Network Time Protocol (NTP) General Overview

Network Time Protocol (NTP)General OverviewDavid L. MillsUniversity of Delawarehttp://www.eecis.udel.edu/ millsmailto:mills@udel.edualautun, Maya glyph2-Aug-041

IntroductionzNetwork Time Protocol (NTP) synchronizes clocks of hosts and routersin the Internet.zNIST estimates 10-20 million NTP servers and clients deployed in theInternet and its tributaries all over the world. Every Windows/XP has anNTP client.zNTP provides nominal accuracies of low tens of milliseconds on WANs,submilliseconds on LANs, and submicroseconds using a precision timesource such as a cesium oscillator or GPS receiver.zNTP software has been ported to almost every workstation and serverplatform available today - from PCs to Crays - Unix, Windows, VMSand embedded systems, even home routers.zThe NTP architecture, protocol and algorithms have been evolved overthe last two decades to the latest NTP Version 4 described in this andrelated briefings.2-Aug-042

The Sun never sets on NTPzNTP is argueably the longest running, continuously operating,ubiquitously available protocol in the Internet– USNO and NIST, as well as equivalents in other countries, provide multipleNTP primary servers directly synchronized to national standard cesiumclock ensembles and GPS– Over 230 Internet primary serversare in Australia, Canada, Chile, France,Germany, Isreal, Italy, Holland, Japan, Norway, Sweden, Switzerland, UK,and US.zWell over a million Internet servers and clients all over the world– National and regional service providers BBN, MCI, Sprint, Alternet, etc.– Agencies and organizations: US Weather Service, US Treasury Service,IRS, PBS, Merrill Lynch, Citicorp, GTE, Sun, DEC, HP, etc.– Private networks are reported to have over 10,000 NTP servers and clientsbehind firewalls; one (GTE) reports in the order of 30,000 NTP workstationsand PCs.– NTP has been on the NASA Shuttle and in Antarctica and planned for theMars Internet.2-Aug-043

Needs for precision timezDistributed database transaction journalling and loggingzStock market buy and sell orderszSecure document timestamps (with cryptographic certification)zAviation traffic control and position reportingzRadio and TV programming launch and monitoringzIntruder detection, location and reportingzMultimedia synchronization for real-time teleconferencingzInteractive simulation event synchronization and orderingzNetwork monitoring, measurement and controlzEarly detection of failing network infrastructure devices and airconditioning equipmentzDifferentiated services traffic engineeringzDistributed network gaming and training2-Aug-044

NTP capsule summaryzPrimary (stratum 1) servers synchronize to national time standards viaradio, satellite and modem.zSecondary (stratum 2, .) servers and clients synchronize to primaryservers via hierarchical subnet.zClients and servers operate in master/slave, symmetric and multicastmodes with or without cryptographic authentication.zReliability assured by redundant servers and diverse network paths.zEngineered algorithms reduce jitter, mitigate multiple sources and avoidimproperly operating servers.zThe system clock is disciplined in time and frequency using an adaptivealgorithm responsive to network time jitter and clock oscillatorfrequency wander.2-Aug-045

NTP architecture overviewPeer 1Clock DisciplineAlgorithmFilter 1Peer 2Filter 2Peer 3Filter mLoop FilterP/F-Lock LoopTimestampsNTP MessagesVFOzMultiple servers/peers provide redundancy and diversity.zClock filters select best from a window of eight time offset samples.zIntersection and clustering algorithms pick best truechimers anddiscard falsetickers.zCombining algorithm computes weighted average of time offsets.zLoop filter and variable frequency oscillator (VFO) implement hybridphase/frequency-lock (P/F) feedback loop to minimize jitter andwander.2-Aug-046

2-Aug-04 2 Introduction zNetwork Time Protocol (NTP) synchronizes clocks of hosts and routers in the Internet. zNIST estimates 10-20 million NTP servers and clients deployed in the Internet and its tributaries all over the world. Every Windows/XP has an NTP client. zNTP provides nominal accuracies of low tens of milliseconds on WANs, submilliseconds on LANs, and submicroseconds using a .