Re: Time synchronization and distribution plan


Mills@UDEL.EDU
Sun, 24 Jan 88 15:46:15 EST


jqj,

While I can understand your concern about Byzantine agreement, there may
never be enough primary servers that everybody can play with very many of
them, so I think we are stuck with a hierarchy, even if we quibble on the
number of strata (to borrow the telephonic term). I am concerned about
broken radio clocks, hosts, networks and leap-seconds, as witness the
experiments I reported in RFCs 956-958 which were repeated recently with
interesting results. I have no problem in organizing subsets of clocks
which might run Byzantine algorithms in order to determine the truthtellers
and falsehood speakers; however, my main concern is the accuracy and
stability of the basic time service itself. I have taken a statistical
approach which attempts to maximize the quality of the data using signal
filtering and smoothing techniques which detect and discard outlyers
due to broken clocks and are based on the assumption that at least half
the clocks are tracking the same random variable and the rest are uniformly
distributed over the observation interval (see RFC-956). These algorithms
have been extensively simulated and tested in prototype implementations
now running and have proved extremely resilient to noisy networks, broken
clocks and jittery gateways but not, I admit, to broken programmers.

Dave



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