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RFC 1589 - A Kernel Model for Precision Timekeeping


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Network Working Group                                           D. Mills
Request for Comments: 1589                        University of Delaware
Category: Informational                                       March 1994

                A Kernel Model for Precision Timekeeping

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

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) often found in the engineering literature. It
   has been implemented in the Unix kernel 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 alternative
   implementations. 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 [1], while the current NTP Version 3 protocol
   specification is given in RFC-1305 [2] and a subset of the protocol,
   the Simple Network Time Protocol (SNTP), in RFC-1361 [4].

   The model has been implemented in three Unix kernels for 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 xntp3.3g.tar.Z, where the version number shown as 3.3g
   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 workstations and servers using the Network Time Protocol
   (NTP) or similar time synchronization protocol. This memorandum
   describes the principles of design and implementation of the model.
   Related technical reports discuss the design approach, engineering
   analysis and performance evaluation of the model as implemented in
   Unix kernels for Sun Microsystems and Digital Equipment 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
   (Sun 4.1.x kernels already do provide 1-us 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 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, the new 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, 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 workstation. As described in
   [1], 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 kernel software and workstation hardware.

   In order to understand how the new software works, it is useful to
   review how most Unix kernels maintain the system time. 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 Ultrix
   kernel adds 64 microseconds 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 ticks. 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 more. 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 considered
      in this memorandum, 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 in 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
      [3], 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 [3]. In this 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
      representions, 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 time 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 time
      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 time 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 backward. 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 depends on the measured dispersion and time
      constant; 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 a 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
      errors 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 the new
      system call ntp_adjtime() mentioned previously, 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 maximum error to grow by an
      amount equal to the 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. 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 room-temperature quartz
      oscillators of 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 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 system can be avoided.

   2.3. Precision Clocks for DECstation 5000/240 and 3000 AXP Alpha

      The stock microtime() routine in the Ultrix kernel 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.

      In both the Ultrix and OSF/1 kernels the gettimeofday() and
      ntp_gettime() system call use the new microtime() routine, which
      returns the actual interpolated value, 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. 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.

   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 system clock oscillator of the
      primary server. As discussed in [5], the traditional interface
      using an RS232 protocol and serial port precludes the full
      accuracy of the radio clock. In addition, the poor 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 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 [5]. In the NTP
      Version 3 distribution, the PPS signal can be used to augment the
      less precise ASCII serial timecode to improve accuracy to the
      order of microseconds. Support is also included in the
      distribution for the TPRO interface as well as the audio codec;
      however, the latter requires a modified kernel audio driver
      contained in the bsd_audio.tar.Z distribution in the same host and
      directory as the NTP Version 3 distribution mentioned previously.

      2.4.1. PPS Signal

         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 above, 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. The kernel routine
         compares the timestamp with a sample of the CPU clock
         oscillator to develop a frequency offset estimate. This offset
         is used to discipline the oscillator frequency, nominally to
         within 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 the SunOS implementation which is very delicate to
         fix. The Sun serial port interrupt routine operates at
         interrupt priority level 12, while the timer interrupt routine
         operates at priority 10. Thus, it is possible that the PPS
         signal interrupt can occur during the timer interrupt routine,
         with result that a tick increment can be missed and the
         returned time early by one tick. It may happen that, if the CPU
         clock oscillator is within a few ppm of the PPS oscillator,
         this condition can persist for two or more successive PPS
         interrupts. A useful workaround has been to use a median filter
         to process the PPS sample offsets. In this filter the sample
         offsets in a window of 20 samples are sorted by offset and the
         six highest and six lowest outlyers discarded. The average of
         the eight samples remaining becomes the output of the filter.

         The problem is not nearly so serious when using the PPS signal
         to discipline the frequency of the CPU clock oscillator. In
         this case the quantity of interest is the contents of the
         microseconds counter only, which does not depend on the kernel
         time variable.

