Meta tags:

Headings (most frequently used words):

edit, time, clock, navigation, tools, expanded, collapsed, network, protocol, contents, history, strata, timestamps, synchronization, algorithm, software, implementations, leap, seconds, security, concerns, see, also, notes, references, further, reading, external, links, menu, reference, implementation, sntp, windows, openntpd, ntimed, ntpsec, chrony, personal, namespaces, variants, views, more, search, contribute, print, export, languages,

Text of the page (most frequently used words):
the (262), time (109), and (102), ntp (85), from (68), retrieved (59), protocol (55), #network (54), for (54), archived (53), original (52), rfc (41), with (35), stratum (33), was (32), version (28), 2017 (27), implementation (27), that (27), servers (26), 2014 (24), server (23), chrony (23), clock (23), edit (22), security (22), reference (21), are (21), 2015 (20), windows (19), more (18), can (17), synchronization (17), internet (15), 2019 (15), 2011 (15), pdf (14), october (14), has (14), mills (13), 2016 (13), this (12), december (12), not (12), service (12), second (12), used (12), david (11), sntp (11), computer (11), been (11), client (11), computers (11), radio (11), wikipedia (10), org (10), packet (10), leap (10), ntpsec (10), june (10), linux (10), also (10), algorithms (10), algorithm (10), which (10), software (9), offset (9), seconds (9), 2010 (9), support (9), september (9), 2018 (9), systems (9), both (9), microsoft (9), accuracy (9), other (9), under (8), 2021 (8), simple (8), 2020 (8), ntpv4 (8), new (8), specification (8), april (8), than (8), first (8), were (8), over (8), timestamp (8), about (7), use (7), page (7), all (7), delay (7), nist (7), nts (7), november (7), project (7), but (7), they (7), system (7), several (7), clocks (7), its (7), have (7), sources (7), khz (7), peer (7), available (6), foundation (6), links (6), round (6), trip (6), where (6), doi (6), attacks (6), 2013 (6), source (6), 2009 (6), red (6), w32time (6), these (6), march (6), unix (6), years (6), number (6), bit (6), using (5), file (5), search (5), history (5), wayback (5), layer (5), accurate (5), one (5), analysis (5), machine (5), august (5), developed (5), com (5), package (5), hat (5), isbn (5), ntimed (5), 2012 (5), clients (5), 5905 (5), 1305 (5), some (5), daemon (5), public (5), between (5), standard (5), synchronized (5), timestamps (5), uses (5), devices (5), documented (5), terms (4), may (4), current (4), articles (4), response (4), compatible (4), ntpd (4), ietf (4), 17487 (4), attack (4), audit (4), secure (4), git (4), july (4), best (4), 978 (4), henning (4), high (4), ipv4 (4), ipv6 (4), called (4), precision (4), two (4), proposed (4), operation (4), 1985 (4), strata (4), currently (4), data (4), such (4), implementations (4), few (4), although (4), designed (4), within (4), usually (4), displaystyle (4), telephone (4), modem (4), published (4), networks (4), milliseconds (4), utc (3), information (3), related (3), changes (3), tools (3), community (3), article (3), events (3), navigation (3), references (3), need (3), january (3), description (3), protocols (3), https (3), group (3), official (3), external (3), how (3), http (3), most (3), nvd (3), product (3), cert (3), update (3), code (3), supported (3), core (3), infrastructure (3), initiative (3), remote (3), cure53 (3), pentest (3), report (3), lichvar (3), miroslav (3), hardware (3), packets (3), sent (3), specified (3), ubuntu (3), enterprise (3), poul (3), kamp (3), technet (3), 4330 (3), osi (3), digital (3), 2006 (3), university (3), delaware (3), various (3), era (3), needs (3), modified (3), marzullo (3), versions (3), provide (3), engineering (3), 1981 (3), dcnet (3), 1992 (3), 1119 (3), 1059 (3), 958 (3), different (3), see (3), query (3), address (3), much (3), amount (3), being (3), dns (3), authentication (3), during (3), tls (3), enough (3), codebase (3), cause (3), significant (3), their (3), into (3), connection (3), when (3), added (3), maintained (3), removal (3), features (3), issues (3), synchronize (3), openntpd (3), ntpv3 (3), implemented (3), required (3), value (3), 2000 (3), since (3), include (3), same (3), state (3), routes (3), common (3), then (3), theta (3), delta (3), request (3), bits (3), part (3), step (3), colorado (3), 100 (3), refid (3), each (3), indicate (3), hierarchy (3), technology (3), local (3), statement (2), developers (2), view (2), contact (2), privacy (2), policy (2), non (2), profit (2), organization (2), license (2), bahasa (2), wikidata (2), link (2), upload (2), what (2), contents (2), main (2), collapsed (2), expanded (2), variants (2), talk (2), personal (2), categories (2), updating (2), short (2), application (2), definition (2), brief (2), 2005 (2), paper (2), guide (2), option (2), further (2), reading (2), netnod (2), amplification (2), command (2), future (2), down (2), attacking (2), bypassing (2), transport (2), open (2), andrew (2), dec (2), apple (2), flaw (2), ics (2), cisco (2), vulnerability (2), google (2), tuxfamily (2), repository (2), wiki (2), full (2), means (2), robust (2), interface (2), versatile (2), universe (2), starting (2), now (2), provided (2), ptp (2), allan (2), replacement (2), boundary (2), 2008 (2), primary (2), subset (2), intended (2), multiple (2), 2002 (2), conference (2), 1109 (2), asymmetric (2), 2036 (2), problems (2), represents (2), richard (2), 2004 (2), ntpq (2), informationweek (2), selection (2), procedure (2), later (2), significantly (2), ordinary (2), compatibility (2), formal (2), correctness (2), 1991 (2), ieee (2), transactions (2), communications (2), timed (2), february (2), 1996 (2), 778 (2), 2030 (2), 1769 (2), 1995 (2), 1361 (2), 7822 (2), 1989 (2), 1988 (2), assigned (2), numbers (2), port (2), light (2), would (2), travel (2), notes (2), irig (2), atomic (2), return (2), target (2), similar (2), increase (2), avoid (2), participating (2), configured (2), queries (2), denial (2), man (2), middle (2), cryptographic (2), services (2), messages (2), identified (2), schemes (2), bgp (2), three (2), capability (2), errors (2), running (2), root (2), only (2), concerns (2), noon (2), event (2), configuration (2), attached (2), must (2), appear (2), any (2), negative (2), repositories (2), aimed (2), unstable (2), mode (2), environment (2), greater (2), chronyc (2), user (2), very (2), scratch (2), fork (2), obsolete (2), able (2), eight (2), including (2), reduce (2), reliably (2), design (2), generic (2), improvements (2), beginning (2), operating (2), conditions (2), became (2), cannot (2), maintain (2), higher (2), prevent (2), less (2), along (2), every (2), runs (2), potential (2), management (2), nominal (2), difference (2), backward (2), candidates (2), note (2), reception (2), transmission (2), defined (2), format (2), fractional (2), rollover (2), goes (2), threshold (2), inter (2), changed (2), ptb (2), usno (2), wwvh (2), fort (2), collins (2), wwv (2), chu (2), jjy (2), msf (2), hbg (2), wwvb (2), pulse (2), per (2), positioning (2), gps (2), satellite (2), spanning (2), themselves (2), quality (2), arrows (2), introduced (2), ntpv2 (2), describe (2), lacking (2), dtss (2), complete (2), ntpv1 (2), experimental (2), fuzzball (2), better (2), atlantic (2), tens (2), ethernet (2), model (2), icmp (2), daytime (2), described (2), area (2), udp (2), variable (2), latency (2), nntp (2), jump (2), web (2), sep (2), cookie, statistics, mobile, disclaimers, text, additional, apply, site, you, agree, registered, trademark, wikimedia, inc, creative, commons, attribution, sharealike, last, edited, yorùbá, tiếng, việt, українська, türkçe, ไทย, svenska, suomi, english, русский, română, português, polski, norsk, bokmål, 日本語, nederlands, монгол, melayu, македонски, magyar, latviešu, עברית, italiano, indonesia, 한국어, français, فارسی, euskara, español, ελληνικά, eesti, deutsch, dansk, čeština, català, български, беларуская, azərbaycanca, العربية, afrikaans, languages, printable, download, print, export, item, cite, permanent, special, pages, here, recent, portal, learn, help, contribute, donate, random, read, views, namespaces, log, create, account, contributions, logged, menu, hidden, needing, detailed, matches, webarchive, template, standards, index, php, title, network_time_protocol, oldid, 1061101414, solving, yields, confessions, timekeeper, survey, microsft, working, list, website, 5908, rfc5908, dhcpv6, 5907, rfc5907, definitions, managed, objects, homepage, langer, martin, weberblog, net, setting, secured, drdos, ntpdc, monlist, lee, dave, bbc, huge, hack, ugly, sign, threats, goodin, dan, ars, technica, dos, taking, game, sites, deliver, crippling, 