Mills@udel.edu (02/27/90)
Folks, For the very few of you who may yet be obsessed with timekeeping nonsense, see the 0.75-Mb PostScript document pub/ntp/ntp.ps on louie.udel.edu for anonymous FTP. It is an engineering revision of RFC- 1119, which describes the specification and implementation of the Network Time Protocol (NTP). This revision is not being proposed for adoption at this time and is presented for information only. It neither changes the protocol in any significant way nor obsoletes previous or current implementations. It corrects a number of minor typographical and drafting errors and clarifies a few points causing some confusion in RFC-1119. However, the main motivation of this revision is to incorporate 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. Specific changes are as follows (from the preface): The local-clock algorithms were 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 algorithms. Since the new algorithms can result in message rates far below the old ones, it is highly recommended that they be used in new implementations. Note that neither the new or old local-clock algorithms are integral to the NTP protocol specification itself and do not affect interoperability with existing implementations; however, in order to insure overall NTP subnet stability in the Internet, it is essential that the local-clock characteristics of all NTP time servers conform to the analytical model presented in RFC-1119 and the revised document. 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 cesium 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 algorithms, implemented and tested in the Fuzzball time servers now running in the Internet. The algorithm requires one new system variable, two new peer variables and two new procedures, which are described in Appendix F. Note that this algorithm is not integral to the NTP protocol specification itself and its use does not affect interoperability with existing implementations. Minor changes have been made in the local-clock procedures to avoid problems observed when leap seconds are introduced in the UTC timescale and also to support an auxiliary atomic clock as a precision timebase. In addition, minor changes were made to some procedures described in Section 3 and in the clock-filter and clock-selection procedures described in Section 4. These are described in Section 5 and in Appendix E, which includes the tutorial material formerly included in Section 2 of RFC-1119, as well as much new material clarifying the interpretation of timescales and leap seconds. While these changes were made to correct minor bugs found as the result of experience and are recommended for new implementations, they do not impact current or previous implementations in any significant way (at least until the next leap second) and should not be construed as any substantial change to the current specification. Dave