      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 to be made to obtain the year
         from the kernel time variable and initialize the driver
         accordingly. This feature is conditionally compiled in the code
         described below only if the EXT_CLOCK option is 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 difference between the microtime() clock and
         kernel time variable and using the difference 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 easy 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.

      2.4.3. 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 caled 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. This
         feature is conditionally compiled in the code described below
         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 [3] and [5]. 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, a
   complete C program kern_ntptime.c which implements the ntp_gettime()
   and ntp_adjtime() functions is provided, but with the vendor-specific
   argument-passing code deleted. 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 a driver program also
   included 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 [3]. 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.

      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. Then, the time since the last update is computed and
         clamped to a maximum (for robustness) 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.

         With the defines given, 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 determined, 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. Previous to this fragment the time_update variable
         has been initialized to 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.

         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_ybar
         (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 discipling the CPU clock oscillator frequency, as
         described below. As long as the actual oscillator frequency is
         a power of two in seconds, 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 fragment operates as a

         first-order, type-I frequency-lock loop (FLL) controlled by the
         difference between the frequency represented by the pps_ybar
         variable and the frequency of the hardware clock oscillator.

         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 can be determined by saving the microseconds
         field of the system time, calling the microtime() routine, and
         subtracting the saved value. If a counter overflow has occured
         during the process, the resulting microseconds value will be
         negative, in which case the caller adds 1000000 to normalize
         the microseconds field.

         The frequency of the hardware oscillator can be 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_ybar 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 used as a control signal sample for the
         FLL.

         Control signal samples which exceed the frequency tolerance are
         discarded, as well as samples resulting from excessive interval
         duration jitter. Surviving samples are then processed by a
         three-stage median filter. The signal which drives the FLL is
         derived from the median sample, while the average of
         differences between the other two samples is used as a measure
         of dispersion. If the dispersion is below the threshold
         pps_dispmax, the median is used to correct the pps_ybar value
         with a weight expressed as a shift PPS_AVG (2). In addition to
         the averaging function, pps_disp is increased by the amount
         pps_dispinc once each second. The result is that, should the
         dispersion be exceptionally high, or if the PPS signal fails
         for some reason, the pps_disp will eventually exceed
         pps_dispmax and raise an alarm.

         Initially, an approximate value for pps_ybar 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 a
         fraction 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 [5].

         Should the PPS signal fail, the current frequency estimate
         pps_ybar 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_ybar 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 clock status of either
      TIME_DEL, if a second is to be deleted, or TIME_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
      occasion, nor is there likely to be one in future. If the value is
      TIME_DEL, the kernel adds one second to the system time
      immediately following second 23:59:58 and resets the clock status
      to TIME_OK. If the value is TIME_INS, the kernel subtracts one
      second from the system time immediately following second 23:59:59
      and resets the clock status to TIME_OOP, in effect causing system

      time to repeat second 59. Immediately following the repeated
      second, the kernel resets the clock status to TIME_OK.

      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 an external clock is present, it is working properly
                and the system clock is derived from it. If no external
                clock is present, 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_BAD  Either (a) the synchronization daemon has declared the
                protocol is not working properly, (b) all sources of
                outside synchronization have been lost or (c) an
                external clock is present and it has just become
                operational following a non-operational condition.

      TIME_ERR  An external clock is present, but is in a non-
                operational condition.

      In all except the TIME_ERR state the system clock is derived from
      either an external clock, if present, or the kernel time variable,
      if not. In the TIME_ERR state the external clock is present, but
      not working properly, so the system clock may be derived from the
      kernel time variable. The following diagram indicates the normal
      transitions of the state machine. Not all valid transitions are
      shown.

          +--------+     +--------+     +--------+     +--------+
          |        |     |        |     |        |     |        |
          |TIME_BAD|---->|TIME_OK |<----|TIME_OOP|<----|TIME_INS|
          |        |     |        |     |        |     |        |
          +--------+     +--------+     +--------+     +--------+
               A              A
               |              |
               |              |
          +--------+     +--------+
          |        |     |        |
          |TIME_ERR|     |TIME_DEL|
          |        |     |        |
          +--------+     +--------+

      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 at each timer interrupt, which may not
      exactly coincide with the actual instant of overflow. This may
      lead to some interesting anomalies, such as a status indication of
      a leap second in progress (TIME_OOP) when actually the leap second
      had already expired.

