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Network Working Group                                         D.L. MillsRequest for Comments: 958                               M/A-COM Linkabit                                                          September 1985Network Time Protocol (NTP)Status of this Memo   This RFC suggests a proposed protocol for the ARPA-Internet   community, and requests discussion and suggestions for improvements.   Distribution of this memo is unlimited.Table of Contents   1.      Introduction   2.      Service Model   3.      Protocol Overview   4.      State Variables and Formats   5.      Protocol Operation   5.1.    Protocol Modes   5.2.    Message Processing   5.3.    Network Considerations   5.4.    Leap Seconds   6.      ReferencesAppendix A. UDP Header FormatAppendix B. NTP Data Format1.  Introduction   This document describes the Network Time Protocol (NTP), a protocol   for synchronizing a set of network clocks using a set of distributed   clients and servers.  NTP is built on the User Datagram Protocol   (UDP) [13], which provides a connectionless transport mechanism.  It   is evolved from the Time Protocol [7] and the ICMP Timestamp message   [6] and is a suitable replacement for both.   NTP provides the protocol mechanisms to synchronize time in principle   to precisions in the order of nanoseconds while preserving a   non-ambiguous date, at least for this century.  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.  However, the protocol itself specifies only the   data representation and message formats and does not specify the   synchronizing algorithms or filtering mechanisms.   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 [8] and IP Timestamp option [9].  The   NTP is not meant to displace either of these mechanisms.  Additional   information on network time synchronization can be found in theMills                                                           [Page 1]

RFC 958                                                        SeptemberNetwork Time Protocol   References at the end of this document.  An earlier synchronization   protocol is discussed in [3] and synchronization algorithms in [2],   [5], [10] and [12]. Experimental results on measured roundtrip delays   and clock offsets in the Internet are discussed in [4] and [11].  A   comprehensive mathematical treatment of clock synchronization can be   found in [1].2.  Service Model   The intent of the service for which this protocol is designed is to   connect a few primary reference clocks, synchronized by wire or radio   to national standards, to centrally accessable resources such as   gateways. These gateways would use NTP between them to cross-check   the primary clocks and mitigate errors due to equipment or   propagation failures. Some number of local-net hosts, serving as   secondary reference clocks, would run NTP with one or more of these   gateways.  In order to reduce the protocol overhead, these hosts   would redistribute time to the remaining local-net hosts.  In the   interest of reliability selected hosts might be equipped with less   accurate but less expensive radio clocks and used for backup in case   of failure of the primary and/or secondary clocks or communication   paths between them.   In the normal configuration a subnetwork of primary and secondary   clocks will assume a hierarchical organization with the more accurate   clocks near the top and the less accurate below.  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.  However, the   NTP protocol does not include a specification of the algorithms for   doing this, which is left as a topic for further study.3.  Protocol Overview   There is no provision for peer discovery, acquisition, or   authentication in NTP.  Data integrity is provided by the IP and UDP   checksums.  No reachability, 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 an unsymmetric mode in which servers need maintain no client   state other than that contained in the client request.  Moreover,   only a single NTP message format is necessary, which simplifies   implementation and can be used in a variety of solicited or   unsolicited polling mechanisms.   In what may be the most common (unsymmetric) mode a client sends anMills                                                           [Page 2]

RFC 958                                                        SeptemberNetwork Time Protocol   NTP message to one or more servers and processes the replies as   received.  The server interchanges addresses and ports, fills in or   overwrites certain fields in the message, recalculates the checksum   and returns it immediately.  Information included in the NTP message   allows each client/server peer to determine the timekeeping   characteristics of its other peers, including the expected accuracies   of their clocks. Using this information each peer is able to select   the best time from possibly several other clocks, update the local   clock and estimate its accuracy.   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 comparisons are usually   adequate to maintain local time to within a second, primarily to   protect against broken hardware or synchronization failure, periods   of hours or days and tens or hundreds of comparisons are required to   maintain local time to within a few tens of milliseconds.   Fortunately, the frequency of comparisons can be quite small and   almost always non-intrusive to normal network operations.4.  State Variables and Formats   NTP timestamps are represented as a 64-bit 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 low-order fraction bit increments at about   0.2-nanosecond intervals, so a free-running one-millisecond clock   will be in error only a small fraction of one part per million, or   less than a second per year.   In some cases a particular timestamp may not be available, such as   when the protocol first starts up.  In these cases the 64-bit field   is set to zero, indicating the value is invalid or undefined.Mills                                                           [Page 3]

