operating conditions over the last three years. In addition, as the
result of experience in operating multiple-server subnets including
radio clocks at several sites in the U.S. and with clients in the U.S.
and Europe, reliable algorithms for selecting good clocks from a
population possibly including broken ones have been developed [DEC89],
[MIL91a] and are described in Section 4.
The accuracies achievable by NTP depend strongly on the precision of the
local-clock hardware and stringent control of device and process
latencies. Provisions must be included to adjust the software logical-
clock time and frequency in response to corrections produced by NTP.
Section 5 describes a local-clock design evolved from the Fuzzball
implementation described in [MIL83b] and [MIL88b]. This design includes
offset-slewing, frequency compensation and deglitching mechanisms
capable of accuracies in the order of a millisecond, even after extended
periods when synchronization to primary reference sources has been lost.
Details specific to NTP packet formats used with the Internet Protocol
(IP) and User Datagram Protocol (UDP) are presented in Appendix A, while
details of a suggested auxiliary NTP Control Message, which may be used
when comprehensive network-monitoring facilities are not available, are
presented in Appendix B. Appendix C contains specification and
implementation details of an optional authentication mechanism which can
be used to control access and prevent unauthorized data modification,
while Appendix D contains a listing of differences between Version 3 of
NTP and previous versions. Appendix E expands on issues involved with
precision timescales and calendar dating peculiar to computer networks
and NTP. Appendix F describes an optional algorithm to improve accuracy
by combining the time offsets of a number of clocks. Appendix G presents
a detailed mathematical model and analysis of the NTP local-clock
algorithms. Appendix H analyzes the sources and propagation of errors
and presents correctness principles relating to the time-transfer
service. Appendix I illustrates C-language code segments for the clock-
filter, clock-selection and related algorithms described in Section 4.
Related Technology
Other mechanisms have been specified in the Internet protocol suite to
record and transmit the time at which an event takes place, including
the Daytime protocol [POS83a], Time Protocol [POS83b], ICMP Timestamp
message [DAR81b] and IP Timestamp option [SU81]. Experimental results on
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