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Encyclopedia > Network Time Protocol
Internet protocol suite
5. Application layer

DHCPDNSFTPHTTPIMAP4IRCMIMEPOP3SIPSMTPSNMPSSHTELNETTLS/SSLBGPRPC • RTP • RTCP • SDPSOAP • … It has been suggested that Internet Protocols be merged into this article or section. ... The application layer is the seventh level of the seven-layer OSI model. ... The Dynamic Host Configuration Protocol (DHCP) is a set of rules used by a communications device (such as a computer, router or networking adapter) to allow the device to request and obtain an Internet address from a server which has a list of addresses available for assignment. ... The domain name system (DNS) stores and associates many types of information with domain names, but most importantly, it translates domain names (computer hostnames) to IP addresses. ... FTP or file transfer protocol is used to connect two computers over the Internet so that the user of one computer can transfer files and perform file commands on the other computer. ... Hypertext Transfer Protocol (HTTP) is a method used to transfer or convey information on the World Wide Web. ... The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol, Interactive Mail Access Protocol [1], and Interim Mail Access Protocol [2] ) is an application layer Internet protocol that allows a local client to access e-mail on a remote server. ... IRC redirects here. ... Multipurpose Internet Mail Extensions (MIME) is an Internet Standard that extends the format of e-mail to support text in character sets other than US-ASCII, non-text attachments, multi-part message bodies, and header information in non-ASCII character sets. ... In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. ... The Session Initiation Protocol (SIP) is an application-layer control (signaling) protocol for creating, modifying, and terminating sessions with one or more participants. ... Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. ... The simple network management protocol (SNMP) forms part of the internet protocol suite as defined by the Internet Engineering Task Force (IETF). ... To meet Wikipedias quality standards, this article or section may require cleanup. ... TELNET is a network protocol used on the Internet or local area network LAN connections. ... Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols which provide secure communications on the Internet for such things as web browsing, e-mail, Internet faxing, and other data transfers. ... The Border Gateway Protocol (BGP) is the core routing protocol of the Internet. ... Remote procedure call (RPC) is a protocol that allows a computer program running on one computer to cause a subroutine on another computer to be executed without the programmer explicitly coding the details for this interaction. ... The Real-time Transport Protocol (or RTP) defines a good standardized packet format for delivering audio and video over the Internet. ... RTP Control Protocol (RTCP) is a sister protocol of the Real-time Transport Protocol (RTP). ... SDP, or Session Description Protocol, is a format for describing streaming media initialization parameters. ... This article is about the computer protocol. ...

4. Transport layer

TCPUDPDCCPSCTP • … In computing and telecommunications, the transport layer is layer four of the seven layer OSI model. ... The Transmission Control Protocol (TCP) is a virtual circuit protocol that is one of the core protocols of the Internet protocol suite, often simply referred to as TCP/IP. Using TCP, applications on networked hosts can create connections to one another, over which they can exchange streams of data. ... The User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... The Datagram Congestion Control Protocol (DCCP) is a message-oriented transport layer protocol that is currently under development in the IETF. Applications that might make use of DCCP include those with timingconstraints on the delivery of data such that reliable in-order delivery, when combined with congestion control, is likely... The Stream Control Transmission Protocol (SCTP) is a transport layer protocol defined in 2000 by the IETF Signaling Transport (SIGTRAN) working group. ...

3. Network layer

IP (IPv4IPv6) • ARPICMPIGMPRSVPIGPRARP • … The network layer is level three of the seven level OSI model. ... The Internet Protocol (IP) is a data-oriented protocol used for communicating data across a packet-switched internetwork. ... Internet Protocol version 4 is the fourth iteration of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. ... Internet Protocol version 6 (IPv6) is a network layer IP standard used by electronic devices to exchange data across a packet-switched internetwork. ... In computer networking, the Address Resolution Protocol (ARP) is the method for finding a hosts hardware address when only its network layer address is known. ... The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. ... The Internet Group Management Protocol is a communications protocol used to manage the membership of Internet Protocol multicast groups. ... The Resource ReSerVation Protocol (RSVP), described in RFC 2205, is a transport layer protocol designed to reserve resources across a network for an integrated services Internet. ... A set of routing protocols that are used within an autonomous system are referred to as interior gateway protocols (IGP). ... Reverse address resolution protocol (RARP) is a protocol used to resolve an IP address from a given hardware address (such as an Ethernet address). ...

