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Abstract

RFC 1589 describes a UNIX kernel implementation model for high- precision time-keeping. This model is meant for use in conjunction with the Network Time Protocol (NTP, RFC 1305), or similar time synchronization protocols. One aspect of this model is an accurate interface to the high-accuracy, one pulse-per-second (PPS) output typically available from precise time sources (such as a GPS or GOES receiver). RFC 1589 did not define an API for managing the PPS facility, leaving implementors without a portable means for using PPS sources. This document specifies such an API.

Table of Contents

   1 Introduction...................................................  2
   2 Data types for representing timestamps.........................  4
   2.1 Resolution...................................................  4
   2.2 Time scale...................................................  5
   3 API............................................................  5
   3.1 PPS abstraction..............................................  6
   3.2 New data structures..........................................  7
   3.3 Mode bit definitions......................................... 10
   3.4 New functions................................................ 12
   3.4.1 New functions: obtaining PPS sources....................... 13
   3.4.2 New functions: setting PPS parameters...................... 14
   3.4.3 New functions: access to PPS timestamps.................... 16
   3.4.4 New functions: disciplining the kernel timebase............ 18
   3.5 Compliance rules............................................. 20
   3.5.1 Functions.................................................. 20
   3.5.2 Mode bits.................................................. 20
   3.6 Examples..................................................... 21
   4 Security Considerations........................................ 24
   5 Acknowledgements............................................... 24
   6 References..................................................... 25
   7 Authors' Addresses............................................. 26
   A. Extensions and related APIs................................... 27
   A.1 Extension: Parameters for the "echo" mechanism............... 27
   A.2 Extension: Obtaining information about external clocks....... 27
   A.3 Extension: Finding a PPS source.............................. 28
   B. Example implementation: PPSDISC Line discipline............... 29
   B.1 Example...................................................... 29
   C. Available implementations..................................... 30
   Full Copyright Statement......................................... 31

1 Introduction

RFC 1589 [4] 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) [3] or similar time synchronization protocol. The model supports the use of external timing sources, such as the precision pulse-per- second (PPS) signals typically available from precise time sources (such as a GPS or GOES receiver).

However, RFC 1589 did not define an application programming interface (API) for the PPS facility. This document specifies such an interface, for use with UNIX (or UNIX-like) operating systems. Such systems often conform to the "Single UNIX Specification" [5], sometimes known as POSIX.

One convenient means to provide a PPS signal to a computer system is to connect that signal to a modem-control pin on a serial-line interface to the computer. The Data Carrier Detect (DCD) pin is frequently used for this purpose. Typically, the time-code output of the time source is transmitted to the computer over the same serial line. The computer detects a signal transition on the DCD pin, usually by receiving an interrupt, and records a timestamp as soon as possible.

Although existing practice has focussed on the use of serial lines and DCD transitions, PPS signals might also be delivered by other kinds of devices. The API specified in this document does not require the use of a serial line, although it may be somewhat biased in that direction.

The typical use of this facility is for the operating system to record ("capture") a high-resolution timestamp as soon as possible after it detects a PPS signal transition (usually indicated by an interrupt). This timestamp can then be made available, with less stringent delay constraints, to time-related software. The software can compare the captured timestamp to the received time-code to accurately discover the offset between the system clock and the precise time source.

   The operating system may also deliver the PPS event to a kernel
   procedure, called the "in-kernel PPS consumer."  One example would be
   the "hardpps()" procedure, described in RFC 1589, which is used to
   discipline the kernel's internal timebase.

The API specified in this document allows for one or more signal sources attached to a computer system to provide PPS inputs, at the option of user-level software. User-level software may obtain signal-transition timestamps for any of these PPS sources. User- level software may optionally specify at most one of these PPS sources to be used to discipline the system's internal timebase.

Although the primary purpose of this API is for capturing true pulse-per-second events, the API may also be used for accurately timestamping events of other periods, or even aperiodic events, when these can be expressed as signal transitions.

This document does not define internal details of how the API must be implemented, and does not specify constraints on the accuracy, resolution, or latency of the PPS feature. However, the utility of this feature is inversely proportional to the delay (and variance of delay), and implementors are encouraged to take this seriously.

In principle, the rate of events to be captured, or the frequency of the signals, can range from once per day (or less often) to several thousand per second. However, since in most implementations the timestamping function will be implemented as a processor interrupt at a relatively high priority, it is prudent to limit the rate of such events. This may be done either by mechanisms in the hardware that generates the signals, or by the operating system.

2 Data types for representing timestamps

Computer systems use various representations of time. Because this API is concerned with the provision of high-accuracy, high-resolution time information, the choice of representation is significant. (Here we consider only binary representations, not human-format representations.)

The two interesting questions are:

1. what is the resolution of the representation?

2. what time scale is represented?

These questions often lead to contentious arguments. Since this API is intended for use with NTP and POSIX-compliant systems, however, we can limit the choices to representations compatible with existing NTP and POSIX practice, even if that practice is considered "wrong" in some quarters.

2.1 Resolution

In the NTP protocol, "timestamps are represented as a 64-bit unsigned fixed-point number, in seconds relative to 0h on 1 January 1900. The integer part is in the first 32 bits and the fraction part in the last 32 bits [...] The precision of this representation is about 200 picoseconds" [3].

