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1、THENETWORKCOMPUTERASPRECISIONTIMEKEEPER*DavidL.MillsElectricalEngineeringDepartmentUniversityofDelawareNewark,Delaware19716,USAmills@udel.edu;URL:www.eecis.udel.edu/-millsAbstractThispaperdescribesalgorithmstodisciplineacomputerclockloasourceofstandardtime,suchasaGPSre
2、ceiveroraMIhercomputersynchronizedtosuchasource.ThealgorithmsaredesignedforuseintheNetworkTimeProtocol(NTP),whichisusedtosynchronizecomputerclocksintheglobalInternet.TheyhavebeenincorporatedintheNTPsowareforUnixandWindowsand,forthehighestaccuracy,intheoperatingsystemke
3、rnelsforSun,DEC,andHPworkstations.RmserrorsonLANsareusuallylessthanI0pandonglobalinternetpathsusuaUylessthan5ms.However,raredisruptionsofonekindoranothercancauseerrorspikesupto100ponLAh'sand100msonInternetpaths.I1INTRODUCTIONGeneralpurposeworkstationcomputersarebewming
4、fastereachyear,withprocessorclocksnowoperatingat300MHzandabove.Computernetworksarebewmingfasteraswell,withspeedsof622Mbpsavailablenowand2.4Gbpsbeinginstalled.UsingavailabletechnologyandexistingworkstationsandInternetpaths,ithasbeendemonstratedthatcomputerscanbereliably
5、synchronizedtobetterthanamillisecondinLANsandbetterthanafewtensofmillisecondsinmostplacesintheglobalInternet.[llThistechnologyincludestheNetworkTimeProtocol(NTP),nowusedinanestimatedtotalofover100,000serversandclientsintheglobalInternet.Over220primaqtimeserversareavail
6、ableinthisnetwork,eachconnectedtoanexternalsourceoftime,suchasaGPSradioclockorACTStelephonemodem.Reliablenetworksynchronizationrequirescraftedalgorithmswhichminimizejitterondiversenetworkpathsbetweenclientsandservers,determinethebestsubsetofredundantservers,anddiscipli
7、nethecomputerclockinbothtimeandfrequency.TheNetworkTimeProtocol(NTP)isdesignedtodothisinUnixandWindowsoperatingsystems.TheNTParchitecture,protocol,andalgorithmshaveevolvedoveralmosttwodecades,withthelatestNTPVersion3designatedanInternet(draft)standardIZ1.Amongthegoalso
8、fthisdesignare:'Sponsoredby:DARPAInformtionTechnologyOfficeContractDABT63-lMC0046,NSFDivisionofNetworkandCommuni-cati