2. Data link layer

ATMBluetooth (PAN-Profile)DTMEthernetFDDIFrame RelayGPRSModemsPPP • Wi-Fi • … To meet Wikipedias quality standards, this article or section may require cleanup. ... Asynchronous Transfer Mode (ATM) is a cell relay network protocol which encodes data traffic into small fixed-sized (53 byte; 48 bytes of data and 5 bytes of header information) cells instead of variable sized packets (sometimes known as frames) as in packet-switched networks (such as the Internet Protocol... Bluetooth logo Bluetooth is an industrial specification for wireless personal area networks (PANs). ... Dynamic synchronous Transfer Mode , or DTM for short, is a network protocol. ... Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks (LANs). ... In computer networking, fiber-distributed data interface (FDDI) is a standard for data transmission in a local area network that can extend in range up to 200 km (124 miles). ... In the context of computer networking, frame relay (also found written as frame-relay) consists of an efficient data transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points. ... General Packet Radio Service (GPRS) is a mobile data service available to users of GSM mobile phones. ... A modem (from modulate and demodulate) is a device that modulates an analogue carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. ... In computing, the Point-to-Point Protocol, or PPP, is commonly used to establish a direct connection between two nodes. ... Official Wi-Fi logo Wi-Fi (also WiFi, wifi, etc. ...

1. Physical layer

Bluetooth RFEthernet physical layerISDNModemsRS232SONET/SDHUSB • Wi-Fi • Power line communication • … The physical layer is level one in the seven level OSI model of computer networking as well as in the five layer TCP/IP reference model. ... Bluetooth logo Bluetooth is an industrial specification for wireless personal area networks (PANs). ... It has been suggested that this article or section be merged with Varieties of Ethernet. ... ISDN is also short for isosorbide dinitrate Integrated Services Digital Network (ISDN) is a type of circuit switched telephone network system, designed to allow digital (as opposed to analog) transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds, than available with analog... A modem (from modulate and demodulate) is a device that modulates an analogue carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. ... RS-232 (also referred to as EIA RS-232C or V.24) is a standard for serial binary data interchange between a DTE (Data terminal equipment) and a DCE (Data communication equipment). ... Synchronous optical networking, is a method for communicating digital information using lasers or light-emitting diodes (LEDs) over optical fiber. ... Note: USB may also mean upper sideband in radio. ... Official Wi-Fi logo Wi-Fi (also WiFi, wifi, etc. ... Power line communication (PLC), also called mains communication, power line telecoms (PLT), powerband or power line networking (PLN), is a term describing several different systems for using power distribution wires for simultaneous distribution of data. ...

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The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. NTP uses UDP port 123 as its transport layer. It is designed particularly to resist the effects of variable latency. Clock synchronization is a problem from computer science which deals with the idea that internal clocks of several computers may differ. ... In computer networking and telecommunications, packet switching is a communications paradigm in which packets (messages or fragments of messages) are individually routed between nodes, with no previously established communication path. ... Latency is a time delay between the moment something is initiated, and the moment one of its effects begins. ... The User Datagram Protocol (UDP) is one of the core protocols of the Internet protocol suite. ... It has been suggested that this article or section be merged into Computer port (software). ... In computing and telecommunications, the transport layer is layer four of the seven layer OSI model. ...


NTP is one of the oldest Internet protocols still in use (since before 1985). NTP was originally designed by Dave Mills of the University of Delaware, who still maintains it, along with a team of volunteers. It has been suggested that Internet Protocols be merged into this article or section. ... David L. Mills (born June 3, 1938) was the first head of the Internet Architecture Board. ... The University of Delaware (UD or UDel) is the largest university in the state of Delaware. ...


NTP is not related to the much simpler DAYTIME (RFC 867) and TIME (RFC 868) protocols. The DAYTIME service is an internet protocol defined in RFC 867. ... The TIME service is an internet protocol defined in RFC 868. ...

Contents

Overview

NTP uses Marzullo's algorithm with the UTC time scale, including support for features such as leap seconds. NTPv4 can usually maintain time to within 10 milliseconds (1/100 s) over the public Internet, and can achieve accuracies of 200 microseconds (1/5000 s) or better in local area networks under ideal conditions. Marzullos algorithm, invented by Keith Marzullo for his Ph. ... Coordinated Universal Time (UTC - see below for explanation) is a high-precision atomic time standard. ... A leap second is a one-second adjustment to civil time in order to keep it close to the mean solar time. ...