However, most computer systems cannot measure time to this resolution (this represents a clock rate of 5 GHz). The POSIX gettimeofday() function returns a "struct timeval" value, with a resolution of 1 microsecond. The POSIX clock_gettime() function returns a "struct timespec" value, with a resolution of 1 nanosecond.

This API uses an extensible representation, but defaults to the "struct timespec" representation.

2.2 Time scale

Several different time scales have been proposed for use in computer systems. UTC and TAI are the two obvious candidates.

Some people would prefer the use of TAI, which is identical to UTC except that it does not correct for leap seconds. Their preference for TAI stems from the difficulty of computing precise time differences when leap seconds are involved, especially when using times in the future (for which the exact number of leap seconds is, in general, unknowable).

However, POSIX and NTP both use UTC, albeit with different base dates. Given that support for TAI would, in general, require other changes to the POSIX specification, this API uses the POSIX base date of 00:00 January 1, 1970 UTC, and conforms to the POSIX use of the UTC time scale.

3 API

A PPS facility can be used in two different ways:

1. An application can obtain a timestamp, using the system's internal timebase, for the most recent PPS event.

2. The kernel may directly utilize PPS events to discipline its internal timebase, thereby providing highly accurate time to all applications.

This API supports both uses, individually or in combination. The timestamping feature may be used on any number of PPS sources simultaneously; the timebase-disciplining feature may be used with at most one PPS source.

Although the proper implementation of this API requires support from the kernel of a UNIX system, this document defines the API in terms of a set of library routines. This gives the implementor some freedom to divide the effort between kernel code and library code (different divisions might be appropriate on microkernels and monolithic kernels, for example).

3.1 PPS abstraction

A PPS signal consists of a series of pulses, each with an "asserted" (logical true) phase, and a "clear" (logical false) phase. The two phases may be of different lengths. The API may capture an "assert timestamp" at the moment of the transition into the asserted phase, and a "clear timestamp" at the moment of the transition into the clear phase.

The specific assignment of the logical values "true" and "false" with specific voltages of a PPS signal, if applicable, is outside the scope of this specification. However, these assignments SHOULD be consistent with applicable standards. Implementors of PPS sources SHOULD document these assignments.

      Reminder to implementors of DCD-based PPS support:  TTL and RS-
      232C (V.24/V.28) interfaces both define the "true" state as the
      one having the highest positive voltage. TTL defines a nominal
      absence of voltage as the "false" state, but RS-232C (V.24/V.28)
      defines the "false" state by the presence of a negative voltage.

The API supports the direct provision of PPS events (and timestamps) to an in-kernel PPS consumer. This could be the function called "hardpps()", as described in RFC 1589 [4], but the API does not require the kernel implementation to use that function name internally. The current version of the API supports at most one in- kernel PPS consumer, and does not provide a way to explicitly name it. The implementation SHOULD impose access controls on the use of this feature.

The API optionally supports an "echo" feature, in which events on the incoming PPS signal may be reflected through software, after the capture of the corresponding timestamp, to an output signal pin. This feature may be used to discover an upper bound on the actual delay between the edges of the PPS signal and the capture of the timestamps; such information may be useful in precise calibration of the system.

The designation of an output pin for the echo signal, and sense and shape of the output transition, is outside the scope of this specification, but SHOULD be documented for each implementation. The output pin MAY also undergo transitions at other times besides those caused by PPS input events.

Note: this allows an implementation of the echo feature to generate an output pulse per input pulse, or an output edge per input pulse, or an output pulse per input edge. It also allows the same signal pin to be used for several purposes simultaneously.

Also, the API optionally provides an application with the ability to specify an offset value to be applied to captured timestamps. This can be used to correct for cable and/or radio-wave propagation delays, or to compensate for systematic jitter in the external signal. The implementation SHOULD impose access controls on the use of this feature.

3.2 New data structures

The data structure declarations and symbol definitions for this API will appear in the header file <sys/timepps.h>. The header file MUST define all constants described in this specification, even if they are not supported by the implementation.

The API includes several implementation-specific types:

      typedef ... pps_handle_t;       /* represents a PPS source */
      
      typedef unsigned ... pps_seq_t; /* sequence number */

The "pps_handle_t" type is an opaque scalar type used to represent a PPS source within the API.

The "pps_seq_t" type is an unsigned integer data type of at least 32 bits.

The precise declaration of the pps_handle_t and pps_seq_t types is system-dependent.

The API imports the standard POSIX definition for this data type:

      struct timespec {
              time_t  tv_sec;         /* seconds */
              long    tv_nsec;        /* nanoseconds */
      };

The API defines this structure as an internal (not "on the wire") representation of the NTP "64-bit unsigned fixed-point" timestamp format [3]:

      typedef struct ntp_fp {
              unsigned int    integral;
              unsigned int    fractional;
      } ntp_fp_t;

The two fields in this structure may be declared as any unsigned integral type, each of at least 32 bits.

The API defines this new union as an extensible type for representing times:

      typedef union pps_timeu {
              struct timespec tspec;
              ntp_fp_t        ntpfp;
              unsigned long   longpad[3];
      } pps_timeu_t;

Future revisions of this specification may add more fields to this union.

Note: adding a field to this union that is larger than 3*sizeof(long) will break binary compatibility.