      The following state transitions are executed automatically by the
      kernel:

      any state -> TIME_ERR   This transition occurs when an external
                              clock is present and an attempt is made to
                              read it when in a non-operational
                              condition.

      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_OK     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_OK     This transition occurs immediately
                              following second 86,399 of the current day
                              when a delete-second event has been
                              declared.

      The following state transitions are executed by specific
      ntp_adjtime() system calls:

      TIME_OK -> TIME_INS     This transition occurs as the result of a
                              ntp_adjtime() system call to declare an
                              insert-second event.

      TIME_OK -> TIME_DEL     This transition occurs as the result of a
                              ntp_adjtime() system call to declare a
                              delete-second event.

      any state -> TIME_BAD   This transition occurs as the result of a
                              ntp_adjtime() system call to declare loss
                              of all sources of synchronization or in
                              other cases of error.

      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.

   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 */
           long maxerror;          /* maximum error (us) */
           long esterror;          /* estimated error (us) */
      };

      The ntp_gettime() system call returns three 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;    /* current time */

         This member returns the current system time, expressed as a
         Unix timeval structure. The timeval structure consists of two
         32-bit words; the first returns the number of seconds past 1
         January 1970, while the second returns the number of
         microseconds.

      long maxerror;          /* maximum error (us) */

         This member returns the time_maxerror kernel variable in
         microseconds. See the entry for this variable in section 5 for
         additional information.

      long esterror;          /* estimated error (us) */

         This member returns the time_esterror kernel variable in
         microseconds. See the entry for this variable in section 5 for
         additional information.

   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.

      /*
       * 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 {
          int mode;                /* mode selector */
          long offset;             /* time offset (us) */
          long frequency;          /* frequency offset (scaled ppm) */
          long maxerror;           /* maximum error (us) */
          long esterror;           /* estimated error (us) */
          int status;              /* clock command/status */
          long time_constant;      /* pll time constant */
          long precision;          /* clock precision (us) (read only)
                                    */
          long tolerance;          /* clock frequency tolerance (scaled
                                    * ppm) (read only) */
          /*
           * The following read-only structure members are implemented
           * only if the PPS signal discipline is configured in the
           * kernel.
           */
          long ybar;               /* frequency estimate (scaled ppm) */
          long disp;               /* dispersion estimate (scaled ppm)
                                    */
          int shift;               /* interval duration (s) (shift) */
          long calcnt;             /* calibration intervals */
          long jitcnt;             /* jitter limit exceeded */
          long discnt;             /* dispersion limit exceeded */
      };

      The ntp_adjtime() system call is used to read and write certain
      time-related kernel variables summarized in this and subsequent
      sections. 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.

      int mode;               /* mode selector */

         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 fragment of the
         timex.h header file. Note that the precision and tolerance are
         determined by the kernel and cannot be changed by
         ntp_adjtime().

         /*
          * Mode codes (timex.mode)
          */
         #define ADJ_OFFSET       0x0001    /* time offset */
         #define ADJ_FREQUENCY    0x0002    /* frequency offset */
         #define ADJ_MAXERROR     0x0004    /* maximum time error */
         #define ADJ_ESTERROR     0x0008    /* estimated time error */
         #define ADJ_STATUS       0x0010    /* clock status */
         #define ADJ_TIMECONST    0x0020    /* pll time constant */

      long offset;            /* time offset (us) */

         If selected, this member replaces the value of the time_offset
         kernel variable in microseconds. The absolute value must be
         less than MAXPHASE microseconds defined in the timex.h header
         file. See the entry for this variable in section 5 for
         additional information.

