rfc1119.ps

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300 2841 42 0 (1.)fjt
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300 2751 1950 13 (This document constitutes a formal specification of the Network Time Protocol \(NTP\), which is)fjt
300 2692 1903 15 (used to synchronize timekeeping among a set of distributed time servers and clients. It defines t)fjt
2203 2692 47 0 (he)fjt
300 2634 1895 13 (architectures, algorithms, entities and protocols used by NTP and is intended primarily for i)fjt
2195 2634 39 0 (m)fjt
2234 2634 17 0 (-)fjt
300 2576 1934 10 (plementors. A companion document [44] summarizes the requirements, analytical models, algo)fjt
2234 2576 17 0 (-)fjt
300 2518 1925 12 (rithmic analysis and performance under typical Internet conditions. NTP was first described i)fjt
2225 2518 25 0 (n)fjt
300 2459 1950 15 (RFC-958 [30], but has evolved in significant ways, culminating in the most recent NTP Version 1)fjt
300 2401 1889 16 (described in RFC-1059 [42]. It is built on the Internet Protocol \(IP\) [14] and User Datagram Proto)fjt
2189 2401 61 0 (col)fjt
300 2343 1914 12 (\(UDP\) [9], which provide a connectionless transport mechanism; however, it is readily adaptab)fjt
2214 2343 36 0 (le)fjt
300 2285 1950 15 (to other protocol suites. NTP is evolved from the Time Protocol [19] and the ICMP Timestamp)fjt
300 2226 1950 13 (message [15], but is specifically designed to maintain accuracy and robustness, even when used)fjt
300 2168 1818 11 (over typical Internet paths involving multiple gateways, highly dispersive delays and unr)fjt
2118 2168 132 0 (eliable)fjt
300 2110 93 0 (nets.)fjt
300 2017 1950 11 (The service environment consists of the implementation model, service model and timescale)fjt
300 1959 1876 13 (described in Section 2. The implementation model is based on a multiple-process operating sys)fjt
2176 1959 75 0 (tem)fjt
300 1901 1888 14 (architecture, although other architectures could be used as well. The service model is based o)fjt
2188 1901 62 1 (n a)fjt
300 1842 1914 13 (returnable-time design which depends only on measured clock offsets, but does not require reliab)fjt
2214 1842 36 0 (le)fjt
300 1784 1928 8 (message delivery. The synchronization subnet uses a self-organizing, hierarchical-master-slav)fjt
2228 1784 22 0 (e)fjt
300 1726 1950 10 (configuration, with synchronization paths determined by a minimum-weight spanning tree. While)fjt
300 1668 1854 13 (multiple masters \(primary servers\) may exist, there is no requirement for an election protocol)fjt
2154 1668 13 0 (.)fjt
300 1575 1950 15 (NTP itself is described in Section 3. It provides the protocol mechanisms to synchronize time in)fjt
300 1517 1911 14 (principle to precisions in the order of nanoseconds while preserving a non-ambiguous date well in)fjt
2211 1517 39 0 (to)fjt
300 1458 1889 13 (the next century. The protocol includes provisions to specify the characteristics and estimate )fjt
2189 1458 61 0 (the)fjt
300 1400 1950 17 (error of the local clock and the time server to which it may be synchronized. It also includes)fjt
300 1342 1834 11 (provisions for operation with a number of mutually suspicious, hierarchically distributed pr)fjt
2134 1342 116 0 (imary)fjt
300 1284 770 5 (reference sources such as radio clocks.)fjt
300 1191 1845 11 (Section 4 describes algorithms useful for deglitching and smoothing clock-offset samples coll)fjt
2145 1191 105 0 (ected)fjt
300 1132 1950 15 (on a continuous basis. These algorithms evolved from with suggested in [28], were refined as the)fjt
300 1074 1808 12 (results of experiments described in [29] and further evolved under typical operating conditio)fjt
2108 1074 143 1 (ns over)fjt
300 1016 1820 14 (the last three years. In addition, as the result of experience in operating multiple-server s)fjt
2120 1016 130 0 (ubnets)fjt
300 958 1950 15 (including radio-synchronized clocks at several sites in the U.S. and with clients in the U.S. and)fjt