100gbps, floods, www, edu, aanchal, malhotra, isaac, cohen, erik, brakke, sharon, goldberg, ndss, jose, selvi, strict, 5906, rfc5906, autokey, cii, identifies, securitynotice, fairhead, harry, programmer, latest, problem, cunningham, arstechnica, apr, automatically, patches, macs, fix, severe, gov, vulnerabilities, ios, notice, smear, timescale, freebsd, netbsd, macos, solaris, introduction, sum, stands, solid, seen, trustworthy, collaborative, securing, heiderich, mario, mozilla, aka, mozillawiki, wikimo, withstanding, eleven, days, testing, strong, mind, team, directive, enables, timestamping, received, conf, opensource, manage, lichtenheld, frank, via, blog, combining, get, worlds, liska, apress, 4842, 2412, quick, start, distribution, github, readme, phk, bikeshed, 20140926, playing, again, dahavey, docs, ned, pyle, 2007, requirements, configure, environments, works, technical, rfc4330, complying, sntpv4, implement, mitigation, fully, upstream, downstream, considerations, completely, interoperable, intermixed, gotoh, imamura, kaneko, electromagnetic, measurements, 448, 449, 7803, 7242, cpem, 1034915, improvement, double, method, seminar, presentation, look, year, 2038, proofness, stevens, bill, fenner, rudoff, addison, wesley, professional, 582, 141155, programming, numbering, nlug, ml1, output, practices, white, fate, hinges, father, money, answer, plans, research, crc, press, 377, 4398, 1464, earth, space, edition, remove, sorting, discarding, steps, replace, incorporated, affect, basis, statements, taskforce, 1482, 1493, 103043, manager, manual, hosts, extension, fields, authority, iana, 2001, faq, executive, summary, taylor, francis, 8493, 5805, picometers, approximately, 585, billion, bohr, radius, zeptoseconds, telecommunication, ntpdate, pool, nitz, timecode, international, variance, improve, small, responds, larger, reply, allows, attacker, substantially, upgraded, ignore, spoofed, distributed, susceptible, unless, cryptographically, signed, computational, overhead, involved, make, impractical, busy, particularly, message, spoofing, move, allow, based, key, expiration, affected, fake, caching, cache, bitcoin, persistent, login, dnssec, conducted, behalf, suggested, problematic, standpoint, exploit, discovered, patched, concerned, auto, basic, missing, routine, lead, unlimited, access, bsd, subject, stack, buffer, overflow, those, appeared, concern, undergoing, revision, review, entire, undergone, audits, misuse, abuse, alternative, smearing, consists, introducing, incrementally, period, hours, internally, amazon, aws, day, receives, notification, either, actually, halted, because, requirement, tiny, preserving, order, should, ever, become, necessary, deleted, sequence, skipping, processes, strictly, increasing, comes, default, distributions, sleep, intermittent, virtual, machines, characterized, low, resource, consumption, cost, supports, components, chronyd, executed, starts, line, evaluated, safe, just, incidents, advantage, versatility, written, unnecessary, complexity, created, 1997, curnow, gnu, general, showing, activity, arch, terminal, window, systematically, point, rash, compromises, production, release, shipped, unsafe, pare, away, making, remainder, auditable, showed, present, did, suffer, remained, specify, hardened, started, abandoned, sponsored, determined, easier, write, size, officially, released, brauer, presented, focus, encompassing, privilege, separated, whilst, closely, simpler, users, includes, while, still, existing, portable, openbsd, 1607, reach, certain, states, desired, recommends, newer, vista, 2003, originally, purpose, minutes, correct, implements, violates, aspects, replay, kerberos, ability, complex, without, requiring, storage, extended, periods, applications, embedded, continuously, backwards, contains, sensitive, especially, discipline, misbehave, almost, computing, platform, offsets, filtered, analysed, way, polled, frequently, audited, finding, numerous, ported, utility, achieved, incoming, outgoing, symmetrical, exists, half, forward, times, systematic, bias, values, passed, through, filters, subjected, statistical, discarded, estimate, derived, remaining, frequency, adjusted, gradually, creating, feedback, loop, outliers, derive, expression, frac, typical, regularly, compute, positive, absolute, polls, introduces, 