rfc1129.ps

RFC 的详细文档！

2190 2345 61 0 (ear)fjt
300 2287 1842 13 (period. It became evident that accurate and reliable internet time synchronization can be ach)fjt
2142 2287 108 0 (ieved)fjt
300 2228 1850 12 (only through a integrated approach to system design including the primary reference sources,)fjt
2150 2228 100 1 ( time)fjt
300 2170 1950 11 (servers, synchronization subnet, protocols and synchronization mechanisms which are at the heart)fjt
300 2112 1873 16 (of this paper. From the analytical point of view the distributed system of NTP time servers oper)fjt
2173 2112 77 0 (ates)fjt
300 2054 1906 13 (as a set of mutually coupled, phase-locked oscillators with phase comparisons exchanged by mea)fjt
2206 2054 44 0 (ns)fjt
300 1995 1794 15 (of update messages and a local clock at each time server functioning as a disciplined oscilla)fjt
2094 1995 156 1 (tor. The)fjt
300 1937 1871 15 (principal features of this design, described in more detail later in this paper, can be summarize)fjt
2171 1937 79 1 (d as)fjt
300 1879 163 0 (follows:)fjt
300 1788 38 0 (1.)fjt
375 1788 1817 11 (The synchronization subnet consists of a self-organizing, hierarchical network of time serv)fjt
2192 1788 58 0 (ers)fjt
375 1730 1212 8 (configured on the basis of estimated accuracy and reliability.)fjt
300 1639 38 0 (2.)fjt
375 1639 1875 11 (The synchronization protocol operates in connectionless mode in order to minimize latencies,)fjt
375 1581 1264 5 (simplify implementations and provide ubiquitous interworking.)fjt
300 1490 38 0 (3.)fjt
375 1490 1875 10 (The synchronization mechanism uses a returnable-time design which tolerates packet loss,)fjt
375 1432 1875 9 (duplication and misordering, together with filtering algorithms based on maximum-likelihood)fjt
375 1374 209 0 (principles.)fjt
300 1283 38 0 (4.)fjt
375 1283 1875 12 (The local clock design is based on a first-order, adaptive-parameter phase-lock loop with)fjt
375 1225 1875 11 (corrections computed using timestamps exchanged along the arcs of the synchronization subnet.)fjt
300 1134 38 0 (5.)fjt
375 1134 1875 12 (Multiply redundant time servers and multiply diverse transmission paths are used in the)fjt
375 1076 1875 11 (synchronization subnet, as well as engineered algorithms which select the most reliable)fjt
375 1017 1357 8 (synchronization source and path using a weighted voting procedure.)fjt
300 927 38 0 (6.)fjt
375 927 1875 14 (System overhead is reduced through the use of dynamic control of polling rates and association)fjt
375 868 254 0 (management)fjt
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300 779 42 0 (2.)fjt
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300 690 1876 15 (Since 1972 the time and frequency standards of the world have been based on International Ato)fjt
2176 690 75 0 (mic)fjt
300 632 1928 14 (Time \(TAI\), which is currently maintained using multiple cesium-beam clocks to an accuracy of )fjt
2228 632 22 0 (a)fjt
300 574 294 3 (few parts in 10)fjt
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594 598 42 0 (12)fjt
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635 574 1599 10 ( [BLA74]. The Bureau International de l'Heure \(BIH\) uses astronomical observa)fjt
2234 574 17 0 (-)fjt
300 516 1880 12 (tions provided by the U.S. Naval Observatory and other observatories to determine corrections)fjt
2180 516 70 1 ( for)fjt
300 457 1895 15 (small changes in the mean solar rotation period of the Earth, which results in Coordinated Univer)fjt
2195 457 55 0 (sal)fjt
300 399 1950 17 (Time \(UTC\). UTC is presently decreasing relative to TAI at a fraction of a second per year, so)fjt
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300 2979 227 1 (RFC 1129)fjt
938 2979 676 2 (Network Time Synchronization)fjt
1949 2979 302 1 (October 1989)fjt
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300 2841 1950 17 (corrections in the form of leap seconds must be inserted from time to time in order to maintain)fjt
300 2782 1950 12 (agreement. The U.S. and many other countries operate standard time and frequency broadcast)fjt
300 2724 1854 15 (stations covering most areas of the world, although only a few utilize a broadcast timecode sui)fjt
