Experiments in network clock synchronization
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Network Working Group                                         D.L. Mills
Request for Comments: 957                               M/A-COM Linkabit
                                                          September 1985

              Experiments in Network Clock Synchronization

Status of this Memo

   This RFC discusses some experiments in clock synchronization in the
   ARPA-Internet community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Table of Contents

   1.      Introduction
   2.      Design of the Synchronization Algorithm
   2.1.    The Logical Clock
   2.2.    Linear Phase Adjustments
   2.3.    Nonlinear Phase Adjustments
   3.      Synchronizing Network Clocks
   3.1.    Reference Clocks and Reference Hosts
   3.2.    Distribution of Timing Information
   4.      Experimental Validation of the Design
   4.1.    Experiment Design
   4.2.    Experiment Execution
   4.3.    Discussion of Results
   4.3.1.  On Power-Grid Clocks
   4.3.2.  On Clocks Synchronized via Network Links
   4.3.3.  On the Accuracy of Radio Clocks
   4.3.3.1. The Spectracom 8170 WWVB Radio Clock
   4.3.3.2. The True Time 468-DC GOES Radio Clock
   4.3.3.3. The Heath GC-1000 WWV Radio Clock
   4.3.4.  On Handling Disruptions
   4.4.    Additional Experiments
   5.      Summary and Conclusions
   6.      References

List of Figures

   Figure 1. Clock Registers
   Figure 2. Network Configuration

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RFC 957                                                   September 1985
Experiments in Network Clock Synchronization

List of Tables

   Table 1. Experiment Hosts
   Table 2. Link Measurements
   Table 3. First Derivative of Delay
   Table 4. GOES Radio Clock Offsets
   Table 5. WWV Radio Clock Offsets
   Table 6. ISI-MCON-GW Clock Statistics
   Table 7. LL-GW Clock Statistics
   Table 8. LL-GW Clock Statistics

1.  Introduction

   One of the services frequently neglected in computer network design
   is a high-quality, time-of-day clock capable of generating accurate
   timestamps with small residual errors compared to intrinsic one-way
   network delays.  Such a service would be useful for tracing the
   progress of complex transactions, synchronizing cached data bases,
   monitoring network performance and isolating problems.

   Several mechanisms have been specified in the Internet protocol suite
   to record and transmit the time at which an event takes place,
   including the ICMP Timestamp message [6], Time Protocol [7], Daytime
   protocol [8] and IP Timestamp option [9].  A new Network Time
   Protocol [12] has been proposed as well.  Additional information on
   network time synchronization can be found in the References at the
   end of this document.  Synchronization protocols are described in [3]
   and [12] and synchronization algorithms in [2], [5], [10] and [11].
   Experimental results on measured roundtrip delays in the Internet are
   discussed in [4].  A comprehensive mathematical treatment of clock
   synchronization can be found in [1].

   Several mechanisms have been specified in the Internet protocol suite
   to record and transmit the time at which an event takes place,
   including the ICMP Timestamp message [6], Time protocol [7], Daytime
   protocol [8] and IP Timestamp option [9].  Issues on time
   synchronization are discussed in [4] and synchronization algorithms
   in [2] and [5].  Experimental results on measured roundtrip delays in
   the Internet are discussed in [2].  A comprehensive mathematical
   treatment of the subject can be found in [1], while an interesting
   discussion on mutual-synchonization techniques can be found in [10].

   There are several ways accurate timestamps can be generated.  One is
   to provide at every service point an accurate, machine-readable clock
   synchronized to a central reference, such as the National Bureau of
   Standards (NBS).  Such clocks are readily available in several models
   ranging in accuracies of a few hundred milliseconds to less than a

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RFC 957                                                   September 1985
Experiments in Network Clock Synchronization

   millisecond and are typically synchronized to special ground-based or
   satellite-based radio broadcasts.  While the expense of the clocks
   themselves, currently in the range $300 to $3000, can often be
   justified, all require carefully sited antennas well away from
   computer-generated electromagnetic noise, as well as shielded
   connections to the clocks.  In addition, these clocks can require a
   lengthy synchonization period upon power-up, so that a battery-backup
   power supply is required for reliable service in the event of power
   interruptions.

   If the propagation delays in the network are stable or can be
   predicted accurately, timestamps can be generated by a central