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
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