The NTP Unix daemon is a user-level process that runs continuously on a machine that supports NTP, and most of the protocol is implemented in this user process. To get the best performance from NTP, it is important to have the standard NTP clock phase-locked loop implemented in the operating system kernel, rather than using only the intervention of the external NTP daemon: all recent versions of the Linux and Solaris operating systems have this support. In Unix and other computer multitasking operating systems, a daemon is a computer program that runs in the background, rather than under the direct control of a user; they are usually instantiated as processes. ... In electronics, a phase-locked loop (PLL) is a closed-loop feedback control system that generates and outputs a signal in relation to the frequency and phase of an input (reference) signal. ... An operating system (OS) is a computer program that manages the hardware and software resources of a computer. ... Linux, or GNU/Linux, refers to any Unix-like computer operating system which uses the Linux kernel. ... Solaris is a computer operating system developed by Sun Microsystems. ...


Windows 2000/XP includes the ability to sync the computer clock to a NTP server.


The operational details of NTP are illustrated in RFC 778, RFC 891, RFC 956, RFC 958, and RFC 1305. The current reference implementation is version 4 (NTPv4); however, as of 2005, only versions up to 3 (1992) have been documented in RFCs. The IETF NTP Working Group has formed to standardize the work of the NTP community since RFC 1305 et al. 2005 (MMV) was a common year starting on Saturday of the Gregorian calendar. ... In internetworking and computer network engineering, Request for Comments (RFC) documents are a series of memoranda encompassing new research, innovations, and methodologies applicable to Internet technologies. ... The Internet Engineering Task Force (IETF) develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standard bodies; and dealing in particular with standards of the TCP/IP and Internet protocol suite. ...


A less complex form of NTP that does not require storing information about previous communications is known as the Simple Network Time Protocol or SNTP. It is used in some embedded devices and in applications where high accuracy timing is not required. See RFC 1361, RFC 1769, RFC 2030 and RFC 4330.


Clock strata

(Note that this is different from the notion of clock strata used in telecommunications systems.)


NTP uses a hierarchical system of "clock strata". The stratum levels define the distance from the reference clock and the associated accuracy.

  • Stratum 0

These are devices such as atomic (cæsium, rubidium) clocks, GPS clocks or other radio clocks. Stratum-0 devices are not attached to the network; instead they are locally connected to computers (e.g. via an RS-232 connection using a Pulse per second signal). Atomic clock Chip-Scale Atomic Clock Unveiled by NIST An atomic clock is a type of clock that uses an atomic resonance frequency standard to feed its counter. ... A radio clock is a clock that is synchronized by a time code transmitted by a radio transmitter connected to a time standard such as an atomic clock. ... A radio clock A radio clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock. ... A Pulse per second (PPS) is an electrical signal that very precisely indicates the start of a second. ...

  • Stratum 1

These are computers attached to Stratum 0 devices. Normally they act as servers for timing requests from Stratum 2 servers via NTP. These computers are also referred to as timeservers. Many Stratum 1 servers (for NTP v3 and earlyer versions) may not actually be operating with Stratum 1 percsion. As the NTP protocol is devloped, it will become less and less possble for misleading Stratum 1 servers to run -- instead the protocol would automatically bump the server Stratum level down accordingly. Meinberg LANTIME timeservers A timeserver is a computer networking device that reads the actual time from a reference clock and distributes this information to its clients using a computer network. ...

  • Stratum 2

These are computers that send NTP requests to Stratum 1 servers. Normally a Stratum 2 computer will reference a number of Stratum 1 servers and use the NTP algorithm to gather the best data sample, dropping any Stratum 1 servers that seem obviously wrong. Stratum 2 computers will peer with other Stratum 2 computers to provide more stable and robust time for all devices in the peer group. Stratum 2 computers normally act as servers for Stratum 3 NTP requests.

  • Stratum 3 and higher

These computers employ exactly the same NTP functions of peering and data sampling as Stratum 2, and can themselves act as servers for higher strata, potentially up to 16 levels. NTP (depending on what version of NTP protocol in use) supports up to 256 strata. It is hoped that in NTP 5 (a protocol still in devlopment) that only 8 or 16 stratas will be permitted.


