rfc958.txt

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   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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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


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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,


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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.