Network Working Group D. Mills Request for Comments: 1059 University of Delaware July 1988 Network Time Protocol (Version 1) Specification and Implementation Status of this Memo This memo describes the Network Time Protocol (NTP), specifies its formal structure and summarizes information useful for its implementation. NTP provides the mechanisms to synchronize time and coordinate time distribution in a large, diverse internet operating at rates from mundane to lightwave. It uses a returnable-time design in which a distributed subnet of time servers operating in a self- organizing, hierarchical master-slave configuration synchronizes logical clocks within the subnet and to national time standards via wire or radio. The servers can also redistribute reference time via local routing algorithms and time daemons. The NTP architectures, algorithms and protocols which have evolved over several years of implementation and refinement are described in this document. The prototype system, which has been in regular operation in the Internet for the last two years, is described in an Appendix along with performance data which shows that timekeeping accuracy throughout most portions of the Internet can be ordinarily maintained to within a few tens of milliseconds, even in cases of failure or disruption of clocks, time servers or nets. This is a Draft Standard for an Elective protocol. Distribution of this memo is unlimited. Table of Contents 1. Introduction 3 1.1. Related Technology 4 2. System Architecture 6 2.1. Implementation Model 7 2.2. Network Configurations 9 2.3. Time Scales 10 3. Network Time Protocol 12 3.1. Data Formats 12 3.2. State Variables and Parameters 13 3.2.1. Common Variables 15 3.2.2. System Variables 17 3.2.3. Peer Variables 18 3.2.4. Packet Variables 19 3.2.5. Clock Filter Variables 19 3.2.6. Parameters 20
3.3. Modes of Operation 21 3.4. Event Processing 22 3.4.1. Timeout Procedure 23 3.4.2. Receive Procedure 24 3.4.3. Update Procedure 27 3.4.4. Initialization Procedures 29 4. Filtering and Selection Algorithms 29 4.1. Clock Filter Algorithm 29 4.2 Clock Selection Algorithm 30 4.3. Variable-Rate Polling 32 5. Logical Clocks 33 5.1. Uniform Phase Adjustments 35 5.2. Nonuniform Phase Adjustments 36 5.3. Maintaining Date and Time 37 5.4. Calculating Estimates 37 6. References 40 Appendices Appendix A. UDP Header Format 43 Appendix B. NTP Data Format 44 Appendix C. Timeteller Experiments 47 Appendix D. Evaluation of Filtering Algorithms 49 Appendix E. NTP Synchronization Networks 56 List of Figures Figure 2.1. Implementation Model 8 Figure 3.1. Calculating Delay and Offset 26 Figure 5.1. Clock Registers 34 Figure D.1. Calculating Delay and Offset 50 Figure E.1. Primary Service Network 57 List of Tables Table 2.1. Dates of Leap-Second Insertion 11 Table 3.1. System Variables 14 Table 3.2. Peer Variables 14 Table 3.3. Packet Variables 15 Table 3.4. Parameters 15 Table 4.1. Outlyer Selection Procedure 32 Table 5.1. Clock Parameters 35 Table C.1. Distribution Functions 47 Table D.1. Delay and Offset Measurements (UMD) 52 Table D.2.a Delay and Offset Measurements (UDEL) 52 Table D.2.b Offset Measurements (UDEL) 53 Table D.3. Minimum Filter (UMD - NCAR) 54 Table D.4. Median Filter (UMD - NCAR) 54 Table D.5. Minimum Filter (UDEL - NCAR) 55 Table E.1. Primary Servers 56
1. Introduction This document describes the Network Time Protocol (NTP), including the architectures, algorithms and protocols to synchronize local clocks in a set of distributed clients and servers. The protocol was first described in RFC-958 , but has evolved in significant ways since publication of that document. NTP is built on the Internet Protocol (IP)  and User Datagram Protocol (UDP) , which provide a connectionless transport mechanism; however, it is readily adaptable to other protocol suites. It is evolved from the Time Protocol  and the ICMP Timestamp message , but is specifically designed to maintain accuracy and robustness, even when used over typical Internet paths involving multiple gateways and unreliable nets. The service environment consists of the implementation model, service model and time scale described in Section 2. The implementation model is based on a multiple-process operating system architecture, although other architectures could be used as well. The service model is based on a returnable-time design which depends only on measured offsets, or skews, but does not require reliable message delivery. The subnet is a self-organizing, hierarchical master-slave configuration, with synchronization paths determined by a minimum- weight spanning tree. While multiple masters (primary servers) may exist, there is no requirement for an election protocol. NTP itself is described in Section 3. It provides the protocol mechanisms to synchronize time in principle to precisions in the order of nanoseconds while preserving a non-ambiguous date well into the next century. The protocol includes provisions to specify the characteristics and estimate the error of the local clock and the time server to which it may be synchronized. It also includes provisions for operation with a number of mutually suspicious, hierarchically distributed primary reference sources such as radio clocks. Section 4 describes algorithms useful for deglitching and smoothing clock-offset samples collected on a continuous basis. These algorithms began with those suggested in , were refined as the results of experiments described in  and further evolved under typical operating conditions over the last two years. In addition, as the result of experience in operating multiple-server nets including radio-synchronized clocks at several sites in the US and with clients in the US and Europe, reliable algorithms for selecting good clocks from a population possibly including broken ones have been developed and are described in Section 4. The accuracies achievable by NTP depend strongly on the precision of