The API defines these new data structures:

      typedef struct {
          pps_seq_t   assert_sequence;        /* assert event seq # */
          pps_seq_t   clear_sequence;         /* clear event seq # */
          pps_timeu_t assert_tu;
          pps_timeu_t clear_tu;
          int         current_mode;           /* current mode bits */
      } pps_info_t;
      
      #define assert_timestamp        assert_tu.tspec
      #define clear_timestamp         clear_tu.tspec
      
      #define assert_timestamp_ntpfp  assert_tu.ntpfp
      #define clear_timestamp_ntpfp   clear_tu.ntpfp
      
      typedef struct {
          int         api_version;            /* API version # */
          int         mode;                   /* mode bits */
          pps_timeu_t assert_off_tu;
          pps_timeu_t clear_off_tu;
      } pps_params_t;
      
      #define assert_offset   assert_off_tu.tspec
      #define clear_offset    clear_off_tu.tspec
      
      #define assert_offset_ntpfp     assert_off_tu.ntpfp
      #define clear_offset_ntpfp      clear_off_tu.ntpfp

The "pps_info_t" type is returned on an inquiry to PPS source. It contains the timestamps for the most recent assert event, and the most recent clear event. The order in which these events were actually received is defined by the timetamps, not by any other aspect of the specification. Each timestamp field represents the value of the operating system's internal timebase when the timestamped event occurred, or as close as possible to that time (with the optional addition of a specified offset). The current_mode field contains the value of the mode bits (see section 3.3) at the time of the most recent transition was captured for this PPS source. An application can use current_mode to discover the format of the timestamps returned.

The assert_sequence number increases once per captured assert timestamp. Its initial value is undefined. If incremented past the largest value for the type, the next value is zero. The clear_sequence number increases once per captured clear timestamp. Its initial value is undefined, and may be different from the initial value of assert_sequence. If incremented past the largest value for the type, the next value is zero. Due to possible signal loss or excessive signal noise, the assert-sequence number and the clear- sequence number might not always increase in step with each other.

Note that these sequence numbers are most useful in applications where events other than PPS transitions are to be captured, which might be involved in a precision stopwatch application, for example. In such cases, the sequence numbers may be used to detect overruns, where the application has missed one or more events. They may also be used to detect an excessive event rate, or to detect that an event has failed to occur between two calls to the time_pps_fetch() function (defined later).

In order to obtain an uninterrupted series of sequence numbers (and hence of event timestamps), it may be necessary to sample the pps_info_t values at a rate somewhat faster than the underlying event rate. For example, an application interested in both assert and clear timestamps may need to sample at least twice per second. Proper use of the sequence numbers allows an application to discover if it has missed any event timestamps due to an insufficient sampling rate.

The pps_params_t data type is used to discover and modify parameters of a PPS source. The data type includes a mode field, described in section 3.3. It also includes an api_version field, a read-only value giving the version of the API. Currently, the only defined value is:

      #define PPS_API_VERS_1  1

This field is present to enable binary compatibility with future versions of the API.

Note: the term "read-only" in this specification means that an application cannot modify the relevant data item; only the implementation can modify the value. The implementation MUST ignore attempts by the application to modify a read-only field.

As an OPTIONAL feature of the API, the implementation MAY support adding offsets to the timestamps that are captured. (Values of type "struct timespec" can represent negative offsets.) The assert_offset field of a pps_params_t value specifies a value to be added to generate a captured assert_timestamp. The clear_offset of a pps_params_t value field specifies a value to be added to generate a captured clear_timestamp. Since the offsets, if any, apply to all users of a given PPS source, the implementation SHOULD impose access controls on the use of this feature; for example, allowing only the super-user to set the offset values. The default value for both offsets is zero.

3.3 Mode bit definitions

A set of mode bits is associated with each PPS source.

The bits in the mode field of the pps_params_t type are:

      /* Device/implementation parameters */
      #define PPS_CAPTUREASSERT       0x01
      #define PPS_CAPTURECLEAR        0x02
      #define PPS_CAPTUREBOTH         0x03
      
      #define PPS_OFFSETASSERT        0x10
      #define PPS_OFFSETCLEAR         0x20
      
      #define PPS_CANWAIT             0x100
      #define PPS_CANPOLL             0x200
      
      /* Kernel actions */
      #define PPS_ECHOASSERT          0x40
      #define PPS_ECHOCLEAR           0x80
      
      /* Timestamp formats */
      #define PPS_TSFMT_TSPEC         0x1000
      #define PPS_TSFMT_NTPFP         0x2000

These mode bits are divided into three categories:

      1. Device/implementation parameters:  These are parameters either
         of the device or of the implementation.  If the implementation
         allows these to be changed, then these bits are read/write for
         users with sufficient privilege (such as the super-user), and

read-only for other users. If the implementation does not

allow these bits to be changed, they are read-only.

      2. Kernel actions:  These bits specify certain kernel actions to
         be taken on arrival of a signal.  If the implementation
         supports one of these actions, then the corresponding bit is
         read/write for users with sufficient privilege (such as the
         super-user), and read-only for other users.  If the
         implementation does not support the action, the corresponding
         bit is always zero.
      
      3. Timestamp formats:  These bits indicate the set of timestamp
         formats available for the device.  They are always read-only.

In more detail, the meanings of the Device/implementation parameter mode bits are:

PPS_CAPTUREASSERT

If this bit is set, the assert timestamp for the associated PPS source will be captured.

