Network Time Protocol (Version 3) Specification, Implementation and Analysis
RFC 1305

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Network Working Group                           David L. Mills
Request for Comments: 1305                      University of Delaware
Obsoletes RFC-1119, RFC-1059, RFC-958           March 1992

                   Network Time Protocol (Version 3)
               Specification, Implementation and Analysis

Note: This document consists of an approximate rendering in ASCII of the
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for use in searches, etc. However, most tables, figures, equations and
captions have not been rendered and the pagination and section headings
are not available.

Abstract

This document describes the Network Time Protocol (NTP), specifies its
formal structure and summarizes information useful for its
implementation. NTP provides the mechanisms to synchronize time and
coordinate time distribution in a large, diverse internet operating at
rates from mundane to lightwave. It uses a returnable-time design in
which a distributed subnet of time servers operating in a self-
organizing, hierarchical-master-slave configuration synchronizes local
clocks within the subnet and to national time standards via wire or
radio. The servers can also redistribute reference time via local
routing algorithms and time daemons.

Status of this Memo

This RFC specifies an IAB standards track protocol for the Internet
community and requests discussion and suggestions for improvements.
Please refer to the current edition of the <169>IAB Official Protocol
Standards<170> for the standardization state and status of this
protocol. Distribution of this memo is unlimited.

Keywords: network clock synchronization, standard time distribution,
fault-tolerant architecture, maximum-likelihood estimation, disciplined
oscillator, internet protocol, high-speed networks, formal
specification.

Preface

This document describes Version 3 of the Network Time Protocol (NTP). It
supersedes Version 2 of the protocol described in RFC-1119 dated
September 1989. However, it neither changes the protocol in any
significant way nor obsoletes previous versions or existing
implementations. The main motivation for the new version is to refine
the analysis and implementation models for new applications at much
higher network speeds to the gigabit-per-second regime and to provide
for the enhanced stability, accuracy and precision required at such
speeds. In particular, the sources of time and frequency errors have
been rigorously examined and error bounds established in order to
improve performance, provide a model for correctness assertions and
indicate timekeeping quality to the user. The revision also incorporates
two new optional features, (1) an algorithm to combine the offsets of a
number of peer time servers in order to enhance accuracy and (2)
improved local-clock algorithms which allow the poll intervals on all
synchronization paths to be substantially increased in order to reduce
network overhead. An overview of the changes, which are described in
detail in Appendix D, follows:

1.
In Version 3 The local-clock algorithm has been overhauled to improve
stability and accuracy. Appendix G presents a detailed mathematical
model and design example which has been refined with the aid of
feedback-control analysis and extensive simulation using data collected
over ordinary Internet paths. Section 5 of RFC-1119 on the NTP local
clock has been completely rewritten to describe the new algorithm. Since
the new algorithm can result in message rates far below the old ones, it
is highly recommended that they be used in new implementations. Note
that use of the new algorithm does not affect interoperability with
previous versions or existing implementations.

2.

In Version 3 a new algorithm to combine the offsets of a number of peer
time servers is presented in Appendix F. This algorithm is modelled on
those used by national standards laboratories to combine the weighted
offsets from a number of standard clocks to construct a synthetic
laboratory timescale more accurate than that of any clock separately. It
can be used in an NTP implementation to improve accuracy and stability
and reduce errors due to asymmetric paths in the Internet. The new
algorithm has been simulated using data collected over ordinary Internet
paths and, along with the new local-clock algorithm, implemented and
tested in the Fuzzball time servers now running in the Internet. Note
that use of the new algorithm does not affect interoperability with
previous versions or existing implementations.

3.

Several inconsistencies and minor errors in previous versions have been
corrected in Version 3. The description of the procedures has been
rewritten in pseudo-code augmented by English commentary for clarity and
to avoid ambiguity. Appendix I has been added to illustrate C-language
implementations of the various filtering and selection algorithms
suggested for NTP. Additional information is included in Section 5 and
in Appendix E, which includes the tutorial material formerly included in