the local clock hardware and stringent control of device and process latencies. Provisions must be included to adjust the software logical clock time and frequency in response to corrections produced by NTP. Section 5 describes a logical clock design evolved from the Fuzzball implementation described in . This design includes offset-slewing, drift-compensation and deglitching mechanisms capable of accuracies in order of a millisecond, even after extended periods when synchronization to primary reference sources has been lost. The UDP and NTP packet formats are shown in Appendices A and B. Appendix C presents the results of a survey of about 5500 Internet hosts showing how their clocks compare with primary reference sources using three different time protocols, including NTP. Appendix D presents experimental results using several different deglitching and smoothing algorithms. Appendix E describes the prototype NTP primary service net, as well as proposed rules of engagement for its use. 1.1. Related Technology Other mechanisms have been specified in the Internet protocol suite to record and transmit the time at which an event takes place, including the Daytime protocol , Time Protocol , ICMP Timestamp message  and IP Timestamp option . Experimental results on measured times and roundtrip delays in the Internet are discussed in ,  and . Other synchronization protocols are discussed in , ,  and . NTP uses techniques evolved from both linear and nonlinear synchronization methodology. Linear methods used for digital telephone network synchronization are summarized in , while nonlinear methods used for process synchronization are summarized in . The Fuzzball routing protocol , sometimes called Hellospeak, incorporates time synchronization directly into the routing protocol design. One or more processes synchronize to an external reference source, such as a radio clock or NTP daemon, and the routing algorithm constructs a minimum-weight spanning tree rooted on these processes. The clock offsets are then distributed along the arcs of the spanning tree to all processes in the system and the various process clocks corrected using the procedure described in Section 5 of this document. While it can be seen that the design of Hellospeak strongly influenced the design of NTP, Hellospeak itself is not an Internet protocol and is unsuited for use outside its local-net environment. The Unix 4.3bsd model  uses a single master time daemon to measure offsets of a number of slave hosts and send periodic corrections to them. In this model the master is determined using an election algorithm  designed to avoid situations where either no
master is elected or more than one master is elected. The election process requires a broadcast capability, which is not a ubiquitous feature of the Internet. While this model has been extended to support hierarchical configurations in which a slave on one network serves as a master on the other , the model requires handcrafted configuration tables in order to establish the hierarchy and avoid loops. In addition to the burdensome, but presumably infrequent, overheads of the election process, the offset measurement/correction process requires twice as many messages as NTP per update. A good deal of research has gone into the issue of maintaining accurate time in a community where some clocks cannot be trusted. A truechimer is a clock that maintains timekeeping accuracy to a previously published (and trusted) standard, while a falseticker is a clock that does not. Determining whether a particular clock is a truechimer or falseticker is an interesting abstract problem which can be attacked using methods summarized in  and . A convergence function operates upon the offsets between the clocks in a system to increase the accuracy by reducing or eliminating errors caused by falsetickers. There are two classes of convergence functions, those involving interactive convergence algorithms and those involving interactive consistency algorithms. Interactive convergence algorithms use statistical clustering techniques such as the fault-tolerant average algorithm of , the CNV algorithm of , the majority-subset algorithm of , the egocentric algorithm of  and the algorithms in Section 4 of this document. Interactive consistency algorithms are designed to detect faulty clock processes which might indicate grossly inconsistent offsets in successive readings or to different readers. These algorithms use an agreement protocol involving successive