128, date, resolves, ambiguity, cases, according, fraction, resolve, takes, photon, pass, electron, speed, unambiguous, representation, until, dim, consist, giving, scale, 136, theoretical, resolution, 233, picoseconds, 1900, therefore, occurs, epoch, rolls, change, panic, 1000, 125, face, association, lost, xfac, multi, mrs, german, acts, kauai, hawaii, ottawa, ontario, allouis, 162, tdf, station, loran, lorc, fukushima, saga, anthorn, prangins, ceased, dcf77, mainflingen, dcf, range, instrumentation, pps, galileo, gal, global, geosynchronous, orbit, identifiers, codes, addition, identify, identifier, upper, limit, device, unsynchronized, interact, construct, shortest, path, minimize, accumulated, tree, bellman, ford, employ, peering, sampling, act, often, stable, whose, microseconds, backup, referred, sanity, check, timekeeping, generate, signal, triggers, connected, known, advertise, field, set, indicates, unspecified, interrupt, hierarchical, semi, layered, level, termed, zero, top, distance, cyclical, dependencies, always, indication, reliability, find, below, yellow, direct, alternate, master, schriever, afb, naval, observatory, subsequent, made, apparent, containing, moved, updated, yet, following, retirement, led, harlan, stenn, included, error, final, enabled, calculation, metric, helps, choose, disagree, broadcast, defining, scheme, survived, bulk, however, criticized, incorporate, onwards, pseudocode, associated, drew, results, filter, modes, architecture, brought, attention, wider, publication, 956, ntpv0, header, calculations, persisted, despite, relatively, slow, obtained, recording, well, disciplining, election, appoint, 781, 1979, possibly, demonstration, trans, york, ien, 173, deployed, hello, routing, prototyping, ran, many, router, national, 1990, 1980, evolution, 8915, aead, easily, relationships, peers, consider, send, receive, 123, passively, listen, updates, after, initial, calibrating, exchange, supplies, warning, impending, adjustment, transmitted, daylight, saving, zones, multicasting, broadcasting, datagram, select, mitigate, effects, achieve, millisecond, ideal, congestion, intersection, coordinated, universal, before, oldest, switched, networking, fddi, isdn, dsl, mac, ppp, l2tp, tunnels, ipsec, igmp, ndp, arp, ecn, rsvp, sctp, dccp, tcp, xmpp, ssl, telnet, ssh, snmp, smtp, sip, rip, rtsp, rtp, onc, rpc, pop, ospf, mqtt, mgcp, ldap, imap, ftp, dhcp, suite, confused, synchronizing, across, redirected, free, encyclopedia, archive, 20211220084846, ntp_server, capture, fail, success, 2022, jan, nov, 2025, captures,

Text of the page (random words):
ransport layer tcp udp dccp sctp rsvp more internet layer ip ipv4 ipv6 icmp v6 ecn arp ndp igmp ipsec more link layer tunnels l2tp ppp mac ethernet wi fi dsl isdn fddi more v t e the network time protocol ntp is a networking protocol for clock synchronization between computer systems over packet switched variable latency data networks in operation since before 1985 ntp is one of the oldest internet protocols in current use ntp was designed by david l mills of the university of delaware ntp is intended to synchronize all participating computers to within a few milliseconds of coordinated universal time utc 1 3 it uses the intersection algorithm a modified version of marzullo s algorithm to select accurate time servers and is designed to mitigate the effects of variable network latency ntp can usually maintain time to within tens of milliseconds over the public internet and can achieve better than one millisecond accuracy in local area networks under ideal conditions asymmetric routes and network congestion can cause errors of 100 ms or more 2 3 the protocol is usually described in terms of a client server model but can as easily be used in peer to peer relationships where both peers consider the other to be a potential time source 1 20 implementations send and receive timestamps using the user datagram protocol udp on port number 123 4 5 they can also use broadcasting or multicasting where clients passively listen to time updates after an initial round trip calibrating exchange 3 ntp supplies a warning of any impending leap second adjustment but no information about local time zones or daylight saving time is transmitted 2 3 the current protocol is version 4 ntpv4 which is a proposed standard as documented in rfc 5905 it is backward compatible with version 3 specified in rfc 1305 network time security nts a secure