2154 2724 97 0 (table)fjt
300 2666 1950 12 (for computer use. The U.S. and Canadian timecodes provide UTC time-of-day, day-of-year and)fjt
300 2608 1950 15 (related information, but not either the year or advance notice of leap seconds, which must be)fjt
300 2549 622 3 (determined from other sources.)fjt
300 2459 1950 13 (A synchronization subnet is a connected network of primary and secondary time servers, clients)fjt
300 2401 1793 13 (and interconnecting transmission paths. A primary time server is directly synchronized to a )fjt
2093 2401 158 0 (primary)fjt
300 2343 1938 11 (reference source, usually a timecode receiver. A secondary time server derives synchronization)fjt
2238 2343 13 0 (,)fjt
300 2285 1805 13 (possibly via other secondary servers, from a primary server over network paths possibly shar)fjt
2105 2285 145 1 (ed with)fjt
300 2226 1793 13 (other services. Under normal circumstances it is intended that the synchronization subnet of )fjt
2093 2226 158 0 (primary)fjt
300 2168 1892 11 (and secondary servers assumes a hierarchical master-slave configuration with the primary serv)fjt
2192 2168 58 0 (ers)fjt
300 2110 1861 14 (at the root and secondary servers of decreasing accuracy at successive levels toward the leave)fjt
2161 2110 32 0 (s.)fjt
300 2017 1886 12 (Following conventions established by the telephone industry, the accuracy of each time serve)fjt
2186 2017 64 1 (r is)fjt
300 1958 1950 16 (defined by a number called the stratum, with the root level \(primary servers\) assigned as one and)fjt
300 1900 1950 13 (each succeeding level towards the leaves \(secondary servers\) assigned as one greater than the)fjt
300 1842 1950 9 (preceding level. Using existing stations, available timecode receivers with propagation-delay)fjt
300 1784 1912 15 (corrections and allowing for sample accumulations up to a week or more, accuracies in the ord)fjt
2212 1784 39 0 (er)fjt
300 1725 1950 16 (of a millisecond can be achieved at the network interface of a primary server [MIL89b]. As the)fjt
300 1667 1906 13 (stratum increases from one, the accuracies achievable will degrade depending on the network pat)fjt
2206 1667 44 0 (hs)fjt
300 1609 1840 14 (and local clock stabilities. In order to avoid the tedious calculations [BRA80] necessary to est)fjt
2140 1609 110 0 (imate)fjt
300 1551 1879 13 (errors in each specific configuration, it is useful to assume the measurement errors accumu)fjt
2179 1551 72 0 (late)fjt
300 1492 1950 14 (approximately in proportion to the total roundtrip path delay to the root of the synchronization)fjt
300 1434 1009 6 (subnet, which is called the synchronizing distance.)fjt
300 1341 1950 13 (Again drawing from the experience of the telephone industry, which learned such lessons at)fjt
300 1283 1852 12 (considerable cost, the synchronization subnet should be organized to produce the highest accu)fjt
2152 1283 98 0 (racy,)fjt
300 1224 1825 14 (but must never be allowed to form a loop, regardless of synchronizing distance. An additional)fjt
2125 1224 125 1 ( factor)fjt
300 1166 1812 13 (is that each increment in stratum involves a potentially unreliable time server which intr)fjt
2112 1166 138 0 (oduces)fjt
300 1108 1820 14 (additional measurement errors. The selection algorithm used in NTP uses a variant of the Be)fjt
2120 1108 130 0 (llman-)fjt
300 1050 1903 11 (Ford distributed routing algorithm [BER87] to compute the minimum-weight spanning trees root)fjt
2203 1050 47 0 (ed)fjt
300 991 1817 16 (on the primary servers. With the foregoing factors in mind, the distance metric used by the alg)fjt
2117 991 133 0 (orithm)fjt
300 933 1950 14 (was chosen using the stratum number as the high-order bits and synchronizing distance as the)fjt
300 875 293 1 (low-order bits.)fjt
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300 781 42 0 (3.)fjt
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300 690 1950 16 (The Network Time Protocol \(NTP\) is used to construct and maintain a set of time servers and)fjt
300 632 1950 12 (transmission paths as a synchronization subnet. The protocol was first described in [MIL85c],)fjt
300 574 1758 11 (extensively revised in successive versions and recently established as a formal Internet)fjt
