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