PPS_CAPTURECLEAR

If this bit is set, the clear timestamp for the associated PPS source will be captured.

PPS_CAPTUREBOTH Defined as the union of PPS_CAPTUREASSERT and

PPS_CAPTURECLEAR, for convenience.

PPS_OFFSETASSERT

If set, the assert_offset value is added to the current value of the operating system's internal timebase in order to generate the captured assert_timestamp.

PPS_OFFSETCLEAR If set, the clear_offset value is added to the

current value of the operating system's internal timebase in order to generate the captured clear_timestamp.

   PPS_CANWAIT     If set, the application may request that the
                   time_pps_fetch() function (see section 3.4.3) should
                   block until the next timestamp arrives.  Note: this
                   mode bit is read-only.
   
   PPS_CANPOLL     This bit is reserved for future use.  An application
                   SHOULD NOT depend on any functionality implied either
                   by its presence or by its absence.

If neither PPS_CAPTUREASSERT nor PPS_CAPTURECLEAR is set, no valid timestamp will be available via the API.

The meanings of the Kernel action mode bits are:

   PPS_ECHOASSERT   If set, after the capture of an assert timestamp,
                   the implementation generates a signal transition as
                   rapidly as possible on an output signal pin.  This
                   MUST NOT affect the delay between the PPS source's
                   transition to the asserted phase and the capture of
                   the assert timestamp.
   
   PPS_ECHOCLEAR    If set, after the capture of a clear timestamp, the
                   implementation generates a signal transition as
                   rapidly as possible on an output signal pin.  This
                   MUST NOT affect the delay between the PPS source's
                   transition to the clear phase and the capture of the
                   clear timestamp.

The timestamp formats are:

PPS_TSFMT_TSPEC Timestamps and offsets are represented as values of

type "struct timespec". All implementations MUST support this format, and this format is the default unless an application specifies otherwise.

PPS_TSFMT_NTPFP Timestamps and offsets are represented as values of

type "ntp_fp_t", which corresponds to the NTP "64-bit unsigned fixed-point" timestamp format [3]. Support for this format is OPTIONAL.

Other timestamp format bits may be defined as fields are added to the "pps_timeu_t" union.

The operating system may implement all of these mode bits, or just a subset of them. If an attempt is made to set an unsupported mode bit, the API will return an error. If an attempt is made to modify a read-only mode bit, the API will return an error.

3.4 New functions

In the description of functions that follows, we use the following function parameters:

   filedes         A file descriptor (type: int), for a serial line or
                   other source of PPS events.
   
   ppshandle       A variable of type "pps_handle_t", as defined in
                   section 3.2.
   
   ppsinfobuf      A record of type "pps_info_t", as defined in
                   section 3.2.
   
   ppsparams       A record of type "pps_params_t", as defined in
                   section 3.2.
   
   tsformat        An integer with exactly one of the timestamp format
                   bits set.

3.4.1 New functions: obtaining PPS sources

The API includes functions to create and destroy PPS source "handles".

   SYNOPSIS
   
      int time_pps_create(int filedes, pps_handle_t *handle);
      int time_pps_destroy(pps_handle_t handle);
   
   DESCRIPTION

All of the other functions in the PPS API operate on PPS handles (type: pps_handle_t). The time_pps_create() is used to convert an already-open UNIX file descriptor, for an appropriate special file, into a PPS handle.

The definition of what special files are appropriate for use with the PPS API is outside the scope of this specification, and may vary based on both operating system implementation, and local system configuration. One typical case is a serial line, whose DCD pin is connected to a source of PPS events.

The mode in which the UNIX file descriptor was originally opened affects what operations are allowed on the PPS handle. The time_pps_setparams() and time_pps_kcbind() functions (see sections 3.4.2 and 3.4.4) SHOULD be prohibited by the implementation if the descriptor is open only for reading (O_RDONLY).

Note: operations on a descriptor opened with an inappropriate mode might fail with EBADF.

The time_pps_destroy() function makes the PPS handle unusable, and frees any storage that might have been allocated for it. It does not close the associated file descriptor, nor does it change any of the parameter settings for the PPS source.

Note: If this API is adapted to an operating system that does not follow UNIX conventions for representing an accessible PPS source as an integer file descriptor, the time_pps_create() function may take different parameters from those shown here.

   RETURN VALUES

On successful completion, the time_pps_create() function returns 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.

If called with a valid handle parameter, the time_pps_destroy() function returns 0. Otherwise, it returns -1.

   ERRORS

If the time_pps_create() function fails, errno may be set to one of the following values:

   [EBADF]         The filedes parameter is not a valid file descriptor.
   
   [EOPNOTSUPP]    The use of the PPS API is not supported for the file
                   descriptor.
   
   [EPERM]         The process's effective user ID does not have the
                   required privileges to use the PPS API.

3.4.2 New functions: setting PPS parameters

The API includes several functions use to set or obtain the parameters of a PPS source.

   SYNOPSIS
   
      int time_pps_setparams(pps_handle_t handle,
                              const pps_params_t *ppsparams);
      int time_pps_getparams(pps_handle_t handle,
                              pps_params_t *ppsparams);
      int time_pps_getcap(pps_handle_t handle, int *mode);
   
   DESCRIPTION

A suitably privileged application may use time_pps_setparams() to set the parameters (mode bits and timestamp offsets) for a PPS source. The pps_params_t type is defined in section 3.2; mode bits are defined in section 3.3. An application may use time_pps_getparams() to discover the current settings of the PPS parameters. An application that needs to change only a subset of the existing parameters must first call time_pps_getparams() to obtain the current parameter values, then set the new values using time_pps_setparams().

