rfc1059.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

   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 [2] 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.








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RFC 1059                 Network Time Protocol                 July 1988


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



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RFC 1059                 Network Time Protocol                 July 1988


   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.















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RFC 1059                 Network Time Protocol                 July 1988


        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



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RFC 1059                 Network Time Protocol                 July 1988


        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)



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RFC 1059                 Network Time Protocol                 July 1988


      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: