rfc1128.ps

RFC 的详细文档！

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300 2979 227 1 (RFC 1128)fjt
805 2979 941 5 (Performance of the Network Time Protocol)fjt
1948 2979 302 1 (October 1989)fjt
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300 2841 42 0 (1.)fjt
(n)30 (o)12 (i)16 (t)27 (c)29 (u)30 (d)29 (o)18 (r)16 (t)29 (n)13 (I)13 ( )0 13 350 2841 fet
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300 2744 1950 16 (How do hosts and gateways in a large, dispersed networking community know what time it is? How)fjt
300 2686 1804 15 (accurate are their clocks? In a 1988 survey involving 5,722 hosts and gateways of the Internet)fjt
2104 2686 146 1 ( system)fjt
300 2628 1950 15 ([MIL88a], 1158 provided their local time via the network. Sixty percent of the replies had errors)fjt
300 2569 1931 17 (greater than one minute, while ten percent had errors greater than 13 minutes. A few had errors a)fjt
2231 2569 19 0 (s)fjt
300 2511 1950 16 (much as two years. Most host clocks are set by eyeball-and-wristwatch to within a minute or two)fjt
300 2453 1950 14 (and rarely checked after that. Many of these are maintained by some sort of battery-backed)fjt
300 2395 1900 14 (clock/calender device using a room-temperature quartz oscillator that may drift as much as a seco)fjt
2200 2395 50 0 (nd)fjt
300 2336 1950 13 (per day and can go for weeks between manual corrections. For many applications, especially)fjt
300 2278 1631 9 (distributed internet applications, much greater accuracy and reliability is required.)fjt
300 2172 1950 14 (The Network Time Protocol \(NTP\) is designed to distribute standard time using the hosts and)fjt
300 2114 1920 15 (gateways of the Internet system. The Internet consists of over 100,000 hosts on over 800 packe)fjt
2220 2114 30 0 (t-)fjt
300 2056 1950 11 (switching networks interconnected by a comparable number of gateways. While the Internet)fjt
300 1998 1950 12 (backbone networks and gateways are engineered and managed for good service, operating speeds)fjt
300 1939 1950 12 (and service reliabilities vary considerably throughout the regional and campus networks of the)fjt
300 1881 1928 14 (system. This places severe demands on NTP, which must deliver accurate and reliable standard tim)fjt
2228 1881 22 0 (e)fjt
300 1823 1837 10 (in spite of component failures, service disruptions and possibly mis-engineered implementat)fjt
2137 1823 96 0 (ions.)fjt
300 1717 1950 14 (NTP and its forebears were developed and tested on PDP11 computers and the Fuzzball operating)fjt
300 1659 1950 12 (system, which was designed specifically for timekeeping precisions of a millisecond or better)fjt
300 1601 1601 12 ([MIL88b]. An implementation of NTP as a Unix 4.3bsd system daemon called )fjt
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1901 1601 89 0 (ntpd)fjt
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1990 1601 261 3 ( was built by)fjt
(e)21 (s)27 (o)26 (p)19 (r)26 (u)27 (p)18 (-)16 (l)24 (a)15 (i)24 (c)24 (e)26 (p)21 (s)25 ( )38 (A)25 ( )14 (.)27 (d)27 (n)24 (a)15 (l)27 (y)18 (r)24 (a)46 (M)25 ( )18 (f)27 (o)25 ( )27 (y)15 (t)16 (i)21 (s)18 (r)24 (e)26 (v)16 (i)27 (n)37 (U)25 ( )24 (e)27 (h)15 (t)25 ( )16 (t)24 (a)25 ( )21 (s)26 (o)27 (k)24 (a)40 (m)24 (a)46 (M)25 ( )21 (s)16 (i)27 (u)27 (o)32 (L)25 ( )27 (d)27 (n)24 (a)25 ( )27 (y)19 (r)15 (t)24 (e)30 (P)25 ( )16 (l)24 (e)24 (a)27 (h)24 (c)16 (i)46 (M)0 80 300 1542 fet
300 1484 1950 12 (hardware/software implementation of NTP was built be Dennis Ferguson at the University of)fjt
300 1426 1936 15 (Toronto. Over a dozen NTP primary time servers are synchronized by radio or satellite to nationa)fjt
2236 1426 14 0 (l)fjt
300 1368 1864 15 (time standards in the U.S. and Canada. About half of these are connected directly to internati)fjt
2164 1368 86 0 (onal)fjt
300 1309 1911 13 (backbone networks and are intended for ubiquitous access, while the remainder are connected )fjt
2211 1309 39 0 (to)fjt
300 1251 1837 15 (regional networks and intended for regional and local access. It is estimated that there are wel)fjt
2137 1251 113 1 (l over)fjt
300 1193 1950 14 (2000 secondary servers in North America, Europe and the Pacific synchronized by NTP directly or)fjt
300 1135 692 4 (indirectly to these primary servers.)fjt
300 1029 1881 12 (This paper describes several large scale experiments designed to evaluate the availability, accur)fjt
2181 1029 69 0 (acy)fjt
300 971 1902 15 (and reliability of standard time distribution using NTP and the hosts and gateways of the Intern)fjt
2202 971 48 0 (et.)fjt
300 912 1892 17 (The first is designed to locate hosts that support at least one of three time protocols specified )fjt
2192 912 58 0 (for)fjt
300 854 1787 14 (use in the Internet, including NTP. Since Internet hosts are not centrally administered and )fjt