Note: a call to time_pps_setparams() replaces the current values of all mode bits with those specified via the ppsparams argument, except those bits whose state cannot be changed. Bits might be read-only due to access controls, or because they are fixed by the implementation.

The timestamp format of the assert_offset and clear_offset fields is defined by the mode field. That is, on a call to time_pps_setparams(), the kernel interprets the supplied offset values using the timestamp format given in the mode field of the ppsparams argument. If the requested timestamp format is not supported, the time_pps_setparams() function has no effect and returns an error value. On a call to time_pps_getparams(), the kernel provides the timestamp format of the offsets by setting one of the timestamp format bits in the mode field.

Note: an application that uses time_pps_getparams() to read the current offset values cannot specify which format is used. The implementation SHOULD return the offsets using the same timestamp format as was used when the offsets were set.

An application wishing to discover which mode bits it may set, with its current effective user ID, may call time_pps_getcap(). This function returns the set of mode bits that may be set by the application, without generating an EINVAL or EPERM error, for the specified PPS source. It does not return the current values for the mode bits. A call to time_pps_getcap() returns the mode bits corresponding to all supported timestamp formats.

The time_pps_getcap() function MAY ignore the mode in which the associated UNIX file descriptor was opened, so the application might still receive an EBADF error on a call to time_pps_setparams(), even if time_pps_getcap() says that the chosen mode bits are allowed.

The mode bits returned by time_pps_getcap() for distinct PPS handles may differ, reflecting the specific capabilities of the underlying hardware connection to the PPS source, or of the source itself.

   RETURN VALUES

On successful completion, the time_pps_setparams(), time_pps_getparams(), and time_pps_getcap() functions return 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.

   ERRORS

If the time_pps_setparams(), time_pps_getparams(), or time_pps_getcap() function fails, errno may be set to one of the following values:

   [EBADF]         The handle parameter is not associated with a valid
                   file descriptor, or the descriptor is not open for
                   writing.
   
   [EFAULT]        A parameter points to an invalid address.
   
   [EOPNOTSUPP]    The use of the PPS API is not supported for the
                   associated file descriptor.
   
   [EINVAL]        The operating system does not support all of the
                   requested mode bits.
   
   [EPERM]         The process's effective user ID does not have the
                   required privileges to use the PPS API, or to set the
                   given mode bits.

3.4.3 New functions: access to PPS timestamps

The API includes one function that gives applications access to PPS timestamps. As an implementation option, the application may request the API to block until the next timestamp is captured. (The API does not directly support the use of the select() or poll() system calls to wait for PPS events.)

   SYNOPSIS

int time_pps_fetch(pps_handle_t handle,

                              const int tsformat,
                              pps_info_t *ppsinfobuf,
                              const struct timespec *timeout);
   
   DESCRIPTION

An application may use time_pps_fetch() to obtain the most recent timestamps captured for the PPS source specified by the handle parameter. The tsformat parameter specifies the desired timestamp format; if the requested timestamp format is not supported, the call fails and returns an error value. The application MUST specify exactly one timestamp format.

This function blocks until either a timestamp is captured from the PPS source, or until the specified timeout duration has expired. If the timeout parameter is a NULL pointer, the function simply blocks until a timestamp is captured. If the timeout parameter specifies a delay of zero, the function returns immediately.

Support for blocking behavior is an implementation option. If the PPS_CANWAIT mode bit is clear, and the timeout parameter is either NULL or points to a non-zero value, the function returns an EOPNOTSUPP error. An application can discover whether the feature is implemented by using time_pps_getcap() to see if the PPS_CANWAIT mode bit is set.

The result is stored in the ppsinfobuf parameter, whose fields are defined in section 3.2. If the function returns as the result of a timeout or error, the contents of the ppsinfobuf are undefined.

If this function is invoked before the system has captured a timestamp for the signal source, the ppsinfobuf returned will have its timestamp fields set to the time format's base date (e.g., for PPS_TSFMT_TSPEC, both the tv_sec and tv_nsec fields will be zero).

   RETURN VALUES

On successful completion, the time_pps_fetch() function returns 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.

   ERRORS

If the time_pps_fetch() function fails, errno may be set to one of the following values:

   [EBADF]         The handle parameter is not associated with a valid
                   file descriptor.
   
   [EFAULT]        A parameter points to an invalid address.
   
   [EINTR]         A signal was delivered before the time limit
                   specified by the timeout parameter expired and before
                   a timestamp has been captured.
   
   [EINVAL]        The requested timestamp format is not supported.
   
   [EOPNOTSUPP]    The use of the PPS API is not supported for the
                   associated file descriptor.
   
   [ETIMEDOUT]     The timeout duration has expired.

3.4.4 New functions: disciplining the kernel timebase

The API includes one OPTIONAL function to specify if and how a PPS source is provided to a kernel consumer of PPS events, such as the code used to discipline the operating system's internal timebase.

   SYNOPSIS
   
      int time_pps_kcbind(pps_handle_t handle,
                              const int kernel_consumer,
                              const int edge,
                              const int tsformat);
   DESCRIPTION

An application with appropriate privileges may use time_pps_kcbind() to bind a kernel consumer to the PPS source specified by the handle.

