ntptimeval *tptr pointer to ntptimeval structure
*
* NTP user interface - used to read kernel clock values
* Note: maximum error = NTP synch distance = dispersion + delay /
* 2
* estimated error = NTP dispersion.
*/
struct ntptimeval {
struct timeval time; /* current time (ro) */
long maxerror; /* maximum error (us) (ro) */
long esterror; /* estimated error (us) (ro) */
};
The ntp_gettime() system call returns three read-only (ro) values in the
ntptimeval structure: the current time in unix timeval format plus the
maximum and estimated errors in microseconds. While the 32-bit long data
type limits the error quantities to something more than an hour, in
practice this is not significant, since the protocol itself will declare
an unsynchronized condition well below that limit. In the NTP Version 3
specification, if the protocol computes either of these values in excess
of 16 seconds, they are clamped to that value and the system clock
declared unsynchronized.
Following is a detailed description of the ntptimeval structure members.
struct timeval time (ro)
This member is the current system time expressed as a Unix timeval
structure. The timeval structure consists of two 32-bit words; the
first is the number of seconds past 1 January 1970 assuming no
intervening leap-second insertions or deletions, while the second
is the number of microseconds within the second.
long maxerror (ro)
This member is the value of the time_maxerror kernel variable,
which represents the maximum error of the indicated time relative
to the primary synchronization source, in microseconds. For NTP,
the value is initialized by a ntp_adjtime() call to the
synchronization distance, which is equal to the root dispersion
plus one-half the root delay. It is increased by a small amount
(time_tolerance) each second to reflect the maximum clock frequency
error. This variable is provided bu a ntp-adjtime() system call and
modified by the kernel, but is otherwise not used by the kernel.
long esterror (ro)
This member is the value of the time_esterror kernel variable,
which represents the expected error of the indicated time relative
to the primary synchronization source, in microseconds. For NTP,
the value is determined as the root dispersion, which represents
the best estimate of the actual error of the system clock based on
its past behavior, together with observations of multiple clocks
within the peer group. This variable is provided bu a ntp-adjtime()
system call, but is otherwise not used by the kernel.
4.2. The ntp_adjtime() System Call
The syntax and semantics of the ntp_adjtime() call are given in the
following fragment of the timex.h header file. Note that, as in the
ntp_gettime() system call, the syscall.h system header file must be
modified to define the SYS_ntp_adjtime system call specific to each
system type. In the fragment, rw = read/write, ro = read-only, wo =
write-only.
/*
* NAME
* ntp_adjtime - NTP daemon application interface
*
* SYNOPSIS
* #include <sys/timex.h>
*
* int system call(SYS_ntp_adjtime, mode, tptr)
*
* int SYS_ntp_adjtime defined in syscall.h header file
* struct timex *tptr pointer to timex structure
*
* NTP daemon interface - used to discipline kernel clock
* oscillator
*/
struct timex {
unsigned int mode; /* mode selector (wo) */
long offset; /* time offset (us) (rw) */
long frequency; /* frequency offset (scaled ppm) (rw)
*/
long maxerror; /* maximum error (us) (rw) */
long esterror; /* estimated error (us) (rw) */
int status; /* clock status bits (rw) */
long constant; /* pll time constant (rw) */
long precision; /* clock precision (us) (ro) */
long tolerance; /* clock frequency tolerance (scaled
* ppm) (ro) */
/*
* The following read-only structure members are implemented
* only if the PPS signal discipline is configured in the
* kernel.
*/
long ppsfreq; /* pps frequency (scaled ppm) (ro) */
long jitter; /* pps jitter (us) (ro) */
int shift; /* interval duration (s) (shift) (ro)
*/
long stabil; /* pps stability (scaled ppm) (ro) */
long jitcnt; /* jitter limit exceeded (ro) */
long calcnt; /* calibration intervals (ro) */
long errcnt; /* calibration errors (ro) */
long stbcnt; /* stability limit exceeded (ro) */
};
The ntp_adjtime() system call is used to read and write certain time-
related kernel variables summarized below. Writing these variables can
only be done in superuser mode. To write a variable, the mode structure
member is set with one or more bits, one of which is assigned each of
the following variables in turn. The current values for all variables
are returned in any case; therefore, a mode argument of zero means to
return these values without changing anything.
Following is a description of the timex structure members.
mode (wo)
This is a bit-coded variable selecting one or more structure
members, with one bit assigned each member. If a bit is set, the
value of the associated member variable is copied to the
corresponding kernel variable; if not, the member is ignored. The
bits are assigned as given in the following, with the variable name
indicated in parens. Note that the precision, tolerance and PPS
variables are determined by the kernel and cannot be changed by
ntp_adjtime().