rounds of readings, possibly relayed and possibly augmented by digital signatures. Examples include the fireworks algorithm of  and the optimum algorithm of . However, these algorithms require large numbers of messages, especially when large numbers of clocks are involved, and are designed to detect faults that have rarely been found in the Internet experience. For these reasons they are not considered further in this document. In practice it is not possible to determine the truechimers from the falsetickers on other than a statistical basis, especially with hierarchical configurations and a statistically noisy Internet. Thus, the approach taken in this document and its predecessors involves mutually coupled oscillators and maximum-likelihood estimation and selection procedures. From the analytical point of view, the system of distributed NTP peers operates as a set of coupled phase-locked oscillators, with the update algorithm
functioning as a phase detector and the logical clock as a voltage- controlled oscillator. This similarity is not accidental, since systems like this have been studied extensively ,  and . The particular choice of offset measurement and computation procedure described in Section 3 is a variant of the returnable-time system used in some digital telephone networks . The clock filter and selection algorithms are designed so that the clock synchronization subnet self-organizes into a hierarchical master-slave configuration . What makes the NTP model unique is the adaptive configuration, polling, filtering and selection functions which tailor the dynamics of the system to fit the ubiquitous Internet environment. 2. System Architecture The purpose of NTP is to connect a number of primary reference sources, synchronized to national standards by wire or radio, to widely accessible resources such as backbone gateways. These gateways, acting as primary time servers, use NTP between them to cross-check the clocks and mitigate errors due to equipment or propagation failures. Some number of local-net hosts or gateways, acting as secondary time servers, run NTP with one or more of the primary servers. In order to reduce the protocol overhead the secondary servers distribute time via NTP to the remaining local-net hosts. In the interest of reliability, selected hosts can be equipped with less accurate but less expensive radio clocks and used for backup in case of failure of the primary and/or secondary servers or communication paths between them. There is no provision for peer discovery, acquisition, or authentication in NTP. Data integrity is provided by the IP and UDP checksums. No circuit-management, duplicate-detection or retransmission facilities are provided or necessary. The service can operate in a symmetric mode, in which servers and clients are indistinguishable, yet maintain a small amount of state information, or in client/server mode, in which servers need maintain no state other than that contained in the client request. A lightweight association-management capability, including dynamic reachability and variable polling rate mechanisms, is included only to manage the state information and reduce resource requirements. Since only a single NTP message format is used, the protocol is easily implemented and can be used in a variety of solicited or unsolicited polling mechanisms. It should be recognized that clock synchronization requires by its nature long periods and multiple comparisons in order to maintain accurate timekeeping. While only a few measurements are usually adequate to reliably determine local time to within a second or so,
periods of many hours and dozens of measurements are required to resolve oscillator drift and maintain local time to the order of a millisecond. Thus, the accuracy achieved is directly dependent on the time taken to achieve it. Fortunately, the frequency of measurements can be quite low and almost always non-intrusive to normal net operations. 2.1. Implementation Model In what may be the most common client/server model a client sends an NTP message to one or more servers and processes the replies as received. The server interchanges addresses and ports, overwrites certain fields in the message, recalculates the checksum and returns the message immediately. Information included in the NTP message allows the client to determine the server time with respect to local time and adjust the logical clock accordingly. In addition, the message includes information to calculate the expected timekeeping accuracy and reliability, thus select the best from possibly several servers. While the client/server model may suffice for use on local nets involving a public server and perhaps many workstation clients, the full generality of NTP requires distributed participation of a number of client/servers or peers arranged in a dynamically reconfigurable, hierarchically distributed configuration. It also requires sophisticated algorithms for association management, data manipulation and logical clock control. Figure 2.1 shows a possible implementation model including four processes sharing a partitioned data base, with a partition dedicated to each peer and interconnected by a message-passing system.