RFC 958                                                        SeptemberNetwork Time Protocol   Following is a description of the various data items used in the   protocol.  Details of packet formats are presented in the Appendices.   Leap Indicator      This is a two-bit code warning of an impending leap-second to be      inserted in the internationally coordinated Standard Time      broadcasts.  A leap-second is occasionally added or subtracted      from Standard Time, which is based on atomic clocks, to maintain      agreement with Earth rotation.  When necessary, the corrections      are notified in advance and executed at the end of the last day of      the month in which notified, usually June or December.  When a      correction is executed the first minute of the following day will      have either 59 or 61 seconds.   Status      This is a six-bit code indicating the status of the local clock.      Values are assigned to indicate whether it is operating correctly      or in one of several error states.   Reference Clock Type      This is an eight-bit code identifying the type of reference clock      used to set the local clock.  Values are assigned for primary      clocks (locally synchronized to Standard Time), secondary clocks      (remotely synchronized via various network protocols) and even      eyeball-and-wristwatch.   Precision      This is a 16-bit signed integer indicating the precision of the      local 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 clock would be assigned the value -10.   Estimated Error      This is a 32-bit fixed-point number indicating the estimated error      of the local clock at the time last set.  The value is in seconds,      with fraction point between bits 15 and 16, and is computed by the      sender based on the reported error of the reference clock, the      precision and drift rate of the local clock and the time the local      clock was last set.  For statistical purposes this quantity can be      assumed equal to the estimated or computed standard deviation, as      described in [12].Mills                                                           [Page 4]

RFC 958                                                        SeptemberNetwork Time Protocol   Estimated Drift Rate      This is a 32-bit signed fixed-point number indicating the      estimated drift rate of the local clock.  The value is      dimensionless, with fraction point to the left of the high-order      bit.  While for most purposes this value can be estimated based on      the hardware characteristics, it is possible to compute it quite      accurately, as described in [12].   Reference Clock Identifier      This is a 32-bit code identifying the particular reference clock.      The interpretation of its value depends on value of Reference      Clock Type.  In the case of a primary clock locally synchronized      to Standard Time (type 1), the value is an ASCII string      identifying the clock.  In the case of a secondary clock remotely      synchronized to an Internet host via NTP (type 2), the value is      the 32-bit Internet address of that host.  In other cases the      value is undefined.   Reference Timestamp      This is a 64-bit timestamp established by the server or client      host as the timestamp (presumably obtained from a reference clock)      most recently used to update the local clock.  If the local clock      has never been synchronized, the value is zero.   Originate Timestamp      This is a 64-bit timestamp established by the client host and      specifying the local time at which the request departed for the      service host.  It will always have a nonzero value.   Receive Timestamp      This is a 64-bit timestamp established by the server host and      specifying the local time at which the request arrived from the      client host.  If no request has ever arrived from the client the      value is zero.   Transmit Timestamp      This is a 64-bit timestamp established by the server host and      specifying the local time at which the reply departed for the      client host.  If no request has ever arrived from the client the      value is zero.Mills                                                           [Page 5]