MOD_OFFSET 0x0001 time offset (offset)
MOD_FREQUENCY 0x0002 frequency offset (frequency)
MOD_MAXERROR 0x0004 maximum time error (maxerror)
MOD_ESTERROR 0x0008 estimated time error (esterror)
MOD_STATUS 0x0010 clock status (status)
MOD_TIMECONST 0x0020 pll time constant (constant)
MOD_CLKB 0x4000 set clock B
MOD_CLKA 0x8000 set clock A
Note that the MOD_CLK0 and MOD_CLK1 bits are intended for those
systems where more than one hardware clock is available for backup,
such as in Tandem Non-Stop computers. Presumably, in such cases
each clock would have its own oscillator and require a separate PLL
for each. Refinements to this model are for further study. The
interpretation of these bits is as follows:
offset (rw)
If selected, this member specifies the time adjustment, in
microseconds. The absolute value must be less than MAXPHASE
(128000) microseconds defined in the timex.h header file. On
return, this member contains the residual offset remaining between
a previously specified offset and the current system time, in
microseconds.
frequency (rw)
If selected, this member replaces the value of the time_frequency
kernel variable. The value is in ppm, with the integer part in the
high order 16 bits and fraction in the low order 16 bits. The
absolute value must be in the range less than MAXFREQ (100) ppm
defined in the timex.h header file.
The time_freq variable represents the frequency offset of the CPU
clock oscillator. It is recalculated as each update to the system
clock is determined by the offset member of the timex structure. It
is usually set from a value stored in a file when the
synchronization daemon is first started. The current value is
usually retrieved via this member and written to the file about
once per hour.
maxerror (rw)
If selected, this member replaces the value of the time_maxerror
kernel variable, in microseconds. This is the same variable as in
the ntp_getime() system call.
esterror (rw)
If selected, this member replaces the value of the time_esterror
kernel variable, in microseconds. This is the same variable as in
the ntp_getime() system call.
int status (rw)
If selected, this member replaces the value of the time_status
kernel variable. This variable controls the state machine used to
insert or delete leap seconds and shows the status of the
timekeeping system, PPS signal and external oscillator, if
configured.
STA_PLL 0x0001 enable PLL updates (r/w)
STA_PPSFREQ 0x0002 enable PPS freq discipline (r/w)
STA_PPSTIME 0x0004 enable PPS time discipline (r/w)
STA_INS 0x0010 insert leap (r/w)
STA_DEL 0x0020 delete leap (r/w)
STA_UNSYNC 0x0040 clock unsynchronized (r/w)
STA_PPSSIGNAL 0x0100 PPS signal present (r)
STA_PPSJITTER 0x0200 PPS signal jitter exceeded (r)
STA_PPSWANDER 0x0400 PPS signal wander exceeded (r)
STA_PPSERROR 0x0800 PPS signal calibration error (r)
STA_CLOCKERR 0x1000 clock hardware fault (r)
The interpretation of these bits is as follows:
STA_PLL set/cleared by the caller to enable PLL updates
STA_PPSFREQ set/cleared by the caller to enable PPS frequency
discipline
STA_PPSTIME set/cleared by the caller to enable PPS time
discipline
STA_INS set by the caller to insert a leap second at the end
of the current day; cleared by the caller after the
event
STA_DEL set by the caller to delete a leap second at the end
of the current day; cleared by the caller after the
event
STA_UNSYNC set/cleared by the caller to indicate clock
unsynchronized (e.g., when no peers are reachable)
STA_PPSSIGNAL set/cleared by the hardpps() fragment to indicate
PPS signal present
STA_PPSJITTER set/cleared by the hardpps() fragment to indicates
PPS signal jitter exceeded
STA_PPSWANDER set/cleared by the hardpps() fragment to indicates
PPS signal wander exceeded
STA_PPSERROR set/cleared by the hardpps() fragment to indicates
PPS signal calibration error
STA_CLOCKERR set/cleared by the external hardware clock driver to
indicate hardware fault
An error condition is raised when (a) either STA_UNSYNC or
STA_CLOCKERR is set (loss of synchronization), (b) STA_PPSFREQ or
STA_PPSTIME is set and STA_PPSSIGNAL is clear (loss of PPS signal),
(c) STA_PPSTIME and STA_PPSJITTER are both set (jitter exceeded),
(d) STA_PPSFREQ is set and either STA_PPSWANDER or STA_PPSERROR is
set (wander exceeded). An error condition results in a system call
return code of TIME_ERROR.
constant (rw)
If selected, this member replaces the value of the time_constant
kernel variable. The value must be between zero and MAXTC (6)
defined in the timex.h header file.
The time_constant variable determines the bandwidth or "stiffness"
of the PLL. The value is used as a shift between zero and MAXTC
(6), with the effective PLL time constant equal to a multiple of (1
<< time_constant), in seconds. For room-temperature quartz
oscillators, the recommended default value is 2, which corresponds
to a PLL time constant of about 900 s and a maximum update interval
of about 64 s. The maximum update interval scales directly with the
time constant, so that at the maximum time constant of 6, the
update interval can be as large as 1024 s.