version of ntp with tls and aead is currently a proposed standard and documented in rfc 8915 contents 1 history 2 clock strata 3 timestamps 4 clock synchronization algorithm 5 software implementations 5 1 reference implementation 5 2 sntp 5 3 windows time 5 4 openntpd 5 5 ntimed 5 6 ntpsec 5 7 chrony 6 leap seconds 7 security concerns 8 see also 9 notes 10 references 11 further reading 12 external links history edit ntp was designed by david l mills needs update rfc evolution for ntp 1980 1985 1990 1995 2000 2005 2010 2015 2020 rfc 958 6 rfc 1059 7 rfc 1119 8 rfc 1305 9 rfc 5905 10 rfc 7822 11 rfc 1361 12 rfc 1769 13 rfc 2030 14 rfc 4330 15 dcnet internet clock service 16 sntp in 1979 network time synchronization technology was used in what was possibly the first public demonstration of internet services running over a trans atlantic satellite network at the national computer conference in new york the technology was later described in the 1981 internet engineering note ien 173 17 and a public protocol was developed from it that was documented in rfc 778 the technology was first deployed in a local area network as part of the hello routing protocol and implemented in the fuzzball router an experimental operating system used in network prototyping where it ran for many years other related network tools were available both then and now they include the daytime and time protocols for recording the time of events as well as the icmp timestamp messages and ip timestamp option rfc 781 more complete synchronization systems although lacking ntp s data analysis and clock disciplining algorithms include the unix daemon timed which uses an election algorithm to appoint a server for all the clients 18 and the digital time synchronization service dtss which uses a hierarchy of servers similar to the ntp stratum model in 1985 ntp version 0 ntpv0 was implemented in both fuzzball and unix and the ntp packet header and round trip delay and offset calculations which have persisted into ntpv4 were documented in rfc 958 despite the relatively slow computers and networks available at the time accuracy of better than 100 milliseconds was usually obtained on atlantic spanning links with accuracy of tens of milliseconds on ethernet networks in 1988 a much more complete specification of the ntpv1 protocol with associated algorithms was published in rfc 1059 it drew on the experimental results and clock filter algorithm documented in rfc 956 and was the first version to describe the client server and peer to peer modes in 1991 the ntpv1 architecture protocol and algorithms were brought to the attention of a wider engineering community with the publication of an article by david l mills in the ieee transactions on communications 19 in 1989 rfc 1119 was published defining ntpv2 by means of a state machine with pseudocode to describe its operation it introduced a management protocol and cryptographic authentication scheme which have both survived into ntpv4 along with the bulk of the algorithm however the design of ntpv2 was criticized for lacking formal correctness by the dtss community and the clock selection procedure was modified to incorporate marzullo s algorithm for ntpv3 onwards 20 in 1992 rfc 1305 defined ntpv3 the rfc included an analysis of all sources of error from the reference clock down to the final client which enabled the calculation of a metric that helps choose the best server where several candidates appear to disagree broadcast mode was introduced in subsequent years as new features were added and algorithm improvements were made it became apparent that a new protocol version was required 21 in 2010 rfc 5905 was published containing a proposed specification for ntpv4 the protocol has significantly moved on since then and as of 2014 an updated rfc has yet to be published 22 following the retirement of mills from the university of delaware the reference implementation is currently maintained as an open source project led by harlan stenn 23 24 clock strata edit the u s naval observatory alternate master clock at schriever afb colorado is a stratum 0 source for ntp yellow arrows indicate a direct connection red arrows indicate a network connection ntp uses a hierarchical semi layered system of time sources each level of this hierarchy is termed a stratum and is assigned a number starting with zero for the reference