rfc1305.txt

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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
PostScript document of the same name. It is provided for convenience and
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
Section 2 of RFC-1119, as well as much new material clarifying the
interpretation of timescales and leap seconds.

4.

Minor changes have been made in the Version-3 local-clock algorithms to
avoid problems observed when leap seconds are introduced in the UTC
timescale and also to support an auxiliary precision oscillator, such as
a cesium clock or timing receiver, as a precision timebase. In addition,
changes were made to some procedures described in Section 3 and in the
clock-filter and clock-selection procedures described in Section 4.
While these changes were made to correct minor bugs found as the result
of experience and are recommended for new implementations, they do not
affect interoperability with previous versions or existing
implementations in other than minor ways (at least until the next leap
second).

5.

In Version 3 changes were made to the way delay, offset and dispersion
are defined, calculated and processed in order to reliably bound the
errors inherent in the time-transfer procedures. In particular, the
error accumulations were moved from the delay computation to the
dispersion computation and both included in the clock filter and
selection procedures. The clock-selection procedure was modified to
remove the first of the two sorting/discarding steps and replace with an
algorithm first proposed by Marzullo and later incorporated in the
Digital Time Service. These changes do not significantly affect the
ordinary operation of or compatibility with various versions of NTP, but
they do provide the basis for formal statements of correctness as
described in Appendix H.
Table of Contents

1.       Introduction   1

1.1.     Related Technology     2

2.       System Architecture    4

2.1.     Implementation Model   6

2.2.     Network Configurations 7

3.       Network Time Protocol  8

3.1.     Data Formats   8

3.2.     State Variables and Parameters 9

3.2.1.   Common Variables       9

3.2.2.   System Variables       12

3.2.3.   Peer Variables 12

3.2.4.   Packet Variables       14

3.2.5.   Clock-Filter Variables 14

3.2.6.   Authentication Variables       15

3.2.7.   Parameters     15

3.3.     Modes of Operation     17

3.4.     Event Processing       19

3.4.1.   Notation Conventions   19

3.4.2.   Transmit Procedure     20

3.4.3.   Receive Procedure      22

3.4.4.   Packet Procedure       24

3.4.5.   Clock-Update Procedure 27

3.4.6.   Primary-Clock Procedure        28

3.4.7.   Initialization Procedures      28

3.4.7.1.         Initialization Procedure       29

3.4.7.2.         Initialization-Instantiation Procedure 29

3.4.7.3.         Receive-Instantiation Procedure        30

3.4.7.4.         Primary Clock-Instantiation Procedure  31

3.4.8.   Clear Procedure        31

3.4.9.   Poll-Update Procedure  32

3.5.     Synchronization Distance Procedure     32

3.6.     Access Control Issues  33

4.       Filtering and Selection Algorithms     34

4.1.     Clock-Filter Procedure 35

4.2.     Clock-Selection Procedure      36

4.2.1.   Intersection Algorithm 36

5.       Local Clocks   40

5.1.     Fuzzball Implementation        41

5.2.     Gradual Phase Adjustments      42

5.3.     Step Phase Adjustments 43

5.4.     Implementation Issues  44

6.       Acknowledgments        45

7.       References     46

A.       Appendix A. NTP Data Format - Version 3        50

B.       Appendix B. NTP Control Messages       53

B.1.     NTP Control Message Format     54

B.2.     Status Words   56

B.2.1.   System Status Word     56

B.2.2.   Peer Status Word       57

B.2.3.   Clock Status Word      58

B.2.4.   Error Status Word      58

B.3.     Commands       59

C.       Appendix C. Authentication Issues      61

C.1.     NTP Authentication Mechanism   62

C.2.     NTP Authentication Procedures  63

C.2.1.   Encrypt Procedure      63

4.2.2.   Clustering Algorithm   38

C.2.2.   Decrypt Procedure      64

C.2.3.   Control-Message Procedures     65

D.       Appendix D. Differences from Previous Versions.        66

E.       Appendix E. The NTP Timescale and its Chronometry      70

E.1.     Introduction   70

E.2.     Primary Frequency and Time Standards   70

E.3.     Time and Frequency Dissemination       72

E.4.     Calendar Systems       74

E.5.     The Modified Julian Day System 75

E.6.     Determination of Frequency     76

E.7.     Determination of Time and Leap Seconds 76

E.8.     The NTP Timescale and Reckoning with UTC       78

F.       Appendix F. The NTP Clock-Combining Algorithm  80

F.1.     Introduction   80

F.2.     Determining Time and Frequency 80