+---------+ | Update | +--------->| +----------+ | |Algorithm| | | +----+----+ | | | | | V V +----+----+ +---------+ +---------+ | | | Local | | | | Receive | | +---->| Timeout | | | | Clock | | | +---------+ +---------+ +-+-----+-+ A A | | | | V V =========================================== Peers Network Peers Figure 2.1. Implementation Model The timeout process, driven by independent timers for each peer, collects information in the data base and sends NTP messages to other peers in the net. Each message contains the local time the message is sent, together with previously received information and other information necessary to compute the estimated error and manage the association. The message transmission rate is determined by the accuracy expected of the local system, as well as its peers. The receive process receives NTP messages and perhaps messages in other protocols as well, including ICMP, other UDP or TCP time protocols, local-net protocols and directly connected radio clocks. When an NTP message is received the offset between the sender clock and the local clock is computed and incorporated into the data base along with other information useful for error estimation and clock selection. The update algorithm is initiated upon receipt of a message and at other times. It processes the offset data from each peer and selects the best peer using algorithms such as those described in Section 4. This may involve many observations of a few clocks or a few observations of many clocks, depending on the accuracies required. The local clock process operates upon the offset data produced by the update algorithm and adjusts the phase and frequency of the logical clock using mechanisms such as described in Section 5. This may result in either a step change or a gradual slew adjustment of the logical clock to reduce the offset to zero. The logical clock provides a stable source of time information to other users of the system and for subsequent reference by NTP itself.
2.2. Network Configurations A primary time server is connected to a primary reference source, usually a radio clock synchronized to national standard time. A secondary time server derives time synchronization, possibly via other secondary servers, from a primary server. Under normal circumstances it is intended that a subnet of primary and secondary servers assumes a hierarchical master-slave configuration with the more accurate servers near the top and the less accurate below. Following conventions established by the telephone industry, the accuracy of each server is defined by a number called its stratum, with the stratum of a primary server assigned as one and each level downwards in the hierarchy assigned as one greater than the preceding level. With current technology and available receiving equipment, single-sample accuracies in the order of a millisecond can be achieved at the radio clock interface and in the order of a few milliseconds at the packet interface to the net. Accuracies of this order require special care in the design and implementation of the operating system, such as described in , and the logical clock mechanism, such as described in Section 5. As the stratum increases from one, the single-sample accuracies achievable will degrade depending on the communication paths and local clock stabilities. In order to avoid the tedious calculations  necessary to estimate errors in each specific configuration, it is useful to assume the errors accumulate approximately in proportion to the minimum total roundtrip path delay between each server and the primary reference source to which it is synchronized. This is called the synchronization distance. Again drawing from the experience of the telephone industry, who learned such lessons at considerable cost, the synchronization paths should be organized to produce the highest accuracies, but must never be allowed to form a loop. The clock filter and selection algorithms used in NTP accomplish this by using a variant of the Bellman-Ford distributed routing algorithm  to compute the minimum-weight spanning trees rooted on the primary servers. This results in each server operating at the lowest stratum and, in case of multiple peers at the same stratum, at the lowest synchronization distance. As a result of the above design, the subnet reconfigures automatically in a hierarchical master-slave configuration to produce the most accurate time, even when one or more primary or secondary servers or the communication paths between them fail. This includes the case where all normal primary servers (e.g., backbone WWVB clocks) on a possibly partitioned subnet fail, but one or more backup primary servers (e.g., local WWV clocks) continue operation.
However, should all primary servers throughout the subnet fail, the remaining secondary servers will synchronize among themselves for some time and then gradually drop off the subnet and coast using their last offset and frequency computations. Since these computations are expected to be very precise, especially in frequency, even extend outage periods of a day or more should result in timekeeping errors of not over a few tens of milliseconds. In the case of multiple primary servers, the spanning-tree computation will usually select the server at minimum synchronization distance. However, when these servers are at approximately the same distance, the computation may result in random selections among them as the result of normal dispersive delays. Ordinarily this does not degrade accuracy as long as any discrepancy between the primary servers is small compared to the synchronization distance. If not, the filter and selection algorithms will select the best of the available servers and cast out outlyers as intended. 2.3. Time Scales Since 1972 the various national time scales have been based on International Atomic Time (TA), which is currently maintained using multiple cesium-beam clocks to an accuracy of a few parts in 10^12. The Bureau International de l'Heure (BIH) uses astronomical observations provided by the US Naval Observatory and other observatories to determine corrections for small changes in the mean rotation period of the Earth. This results in Universal Coordinated Time (UTC), which is presently decreasing from TA at a fraction of a second per year. When the magnitude of the correction approaches 0.7 second, a leap second is inserted or deleted in the UTC time scale on the last day of June or December. Further information on time scales can be found in . For the most precise coordination and timestamping of events since 1972 it is necessary to know when leap seconds were inserted or deleted in UTC and how the seconds are numbered. A leap second is inserted following second 23:59:59 on the last day of June or December and becomes second 23:59:60 of that day. A leap second would be deleted by omitting second 23:59:59 on one of these days, although this has never happened. Leap seconds were inserted on the following fourteen occasions prior to January 1988 (courtesy US Naval Observatory):
