rfc1129.ps

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

300 2841 83 0 (1.1.)fjt
(s)16 (t)29 (n)27 (e)43 (m)27 (e)18 (r)13 (i)30 (u)29 (q)27 (e)35 (R)13 ( )26 (e)27 (c)30 (n)26 (a)44 (m)18 (r)29 (o)16 (f)18 (r)27 (e)32 (P)13 ( )0 25 383 2841 fet
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300 2750 1867 18 (In this paper to synchronize frequency means to adjust the clocks in the network to run at the s)fjt
2167 2750 83 0 (ame)fjt
300 2692 1950 16 (frequency, while to synchronize time means to set the clocks so that all agree upon a particular)fjt
300 2634 1950 13 (epoch with respect to Coordinated Universal Time \(UTC\), as provided by national standards, and)fjt
300 2576 1934 14 (to synchronize clocks means to synchronize them in both frequency and time. A clock synchroniza)fjt
2234 2576 17 0 (-)fjt
300 2517 1866 14 (tion subnet operates by measuring clock offsets between the various time servers in the subnet)fjt
2166 2517 84 1 ( and)fjt
300 2459 1778 14 (so is vulnerable to statistical delay variations on the various transmission paths. In the Int)fjt
2078 2459 173 1 (ernet the)fjt
300 2401 1879 14 (paths involved can have wide variations in delay and reliability. The routing algorithms can se)fjt
2179 2401 72 0 (lect)fjt
300 2343 1878 14 (landline or satellite paths, public network or dedicated links or even suspend service without p)fjt
2178 2343 72 0 (rior)fjt
300 2284 134 0 (notice.)fjt
300 2191 1925 13 (It should be noted that stable frequency synchronization in large subnets requires finely tune)fjt
2225 2191 25 0 (d)fjt
300 2133 1867 13 (tracking loops and multiple phase comparisons over relatively long periods of time, while reli)fjt
2167 2133 83 0 (able)fjt
300 2075 1864 11 (time synchronization requires carefully engineered selection algorithms and the use of redun)fjt
2164 2075 86 0 (dant)fjt
300 2016 1925 13 (resources and diverse transmission paths. For instance, while only a few comparisons are usuall)fjt
2225 2016 25 0 (y)fjt
300 1958 1934 16 (adequate to determine local time in the Internet to within a few tens of milliseconds, dozens o)fjt
2234 1958 17 0 (f)fjt
300 1900 1751 13 (measurements over many days are required to reliably resolve oscillator drift and stabilize )fjt
2051 1900 199 0 (frequency)fjt
300 1842 809 6 (to a few milliseconds per day [MIL89b].)fjt
300 1748 1821 11 (Thus, the performance requirements of an internet-based time synchronization system are part)fjt
2121 1748 113 0 (icular)fjt
2234 1748 17 0 (-)fjt
300 1690 1403 10 (ly demanding. A basic set of requirements must include the following:)fjt
300 1595 38 0 (1.)fjt
375 1595 1875 13 (The primary time reference source\(s\) must be synchronized to national standards by wire, radio)fjt
375 1537 1875 14 (or portable clock. The system of time servers and clients must deliver continuous local time)fjt
375 1478 1485 12 (based on UTC, even when leap seconds are inserted in the UTC timescale.)fjt
300 1383 38 0 (2.)fjt
375 1383 1875 13 (The time servers must provide accurate and precise time, even with relatively large stochastic)fjt
375 1325 1875 13 (delays on the transmission paths. This requires careful design of the data smoothing and)fjt
375 1267 1798 12 (deglitching algorithms, as well as an extremely stable local clock oscillator and synchroniza)fjt
2173 1267 78 0 (tion)fjt
375 1209 239 0 (mechanism.)fjt
300 1114 38 0 (3.)fjt
375 1114 1825 12 (The synchronization subnet must be reliable and survivable, even under unstable conditions a)fjt
2200 1114 50 0 (nd)fjt
375 1055 1875 15 (where connectivity may be lost for periods up to days. This requires redundant time servers and)fjt
375 997 1740 10 (diverse transmission paths, as well as a dynamically reconfigurable subnet architecture.)fjt