RFC 958                                                        SeptemberNetwork Time Protocol5.  Protocol Operation   The intent of this document is to specify a standard for data   representation and message format which can be used for a variety of   synchronizing algorithms and filtering mechanisms.  Accordingly, the   information in this section should be considered a guide, rather than   a concise specification.  Nevertheless, it is expected that a   standard Internet distributed timekeeping protocol with concisely   specified synchronizing and filtering algorithms can be evolved from   the information in this section.   5.1.  Protocol Modes      The distinction between client and server is significant only in      the way they interact in the request/response interchange.  The      same NTP message format is used by each peer and contains the same      data relative to the other peer.  In the unsymmetric mode the      client periodically sends an NTP message to the server, which then      responds within some interval.  Usually, the server simply      interchanges addresses and ports, fills in the required      information and sends the message right back. Servers operating in      the unsymmetric mode then need retain no state information between      client requests.      In the symmetric mode the client/server distinction disappears.      Each peer maintains a table with as many entries as active peers,      each entry including a code uniquely identifying the peer (e.g.      Internet address), together with status information and a copy of      the Originate Timestamp and Receive Timestamp values last received      from that peer. The peer periodically sends an NTP message to each      of these peers including the latest copy of these timestamps.  The      interval between sending NTP messages is managed solely by the      sending peer and is unaffected by the arrival of NTP messages from      other peers.      The mode assumed by a peer can be determined by inspection of the      UDP Source Port and Destination Port fields (seeAppendix A).  If      both of these fields contain the NTP service-port number 123, the      peer is operating in symmetric mode.  If they are different and      the Destination Port field contains 123, this is a client request      and the receiver is expected to reply in the manner described      above.  If they are different and the Source Port field contains      123, this is a server reply to a previously sent client request.Mills                                                           [Page 6]

RFC 958                                                        SeptemberNetwork Time Protocol   5.2.  Message Processing      The significant events of interest in NTP occur usually near the      times the NTP messages depart and arrive the client/server.  In      order to maintain the highest accuracy it is important that the      timestamps associated with these events be computed as close as      possible to the hardware or software driver associated with the      communications link and, in particular, that departure timestamps      be recomputed for each retransmission, if used at the link level.      An NTP message is constructed as follows (seeAppendix B).  The      source peer constructs the UDP header and the LI, Status,      Reference Clock Type and Precision fields in the NTP data portion.      Next, it determines the current synchronizing source and      constructs the Type and Reference Clock Identifier fields.  From      its timekeeping algorithm (see [12] for examples) it determines      the Reference Timestamp, Estimated Error and Estimated Drift Rate      fields.  Then it copies into the Receive Timestamp and Transmit      Timestamp fields the data saved from the latest message received      from the destination peer and, finally, computes the Originate      Timestamp field.      The destination peer calculates the roundtrip delay and clock      offset relative to the source peer as follows.  Let t1, t2 and t3      represent the contents of the Originate Timestamp, Receive      Timestamp and Transmit Timestamp fields and t4 the local time the      NTP message is received.  Then the roundtrip delay d and clock      offset c is:         d = (t4 - t1) - (t3 - t2)  and  c = (t2 - t1 + t3 - t4)/2 .      The implicit assumption in the above is that the one-way delay is      statistically half the roundtrip delay and that the intrinsic      drift rates of both the client and server clocks are small and      close to the same value.   5.3.  Network Considerations      The client/server peers have an opportunity to learn a good deal      about each other in the NTP message exchange.  For instance, each      can learn about the characteristics of the other clocks and select      among them the most accurate to use as reference clock, compute      the estimated error and drift rate and use this information to      manage the dynamics of the subnetwork of clocks.  An outline of a      suggested mechanism is as follows:      Included in the table of timestamps for each peer are stateMills                                                           [Page 7]