Strata illustration


NTP Stratum Levels
NTP Stratum Levels


Image File history File links Download high-resolution version (1008x783, 93 KB) NTP Stratum Diagram File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Image File history File links Download high-resolution version (1008x783, 93 KB) NTP Stratum Diagram File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...


NTP timestamps

The 64-bit timestamps used by NTP consist of a 32-bit seconds part and a 32-bit fractional second part with an epoch of January 1, 1900, giving NTP a time scale of 232 seconds (136 years) and a theoretical resolution of 2−32 seconds (0.233 nanoseconds). Although the NTP timescale wraps around every 232 seconds, implementations should disambiguate NTP time using a knowledge of the approximate time from other sources. Since this only requires time accurate to a few decades, this is unlikely to ever be a problem in general use. January 1 is the first day of the calendar year in both the Julian and Gregorian calendars. ... 1900 (MCM) was an exceptional common year starting on Monday of the Gregorian calendar, but a leap year starting on Saturday of the Julian calendar. ...


Even so, future versions of NTP will extend the time representation to 128 bits: 64 bits for the second and 64 bits for the fractional-second. According to Mills, "The 64 bit value for the fraction is enough to resolve the amount of time it takes a proton to pass an electron. The 64 bit second value is enough to provide unambiguous time representation until the universe goes dim."[1] Indeed, 2−64 seconds is about 54 zettoseconds, and 264 seconds is about 585 billion years. // For alternative meanings see proton (disambiguation). ... The electron is a fundamental subatomic particle that carries an electric charge. ... The heat death is a possible final state of the universe, in which it has run down to a state of no free energy to sustain motion or life. ... To help compare orders of magnitude of different times this page lists times between 10-21 seconds and 10-20 seconds (1 zeptosecond to 10 zeptoseconds) See also times of other orders of magnitude. ... To help compare orders of magnitude of different times, this page lists times longer than 1019 seconds (317 billion years) See also times of other orders of magnitude. ...


References

  1. ^ Digital Systems Seminar presen tation by David Mills, April 26, 2006

Bibliography

  • Computer Network Time Synchronization: The Network Time Protocol, David L. Mills, Taylor & Francis / CRC Press, ISBN 0849358051

See also

The Allan variance, named after David W. Allen, also known as two-sample variance, is a measurement of accuracy in clocks. ... Clock synchronization is a problem from computer science which deals with the idea that internal clocks of several computers may differ. ... // Description The ntpd (Network Time Protocol daemon) program is an operating system daemon which sets and maintains the system time of day in synchronism with time servers [Adapted from Mills (2005)]. Implementations ISC NTP.Dev Web - The Internet Systems Consortium ISC development site. ... OpenNTPD is a Unix system daemon implementing the Network Time Protocol to synchronize the local clock of a computer system with remote NTP servers. ... Synchronization is a problem in timekeeping which requires the coordination of events to operate a system in unison. ... International Atomic Time (TAI, from the French name Temps Atomique International) is a high-precision atomic time standard that tracks proper time on Earths geoid. ... Meinberg LANTIME timeservers A timeserver is a computer networking device that reads the actual time from a reference clock and distributes this information to its clients using a computer network. ... NTP server misuse and abuse covers a number of practices which cause damage or degradation to an NTP server, ranging from flooding it with traffic (effectively a DDoS attack) or violating the servers access policy or the NTP rules of engagement. ... The NTP pool is a dynamic collection of networked computers that volunteer to provide highly accurate time via the Network Time Protocol to clients worldwide. ...

External links

Other time synchronization protocols


  Results from FactBites:
 
RFC 958 (rfc958) - Network Time Protocol (NTP) (3532 words)
The protocol includes provisions to specify the precision and estimated error of the local clock and the characteristics of the reference clock to which it may be synchronized.
NTP provides information that can be used to organize this hierarchy on the basis of precision or estimated error and even to serve as a rudimentary routing algorithm to organize the subnetwork itself.
An outline of a suggested mechanism is as follows: Included in the table of timestamps for each peer are state RFC 958 September Network Time Protocol variables to indicate the precision, as well as the current estimated delay, offset, error and drift rate of its local clock.
  More results at FactBites »

 
 

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