1 June 1972 8 December 1978 2 December 1972 9 December 1979 3 December 1973 10 June 1981 4 December 1974 11 June 1982 5 December 1975 12 June 1983 6 December 1976 13 June 1985 7 December 1977 14 December 1987 Table 2.1. Dates of Leap-Second Insertion Like UTC, NTP operates with an abstract oscillator synchronized in frequency to the TA time scale. At 0000 hours on 1 January 1972 the NTP time scale was set to 2,272,060,800, representing the number of TA seconds since 0000 hours on 1 January 1900. The insertion of leap seconds in UTC does not affect the oscillator itself, only the translation between TA and UTC, or conventional civil time. However, since the only institutional memory assumed by NTP is the UTC radio broadcast service, the NTP time scale is in effect reset to UTC as each offset estimate is computed. When a leap second is inserted in UTC and subsequently in NTP, knowledge of all previous leap seconds is lost. Thus, if a clock synchronized to NTP in early 1988 was used to establish the time of an event that occured in early 1972, it would be fourteen seconds early. When NTP is used to measure intervals between events that straddle a leap second, special considerations apply. When it is necessary to determine the elapsed time between events, such as the half life of a proton, NTP timestamps of these events can be used directly. When it is necessary to establish the order of events relative to UTC, such as the order of funds transfers, NTP timestamps can also be used directly; however, if it is necessary to establish the elapsed time between events relative to UTC, such as the intervals between payments on a mortgage, NTP timestamps must be converted to UTC using the above table and its successors. The current formats used by NBS radio broadcast services  do not include provisions for advance notice of leap seconds, so this information must be determined from other sources. NTP includes provisions to distribute advance warnings of leap seconds using the Leap Indicator bits described in Section 3. The protocol is designed so that these bits can be set manually at the primary clocks and then automatically distributed throughout the system for delivery to all logical clocks and then effected as described in Section 5.
3. Network Time Protocol This section consists of a formal definition of the Network Time Protocol, including its data formats, entities, state variables, events and event-processing procedures. The specification model is based on the implementation model illustrated in Figure 2.1, but it is not intended that this model is the only one upon which a specification can be based. In particular, the specification is intended to illustrate and clarify the intrinsic operations of NTP and serve as a foundation for a more rigorous, comprehensive and verifiable specification. 3.1. Data Formats All mathematical operations expressed or implied herein are in two's-complement arithmetic. Data are specified as integer or fixed-point quantities. Since various implementations would be expected to scale externally derived quantities for internal use, neither the precision nor decimal-point placement for fixed-point quantities is specified. Unless specified otherwise, all quantities are unsigned and may occupy the full field width, if designated, with an implied zero preceding the most significant (leftmost) bit. Hardware and software packages designed to work with signed quantities will thus yield surprising results when the most significant (sign) bit is set. It is suggested that externally derived, unsigned fixed-point quantities such as timestamps be shifted right one bit for internal use, since the precision represented by the full field width is seldom justified. Since NTP timestamps are cherished data and, in fact, represent the main product of the protocol, a special timestamp format has been established. NTP timestamps are represented as a 64-bit unsigned fixed-point number, in seconds relative to 0000 UT on 1 January 1900. The integer part is in the first 32 bits and the fraction part in the last 32 bits, as shown in the following diagram. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Integer Part | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Fraction Part | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This format allows convenient multiple-precision arithmetic and conversion to Time Protocol representation (seconds), but does complicate the conversion to ICMP Timestamp message representation (milliseconds). The precision of this representation is about 0.2
nanosecond, which should be adequate for even the most exotic requirements. Timestamps are determined by copying the current value of the logical clock to a timestamp variable when some significant event, such as the arrival of a message, occurs. In order to maintain the highest accuracy, it is important that this be done as close to the hardware or software driver associated with the event as possible. In particular, departure timestamps should be redetermined for each link-level retransmission. In some cases a particular timestamp may not be available, such as when the host is rebooted or the protocol first starts up. In these cases the 64-bit field is set to zero, indicating the value is invalid or undefined. Note that since some time in 1968 the most significant bit (bit 0 of the Integer Part) has been set and that the 64-bit field will overflow some time in 2036. Should NTP be in use in 2036, some external means will be necessary to qualify time relative to 1900 and time relative to 2036 (and other multiples of 136 years). Timestamped data requiring such qualification will be so precious that appropriate means should be readily available. There will exist an 0.2-nanosecond interval, henceforth ignored, every 136 years when the 64-bit field will be zero and thus considered invalid. 3.2. State Variables and Parameters Following is a tabular summary of the various state variables and parameters used by the protocol. They are separated into classes of system variables, which relate to the operating system environment and logical clock mechanism; peer variables, which are specific to each peer operating in symmetric mode or client mode; packet variables, which represent the contents of the NTP message; and parameters, which are fixed in all implementations of the current version. For each class the description of the variable is followed by its name and the procedure or value which controls it. Note that variables are in lower case, while parameters are in upper case.