RFC 958                                                        SeptemberNetwork Time Protocol      variables to indicate the precision, as well as the current      estimated delay, offset, error and drift rate of its local clock.      These variables are updated for each NTP message received from the      peer, after which the estimated error is periodically recomputed      on the basis of elapsed time and estimated drift rate.      Assuming symmetric mode, a polling interval is established for      each peer, depending upon its normal synchronization source,      precision and intrinsic accuracy, which might be determined in      advance or even as the result of observation.  The delay and      clock-offset samples obtained can be filtered using      maximum-likelihood techniques and algorithms described in [12].      From time to time a local-clock correction is computed from the      offset data accumulated as above, perhaps using algorithms      described in [10] and [12].  The correction causes the local clock      to run slightly fast or slow to the corrected time or to jump      instantaneously to the correct time, depending on the magnitude of      the correction.  See [5] and [11] for a discussion of local-clock      implementation models and synchronizing algorithms.  Note that the      expectation here is that all network clocks are maintained by      these algorithms, so that manual intervention is not normally      required.      As a byproduct of the above operations an estimate of local-clock      error and drift rate can be computed.  Note that the magnitude of      the error estimate must always be greater than that of the      selected reference clock by at least the inherent precision of the      local clock. It does not take a leap of imagination to see that      the estimated error, delay or precision, or some combination of      them, can be used as a metric for a simple min-hop-type routing      algorithm to organize the subnetwork so as to provide the most      accurate time to all peers and to provide automatic fallback to      alternate sources in case of failures.      A variety of network configurations can be included in the above      scenario.  In the case of networks supporting a broadcast      function, for example, NTP messages can be broadcast from one or      more server hosts and picked up by client hosts sharing the same      cable.  Since typical networks of this type have a very low      propagation delay, the roundtrip-delay calculation can be omitted      and the clients need not broadcast in return.  Thus, the      requirement to save per-peer timestamps is removed, so that the      Receive Timestamp and Transmit Timestamp fields can be set to zero      and the local-clock offset becomes simply the difference between      the Originate Timestamp and the local time upon arrival.  In the      case of long-delay satellite networks with broadcast capabilities,Mills                                                           [Page 8]

RFC 958                                                        SeptemberNetwork Time Protocol      an accurate measure of roundtrip delay is usually available from      the channel-scheduling algorithm, so the per-peer timestamps again      can be avoided.   5.4.  Leap Seconds      A standard mechanism to effect leap-second correction is not a      part of this specification.  It is expected that the Leap      Indicator bits would be set by hand in the primary reference      clocks, then trickle down to all other clocks in the network,      which would execute the correction at the specified time and reset      the bits.Mills                                                           [Page 9]

RFC 958                                                        SeptemberNetwork Time Protocol6.  References   1.  Lindsay, W.C., and A.V.  Kantak.  Network Synchronization of       Random Signals.  IEEE Trans.  Comm.  COM-28, 8 (August 1980),       1260-1266.   2.  Mills, D.L.  Time Synchronization in DCNET Hosts.  DARPA Internet       Project Report IEN-173, COMSAT Laboratories, February 1981.   3.  Mills, D.L.  DCNET Internet Clock Service.  DARPA Network Working       Group ReportRFC-778, COMSAT Laboratories, April 1981.   4.  Mills, D.L.  Internet Delay Experiments.  DARPA Network Working       Group ReportRFC-889, M/A-COM Linkabit, December 1983.   5.  Mills, D.L.  DCN Local-Network Protocols.  DARPA Network Working       Group ReportRFC-891, M/A-COM Linkabit, December 1983.   6.  Postel, J.  Internet Control Message Protocol.  DARPA Network       Working Group ReportRFC-792, USC Information Sciences Institute,       September 1981.   7.  Postel, J.  Time Protocol.  DARPA Network Working Group ReportRFC-868, USC Information Sciences Institute, May 1983.   8.  Postel, J.  Daytime Protocol.  DARPA Network Working Group ReportRFC-867, USC Information Sciences Institute, May 1983.   9.  Su, Z.  A Specification of the Internet Protocol (IP) Timestamp       Option.  DARPA Network Working Group ReportRFC-781.  SRI       International, May 1981.   10. Marzullo, K., and S.  Owicki.  Maintaining the Time in a       Distributed System.  ACM Operating Systems Review 19, 3 (July       1985), 44-54.   11. Mills, D.L.  Experiments in Network Clock Synchronization.  DARPA       Network Working Group ReportRFC-957, M/A-COM Linkabit, August       1985.   12. Mills, D.L.  Algorithms for Synchronizing Network Clocks.  DARPA       Network Working Group ReportRFC-956, M/A-COM Linkabit, September       1985.   13. Postel, J.  User Datagram Protocol.  DARPA Network Working Group       ReportRFC-768, USC Information Sciences Institute, August 1980.Mills                                                          [Page 10]