         If within range and the PPS signal and/or external oscillator
         are configured and operating properly, the clock status is
         automatically set to TIME_OK.

      long time_constant;     /* pll time constant */

         If selected, this member replaces the value of the
         time_constant kernel variable. The value must be between zero
         and MAXTC defined in the timex.h header file. See the entry for
         this variable in section 5 for additional information.

      long frequency;         /* frequency offset (scaled ppm) */

         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 ppm defined in the timex.h header file. See the
         entry for this variable in section 5 for additional
         information.

      long maxerror;          /* maximum error (us) */

         If selected, this member replaces the value of the
         time_maxerror kernel variable in microseconds. See the entry
         for this variable in section 5 for additional information.

      long esterror;          /* estimated error (us) */

         If selected, this member replaces the value of the
         time_esterror kernel variable in microseconds. See the entry
         for this variable in section 5 for additional information.

      int status;             /* clock command/status */

         If selected, this member replaces the value of the time_status
         kernel variable. See the entry for this variable in section 5
         for additional information.

         In order to set this variable by ntp_adjtime(), either (a) the
         current clock status must be TIME_OK or (b) the member value is
         TIME_BAD; that is, the ntp_adjtime() call can always set the
         clock to the unsynchronized state or, if the clock is running
         correctly, can set it to any state. In any case, the
         ntp_adjtime() call always returns the current state in this
         member, so the caller can determine whether or not the request
         succeeded.

      long time_constant;     /* pll time constant */

         If selected, this member replaces the value of the
         time_constant kernel variable. The value must be between zero
         and MAXTC defined in the timex.h header file. See the entry for
         this variable in section 5 for additional information.

      long precision;         /* clock precision (us) (read only) */

         This member returns the time_precision kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      long tolerance;         /* clock frequency tolerance (scaled ppm)
                               */

         This member returns the time_tolerance kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      long ybar;              /* frequency estimate (scaled ppm) */

         This member returns the pps_ybar kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      long disp;              /* dispersion estimate (scaled ppm) */

         This member returns the pps_disp kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      int shift;              /* interval duration (s) (shift) */

         This member returns the pps_shift kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      long calcnt;            /* calibration intervals */

         This member returns the pps_calcnt kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      long jitcnt;            /* jitter limit exceeded */

         This member returns the pps_jittcnt kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

      long discnt;            /* dispersion limit exceeded */

         This member returns the pps_discnt kernel variable in
         microseconds. The variable can be written only by the kernel.
         See the entry for this variable in section 5 for additional
         information.

   4.3. Command/Status Codes

      The kernel routines use the system clock status variable
      time_status, 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. In addition, it can be explicitly read
      and written using the ntp_adjtime() system call, but can be
      written only in superuser mode. Values presently defined in the
      timex.h header file are as follows:

      /*
       * Clock command/status codes (timex.status)
       */
      #define TIME_OK    0    /* clock synchronized */
      #define TIME_INS   1    /* insert leap second */
      #define TIME_DEL   2    /* delete leap second */
      #define TIME_OOP   3    /* leap second in progress */
      #define TIME_BAD   4    /* kernel clock not synchronized */
      #define TIME_ERR   5    /* external oscillator not
                                 synchronized */

      A detailed description of these codes as used by the leap-second
      state machine is given later in this memorandum. 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.

5. Kernel Variables

   This section contains a list of kernel variables and a detailed
   description of their function, initial value, scaling and limits.

   5.1. Interface Variables

      The following variables are read and set by the ntp_adjtime()
      system call. Additional automatic variables are used as
      temporaries as described in the code fragments.

      int time_status = TIME_BAD;

         This variable controls the state machine used to insert or
         delete leap seconds and show the status of the timekeeping
         system, PPS signal and external oscillator, if configured.

      long time_offset = 0;

         This variable is used by the PLL to adjust the system time in
         small increments. It is scaled by (1 << SHIFT_UPDATE) (12) in
         microseconds. The maximum value that can be represented is
         about +-512 ms and the minimum value or precision is a few
         parts in 10^10 s.

      long time_constant = 0;      /* pll time constant */

         This 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
         oscillator 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 external oscillator is
         present.

      long time_tolerance = MAXFREQ; /* frequency tolerance (ppm) */

         This variable represents the maximum frequency error or
         tolerance in ppm of the particular CPU clock oscillator and is
         a property of the architecture; however, in principle it could
         change as result of the presence of external discipline
         signals, for instance. It is expressed as a positive number
         greater than zero in parts-per-million (ppm).