The kernel consumer is identified by the kernel_consumer parameter. In the current version of the API, the possible values for this parameter are:

      #define PPS_KC_HARDPPS          0
      #define PPS_KC_HARDPPS_PLL      1
      #define PPS_KC_HARDPPS_FLL      2

with these meanings:

   PPS_KC_HARDPPS  The kernel's hardpps() function (or equivalent).

PPS_KC_HARDPPS_PLL

A variant of hardpps() constrained to use a phase-locked loop.

PPS_KC_HARDPPS_FLL

A variant of hardpps() constrained to use a frequency-locked loop.

Implementation of any or all of these values is OPTIONAL.

The edge parameter indicates which edge of the PPS signal causes a timestamp to be delivered to the kernel consumer. It may have the value PPS_CAPTUREASSERT, PPS_CAPTURECLEAR, or PPS_CAPTUREBOTH, depending on particular characteristics of the PPS source. It may also be zero, which removes any binding between the PPS source and the kernel consumer.

The tsformat parameter specifies the format for the timestamps delivered to the kernel consumer. If this value is zero, the implementation MAY choose the appropriate format, or return EINVAL. The implementation MAY ignore a non-zero value for this parameter.

The binding created by this call persists until it is changed by a subsequent call specifying the same kernel_consumer. In particular, a subsequent call to time_pps_destroy() for the specified handle does not affect the binding.

The binding is independent of any prior or subsequent changes to the PPS_CAPTUREASSERT and PPS_CAPTURECLEAR mode bits for the device. However, if either the edge or the tsformat parameter values are inconsistent with the capabilities of the PPS source, an error is returned. The implementation MAY also return an error if the tsformat value is unsupported for time_pps_kcbind(), even if it is supported for other uses of the API.

The operating system may enforce two restrictions on the bindings created by time_pps_kcbind():

1. the kernel MAY return an error if an attempt is made to bind a kernel consumer to more than one PPS source a time.

2. the kernel MAY restrict the ability to set bindings to processes with sufficient privileges to modify the system's internal timebase. (On UNIX systems, such modification is normally done using settimeofday() and/or adjtime(), and is restricted to users with superuser privilege.)

Warning: If this feature is configured for a PPS source that does not have an accurate 1-pulse-per-second signal, or is otherwise inappropriately configured, use of this feature may result in seriously incorrect timekeeping for the entire system. For best results, the 1-PPS signal should have much better frequency stability than the system's internal clock source (usually a crystal-controlled oscillator), and should have jitter (variation in interarrival time) much less than the system's clock-tick interval.

See RFC 1589 [4] for more information about how the system's timebase may be disciplined using a PPS signal.

   RETURN VALUES

On successful completion, the time_pps_kcbind() function returns 0. Otherwise, a value of -1 is returned and errno is set to indicate the error.

   ERRORS

If the time_pps_kcbind() function fails, errno may be set to one of the following values:

   [EBADF]         The handle parameter is not associated with a valid
                   file descriptor, or the descriptor is not open for
                   writing.
   
   [EFAULT]        A parameter points to an invalid address.
   
   [EINVAL]        The requested timestamp format is not supported.
   
   [EOPNOTSUPP]    The use of the PPS API is not supported for the
                   associated file descriptor, or this OPTIONAL
                   function is not supported.
   
   [EPERM]         The process's effective user ID does not have the
                   required privileges to set the binding.

3.5 Compliance rules

   The key words "MUST", "MUST NOT", "REQUIRED","SHOULD", SHOULD NOT",
   "MAY", and "OPTIONAL" in this document are to be interpreted as
   described in RFC 2119 [1].

Some features of this specification are OPTIONAL, but others are REQUIRED.

3.5.1 Functions

An implementation MUST provide these functions:

      - time_pps_create()
      - time_pps_destroy()
      - time_pps_setparams()
      - time_pps_getparams()
      - time_pps_getcap()
      - time_pps_fetch()

An implementation MUST provide this function, but it may be implemented as a function that always return an EOPNOTSUPP error, possibly on a per-source basis:

• time_pps_kcbind()

3.5.2 Mode bits

An implementation MUST support at least one of these mode bits for each PPS source:

- PPS_CAPTUREASSERT
- PPS_CAPTURECLEAR

and MAY support both of them. If an implementation supports both of these bits for a PPS source, it SHOULD allow them to be set simultaneously.

An implementation MUST support this timestamp format:

• PPS_TSFMT_TSPEC

An implementation MAY support these mode bits:

      - PPS_ECHOASSERT
      - PPS_ECHOCLEAR
      - PPS_OFFSETASSERT
      - PPS_OFFSETCLEAR

An implementation MAY support this timestamp format:

• PPS_TSFMT_NTPFP

3.6 Examples

A very simple use of this API might be:

      int fd;
      pps_handle_t handle;
      pps_params_t params;
      pps_info_t infobuf;
      struct timespec timeout;
      
      /* Open a file descriptor and enable PPS on rising edges */
      fd = open(PPSfilename, O_RDWR, 0);
      time_pps_create(fd, &handle);
      time_pps_getparams(handle, &params);
      if ((params.mode & PPS_CAPTUREASSERT) == 0) {
          fprintf(stderr, "%s cannot currently CAPTUREASSERT\n",
                PPSfilename);
          exit(1);
      }
      
      /* create a zero-valued timeout */
      timeout.tv_sec = 0;
      timeout.tv_nsec = 0;
      
      /* loop, printing the most recent timestamp every second or so */
      while (1) {
          sleep(1);
          time_pps_fetch(handle, PPS_TSFMT_TSPEC, &infobuf, &timeout);
          printf("Assert timestamp: %d.%09d, sequence: %ld\n",
                      infobuf.assert_timestamp.tv_sec,
                      infobuf.assert_timestamp.tv_nsec,
                      infobuf.assert_sequence);
      }

Note that this example omits most of the error-checking that would be expected in a reliable program.