RFC 958                                                        SeptemberNetwork Time ProtocolAppendix A.  UDP Header Format   An NTP packet consists of the UDP header followed by the NTP data   portion.  The format of the UDP header and the interpretation of its   fields are described in [13] and are not part of the NTP   specification.  They are shown below for completeness.    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |          Source Port          |       Destination Port        |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |            Length             |           Checksum            |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Source Port      UDP source port number. In the case of unsymmetric mode and a      client request this field is assigned by the client host, while      for a server reply it is copied from the Destination Port field of      the client request.  In the case of symmetric mode, both the      Source Port and Destination Port fields are assigned the NTP      service-port number 123.   Destination Port      UDP destination port number. In the case of unsymmetric mode and a      client request this field is assigned the NTP service-port number      123, while for a server reply it is copied form the Source Port      field of the client request.  In the case of symmetric mode, both      the Source Port and Destination Port fields are assigned the NTP      service-port number 123.   Length      Length of the request or reply, including UDP header, in octets.   Checksum      Standard UDP checksum.Mills                                                          [Page 11]

RFC 958                                                        SeptemberNetwork Time ProtocolAppendix B.  NTP Data Format   The format of the NTP data portion, which immediately follows the UDP   header, is shown below along with a description of its fields.    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |LI |   Status  |      Type     |           Precision           |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                       Estimated Error                         |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                     Estimated Drift Rate                      |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                  Reference Clock Identifier                   |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   |                 Reference Timestamp (64 bits)                 |   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   |                 Originate Timestamp (64 bits)                 |   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   |                  Receive Timestamp (64 bits)                  |   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |                                                               |   |                  Transmit Timestamp (64 bits)                 |   |                                                               |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Leap Indicator (LI)      Code warning of impending leap-second to be inserted at the end of      the last day of the current month. Bits are coded as follows:         00      no warning         01      +1 second (following minute has 61 seconds)         10      -1 second (following minute has 59 seconds)         11      reserved for future use   Status      Code indicating status of local clock. Values are defined as      follows:Mills                                                          [Page 12]

RFC 958                                                        SeptemberNetwork Time Protocol         0       clock operating correctly         1       carrier loss         2       synch loss         3       format error         4       interface (Type 1) or link (Type 2) failure         (additional codes reserved for future use)   Reference Clock Type   (Type)      Code identifying the type of reference clock. Values are defined      as follows:         0       unspecified         1       primary reference (e.g. radio clock)         2       secondary reference using an Internet host via NTP         3       secondary reference using some other host or protocol         4       eyeball-and-wristwatch         (additional codes reserved for future use)   Precision      Signed integer in the range +32 to -32 indicating the precision of      the local clock, in seconds to the nearest power of two.   Estimated Error      Fixed-point number indicating the estimated error of the local      clock at the time last set, in seconds with fraction point between      bits 15 and 16.   Estimated Drift Rate      Signed fixed-point number indicating the estimated drift rate of      the local clock, in dimensionless units with fraction point to the      left of the high-order bit.   Reference Clock   Identifier      Code identifying the particular reference clock. In the case of      type 1 (primary reference), this is a left-justified, zero-filled      ASCII string identifying the clock, for example:         WWVB    WWVB radio clock (60 KHz)Mills                                                          [Page 13]

RFC 958                                                        SeptemberNetwork Time Protocol         GOES    GOES satellite clock (468 HMz)         WWV     WWV radio clock (2.5/5/10/15/20 MHz)         (and others as necessary)      In the case of type 2 (secondary reference) this is the 32-bit      Internet address of the reference host. In other cases this field      is reserved for future use and should be set to zero.   Reference Timestamp      Local time at which the local clock was last set or corrected.   Originate Timestamp      Local time at which the request departed the client host for the      service host.   Receive Timestamp      Local time at which the request arrived at the service host.   Transmit Timestamp      Local time at which the reply departed the service host for the      client host.Mills                                                          [Page 14]