         The recommended value of MAXFREQ is 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.

      long time_precision = 1000000 / HZ; /* clock precision (us) */

         This variable represents the maximum error in reading the
         system clock in microseconds. It is usually based on the number
         of microseconds between timer interrupts, 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 unmodified 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.

      long time_maxerror = MAXPHASE; /* maximum error */

         This variable establishes 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 clock frequency tolerance. This variable is
         computed by the synchronization daemon and the kernel, but is
         otherwise not used by the kernel.

      long time_esterror = MAXPHASE; /* estimated error */

         This variable establishes 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 computed by the synchronization daemon and
         returned in system calls, but is otherwise not used by the
         kernel.

   5.2. Phase-Lock Loop Variables

      The following variables establish the state of the PLL and the
      residual time and frequency offset of the system clock. Additional
      automatic variables are used as temporaries as described in the
      code fragments.

      long time_phase = 0;         /* phase offset (scaled us) */

         The time_phase variable represents the phase of the kernel time
         variable at each tick of the clock. This variable is scaled by
         (1 << SHIFT_SCALE) (23) in microseconds, giving a maximum
         adjustment of about +-256 us/tick and a resolution less than
         one part in 10^12.

      long time_offset = 0;        /* time offset (scaled us) */

         The time_offset variable represents the time offset of the CPU
         clock oscillator. It is recalculated as each update to the
         system clock is received via the hardupdate() routine and at
         each second in the seconds_overflow routine. This variable is
         scaled by (1 << SHIFT_UPDATE) (12) in microseconds, giving a
         maximum adjustment of about +-512 ms and a resolution of a few
         parts in 10^10 s.

      long time_freq = 0;          /* frequency offset (scaled ppm) */

         The time_freq variable represents the frequency offset of the
         CPU clock oscillator. It is recalculated as each update to the
         system clock is received via the hardupdate() routine. It can
         also be set via ntp_adjtime() from a value stored in a file
         when the synchronization daemon is first started. It can be
         retrieved via ntp_adjtime() and written to the file about once
         per hour by the daemon. The time_freq variable is scaled by (1
         << SHIFT_KF) (16) ppm, giving it a maximum value well in excess
         of the limit of +-256 ppm imposed by other constraints. The
         precision of this representation (frequency resolution) is
         parts in 10^11, which is adequate for all but the best external
         oscillators.

      time_adj = 0;                /* tick adjust (scaled 1 / HZ) */

         The time_adj variable is the adjustment added to the value of
         tick at each timer interrupt. It is computed once each second
         from the time_offset, time_freq and, if the PPS signal is
         present, the ps_ybar variable once each second.

      long time_reftime = 0;       /* time at last adjustment (s) */

         This variable is the seconds portion of the system time on the
         last update received by the hardupdate() routine. It is used to
         compute the time_freq variable as the time since the last
         update increases.

      int fixtick = 1000000 % HZ;  /* amortization factor */

         In the Ultrix and OSF/1 kernels, the interval between timer
         interrupts does not evenly divide the number of microseconds in
         the second. In order that the clock runs at a precise rate, it
         is necessary to introduce an amortization factor into the local
         timescale. In the original Unix code, the value of fixtick is
         amortized once each second, introducing an additional source of
         jitter; in the new model the value is amortized at each tick of
         the system clock, reducing the jitter by the reciprocal of the
         clock oscillator frequency. This is not a new kernel variable,
         but a new use of an existing kernel variable.

