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A Kernel Model for Precision Timekeeping (RFC1589) Disclosure Number: IPCOM000002423D
Original Publication Date: 1994-Mar-01
Included in the Prior Art Database: 2000-Sep-12
Document File: 30 page(s) / 86K

Publishing Venue

Internet Society Requests For Comment (RFCs)

Related People

D. Mills: AUTHOR


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.

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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.


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