clock at the top a server synchronized to a stratum n server runs at stratum n 1 the number represents the distance from the reference clock and is used to prevent cyclical dependencies in the hierarchy stratum is not always an indication of quality or reliability it is common to find stratum 3 time sources that are higher quality than other stratum 2 time sources a a brief description of strata 0 1 2 and 3 is provided below stratum 0 these are high precision timekeeping devices such as atomic clocks gps or other radio clocks they generate a very accurate pulse per second signal that triggers an interrupt and timestamp on a connected computer stratum 0 devices are also known as reference clocks ntp servers cannot advertise themselves as stratum 0 a stratum field set to 0 in ntp packet indicates an unspecified stratum 25 stratum 1 these are computers whose system time is synchronized to within a few microseconds of their attached stratum 0 devices stratum 1 servers may peer with other stratum 1 servers for sanity check and backup 26 they are also referred to as primary time servers 2 3 stratum 2 these are computers that are synchronized over a network to stratum 1 servers often a stratum 2 computer queries several stratum 1 servers stratum 2 computers may also peer with other stratum 2 computers to provide more stable and robust time for all devices in the peer group stratum 3 these are computers that are synchronized to stratum 2 servers they employ the same algorithms for peering and data sampling as stratum 2 and can themselves act as servers for stratum 4 computers and so on the upper limit for stratum is 15 stratum 16 is used to indicate that a device is unsynchronized the ntp algorithms on each computer interact to construct a bellman ford shortest path spanning tree to minimize the accumulated round trip delay to the stratum 1 servers for all the clients 1 20 in addition to stratum the protocol is able to identify the synchronization source for each server in terms of a reference identifier refid common time reference identifiers refid codes refid 27 clock source goes geosynchronous orbit environment satellite gps global positioning system gal galileo positioning system pps generic pulse per second irig inter range instrumentation group wwvb lf radio wwvb fort collins colorado 60 khz dcf lf radio dcf77 mainflingen de 77 5 khz hbg lf radio hbg prangins hb 75 khz ceased operation msf lf radio msf anthorn uk 60 khz jjy lf radio jjy fukushima jp 40 khz saga jp 60 khz lorc mf radio loran c station 100 tdf mf radio allouis fr 162 khz chu hf radio chu ottawa ontario wwv hf radio wwv fort collins colorado wwvh hf radio wwvh kauai hawaii nist nist telephone modem acts nist telephone modem usno usno telephone modem ptb german ptb time standard telephone modem mrs multi reference sources xfac inter face association changed ip address changed or lost step step time change the offset is less than the panic threshold 1000 s but greater than the step threshold 125 ms timestamps edit the 64 bit timestamps used by ntp consist of a 32 bit part for seconds and a 32 bit part for fractional second giving a time scale that rolls over every 2 32 seconds 136 years and a theoretical resolution of 2 32 seconds 233 picoseconds ntp uses an epoch of january 1 1900 therefore the first rollover occurs on february 7 2036 28 29 ntpv4 introduces a 128 bit date format 64 bits for the second and 64 bits for the fractional second the most significant 32 bits of this format is the era number which resolves rollover ambiguity in most cases 30 31 according to mills the 64 bit value for the fraction is enough to resolve the amount of time it takes a photon to pass an electron at the speed of light the 64 bit second value is enough to provide unambiguous time representation until the universe goes dim 32 b clock synchronization algorithm edit round trip delay time δ a typical ntp client regularly polls one or more ntp servers the client must compute its time offset and round trip delay time offset θ is positive or negative client time server time difference in absolute time between the two clocks is defined by θ t 1 t 0 t 2 t 3 2 displaystyle theta frac t_ 1 t_ 0 t_ 2 t_ 3 2 and the round trip delay δ by δ t 3 t 0 t 2 t 1 displaystyle delta t_ 3 t_ 0 t_ 2 t_ 1 where t 0 is the client s timestamp of the request packet transmission t 1 is the