Time bombs

By Hal Stern, Unix Insider |  Operating Systems

About ten years ago, Dennis Ritchie posted the
question "What time is it?" on
net.general, then the catch-all for USENET
traffic of interest to the masses. Dennis was poking fun at messages
(and their authors) that arrived several days after bearing any sense
of relevance. The "what time is it?" probe demonstrated that USENET
isn't a good source for a watch-setting consensus. The underlying
problem, however, is not setting just one clock on one machine, but
instead keeping an entire network of machines synchronized with
respect to a global, reliable time source and with respect to each
other.

Accurate time-keeping affects many day-to-day functions as well
as issues critical to system administrators:


  • File modification times are consulted by make, so
    skewed desktop clocks may result in erratic actions.

  • Secure RPC, NIS+, and Secure NFS rely on timestamps for verifying
    requests. Client clocks that stray into network tardiness result
    in rejected requests.
  • Digital signatures or transaction timestamps that
    are used to sequence events must refer to a time base that is not
    machine dependent.
  • Attempts to correlate network activity, performance data, and system
    log messages from multiple hosts require a single, global timepiece
    for the network. You can't track a performance problem across several
    machines if you can't lay out events on a single, absolute timeline.
  • Put simply: how do you keep the software watches of hundreds or
    thousands of systems synchronized, particularly when they are spread
    around the world, and how do you make sure that your sense of time
    matches that of the rest of the computing world? This month, we
    address the question of what to do when your sense of time bombs.
    We'll start with an overview of how Unix systems keep track of time,
    We'll look a simple method for synchronizing Unix system clocks, and take
    a peek at the Network Time Protocol, a small dose of Swiss perfection
    you can bring to most Unix and Windows NT machines.

    A brief history of time, in 64 bits or less

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