clock offsets are then distributed along the arcs of the spanning tree
to all processes in the system and the various process clocks corrected
using the procedure described in Section 5 of this document. While it
can be seen that the design of Hellospeak strongly influenced the design
of NTP, Hellospeak itself is not an Internet protocol and is unsuited
for use outside its local-net environment.
The Unix 4.3bsd time daemon timed [GUS85a] uses a single master-time
daemon to measure offsets of a number of slave hosts and send periodic
corrections to them. In this model the master is determined using an
election algorithm [GUS85b] designed to avoid situations where either no
master is elected or more than one master is elected. The election
process requires a broadcast capability, which is not a ubiquitous
feature of the Internet. While this model has been extended to support
hierarchical configurations in which a slave on one network serves as a
master on the other [TRI86], the model requires handcrafted
configuration tables in order to establish the hierarchy and avoid
loops. In addition to the burdensome, but presumably infrequent,
overheads of the election process, the offset measurement/correction
process requires twice as many messages as NTP per update.
A scheme with features similar to NTP is described in [KOP87]. This
scheme is intended for multi-server LANs where each of a set of possibly
many time servers determines its local-time offset relative to each of
the other servers in the set using periodic timestamped messages, then
determines the local-clock correction using the Fault-Tolerant Average
(FTA) algorithm of [LUN84]. The FTA algorithm, which is useful where up
to k servers may be faulty, sorts the offsets, discards the k highest
and lowest ones and averages the rest. The scheme, as described in
[SCH86], is most suitable to LAN environments which support broadcast
and would result in unacceptable overhead in an internet environment. In
addition, for reasons given in Section 4 of this paper, the statistical
properties of the FTA algorithm are not likely to be optimal in an
internet environment with highly dispersive delays.
A good deal of research has gone into the issue of maintaining accurate
time in a community where some clocks cannot be trusted. A truechimer is
a clock that maintains timekeeping accuracy to a previously published
(and trusted) standard, while a falseticker is a clock that does not.
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