server s timestamp of the request packet reception t 2 is the server s timestamp of the response packet transmission and t 3 is the client s timestamp of the response packet reception 1 19 to derive the expression for the offset note that for the request packet t 0 θ δ 2 t 1 displaystyle t_ 0 theta delta 2 t_ 1 and for the response packet t 3 θ δ 2 t 2 displaystyle t_ 3 theta delta 2 t_ 2 solving for θ yields the definition of the time offset the values for θ and δ are passed through filters and subjected to statistical analysis outliers are discarded and an estimate of time offset is derived from the best three remaining candidates the clock frequency is then adjusted to reduce the offset gradually creating a feedback loop 1 20 accurate synchronization is achieved when both the incoming and outgoing routes between the client and the server have symmetrical nominal delay if the routes do not have a common nominal delay a systematic bias exists of half the difference between the forward and backward travel times 33 software implementations edit the ntp management protocol utility ntpq being used to query the state of a stratum 2 server reference implementation edit the ntp reference implementation along with the protocol has been continuously developed for over 20 years backwards compatibility has been maintained as new features have been added it contains several sensitive algorithms especially to discipline the clock that can misbehave when synchronized to servers that use different algorithms the software has been ported to almost every computing platform including personal computers it runs as a daemon called ntpd under unix or as a service under windows reference clocks are supported and their offsets are filtered and analysed in the same way as remote servers although they are usually polled more frequently 1 15 19 this implementation was audited in 2017 finding numerous potential security issues 34 sntp edit simple network time protocol sntp is a less complex implementation of ntp using the same protocol but without requiring the storage of state over extended periods of time 35 it is used in some embedded systems and in applications where full ntp capability is not required 36 windows time edit all microsoft windows versions since windows 2000 include the windows time service w32time 37 which has the ability to synchronize the computer clock to an ntp server w32time was originally implemented for the purpose of the kerberos version 5 authentication protocol which required time to be within 5 minutes of the correct value to prevent replay attacks the version in windows 2000 and windows xp only implements sntp and violates several aspects of the ntp version 3 standard 38 beginning with windows server 2003 and windows vista w32time became compatible with a significant subset of ntpv3 39 microsoft states that w32time cannot reliably maintain time synchronization with one second accuracy 40 if higher accuracy is desired microsoft recommends using a newer version of windows or different ntp implementation 41 beginning with windows 10 version 1607 and windows server 2016 w32time can be configured to reach time accuracy of 1 s 50 ms or 1 ms under certain specified operating conditions 42 40 43 openntpd edit in 2004 henning brauer presented openntpd an ntp implementation with a focus on security and encompassing a privilege separated design whilst it is aimed more closely at the simpler generic needs of openbsd users it also includes some protocol security improvements while still being compatible with existing ntp servers a portable version is available in linux package repositories ntimed edit a new ntp client ntimed was started by poul henning kamp in 2014 and abandoned in 2015 44 the new implementation was sponsored by the linux foundation as a replacement for the reference implementation as it was determined to be easier to write a new implementation from scratch than to reduce the size of the reference implementation although it has not been officially released ntimed can synchronize clocks reliably 45 ntpsec edit ntpsec is a fork of the reference implementation that has been systematically security hardened the fork point was in june 2015 and was in response to a rash of compromises in 2014 specify the first production release shipped in october 2017 46 between removal of unsafe features removal o...