300 899 1812 12 (Europe, reliable algorithms for selecting good clocks from a population possibly including )fjt
2112 899 139 0 (broken)fjt
300 841 1145 9 (ones have been developed and are described in Section 4.)fjt
300 749 1950 13 (The accuracies achievable by NTP depend strongly on the precision of the local-clock hardware)fjt
300 690 1928 14 (and stringent control of device and process latencies. Provisions must be included to adjust th)fjt
2228 690 22 0 (e)fjt
300 632 1925 13 (software logical-clock time and frequency in response to corrections produced by NTP. Section )fjt
2225 632 25 0 (5)fjt
300 574 1896 13 (describes a local-clock design evolved from the Fuzzball implementation described in [21] and [4)fjt
2196 574 54 0 (3].)fjt
300 516 1950 9 (This design includes offset-slewing, drift-compensation and deglitching mechanisms capable of)fjt
300 457 1950 13 (accuracies in the order of a millisecond, even after extended periods when synchronization to)fjt
300 399 798 5 (primary reference sources has been lost.)fjt
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300 2979 230 0 (RFC-1119)fjt
1024 2979 504 2 (Network Time Protocol)fjt
1882 2979 369 1 (September 1989)fjt
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300 2841 1950 14 (Details specific to NTP packet formats used with the Internet Protocol \(IP\) and User Datagram)fjt
300 2782 1950 14 (Protocol \(UDP\) are presented in Appendix A, while details of a suggested auxiliary NTP Control)fjt
300 2724 1950 11 (Message, which may be used when comprehensive network-monitoring facilities are not available,)fjt
300 2666 1874 12 (are presented in Appendix B. Appendix C contains specification and implementation details o)fjt
2174 2666 76 1 (f an)fjt
300 2608 1925 12 (optional authentication mechanism which can be used to control access and prevent unauthorize)fjt
2225 2608 25 0 (d)fjt
300 2549 1925 14 (data modification. Appendix D contains a listing of differences between Version 2 of NTP an)fjt
2225 2549 25 0 (d)fjt
300 2491 362 1 (previous versions.)fjt
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300 2398 83 0 (1.1.)fjt
(y)29 (g)30 (o)12 (l)30 (o)29 (n)30 (h)27 (c)26 (e)26 (T)13 ( )30 (d)26 (e)16 (t)27 (a)12 (l)27 (e)35 (R)13 ( )0 19 383 2398 fet
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300 2308 1889 15 (Other mechanisms have been specified in the Internet protocol suite to record and transmit the ti)fjt
2189 2308 61 0 (me)fjt
300 2250 1950 14 (at which an event takes place, including the Daytime protocol [18], Time Protocol [19], ICMP)fjt
300 2191 1950 12 (Timestamp message [15] and IP Timestamp option [13]. Experimental results on measured times)fjt
300 2133 1851 15 (and roundtrip delays in the Internet are discussed in [20], [29], [41] and [42]. Other synchroniz)fjt
2151 2133 100 0 (ation)fjt
300 2075 1828 15 (algorithms are discussed in [4], [22], [23], [24], [25], [27], [28], [29], [30], [31], [33], [35])fjt
2128 2075 122 1 (, [38],)fjt
300 2017 1896 17 ([39], [40], [42] and [44], while protocols based on them are described in [11], [12], [21], [26], [3)fjt
2196 2017 54 0 (0],)fjt
300 1958 1950 14 ([35], [42] and [44]. NTP uses techniques evolved from them and both linear-systems and agreement)fjt
300 1900 1950 10 (methodologies. Linear methods for digital telephone network synchronization are summarized in)fjt
300 1842 1603 10 ([6], while agreement methods for clock synchronization are summarized in [25].)fjt
300 1749 1873 10 (The Fuzzball routing protocol [21], sometimes called Hellospeak, incorporates time synchroniza)fjt
2173 1749 78 0 (tion)fjt
300 1691 1821 13 (directly into the routing-protocol design. One or more processes synchronize to an external ref)fjt
2121 1691 130 0 (erence)fjt
300 1633 1950 15 (source, such as a radio clock or NTP daemon, and the routing algorithm constructs a minimum-)fjt
300 1575 1860 14 (weight spanning tree rooted on these processes. The clock offsets are then distributed along the)fjt
2160 1575 90 1 ( arcs)fjt