   5.3. Pulse-per-second (PPS) Frequency-Lock Loop Variables

      The following variables are used only if a pulse-per-second (PPS)
      signal is available and connected via a modem-control lead, such
      as produced by the optional ppsclock feature incorporated in the
      serial port driver. They establish the design parameters of the
      PPS frequency-lock loop used to discipline the CPU clock
      oscillator to an external PPS signal. Additional automatic
      variables are used as temporaries as described in the code
      fragments.

      long pps_usec;          /* microseconds at last pps */

         The pps_usec variable is latched from a high resolution counter
         or external oscillator at each PPS interrupt. In determining
         this value, only the hardware counter contents are used, not
         the contents plus the kernel time variable, as returned by the
         microtime() routine.

      long pps_ybar = 0;      /* pps frequency offset estimate */

         The pps_ybar variable is the average CPU clock oscillator
         frequency offset relative to the PPS disciplining signal. It is
         scaled in the same units as the time_freq variable.

      pps_disp = MAXFREQ;     /* dispersion estimate (scaled ppm) */

         The pps_disp variable represents the average sample dispersion
         measured over the last three samples. It is scaled in the same
         units as the time_freq variable.

      pps_dispmax = MAXFREQ / 2; /* dispersion threshold */

         The pps_dispmax variable is used as a dispersion threshold. If
         pps_disp is less than this threshold, the median sample is used
         to update the pps_ybar estimate; if not, the sample is
         discarded.

      pps_dispinc = MAXFREQ >> (PPS_SHIFT + 4); /* pps dispersion
      increment/sec */

         The pps_dispinc variable is the increment to add to pps_disp
         once each second. It is computed such that, if no PPS samples
         have arrived for several calibration intervals, the value of
         pps_disp will exceed the pps_dispmax threshold and raise an
         alarm.

      int pps_mf[] = {0, 0, 0};    /* pps median filter */

         The pps-mf[] array is used as a median filter to detect and
         discard jitter in the PPS signal.

      int pps_count = 0;           /* pps calibrate interval counter */

         The pps_count variable measures the length of the calibration
         interval used to calculate the frequency. It normally counts
         from zero to the value 1 << pps_shift.

      pps_shift = PPS_SHIFT;       /* interval duration (s) (shift) */

         The pps_shift variable determines the duration of the
         calibration interval, 1 << pps_shift s.

      pps_intcnt = 0;              /* intervals at current duration */

         The pps_intcnt variable counts the number of calibration
         intervals at the current interval duration. It is reset to zero
         after four intervals and when the interval duration is changed.

      long pps_calcnt = 0;         /* calibration intervals */

         The pps_calcnt variable counts the number of calibration
         intervals.

      long pps_jitcnt = 0;         /* jitter limit exceeded */

         The pps_jitcnt variable counts the number of resets due to
         excessive jitter or frequency offset. These resets are
         usually due to excessive noise in the PPS signal or
         interface.

      long pps_discnt = 0;         /* dispersion limit exceeded */

         The pps_discnt variable counts the number of calibration
         intervals where the dispersion is above the pps_dispmax
         limit.  These resets are usually due to excessive frequency
         wander in the PPS signal source.

   5.4. External Oscillator Variables

      The following variables are used only if an external oscillator
      (HIGHBALL or TPRO) is present. Additional automatic variables are
      used as temporaries as described in the code fragments.

      int clock_count = 0;         /* CPU clock counter */

         The clock_count variable counts the seconds between adjustments
         to the kernel time variable to discipline it to the external
         clock.

      struct timeval clock_offset; /* HIGHBALL clock offset */

         The clock_offset variable defines the offset between system
         time and the HIGHBALL counters.

      long clock_cpu = 0;          /* CPU clock adjust */

         The clock_cpu variable contains the offset between the system
         clock and the HIGHBALL clock for use in disciplining the kernel
         time variable.

6. Architecture Constants

   Following is a list of the important architecture constants that
   establish the response and stability of the PLL and provide maximum
   bounds on behavior in order to satisfy correctness assertions made in
   the protocol specification. Additional definitions are given in the
   timex.h header file.

