rfc957.txt

RFC 相关的技术文档

Mills                                                          [Page 20]



RFC 957                                                   September 1985
Experiments in Network Clock Synchronization


   the intrinsic receiver delay, this was not done in the experiments.
   During periods of lock, the clock indications are claimed to be
   accurate to within 100 ms.

   Table 5 shows a summary of offset statistics for the WWV radio clock
   by day (all day numbers refer to July, 1985).

                 Day     Mean    Dev     Max     Min   
                 ------------------------------------  
                 2       -31     36      110     -119  
                 3       -42     38      184     -141  
                 4       -21     38      61      -133  
                 5       -31     37      114     -136  
                 6       -48     42      53      -160  
                 7       -100    80      86      -315  
                 8       -71     70      115     -339  

                    Table 5. WWV Radio Clock Offsets

   On inspection of the detailed plots of offsets versus time the data
   reveal an interesting sawtooth variation with period about 25 cycles
   per hour and amplitude about 90 ms.  Once the clock has locked for
   some time the variation decreases in frequency and sometimes
   disappears.  This behavior is precisely what would be expected of a
   phase-locked oscillator and accounts for the rather large standard
   deviations in Table 5.

   On inspection of the plots of offsets versus time, it is apparent
   that by far the best accuracies are obtained at or in the periods of
   lock, which is most frequent during periods of darkness over the
   propagation path, which occured roughly between 0800 UT and 1100 UT
   during the experiment period.  Excluding all data except that
   collected during this period, the mean offset is -21.3 ms with
   standard deviation in the range 29-31.  The maximum offset is 59 ms
   and the minimum is -118 ms.

   In order to compute the discrepancy between the WWV and WWVB clocks,
   it is necessary to subtract the total of the propagation delay plus
   WWVB clock delay from the mean offsets computed above.  Thus, the WWV
   clock indications are -21.3 - 10 - 32 = -72.3 ms late (72.3 ms early)
   with respect to the WWVB clock indications.  Considering the large
   standard deviations noted above, it is probably not worthwhile to
   include this correction in the WWV clock indications.

   On exceptional occasions excursions in offset over 300 ms relative to
   the WWVB clock were observed.  Close inspection of the data showed
   that this was due to an extended period (a day or more) in which lock


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RFC 957                                                   September 1985
Experiments in Network Clock Synchronization


   was not achieved on any frequency.  The master oscillator uses a
   3.6-MHz crystal oscillator trimmed by a digital/analog converter and
   register which is loaded by the microprocessor.  The occasional
   excursions in offset were apparently due to incorrect register values
   as the result of noisy reception conditions and excessive intervals
   between lock.  On occasion the oscillator frequency was observed in
   error over 4 ppm due to this cause, which could result in a
   cumulative error of almost 400 ms per day if uncorrected.

4.3.4.  On Handling Disruptions

   The experiment period was intentionally selected to coincide with the
   insertion of a leap second in the worldwide time broadcasts.  The
   intent was to examine the resulting behavior of the various radio
   clocks and the synchronization algorithm when an additional second
   was introduced at 2400 UT on 30 June.

   As it turned out, radio reception conditions at the time of insertion
   were quite poor on all WWV frequencies, the WWVB frequency and the
   GOES frequency.  Thus, all three clocks took varying periods up to
   several hours to resynchonize and correct the indicated time.  In
   fact, the only time signals heard around the time of interest were
   those from Canadian radio CHU, but the time code of the Canadian
   broadcasts is incompatible with the of the US broadcasts.

   As mentioned above, the WWVB clock was used as the master during the
   experiment period.  About two hours after insertion of the leap
   second the clock resynchronized and all hosts in the experimental
   network were corrected shortly afterwards.  Since the magnitude of
   the correction exceeded 128 ms, the correction was of a step nature,
   but was not performed simultaneously in all hosts due to the
   individual timing of the Hello messages.  Thus, if timing-critical
   network operations happened to take place during the correction
   process, inconsistent timestamps could result.

   The lesson drawn from this experience is quite clear.  Accurate time
   synchronization requires by its very nature long integration times,
   so that epochal events which disrupt the process must be predicted in
   advance and applied in all hosts independently.  In principle, this
   would not be hard to do and could even be integrated into the
   operation of the step-correction procedure described earlier, perhaps
   in the form of bits included in Hello messages which trigger a
   one-second correction at the next rollover from 2400 to 0000 hours.