2087 854 163 0 (network)fjt
300 796 1848 14 (time is not a required service in the TCP/IP protocol suite, experimental determination is the)fjt
2148 796 102 1 ( only)fjt
300 738 1779 14 (practical way to estimate the penetration of time service in the Internet. The remaining exp)fjt
2079 738 171 0 (eriments)fjt
300 679 1950 17 (use only NTP and are designed to assess the nominals and extremes in various types of errors that)fjt
300 621 1902 14 (occur in regular system operation, including those due to the network paths between the serve)fjt
2202 621 48 0 (rs,)fjt
300 563 1692 13 (the radio propagation path to the source of synchronization and the radio clock itself.)fjt
300 457 1903 17 (This paper does not describe in detail the architecture or protocols of NTP, nor does it present t)fjt
2203 457 47 0 (he)fjt
300 399 1786 11 (rationale for the particular choice of synchronization method and statistical processing alg)fjt
2086 399 165 0 (orithms.)fjt
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300 2979 227 1 (RFC 1128)fjt
805 2979 941 5 (Performance of the Network Time Protocol)fjt
1948 2979 302 1 (October 1989)fjt
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300 279 100 0 (Mills)fjt
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300 2841 1950 13 (Further information on the background, model and algorithms can be found in [MIL89a], while)fjt
300 2782 1471 11 (details of the latest NTP protocol specification can be found in [MIL89b].)fjt
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300 2676 83 0 (1.1.)fjt
(n)29 (o)13 (i)15 (t)27 (a)30 (n)12 (i)44 (m)26 (e)27 (s)27 (s)13 (i)35 (D)12 ( )27 (y)27 (c)29 (n)27 (e)30 (u)29 (q)27 (e)18 (r)30 (F)12 ( )30 (d)29 (n)27 (a)13 ( )27 (e)43 (m)13 (i)28 (T)13 ( )30 (d)18 (r)27 (a)29 (d)30 (n)26 (a)16 (t)32 (S)13 ( )0 42 383 2676 fet
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300 2571 1853 14 (In order that atomic and civil time can be coordinated throughout the world, national administra)fjt
2153 2571 97 0 (tions)fjt
300 2512 1950 12 (operate primary time and frequency standards and maintain Coordinated Universal Time \(UTC\) by)fjt
300 2454 1887 13 (observing various radio broadcasts and through occasional use of portable atomic clocks. A prim)fjt
2187 2454 64 0 (ary)fjt
300 2396 1829 11 (frequency standard is an oscillator that can maintain extremely precise frequency relativ)fjt
2129 2396 121 2 (e to a)fjt
300 2338 1854 14 (physical phenomenon, such as a transition in the orbital states of an electron. Presently avai)fjt
2154 2338 97 0 (lable)fjt
300 2279 1912 15 (atomic oscillators are based on the transitions of the hydrogen, cesium and rubidium atoms and a)fjt
2212 2279 39 0 (re)fjt
300 2221 929 6 (capable of maintaining UTC frequency to 10)fjt
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1284 2221 966 9 ( and time to 100 ns when operated in multiple)fjt
300 2163 1047 5 (ensembles at various national standards laboratories.)fjt
300 2040 1950 13 (The U.S. National Institute of Standards and Technology \(NIST - formerly National Bureau of)fjt
300 1982 1925 12 (Standards\) operates three radio services for the dissemination of standard time and frequenc)fjt
2225 1982 25 0 (y)fjt
300 1923 1950 13 (information [NBS79]. One of these uses high-frequency \(HF or CCIR band 7\) transmissions from)fjt
300 1865 1950 13 (Fort Collins, CO \(WWV\), and Kauai, HI \(WWVH\). Signal propagation is usually by reflection)fjt
300 1807 1950 14 (from the upper ionospheric layers, which vary in height and composition throughout the day and)fjt
300 1749 1909 14 (season and result in unpredictable delay variations at the receiver \(see Section 3.2\). While the)fjt
2209 1749 41 0 (se)fjt
300 1690 1931 14 (services and those operated by the National Research Council of Canada \(CHU\) and other countrie)fjt
2231 1690 19 0 (s)fjt
300 1632 1925 16 (can be received over large areas of the world, reliable frequency comparisons can be made only t)fjt
2225 1632 25 0 (o)fjt
300 1574 295 3 (the order of 10)fjt
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595 1598 35 0 (-7)fjt
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629 1574 1413 12 ( and time accuracies are limited to the order of a millisecond [BLA74].)fjt
300 1451 1950 15 (A second service operated by NIST is the low-frequency \(LF or CCIR band 5\) transmissions from)fjt
300 1392 1950 14 (Boulder, CO \(WWVB\), which can be received over the continental U.S. and adjacent coastal areas.)fjt
300 1334 1950 13 (Signal propagation is via the lower ionospheric layers, which are relatively stable and have)fjt
300 1276 1823 10 (predictable diurnal variations in height. With appropriate receiving and averaging techniqu)fjt