Also note that, on a system that supports PPS_CANWAIT, the function of these lines:

         sleep(1);
         time_pps_fetch(handle, PPS_TSFMT_TSPEC, &infobuf, &timeout);

might be more reliably accomplished using:

         timeout.tv_sec = 100;
         timeout.tv_nsec = 0;
         time_pps_fetch(handle, PPS_TSFMT_TSPEC, &infobuf, &timeout);

The (arbitrary) timeout value is used to protect against the possibility that another application might disable PPS timestamps, or that the hardware generating the timestamps might fail.

A slightly more elaborate use of this API might be:

      int fd;
      pps_handle_t handle;
      pps_params_t params;
      pps_info_t infobuf;
      int avail_mode;
      struct timespec timeout;
      
      /* Open a file descriptor */
      fd = open(PPSfilename, O_RDWR, 0);
      time_pps_create(fd, &handle);
      
      /*
       * Find out what features are supported
       */
      
      time_pps_getcap(handle, &avail_mode);
      if ((avail_mode & PPS_CAPTUREASSERT) == 0) {
          fprintf(stderr, "%s cannot CAPTUREASSERT\n", PPSfilename);
          exit(1);
      }
      if ((avail_mode & PPS_OFFSETASSERT) == 0) {
          fprintf(stderr, "%s cannot OFFSETASSERT\n", PPSfilename);
          exit(1);
      }
      
      /*
       * Capture assert timestamps, and
       *   compensate for a 675 nsec propagation delay
       */
      
      time_pps_getparams(handle, &params);
      params.assert_offset.tv_sec = 0;
      params.assert_offset.tv_nsec = 675;
      params.mode |= PPS_CAPTUREASSERT | PPS_OFFSETASSERT;
      time_pps_setparams(handle, &params);
      
      /* create a zero-valued timeout */
      timeout.tv_sec = 0;
      timeout.tv_nsec = 0;
      
      /* loop, printing the most recent timestamp every second or so */
      while (1) {
          if (avail_mode & PPS_CANWAIT) {
              time_pps_fetch(handle, PPS_TSFMT_TSPEC, &infobuf, NULL);
                              /* waits for the next event */
          } else {
              sleep(1);
              time_pps_fetch(handle, PPS_TSFMT_TSPEC, &infobuf,
                timeout);
      
          }
          printf("Assert timestamp: %d.%09d, sequence: %ld\n",
                      infobuf.assert_timestamp.tv_sec,
                      infobuf.assert_timestamp.tv_nsec,
                      infobuf.assert_sequence);
      }

Again, most of the necessary error-checking has been omitted from this example.

4 Security Considerations

This API gives applications three capabilities:

• Causing the system to capture timestamps on certain events.

• Obtaining timestamps for certain events.

• Affecting the system's internal timebase.

The first capability should not affect security directly, but might cause a slight increase in interrupt latency and interrupt-handling overhead.

The second capability might be useful in implementing certain kinds of covert communication channels.

In most cases, neither of these first two issues is a significant security threat, because the traditional UNIX file protection facility may be used to to limit access to the relevant special files. Provision of the PPS API adds minimal additional risk.

The final capability is reserved to highly privileged users. In UNIX systems, this means those with superuser privilege. Such users can evade protections based on file permissions; however, such users can in general cause unbounded havoc, and can set the internal timebase (and its rate of change), so this API creates no new vulnerabilities.

5 Acknowledgements

The API in this document draws some of its inspiration from the LBL "ppsclock" distribution [2], originally implemented in 1993 by Steve McCanne, Craig Leres, and Van Jacobson. We also thank Poul-Henning Kamp, Craig Leres, Judah Levine, and Harlan Stenn for helpful comments they contributed during the drafting of this document.

6 References

   1.  Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.

2. Steve McCanne, Craig Leres, and Van Jacobson. PPSCLOCK. ftp://ftp.ee.lbl.gov/ppsclock.tar.Z.

   3.  Mills, D., "Network Time Protocol (Version 3):  Specification,
       Implementation and Analysis", RFC 1305, March 1992.
   
   4.  Mills, D., "A Kernel Model for Precision Timekeeping", RFC 1589,
       March, 1994.

5. The Open Group. The Single UNIX Specification, Version 2 - 6 Vol Set for UNIX 98. Document number T912, The Open Group, February, 1997.

7 Authors' Addresses

Jeffrey C. Mogul
Western Research Laboratory
Compaq Computer Corporation
250 University Avenue
Palo Alto, California, 94305, U.S.A.