Values of time_constant between zero and 2 can be used if quick
convergence is necessary; values between 2 and 6 can be used to
reduce network load, but at a modest cost in accuracy. Values above
6 are appropriate only if an precision external oscillator is
present.
precision (ro)
This is the current value of the time_precision kernel variable in
microseconds.
The time_precision variable represents the maximum error in reading
the system clock, in microseconds. It is usually based on the
number of microseconds between timer interrupts (tick), 10000 us
for the SunOS kernel, 3906 us for the Ultrix kernel, 976 us for the
OSF/1 kernel. However, in cases where the time can be interpolated
between timer interrupts with microsecond resolution, such as in
the stock SunOS kernel and modified Ultrix and OSF/1 kernels, the
precision is specified as 1 us. In cases where a PPS signal or
external oscillator is available, the precision can depend on the
operating condition of the signal or oscillator. This variable is
determined by the kernel for use by the synchronization daemon, but
is otherwise not used by the kernel.
tolerance (ro)
This is the current value of the time_tolerance kernel variable.
The value is in ppm, with the integer part in the high order 16
bits and fraction in the low order 16 bits.
The time_tolerance variable represents the maximum frequency error
in ppm of the particular CPU clock oscillator and is a property of
the hardware; however, in principle it could change as result of
the presence of external discipline signals, for instance.
The recommended value for time_tolerance MAXFREQ (200) ppm is
appropriate for room-temperature quartz oscillators used in typical
workstations. However, it can change due to the operating condition
of the PPS signal and/or external oscillator. With either the PPS
signal or external oscillator, the recommended value for MAXFREQ is
100 ppm.
The following members are defined only if the PPS_SYNC option is
specified in the kernel configuration file. These members are useful
primarily as a monitoring and evalutation tool. These variables can be
written only by the kernel.
ppsfreq (ro)
This is the current value of the pps_freq kernel variable, which is
the CPU clock oscillator frequency offset relative to the PPS
discipline signal. The value is in ppm, with the integer part in
the high order 16 bits and fraction in the low order 16 bits.
jitter (ro)
This is the current value of the pps_jitter kernel variable, which
is the average PPS time dispersion measured by the time-offset
median filter, in microseconds.
shift (ro)
This is the current value of the pps_shift kernel variable, which
determines the duration of the calibration interval as the value of
1 << pps_shift, in seconds.
stabil (ro)
This is the current value of the pps_stabil kernel variable, which
is the average PPS frequency dispersion measured by the frequency-
offset median filter. The value is in ppm, with the integer part in
the high order 16 bits and fraction in the low order 16 bits.
jitcnt (ro)
This is the current value of the pps_jitcnt kernel variable, counts
the number of PPS signals where the average jitter exceeds the
threshold MAXTIME (200 us).
calcnt (ro)
This is the current value of the pps_calcnt kernel variable, which
counts the number of frequency calibration intervals. The duration
of these intervals can range from 4 to 256 seconds, as determined
by the pps_shift kernel variable.
errcnt (ro)
This is the current value of the pps_errcnt kernel variable, which
counts the number of frequency calibration cycles where (a) the
apparent frequency offset is greater than MAXFREQ (100 ppm) or (b)
the interval jitter exceeds tick * 2.
stbcnt (ro)
This is the current value of the pps_discnt kernel variable, which
counts the number of calibration intervals where the average
stability exceeds the threshold MAXFREQ / 4 (25 ppm).
7. References
[MIL91] Mills, D.L. Internet time synchronization: the Network Time
Protocol, IEEE Trans. Communications COM-39, 10 (October 1991),
1482-1493. Also in: Yang, Z., and T.A. Marsland (Eds.). Global
States and Time in Distributed Systems, IEEE Press, Los Alamitos,
CA, 91-102.
[MIL92a] Mills, D.L. Network Time Protocol (Version 3) specification,
implementation and analysis, RFC 1305, University of Delaware, March
1992, 113 pp.
[MIL92b] Mills, D.L. Modelling and analysis of computer network clocks,
Electrical Engineering Department Report 92-5-2, University of Delaware,
May 1992, 29 pp.
[MIL92c] Mills, D.L. Simple Network Time Protocol (SNTP), RFC 1361,
University of Delaware, August 1992, 10 pp.
[MIL93] Mills, D.L. Precision synchronizatin of computer network clocks,
Electrical Engineering Department Report 93-11-1, University of
Delaware, November 1993, 66 pp.
[LEV89] Levine, J., M. Weiss, D. Davis, D. Allan, and D. Sullivan. The
NIST automated computer time service. J. Research National Institute of
Standards and Technology 94, 5 (September-October 1989), 311-321.
David L. Mills <mills@udel.edu>
Electrical Engineering Department
University of Delaware
Newark, DE 19716
302 831 8247 fax 302 831 4316
3 April 1994
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