300 902 38 0 (4.)fjt
375 902 1834 11 (The synchronization protocol must operate continuously and provide update information at rat)fjt
2209 902 41 0 (es)fjt
375 844 1856 11 (sufficient to compensate for the expected wander of the room-temperature crystal oscillator)fjt
2231 844 19 0 (s)fjt
375 785 1776 14 (used in ordinary computer systems. It must operate efficiently with large numbers of time se)fjt
2151 785 99 0 (rvers)fjt
375 727 1875 11 (and clients in continuous-polled and procedure-call modes and in multicast and point-to-point)fjt
375 669 300 0 (configurations.)fjt
300 574 38 0 (5.)fjt
375 574 1875 13 (The system must operate in existing internets including a spectrum of machines ranging from)fjt
375 516 1875 11 (personal workstations to supercomputers, but make minimal demands on the operating system)fjt
375 457 1875 11 (and supporting services. Time-server software and especially client software must be easily)fjt
375 399 494 2 (installed and configured.)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 83 0 (1.2.)fjt
(s)27 (e)29 (h)27 (c)27 (a)29 (o)18 (r)30 (p)29 (p)35 (A)12 ( )16 (f)29 (o)13 ( )29 (n)30 (o)12 (i)27 (s)27 (s)29 (u)27 (c)27 (s)13 (i)35 (D)12 ( )0 25 383 2841 fet
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300 2749 1950 13 (There are many ways that hosts distributed throughout a large geographic area can synchronize)fjt
300 2690 1950 16 (clocks to UTC. In North America the U.S. and Canada operate broadcast radio services with a UTC)fjt
300 2632 1950 13 (timecode modulation which can be decoded by suitable receivers [NBS79]. One approach to time)fjt
300 2574 1950 12 (synchronization is to provide timecode receivers at every site where required. However, these)fjt
300 2516 1925 12 (receivers are specialized, moderately expensive and subject to occasional gross errors due t)fjt
2225 2516 25 0 (o)fjt
300 2457 719 3 (propagation and equipment failures.)fjt
300 2360 1950 12 (The U.S. National Institute of Standards and Technology \(NIST\) \(formerly National Bureau of)fjt
300 2302 1931 13 (Standards\), recently announced a computer time service available to the general public by mean)fjt
2231 2302 19 0 (s)fjt
300 2244 1950 14 (of a standard telephone modem [NBS88]. The service is intended for use by personal workstations)fjt
300 2185 1837 16 (to set clock-calenders, for example, but would not be suitable for a large population of clients c)fjt
2137 2185 113 0 (alling)fjt
300 2127 1146 7 (on a frequent, regular basis without further redistribution.)fjt
300 2030 1809 13 (In principle, it is possible to use special network facilities designed for time synchronizatio)fjt
2109 2030 141 1 (n, such)fjt
300 1972 1950 16 (as a dedicated FM or TV subcarrier or timecode rebroadcast by a cable system. For many years)fjt
300 1914 1950 10 (AT&T has synchronized digital switching equipment to the Basic Synchronization Reference)fjt
300 1855 1950 14 (Frequency \(BSRF\), which consists of a master oscillator synchronized to UTC and a network of)fjt
300 1797 1886 13 (dedicated 2048-kHz links embedded in the transmission plant. AT&T and other carriers are plann)fjt
2186 1797 64 0 (ing)fjt
300 1739 1950 12 (to use the Global Positioning System and the LORAN-C radionavigation system to synchronize)fjt
300 1681 1950 15 (switches in various areas of the country to UTC. However, neither of these methods would be)fjt
300 1622 1653 10 (economically viable for widespread deployment in a large, diverse internet system.)fjt
300 1525 1950 15 (Various mechanisms have been used in the Internet protocol suite to record and transmit the time)fjt
300 1467 1950 13 (at which an event takes place, including the Daytime protocol [POS83a], Time protocol [POS83b],)fjt
