RFC 956 (rfc956) - Page 2 of 26
Algorithms for synchronizing network clocks
Alternative Format: Original Text Document
RFC 956 September 1985 Algorithms for Synchronizing Network Clocks 1. Introduction The recent interest within the Internet community in determining accurate time from a set of mutually suspicious network clocks has been prompted by several occasions in which gross errors were found in usually reliable, highly accurate clock servers after seasonal thunderstorms which disrupted their primary power supply. To these sources of error should be added those due to malfunctioning hardware, defective software and operator mistakes, as well as random errors in the mechanism used to set and/or synchronize the clocks via Internet paths. The results of these errors can range from simple disorientation to major disruption, depending upon the operating system, when files or messages are incorrectly timestamped or the order of critical network transactions is altered. This report suggests a stochastic model based on the principles of maximum-likelihood estimation, together with algorithms for computing a good estimator from a number of time-offset samples measured between one or more clocks connected via network links. The model provides a rational method for detecting and resolving errors due to faulty clocks or excessively noisy links. Included in this report are descriptions of certain experiments conducted with Internet hosts and ARPANET paths which give an indication of the effectiveness of the algorithms. Several mechanisms have been specified in the Internet protocol suite to record and transmit the time at which an event takes place, including the ICMP Timestamp message [6], Time Protocol [7], Daytime protocol [8] and IP Timestamp option [9]. A new Network Time Protocol [12] has been proposed as well. Additional information on network time synchronization can be found in the References at the end of this document. Synchronization protocols are described in [3] and [12] and synchronization algorithms in [2], [5] and [10]. Experimental results on measured roundtrip delays and clock offsets in the Internet are discussed in [4] and [11]. A comprehensive mathematical treatment of clock synchronization can be found in [1]. In [10] the problem of synchronizing a set of mutually suspicious clocks is discussed and algorithms offered which maximize in some sense the expectation that a correct set of "good" clocks can be extracted from the population including also "bad" ones. The technique is based upon overlapping, discrete confidence intervals which are assigned a-priori. The model assumes the reasonable presumption that "bad" clocks display errors far outside these confidence intervals, so can be easily identified and discarded from the voting process. Mills
