Re: Who's got the time?


Mills@UDEL.EDU
Thu, 12 May 88 12:38:33 EDT


Drew,

First, let me point out that I am far from an unbiased source. Probably
the most widely implemented time protocol is UDP/TIME, which is distributed
with vanilla Unix. The 4.3bsd Unix distributions include a time daemon
which manages time distribution within a LAN, but would not be suitable for
use over most multi-net Internet paths. The Network Time Protocol (NTP)
described in RFC-958, as amended, was specifically intended for time
distribution both within a LAN and as far as the Internet eye can see.
A 4.3bsd daemon for NTP is available from MIke Petry (petry@trantor.umd.edu).

There are varying opinions on the robustness and accuracy of various
time-synchronization mechanisms in the literature. The NTP design was
based on models used by digital common carriers and uses maximum-likelihood
estimation and nonlinear-filtering techniques. A survey of different
approaches to the problem, the factors the led to the NTP design choices
and a comprehensive bibliography on the subject can be found in the
~ftp/pub/ntp.doc file in the anonymous directory at louie.udel.edu. This
document has been printed as a departmental report and submitted as an
RFC.

In my opinion it does not make sense to porpose a new service without
extensive experimentation and evaluation of a prototype implementation.
NTP was born three years ago and has been in regular use since that time
with at least three radio-synchronized time servers (now with five).
As the result of this experience the original NTP design was thoroughly
overhauled and tested over the last six months. The resulting NTP design,
including algorithms necessary to improve accuracy and mitigate between
possibly broken or bogus clocks (falsetickers), is described in the cited
document. All five of the radio-synchronized NTP servers, as well as all
Fuzzball secondary servers (e.g. NSFNET Backbone gateways) and many Unix
secondary/retail servers now tick the new version. In most places where
such things can be measured, the service provides time acccurate to within
a few tens of milliseconds and has an incredible degree of redundancy.

Dave

P.S. Truechimers may wish to subscribe to the list ntp@trantor.umd.edu
for additional information.

DLM



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