300 1409 1950 12 (ICMP Timestamp message [DAR81a] and IP Timestamp option [SU81]. In the Hellospeak routing)fjt
300 1350 1950 14 (protocol [MIL83b] one or more processes synchronize to an external reference source, such as a)fjt
300 1292 1950 14 (timecode receiver or time daemon, and the corrections are distributed via routing updates and a)fjt
300 1234 1840 12 (minimum-delay spanning tree rooted on these processes. The Unix 4.3bsd time daemon )fjt
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2140 1234 111 0 (timed)fjt
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300 1176 1950 16 ([GUS85a] uses an elected master host [GUS85b] to measure offsets of a number of slave hosts and)fjt
300 1117 676 4 (send periodic corrections to them.)fjt
300 1021 1950 13 (Experimental results on measured times and delays in the Internet are discussed in [COL88],)fjt
300 962 1950 10 ([MIL83a] and [MIL85b]. Other synchronization algorithms are discussed in [GUS84], [HAL84],)fjt
300 904 1950 8 ([LAM78], [LAM85], [LUN84], [MAR85], [MIL85a], [MIL85b], [MIL89a], [RIC88], [SCH86],)fjt
300 846 1950 13 ([SRI87] and [TRI86] and protocols based on them in [MIL81], [MIL85c], [MIL89a] and [TRI86].)fjt
300 788 1950 12 (NTP uses techniques evolved from them and both linear systems and Byzantine agreement)fjt
300 729 1950 10 (methodologies. Linear methods for digital telephone network synchronization are summarized in)fjt
300 671 1817 10 ([LIN80], while Byzantine methods for clock synchronization are summarized in [LAM85].)fjt
300 574 1892 16 (In an internet system involving many networks and gateways a useful approach is to equip a f)fjt
2192 574 58 0 (ew)fjt
300 516 1889 11 (strategically located time-server hosts \(or gateways\) with timecode receivers and coordinate ti)fjt
2189 516 61 0 (me)fjt
300 457 1792 12 (distribution using a suitable internet protocol. However, the success of this approach requi)fjt
2092 457 159 1 (res very)fjt
300 399 1928 12 (accurate and reliable mechanisms to process and distribute the time information, since timecod)fjt
2228 399 22 0 (e)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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300 281 100 0 (Mills)fjt
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300 2841 1801 13 (receivers, time servers and the internet itself cannot be considered wholly reliable. While )fjt
2101 2841 149 0 (reliable)fjt
300 2782 1950 11 (clock synchronization has been studied using agreement algorithms [LAM85], [SRI87], in practice)fjt
300 2724 710 7 (it is not possible to distinguish the )fjt
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1010 2724 218 0 (truechimer)fjt
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1228 2724 1022 7 ( clocks, which maintain timekeeping accuracy to a)fjt
300 2666 1050 7 (previously published \(and trusted\) standard, from the )fjt
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1350 2666 207 0 (falseticker)fjt
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1556 2666 694 7 ( clocks, which do not, on other than)fjt
300 2608 1840 13 (a statistical basis. In addition, the algorithms and protocols discussed in the literature d)fjt
2140 2608 110 1 (o not)fjt
300 2549 1914 13 (necessarily produce the most accurate time on a statistical basis and may produce unacceptab)fjt
2214 2549 36 0 (le)fjt
300 2491 1397 9 (network overheads and instabilities in a large, diverse internet system.)fjt
300 2403 1950 14 (The above approach was used in the design of the NTP synchronization mechanisms, which were)fjt
300 2345 1890 13 (evolved as the result of numerous experiments, analyses and stepwise refinements over an eight-y)fjt
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