2058 574 193 1 ( Standard)fjt
300 516 1950 13 (protocol [MIL89a]. NTP is built on the Internet Protocol \(IP\) [DAR81b] and User Datagram)fjt
300 457 1950 11 (Protocol \(UDP\) [POS80], which provide a connectionless transport mechanism; however, it is)fjt
300 399 1950 15 (readily adaptable to other protocol suites. It is evolved from the Time Protocol [POS83b] and the)fjt
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300 2979 227 1 (RFC 1129)fjt
938 2979 676 2 (Network Time Synchronization)fjt
1949 2979 302 1 (October 1989)fjt
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300 2001 1950 11 (ICMP Timestamp Message [DAR81a], but is specifically designed to maintain accuracy and)fjt
300 1943 1840 12 (reliability, even when used over typical Internet paths involving multiple gateways and unre)fjt
2140 1943 110 0 (liable)fjt
300 1885 93 0 (nets.)fjt
300 1797 1760 13 (There is no provision for peer discovery, acquisition, or authentication in NTP itself, altho)fjt
2060 1797 190 1 (ugh some)fjt
300 1739 1918 13 (implementations include these features. Data integrity is provided by the IP and UDP checksum)fjt
2218 1739 32 0 (s.)fjt
300 1681 1874 9 (No circuit-management, duplicate-detection or retransmission facilities are provided or necess)fjt
2174 1681 76 0 (ary.)fjt
300 1623 1950 12 (The protocol can operate in several modes appropriate to different scenarios involving private)fjt
300 1564 1912 10 (workstations, public service machines and various network configurations. A lightweight associ)fjt
2212 1564 22 0 (a)fjt
2234 1564 17 0 (-)fjt
300 1506 1938 8 (tion-management capability, including dynamic reachability and variable polling-rate mechanisms)fjt
2238 1506 13 0 (,)fjt
300 1448 1864 12 (is used to manage state information and reduce resource requirements. Optional features inc)fjt
2164 1448 86 0 (lude)fjt
300 1390 1934 11 (message authentication based on crypto-checksums and provisions for remote control and monitor)fjt
2234 1390 17 0 (-)fjt
300 1331 1950 16 (ing. Since only a single NTP message format is used, the protocol is easily implemented and can)fjt
300 1273 1416 9 (be used in a variety of operating-system and networking environments.)fjt
300 1186 1842 12 (The following subsections contain an overview of the data formats, entities, state variable)fjt
2142 1186 108 1 (s and)fjt
300 1127 1829 13 (procedures used in NTP. Further details are contained in the formal specification. The specifi)fjt
2129 1127 122 0 (cation)fjt
300 1069 1831 16 (is based on the implementation model illustrated below, but it is not intended that this model )fjt
2131 1069 119 1 (be the)fjt
300 1011 1925 15 (only one upon which a specification can be based. In particular, the specification is intended t)fjt
2225 1011 25 0 (o)fjt
300 953 1802 16 (illustrate and clarify the intrinsic operations of NTP and serve as a foundation for a more ri)fjt
2102 953 148 0 (gorous,)fjt
300 894 867 3 (comprehensive and verifiable specification.)fjt
300 807 1950 17 (In what may be the most common client/server modes a client sends an NTP message to one or)fjt
300 749 1790 13 (more time servers, which process the replies as received. A server interchanges addresses, ov)fjt
2090 749 160 0 (erwrites)fjt
300 690 1913 12 (certain fields in the message, recalculates the checksum and returns the message immediatel)fjt
2213 690 38 0 (y.)fjt
300 632 1810 15 (Information included in the NTP message allows the client to determine the server time with )fjt
2110 632 141 0 (respect)fjt
300 574 1770 14 (to local time and adjust the local clock accordingly. In addition, the message includes inf)fjt
2070 574 180 0 (ormation)fjt
300 516 1785 14 (to calculate the expected timekeeping accuracy and reliability, so that inferior data can be d)fjt
2085 516 166 0 (iscarded)fjt
300 457 1903 15 (and only the best from possibly several servers can be selected. While the client/server modes m)fjt
2203 457 47 0 (ay)fjt
300 399 1844 15 (suffice for use on LANs involving a public time server and perhaps many private workstation cl)fjt
2144 399 106 0 (ients,)fjt
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