300 1516 1900 16 (of the spanning tree to all processes in the system and the various process clocks corrected usi)fjt
2200 1516 50 0 (ng)fjt
300 1458 1950 17 (the procedure described in Section 5 of this document. While it can be seen that the design of)fjt
300 1400 1828 13 (Hellospeak strongly influenced the design of NTP, Hellospeak itself is not an Internet protoc)fjt
2128 1400 122 1 (ol and)fjt
300 1342 1042 7 (is unsuited for use outside its local-net environment.)fjt
300 1249 623 5 (The Unix 4.3bsd time daemon )fjt
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923 1249 111 0 (timed)fjt
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1033 1249 1217 10 ( [26] uses a single master-time daemon to measure offsets of)fjt
300 1191 1809 17 (a number of slave hosts and send periodic corrections to them. In this model the master is dete)fjt
2109 1191 141 0 (rmined)fjt
300 1132 1909 15 (using an election algorithm [31] designed to avoid situations where either no master is elected )fjt
2209 1132 42 0 (or)fjt
300 1074 1911 15 (more than one master is elected. The election process requires a broadcast capability, which is n)fjt
2211 1074 39 0 (ot)fjt
300 1016 1854 14 (a ubiquitous feature of the Internet. While this model has been extended to support hierarc)fjt
2154 1016 97 0 (hical)fjt
300 958 1950 17 (configurations in which a slave on one network serves as a master on the other [35], the model)fjt
300 899 1879 12 (requires handcrafted configuration tables in order to establish the hierarchy and avoid loops)fjt
2179 899 71 1 (. In)fjt
300 841 1781 12 (addition to the burdensome, but presumably infrequent, overheads of the election process, t)fjt
2081 841 170 1 (he offset)fjt
300 783 1706 10 (measurement/correction process requires twice as many messages as NTP per update.)fjt
300 690 1912 16 (A scheme with features similar to NTP is described in [39]. This scheme is intended for multi-serv)fjt
2212 690 39 0 (er)fjt
300 632 1853 15 (LANs where each of a set of possibly many time servers determines its local-time offset relati)fjt
2153 632 97 1 (ve to)fjt
300 574 1950 14 (each of the other servers in the set using periodic timestamped messages, then determines the)fjt
300 516 1950 11 (local-clock correction using the Fault-Tolerant Average \(FTA\) algorithm of [24]. The FTA)fjt
300 457 812 7 (algorithm, which is useful where up to )fjt
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1112 457 22 0 (k)fjt
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1134 457 1095 10 ( servers may be faulty, sorts the offsets, discards the )fjt
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2228 457 22 0 (k)fjt
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300 399 1950 17 (highest and lowest ones and averages the rest. The scheme, as described in [39], is most suitable to)fjt
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300 2841 1950 12 (LAN environments which support broadcast and would result in unacceptable overhead in an)fjt
300 2782 1879 14 (internet environment. In addition, for reasons given in Section 4 of this paper, the statist)fjt
2179 2782 72 0 (ical)fjt
300 2724 1786 15 (properties of the FTA algorithm are not likely to be optimal in an internet environment wit)fjt
2086 2724 164 1 (h highly)fjt
300 2666 354 1 (dispersive delays.)fjt
300 2577 1912 17 (A good deal of research has gone into the issue of maintaining accurate time in a community whe)fjt
2212 2577 39 0 (re)fjt
300 2518 686 6 (some clocks cannot be trusted. A )fjt
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986 2518 218 0 (truechimer)fjt
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1204 2518 1047 9 ( is a clock that maintains timekeeping accuracy to a)fjt
300 2460 1023 7 (previously published \(and trusted\) standard, while a )fjt
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1323 2460 207 0 (falseticker)fjt
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1529 2460 721 7 ( is a clock that does not. Determining)fjt