   Phone: 1 650 617 3304 (email preferred)
   EMail: mogul@wrl.dec.com

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

   Phone: (302) 831-8247
   EMail: mills@udel.edu
   
   Jan Brittenson
   Sun Microsystems, Inc.
   901 San Antonio Rd  M/S MPK17-202
   Palo Alto, CA 94303
   Email: Jan.Brittenson@Eng.Sun.COM

Jonathan Stone
Stanford Distributed Systems Group
Stanford, CA 94305

   Phone: (650) 723-2513
   EMail: jonathan@dsg.stanford.edu

Ulrich Windl
Universitaet Regensburg, Klinikum

   EMail: ulrich.windl@rz.uni-regensburg.de

A. Extensions and related APIs

The API specified in the main body of this document could be more useful with the provision of several extensions or companion APIs.

At present, the interfaces listed in this appendix are not part of the formal specification in this document.

A.1 Extension: Parameters for the "echo" mechanism

The "echo" mechanism described in the body of this specification leaves most of the details to the implementor, especially the designation of one or more output pins.

It might be useful to extend this API to provide either or both of these features:

• A means by which the application can discover which output pin is echoing the input pin.

• A means by which the application can select which output pin is echoing the input pin.

A.2 Extension: Obtaining information about external clocks

The PPS API may be useful with a wide variety of reference clocks, connected via several different interface technologies (including serial lines, parallel interfaces, and bus-level interfaces). These reference clocks can have many features and parameters, some of which might not even have been invented yet.

We believe that it would be useful to have a mechanism by which an application can discover arbitrary features and parameters of a reference clock. These might include:

- Clock manufacturer, model number, and revision level - Whether the clock is synchronized to an absolute standard - For synchronized clocks,

           * The specific standard
           * The accuracy of the standard
           * The path used (direct connection, shortwave, longwave,
             satellite, etc.)
           * The distance (offset) and variability of this path

- For PPS sources,

* The pulse rate
* The pulse shape
* Which edge of the pulse corresponds to the epoch

• The time representation format

This information might best be provided by an API analogous to the standard "curses" API, with a database analogous to the standard "terminfo" database. That is, a "clockinfo" database would contain a set of (attribute, value) pairs for each type of clock, and the API would provide a means to query this database.

Additional mechanisms would allow an application to discover the clock or clocks connected to the local system, and to discover the clockinfo type of a specific clock device.

A.3 Extension: Finding a PPS source

Although the clockinfo database described in section A.2, together with the discover mechanisms described there, would allow an application to discover the PPS source (or sources) connected to a system, it might be more complex than necessary.

A simpler approach would be to support a single function that provides the identity of one or more PPS sources.

For example, the function might be declared as

int time_pps_findsource(int index,

                              char *path, int pathlen,
                              char *idstring, int idlen);

The index argument implicitly sets up an ordering on the PPS sources attached to the system. An application would use this function to inquire about the Nth source. The function would return -1 if no such source exists; otherwise, it would return 0, and would place the pathname of the associated special file in the path argument. It would also place an identification string in the idstring argument. The identification string could include the clock make, model, version, etc., which could then be used by the application to control its behavior.

This function might simply read the Nth line from a simple database, containing lines such as:

      /dev/tty00      "TrueTime 468-DC"
      /dev/pps1       "Homebrew rubidium frequency standard"

allowing the system administrator to describe the configuration of PPS sources.

B. Example implementation: PPSDISC Line discipline

One possible implementation of the PPS API might be to define a new "line discipline" and then map the API onto a set of ioctl() commands. Here we sketch such an implementation; note that this is not part of the specification of the API, and applications should not expect this low-level interface to be available.

In this approach, the set of line disciplines is augmented with one new line discipline, PPSDISC. This discipline will act exactly the same as the TTYDISC discipline, except for its handling of modem DCD interrupts.

Once the TIOCSETD ioctl() has been used to select this line discipline, PPS-related operations on the serial line may be invoked using new ioctl() commands. For example (values used only for illustration):

   #define PPSFETCH      _IOR('t', 75, pps_info_t)
   #define PPSSETPARAM   _IOW('t', 76, pps_params_t)
   #define PPSGETPARAM   _IOR('t', 77, pps_params_t)
   #define PPSGETCAP     _IOR('t', 78, int)

B.1 Example

A typical use might be:

int ldisc = PPSDISC;
pps_params_t params;
pps_info_t infobuf;

      ioctl(fd, TIOCSETD, &ldisc);    /* set discipline */
      
      /*
       * Check the capabilities of this PPS source to see
       * if it supports what we need.
       */
      ioctl(fd, PPSGETCAP, &params);
      if ((params.mode & PPS_CAPTUREASSERT) == 0) {
          fprintf(stderr, "PPS source is not suitable\n");
          exit(1);
      }
      
      /*
       * Set this line to timestamp on a rising-edge interrupt
      
       */
      ioctl(fd, PPSGETPARAMS, &params);
      params.mode |= PPS_CAPTUREASSERT;
      ioctl(fd, PPSSETPARAMS, &params);
      
      sleep(2);       /* allow time for the PPS pulse to happen */
      
      /* obtain most recent timestamp and sequence # for this line */
      ioctl(fd, PPSFETCH, &infobuf);

Again, this example imprudently omits any error-checking.

C. Available implementations

Several available implementations of this API are listed at <http://www.ntp.org/ppsapi/PPSImpList.html>. Note that not all of these implementations correspond to the current version of the specification.

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