System Variables Name Control ------------------------------------------------------- Logical Clock sys.clock update Clock Source sys.peer selection algorithm Leap Indicator sys.leap update Stratum sys.stratum update Precision sys.precision system Synchronizing Distance sys.distance update Estimated Drift Rate sys.drift system Reference Clock Identifier sys.refid update Reference Timestamp sys.reftime update Table 3.1. System Variables Peer Variables Name Control ------------------------------------------------------- Peer Address peer.srcadr system Peer Port peer.srcport system Local Address peer.dstadr system Local Port peer.dstport system Peer State peer.state receive, transmit Reachability Register peer.reach receive, transmit Peer Timer peer.timer system Timer Threshold peer.threshold system Leap Indicator peer.leap receive Stratum peer.stratum receive Peer Poll Interval peer.ppoll receive Host Poll Interval peer.hpoll receive, transmit Precision peer.precision receive Synchronizing Distance peer.distance receive Estimated Drift Rate peer.drift receive Reference Clock Identifier peer.refid receive Reference Timestamp peer.reftime receive Originate Timestamp peer.org receive Receive Timestamp peer.rec receive Filter Register peer.filter filter algorithm Delay Estimate peer.delay filter algorithm Offset Estimate peer.offset filter algorithm Dispersion Estimate peer.dispersion filter Table 3.2. Peer Variables
Packet Variables Name Control ------------------------------------------------------- Peer Address pkt.srcadr transmit Peer Port pkt.srcport transmit Local Address pkt.dstadr transmit Local Port pkt.dstport transmit Leap Indicator pkt.leap transmit Version Number pkt.version transmit Stratum pkt.stratum transmit Poll pkt.poll transmit Precision pkt.precision transmit Synchronizing Distance pkt.distance transmit Estimated Drift Rate pkt.drift transmit Reference Clock Identifier pkt.refid transmit Reference Timestamp pkt.reftime transmit Originate Timestamp pkt.org transmit Receive Timestamp pkt.rec transmit Transmit Timestamp pkt.xmt transmit Table 3.3. Packet Variables Parameters Name Value ------------------------------------------------------- NTP Version NTP.VERSION 1 NTP Port NTP.PORT 123 Minimum Polling Interval NTP.MINPOLL 6 (64 sec) Maximum Polling Interval NTP.MAXPOLL 10 (1024 sec) Maximum Dispersion NTP.MAXDISP 65535 ms Reachability Register Size PEER.WINDOW 8 Shift Register Size PEER.SHIFT 4/8 Dispersion Threshold PEER.THRESHOLD 500 ms Filter Weight PEER.FILTER .5 Select Weight PEER.SELECT .75 Table 3.4. Parameters Following is a description of the various variables used in the protocol. Additional details on formats and use are presented in later sections and appendices. 3.2.1. Common Variables The following variables are common to the system, peer and packet classes. Peer Address (peer.srcadr, pkt.srcadr) Peer Port (peer.srcport, pkt.srcport)
These are the 32-bit Internet address and 16-bit port number of the remote host. Local Address (peer.dstadr, pkt.dstadr) Local Port (peer.dstport, pkt.dstport) These are the 32-bit Internet address and 16-bit port number of the local host. They are included among the state variables to support multi-homing. Leap Indicator (sys.leap, peer.leap, pkt.leap) This is a two-bit code warning of an impending leap second to be inserted in the NTP time scale. The bits are set before 23:59 on the day of insertion and reset after 00:01 on the following day. This causes the number of seconds (rollover interval) in the day of insertion to be increased or decreased by one. In the case of primary servers the bits are set by operator intervention, while in the case of secondary servers the bits are set by the protocol. The two bits are coded as follows: 00 no warning (day has 86400 seconds) 01 +1 second (day has 86401 seconds) seconds) 10 -1 second (day has 86399 seconds) seconds) 11 alarm condition (clock not synchronized) In all except the alarm condition (11) NTP itself does nothing with these bits, except pass them on to the time-conversion routines that are not part of NTP. The alarm condition occurs when, for whatever reason, the logical clock is not synchronized, such as when first coming up or after an extended period when no outside reference source is available. Stratum (sys.stratum, peer.stratum, pkt.stratum) This is an integer indicating the stratum of the logical clock. A value of zero is interpreted as unspecified, one as a primary clock (synchronized by outside means) and remaining values as the stratum level (synchronized by NTP). For comparison purposes a value of zero is considered greater than any other value. Peer Poll Interval (peer.ppoll, pkt.poll) This is a signed integer used only in symmetric mode and indicating the minimum interval between messages sent to the peer, in seconds as a power of two. For instance, a value of six
