RFC 1589 (rfc1589) - Page 2 of 37

A Kernel Model for Precision Timekeeping

RFC 1589         Kernel Model for Precision Timekeeping       March 1994


   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



Mills