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rfc:rfc958

Network Working Group D.L. Mills Request for Comments: 958 M/A-COM Linkabit

                                                        September 1985
                    Network 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.      References
 Appendix A. UDP Header Format
 Appendix B. NTP Data Format

1. 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 the

Mills [Page 1]

RFC 958 September Network 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 an

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RFC 958 September Network 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.

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 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].

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RFC 958 September Network 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.

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RFC 958 September Network Time Protocol

5. 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 (see Appendix 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.

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RFC 958 September Network 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 (see Appendix 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 state

Mills [Page 7]

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.
    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 September Network 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.

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RFC 958 September Network Time Protocol

6. 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 Report RFC-778, COMSAT Laboratories, April 1981.
 4.  Mills, D.L.  Internet Delay Experiments.  DARPA Network Working
     Group Report RFC-889, M/A-COM Linkabit, December 1983.
 5.  Mills, D.L.  DCN Local-Network Protocols.  DARPA Network Working
     Group Report RFC-891, M/A-COM Linkabit, December 1983.
 6.  Postel, J.  Internet Control Message Protocol.  DARPA Network
     Working Group Report RFC-792, USC Information Sciences Institute,
     September 1981.
 7.  Postel, J.  Time Protocol.  DARPA Network Working Group Report
     RFC-868, USC Information Sciences Institute, May 1983.
 8.  Postel, J.  Daytime Protocol.  DARPA Network Working Group Report
     RFC-867, USC Information Sciences Institute, May 1983.
 9.  Su, Z.  A Specification of the Internet Protocol (IP) Timestamp
     Option.  DARPA Network Working Group Report RFC-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 Report RFC-957, M/A-COM Linkabit, August
     1985.
 12. Mills, D.L.  Algorithms for Synchronizing Network Clocks.  DARPA
     Network Working Group Report RFC-956, M/A-COM Linkabit, September
     1985.
 13. Postel, J.  User Datagram Protocol.  DARPA Network Working Group
     Report RFC-768, USC Information Sciences Institute, August 1980.

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RFC 958 September Network Time Protocol

Appendix 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.

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RFC 958 September Network Time Protocol

Appendix 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:

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RFC 958 September Network 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)

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RFC 958 September Network 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]

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