indicates a minimum interval of 64 seconds. The value of this variable must not be less than NTP.MINPOLL and must not be greater than NTP.MAXPOLL. Precision (sys.precision, peer.precision, pkt.precision) This is a signed integer indicating the precision of the logical clock, in seconds to the nearest power of two. For instance, a 60-Hz line-frequency clock would be assigned the value -6, while a 1000-Hz crystal-derived clock would be assigned the value -10. Synchronizing Distance (sys.distance, peer.distance, pkt.distance) This is a fixed-point number indicating the estimated roundtrip delay to the primary clock, in seconds. Estimated Drift Rate (sys.drift, peer.drift, pkt.drift) This is a fixed-point number indicating the estimated drift rate of the local clock, in dimensionless units. Reference Clock Identifier (sys.refid, peer.refid, pkt.refid) This is a code identifying the particular reference clock or server. The interpretation of the value depends on the stratum. For stratum values of zero (unspecified) or one (primary clock), the value is an ASCII string identifying the reason or clock, respectively. For stratum values greater than one (synchronized by NTP), the value is the 32-bit Internet address of the reference server. Reference Timestamp (sys.reftime, peer.reftime, pkt.reftime) This is the local time, in timestamp format, when the logical clock was last updated. If the logical clock has never been synchronized, the value is zero. 3.2.2. System Variables The following variables are used by the operating system in order to synchronize the logical clock. Logical Clock (sys.clock) This is the current local time, in timestamp format. Local time is derived from the hardware clock of the particular machine and increments at intervals depending on the design used. An
appropriate design, including slewing and drift-compensation mechanisms, is described in Section 5. Clock Source (sys.peer) This is a selector identifying the current clock source. Usually this will be a pointer to a structure containing the peer variables. 3.2.3. Peer Variables Following is a list of state variables used by the peer management and measurement functions. There is one set of these variables for every peer operating in client mode or symmetric mode. Peer State (peer.state) This is a bit-encoded quantity used for various control functions. Host Poll Interval (peer.hpoll) This is a signed integer used only in symmetric mode and indicating the minimum interval between messages expected from the peer, in seconds as a power of two. For instance, a value of six indicates a minimum interval of 64 seconds. The value of this variable must not be less than NTP.MINPOLL and must not be greater than NTP.MAXPOLL. Reachability Register (peer.reach) This is a code used to determine the reachability status of the peer. It is used as a shift register, with bits entering from the least significant (rightmost) end. The size of this register is specified as PEER.SHIFT bits. Peer Timer (peer.timer) This is an integer counter used to control the interval between transmitted NTP messages. Timer Threshold (peer.threshold) This is the timer value which, when reached, causes the timeout procedure to be executed. Originate Timestamp (peer.org, pkt.org) This is the local time, in timestamp format, at the peer when its
latest NTP message was sent. If the peer becomes unreachable the value is set to zero. Receive Timestamp (peer.rec, pkt.rec) This is the local time, in timestamp format, when the latest NTP message from the peer arrived. If the peer becomes unreachable the value is set to zero. 3.2.4. Packet Variables Following is a list of variables used in NTP messages in addition to the common variables above. Version Number (pkt.version) This is an integer indicating the version number of the sender. NTP messages will always be sent with the current version number NTP.VERSION and will always be accepted if the version number matches NTP.VERSION. Exceptions may be advised on a case-by-case basis at times when the version number is changed. Transmit Timestamp (pkt.xmt) This is the local time, in timestamp format, at which the NTP message departed the sender. 3.2.5. Clock Filter Variables When the filter and selection algorithms suggested in Section 4 are used, the following state variables are defined. There is one set of these variables for every peer operating in client mode or symmetric mode. Filter Register (peer.filter) This is a shift register of PEER.WINDOW bits, where each stage is a tuple consisting of the measured delay concatenated with the measured offset associated with a single observation. Delay/offset observations enter from the least significant (rightmost) right and are shifted towards the most significant (leftmost) end and eventually discarded as new observations arrive. The register is cleared to zeros when (a) the peer becomes unreachable or (b) the logical clock has just been reset so as to cause a significant discontinuity in local time.
