Network Time Protocol (Version 3) Specification, Implementation and Analysis
RFC 1305
Document | Type |
RFC - Draft Standard
(March 1992; No errata)
Obsoleted by RFC 5905
Was draft-mills-ntpv3 (individual)
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Last updated | 2013-03-02 | ||
Stream | Legacy | ||
Formats | plain text html pdf htmlized bibtex | ||
Stream | Legacy state | (None) | |
Consensus Boilerplate | Unknown | ||
RFC Editor Note | (None) | ||
IESG | IESG state | RFC 1305 (Draft Standard) | |
Telechat date | |||
Responsible AD | (None) | ||
Send notices to | (None) |
Network Working Group David L. Mills Request for Comments: 1305 University of Delaware Obsoletes RFC-1119, RFC-1059, RFC-958 March 1992 Network Time Protocol (Version 3) Specification, Implementation and Analysis Note: This document consists of an approximate rendering in ASCII of the PostScript document of the same name. It is provided for convenience and for use in searches, etc. However, most tables, figures, equations and captions have not been rendered and the pagination and section headings are not available. Abstract This document describes the Network Time Protocol (NTP), specifies its formal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self- organizing, hierarchical-master-slave configuration synchronizes local clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons. Status of this Memo This RFC specifies an IAB standards track protocol for the Internet community and requests discussion and suggestions for improvements. Please refer to the current edition of the <169>IAB Official Protocol Standards<170> for the standardization state and status of this protocol. Distribution of this memo is unlimited. Keywords: network clock synchronization, standard time distribution, fault-tolerant architecture, maximum-likelihood estimation, disciplined oscillator, internet protocol, high-speed networks, formal specification. Preface This document describes Version 3 of the Network Time Protocol (NTP). It supersedes Version 2 of the protocol described in RFC-1119 dated September 1989. However, it neither changes the protocol in any significant way nor obsoletes previous versions or existing implementations. The main motivation for the new version is to refine the analysis and implementation models for new applications at much higher network speeds to the gigabit-per-second regime and to provide for the enhanced stability, accuracy and precision required at such speeds. In particular, the sources of time and frequency errors have been rigorously examined and error bounds established in order to improve performance, provide a model for correctness assertions and indicate timekeeping quality to the user. The revision also incorporates two new optional features, (1) an algorithm to combine the offsets of a number of peer time servers in order to enhance accuracy and (2) improved local-clock algorithms which allow the poll intervals on all synchronization paths to be substantially increased in order to reduce network overhead. An overview of the changes, which are described in detail in Appendix D, follows: 1. In Version 3 The local-clock algorithm has been overhauled to improve stability and accuracy. Appendix G presents a detailed mathematical model and design example which has been refined with the aid of feedback-control analysis and extensive simulation using data collected over ordinary Internet paths. Section 5 of RFC-1119 on the NTP local clock has been completely rewritten to describe the new algorithm. Since the new algorithm can result in message rates far below the old ones, it is highly recommended that they be used in new implementations. Note that use of the new algorithm does not affect interoperability with previous versions or existing implementations. 2. In Version 3 a new algorithm to combine the offsets of a number of peer time servers is presented in Appendix F. This algorithm is modelled on those used by national standards laboratories to combine the weighted offsets from a number of standard clocks to construct a synthetic laboratory timescale more accurate than that of any clock separately. It can be used in an NTP implementation to improve accuracy and stability and reduce errors due to asymmetric paths in the Internet. The new algorithm has been simulated using data collected over ordinary Internet paths and, along with the new local-clock algorithm, implemented and tested in the Fuzzball time servers now running in the Internet. Note that use of the new algorithm does not affect interoperability with previous versions or existing implementations. 3. Several inconsistencies and minor errors in previous versions have been corrected in Version 3. The description of the procedures has been rewritten in pseudo-code augmented by English commentary for clarity and to avoid ambiguity. Appendix I has been added to illustrate C-language implementations of the various filtering and selection algorithms suggested for NTP. Additional information is included in Section 5 and in Appendix E, which includes the tutorial material formerly included inShow full document text