RFC 1589 (rfc1589) - Page 1 of 37
A Kernel Model for Precision Timekeeping
Alternative Format: Original Text Document
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
Mills