Delay Estimate (peer.delay) This is a signed, fixed-point number indicating the latest delay estimate output from the filter, in seconds. While the number is signed, only those values greater than zero represent valid delay estimates. Offset Estimate (peer.offset) This is a signed, fixed-point number indicating the latest offset estimate output from the filter, in seconds. Dispersion Estimate (peer.dispersion) This is a fixed-point number indicating the latest dispersion estimate output from the filter, in scrambled units. 3.2.6. Parameters Following is a list of parameters assumed for all implementations operating in the Internet system. It is necessary to agree on the values for these parameters in order to avoid unnecessary network overheads and stable peer associations. Version Number (NTP.VERSION) This is the NTP version number, currently one (1). NTP Port (NTP.PORT) This is the port number (123) assigned by the Internet Number Czar to NTP. Minimum Polling Interval (NTP.MINPOLL) This is the minimum polling interval allowed by any peer of the Internet system, currently set to 6 (64 seconds). Maximum Polling Interval (NTP.MAXPOLL) This is the maximum polling interval allowed by any peer of the Internet system, currently set to 10 (1024 seconds). Maximum Dispersion (NTP.MAXDISP) This is the maximum dispersion assumed by the filter algorithms, currently set to 65535 milliseconds.
Reachability Register Size (PEER.WINDOW) This is the size of the Reachability Register (peer.reach), currently set to eight (8) bits. Shift Register Size (PEER.SHIFT) When the filter and selection algorithms suggested in Section 4 are used, this is the size of the Clock Filter (peer.filter) shift register, in bits. For crystal-stabilized oscillators a value of eight (8) is suggested, while for mains-frequency oscillators a value of four (4) is suggested. Additional considerations are given in Section 5. Dispersion Threshold (PEER.THRESHOLD) When the filter and selection algorithms suggested in Section 4 are used, this is the threshold used to discard noisy data. While a value of 500 milliseconds is suggested, the value may be changed to suit local conditions on particular peer paths. Filter Weight (PEER.FILTER) When the filter algorithm suggested in Section 4 is used, this is the filter weight used to discard noisy data. While a value of 0.5 is suggested, the value may be changed to suit local conditions on particular peer paths. Select Weight (PEER.SELECT) When the selection algorithm suggested in Section 4 is used, this is the select weight used to discard outlyers. data. While a value of 0.75 is suggested, the value may be changed to suit local conditions on particular peer paths. 3.3. Modes of Operation An NTP host can operate in three modes: client, server and symmetric. The mode of operation is determined by whether the source port (peer.srcport) or destination port (peer.dstport) peer variables contain the assigned NTP service port number NTP.PORT (123) as shown in the following table.
peer.srcport peer.dstport Mode ------------------------------------------- not NTP.PORT not NTP.PORT not possible not NTP.PORT NTP.PORT server NTP.PORT not NTP.PORT client NTP.PORT NTP.PORT symmetric A host operating in client mode occasionally sends an NTP message to a host operating in server mode. The server responds by simply interchanging addresses and ports, filling in the required information and returning the message to the client. Servers then need retain no state information between client requests. Clients are free to manage the intervals between sending NTP messages to suit local conditions. In symmetric mode the client/server distinction disappears. Each host maintains a table with as many entries as active peers. Each entry includes a code uniquely identifying the peer (e.g., Internet address and port), together with status information and a copy of the timestamps last received. A host operating in symmetric mode periodically sends NTP messages to each peer including the latest copy of the timestamps. The intervals between sending NTP messages are managed jointly by the host and each peer using the polling variables peer.ppoll and peer.hpoll. When a pair of peers operating in symmetric mode exchange NTP messages and each determines that the other is reachable, an association is formed. One or both peers must be in active state; that is, sending messages to the other regardless of reachability status. A peer not in active state is in passive state. If a peer operating in passive state discovers that the other peer is no longer reachable, it ceases sending messages and reclaims the storage and timer resources used by the association. A peer operating in client mode is always in active state, while a peer operating in server mode is always in passive state.