RFC 1059 (rfc1059) - Page 3 of 58
Network Time Protocol (version 1) specification and implementation
Alternative Format: Original Text Document
RFC 1059 Network Time Protocol July 1988
1. Introduction
This document describes the Network Time Protocol (NTP), including
the architectures, algorithms and protocols to synchronize local
clocks in a set of distributed clients and servers. The protocol was
first described in RFC-958 [24], but has evolved in significant ways
since publication of that document. NTP is built on the Internet
Protocol (IP) [10] and User Datagram Protocol (UDP) [6], which
provide a connectionless transport mechanism; however, it is readily
adaptable to other protocol suites. It is evolved from the Time
Protocol [13] and the ICMP Timestamp message [11], but is
specifically designed to maintain accuracy and robustness, even when
used over typical Internet paths involving multiple gateways and
unreliable nets.
The service environment consists of the implementation model, service
model and time scale described in Section 2. The implementation
model is based on a multiple-process operating system architecture,
although other architectures could be used as well. The service
model is based on a returnable-time design which depends only on
measured offsets, or skews, but does not require reliable message
delivery. The subnet is a self-organizing, hierarchical master-slave
configuration, with synchronization paths determined by a minimum-
weight spanning tree. While multiple masters (primary servers) may
exist, there is no requirement for an election protocol.
NTP itself is described in Section 3. It provides the protocol
mechanisms to synchronize time in principle to precisions in the
order of nanoseconds while preserving a non-ambiguous date well into
the next century. The protocol includes provisions to specify the
characteristics and estimate the error of the local clock and the
time server to which it may be synchronized. It also includes
provisions for operation with a number of mutually suspicious,
hierarchically distributed primary reference sources such as radio
clocks.
Section 4 describes algorithms useful for deglitching and smoothing
clock-offset samples collected on a continuous basis. These
algorithms began with those suggested in [22], were refined as the
results of experiments described in [23] and further evolved under
typical operating conditions over the last two years. In addition,
as the result of experience in operating multiple-server nets
including radio-synchronized clocks at several sites in the US and
with clients in the US and Europe, reliable algorithms for selecting
good clocks from a population possibly including broken ones have
been developed and are described in Section 4.
The accuracies achievable by NTP depend strongly on the precision of
Mills