   6.1. Phase-lock loop (PLL) definitions

      The following defines establish the performance envelope of the
      PLL. They establish the maximum phase error (MAXPHASE), maximum
      frequency error (MAXFREQ), minimum interval between updates
      (MINSEC) and maximum interval between updates (MAXSEC). The intent
      of these bounds is to force the PLL to operate within predefined
      limits in order to satisfy correctness assertions of the
      synchronization protocol. An excursion which exceeds these bounds
      is clamped to the bound and operation proceeds normally. In
      practice, this can occur only if something has failed or is
      operating out of tolerance, but otherwise the PLL continues to
      operate in a stable mode.

      MAXPHASE must be set greater than or equal to CLOCK.MAX (128 ms),
      as defined in the NTP specification. CLOCK.MAX establishes the
      maximum time offset allowed before the system time is reset,

      rather than incrementally adjusted. Here, the maximum offset is
      clamped to MAXPHASE only in order to prevent overflow errors due
      to defective programming.

      MAXFREQ reflects the manufacturing frequency tolerance of the CPU
      oscillator plus the maximum slew rate allowed by the protocol. It
      should be set to at least the intrinsic frequency tolerance of the
      oscillator plus 100 ppm for vernier frequency adjustments. If the
      kernel frequency discipline code is installed (PPS_SYNC), the CPU
      oscillator frequency is disciplined to an external source,
      presumably with negligible frequency error.

      #define MAXPHASE 512000      /* max phase error (us) */
      #ifdef PPS_SYNC
      #define MAXFREQ 100          /* max frequency error (ppm) */
      #else
      #define MAXFREQ 200          /* max frequency error (ppm) */
      #endif /* PPS_SYNC */
      #define MINSEC 16            /* min interval between updates (s)
                                    */
      #define MAXSEC 1200          /* max interval between updates (s)
                                    */

   6.2. Pulse-per-second (PPS) Frequency-lock Loop (FLL) Definitions

      The following defines and declarations are used only if a pulse-
      per-second (PPS) signal is available and connected via a modem-
      control lead, such as produced by the optional ppsclock feature
      incorporated in the serial port driver. They establish the design
      parameters of the frequency-lock loop (FLL) used to discipline the
      CPU clock oscillator to the PPS oscillator.

      PPS_AVG is the averaging constant used to update the FLL from
      frequency samples measured for each calibration interval.
      PPS_SHIFT and PPS_SHIFTMAX are the minimum and maximem,
      respectively, of the calibration interval represented as a power
      of two. The PPS_DISPINC is the initial increment to pps_disp at
      each second.

      #define PPS_AVG 2            /* pps averaging constant (shift) */
      #define PPS_SHIFT 2          /* min interval duration (s) (shift)
                                    */
      #define PPS_SHIFTMAX 6       /* max interval duration (s) (shift)
                                    */
      #define PPS_DISPINC 0        /* dispersion increment (us/s) */

   6.3. External Oscillator Definitions

      The following definitions and declarations are used only if an
      external oscillator (HIGHBALL or TPRO) is configured on the
      system.

      #define CLOCK_INTERVAL 30    /* CPU clock update interval (s) */

7. References

   [1] Mills, D., "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.

   [2] Mills, D., "Network Time Protocol (Version 3) specification,
       implementation and analysis", RFC 1305, University of Delaware,
       March 1992, 113 pp.

   [3] Mills, D., "Modelling and analysis of computer network clocks",
       Electrical Engineering Department Report 92-5-2, University of
       Delaware, May 1992, 29 pp.

   [4] Mills, D., "Simple Network Time Protocol (SNTP)", RFC 1361,
       University of Delaware, August 1992, 10 pp.

   [5] Mills, D., "Precision synchronizatin of computer network clocks",
       Electrical Engineering Department Report 93-11-1, University of
       Delaware, November 1993, 66 pp.

Security Considerations

   Security issues are not discussed in this memo.

Author's Address

   David L. Mills
   Electrical Engineering Department
   University of Delaware
   Newark, DE 19716

   Phone: (302) 831-8247
   EMail: mills@udel.edu

 

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