Experiments in network clock synchronization
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(September 1985; No errata)
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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
Mills [Page 1]
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
Mills [Page 2]
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
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