2123 1276 127 1 (es and)fjt
300 1218 1824 11 (corrections for diurnal and seasonal propagation effects, frequency comparisons to within 10)fjt
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2179 1218 71 1 ( are)fjt
300 1159 1950 15 (possible and time accuracies of from a few to 50 microseconds can be obtained [BLA74]. However,)fjt
300 1101 1260 11 (there is only one station and it operates at modest power levels.)fjt
300 978 1950 14 (The third service operated by NIST uses ultra-high frequency \(UHF or CCIR band 9\) transmissions)fjt
300 920 1950 11 (from the Geosynchronous Orbiting Environmental Satellite \(GOES\). There is some speculation on)fjt
300 862 1950 12 (the continued operation of GOES, especially if the LORAN-C [FRA82] and Global Positioning)fjt
300 803 1928 12 (System \(GPS\) [BES82] radiopositioning systems operated by other U.S. agencies continue to evolv)fjt
2228 803 22 0 (e)fjt
300 745 1950 13 (as expected. While the OMEGA [VAS78] radionavigation system operated by the U.S. Navy and)fjt
300 687 1843 12 (other countries can in principle provide worldwide frequency and time distribution, this syst)fjt
2143 687 107 1 (em is)fjt
300 629 1203 8 (unlikely to long survive the operational deployment of GPS.)fjt
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300 504 83 0 (1.2.)fjt
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300 399 1501 9 (An accurate, reliable time distribution protocol must provide the following:)fjt
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300 2979 227 1 (RFC 1128)fjt
805 2979 941 5 (Performance of the Network Time Protocol)fjt
1948 2979 302 1 (October 1989)fjt
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300 2839 38 0 (1.)fjt
375 2839 1875 13 (The primary time reference source\(s\) must be synchronized to national standards by wire, radio)fjt
375 2781 1875 14 (or portable clock. The system of time servers and clients must deliver continuous local time)fjt
375 2722 1485 12 (based on UTC, even when leap seconds are inserted in the UTC timescale.)fjt
300 2628 38 0 (2.)fjt
375 2628 1839 13 (The time servers must provide accurate and precise time, even with relatively large statistic)fjt
2214 2628 36 0 (al)fjt
375 2569 1875 13 (delays on the transmission paths. This requires careful design of the data smoothing and)fjt
375 2511 1798 12 (deglitching algorithms, as well as an extremely stable local clock oscillator and synchroniza)fjt
2173 2511 78 0 (tion)fjt
375 2453 239 0 (mechanism.)fjt
300 2358 38 0 (3.)fjt
375 2358 1825 12 (The synchronization subnet must be reliable and survivable, even under unstable conditions a)fjt
2200 2358 50 0 (nd)fjt
375 2300 1875 15 (where connectivity may be lost for periods up to days. This requires redundant time servers and)fjt
375 2241 1740 10 (diverse transmission paths, as well as a dynamically reconfigurable subnet architecture.)fjt
300 2146 38 0 (4.)fjt
375 2146 1834 11 (The synchronization protocol must operate continuously and provide update information at rat)fjt
2209 2146 41 0 (es)fjt
375 2088 1856 11 (sufficient to compensate for the expected wander of the room-temperature crystal oscillator)fjt
2231 2088 19 0 (s)fjt
375 2030 1776 14 (used in ordinary computer systems. It must operate efficiently with large numbers of time se)fjt
2151 2030 99 0 (rvers)fjt
375 1972 1875 11 (and clients in continuous-polled and procedure-call modes and in multicast and point-to-point)fjt
375 1913 300 0 (configurations.)fjt
300 1818 38 0 (5.)fjt
375 1818 1875 13 (The system must operate with a spectrum of systems ranging from personal workstations to)fjt
375 1760 1875 11 (supercomputers, but make minimal demands on the operating system and supporting services.)fjt
375 1702 1831 11 (Time-server software and especially client software must be easily installed and configured.)fjt
300 1609 1903 14 (In NTP one or more primary time servers synchronize directly to external reference sources su)fjt
2203 1609 47 0 (ch)fjt
300 1551 1950 14 (as radio clocks. Secondary time servers synchronize to the primary servers and others in the)fjt
300 1492 1950 13 (configured subnet using NTP. Subnet peers calculate clock offset and delay between them using)fjt
300 1434 1950 14 (timestamps with 200 picosecond resolution exchanged at intervals of up to about 17 minutes. As)fjt
300 1376 1950 11 (explained in [MIL89a], the protocol uses a distributed Bellman-Ford algorithm [BER87] to)fjt
300 1318 1914 12 (construct minimum-weight spanning trees within the subnet based on hierarchical level and tot)fjt
2214 1318 36 0 (al)fjt
300 1259 766 5 (synchronization path delay to the root.)fjt
300 1166 1950 12 (Besides NTP, there are several protocols designed to distribute time in local-area networks,)fjt