   In order for such an out-of-band correction to be effective, advance
   notice of the leap second must be available.  At present, this
   information is not available in the broadcast format and must be


Mills                                                          [Page 22]



RFC 957                                                   September 1985
Experiments in Network Clock Synchronization


   obtained via the news media.  In fact, there are spare bits in the
   broadcast format that could be adapted for this purpose, but this
   would require reprogramming both the transmitting and receiving
   equipment. Nevertheless, this feature should be considered for future
   systems.

4.4.  Additional Experiments

   A set of experiments was performed using two WIDEBAND/EISN gateways
   equipped with WWVB radio clocks and connected to the ARPANET.  These
   experiments were designed to determine the limits of accuracy when
   comparing these clocks via ARPANET paths.  One of the gateways
   (ISI-MCON-GW) is located at the Information Sciences Institute near
   Los Angeles, while the other (LL-GW) is located at Lincoln
   Laboratories near Boston.  Both gateways consist of PDP11/44
   computers running the EPOS operating system and clock-interface
   boards with oscillators phase-locked to the WWVB clock.

   The clock indications of the WIDEBAND/EISN gateways were compared
   with the DCNet WWVB reference clock using ICMP Timestamp messages
   [6], which record the individual timestamps with a precision of a
   millisecond.  This technique is not as accurate as the one described
   in Section 3, since the protocol implementation involves the
   user-process level, which can be subject to minor delays due to
   process scheduling and interprocess-message queueing.  However,
   calibration measurements made over several of the links shown in
   Figure 2 indicate that the measurement errors are dominated by the
   individual link variations and not by the characteristics of the
   measurement technique itself.

   Measurements were made separately with each gateway by sending an
   ICMP Timestamp Request message from the ARPANET address of DCN1 to
   the ARPANET address of the gateway and computing the round-trip delay
   and clock offset from the ICMP Timestamp Reply message.  This process
   was continued for 1000 message exchanges, which took about seven
   minutes. Table 6 shows the statistics obtained with ISI-MCON-GW and
   Table 7 those with LL-GW (all numbers are milliseconds).












Mills                                                          [Page 23]



RFC 957                                                   September 1985
Experiments in Network Clock Synchronization


            ISI-MCON-GW     Mean    Dev     Max     Min    
             --------------------------------------------  
             Offset          -16     40      126     -908  
             Delay           347     59      902     264   

                 Table 6. ISI-MCON-GW Clock Statistics

             LL-GW (a)       Mean    Dev     Max     Min   
             --------------------------------------------  
             Offset          -23     15      32      -143  
             Delay           310     25      536     252   

                    Table 7. LL-GW Clock Statistics

   The smaller values of standard deviation and extreme for LL-GW are
   probably due to the shorter ARPANET path involved.  The confidence in
   the mean offset can be estimated by dividing the standard deviation
   by the square root of the number of samples (1000), which suggests
   that the mean offsets are accurate to within a couple of miliseconds.
   The mean offsets of the WIDEBAND/EISN clocks as a group relative to
   the DCN1 clock may thus indicate a minor discrepancy in the setting
   of the delay-compensation switches.

   It is well known that ARPANET paths exhibit wide variations in
   delays, with occasional delays reaching surprising values up to many
   seconds.  In order to improve the estimates a few samples were
   removed from both the offset and delay data, including all those with
   magnitude greater than one second.

   The above experiments involve a burst of activity over a relatively
   short time during which the ratio of the measurement traffic to other
   network traffic may be nontrivial.  Another experiment with LL-GW was
   designed with intervals of ten seconds between ICMP messages and
   operated over a period of about three hours.  The results are shown
   in Table 8.

             LL-GW (b)       Mean    Dev     Max     Min  
             -------------------------------------------- 
             Offset          -16     93      990     -874 
             Delay           371     108     977     240  

                    Table 8. LL-GW Clock Statistics







Mills                                                          [Page 24]



RFC 957                                                   September 1985
Experiments in Network Clock Synchronization


   Note that the standard deviations and extrema are higher than in the
   previous experiments, but the mean offset is about the same.

   The results of these experiments suggest that time synchronization
   via ARPANET paths can yield accuracies to the order of a few
   milliseconds, but only if relatively large numbers of samples are
   available.  The number of samples can be reduced and the accuracy
   improved by using the techniques of Section 3 modified for ICMP
   Timestamp messages and the longer, more noisy paths involved.

5.  Summary and Conclusions

   The experiments described above were designed to verify the correct
   operation of the DCnet time-synchronization algorithms and protocols
   under a variety of scenarios, including the use of line-frequency
   clocks, three types of radio clocks and various types of
   interprocessor links.  They involved the collection and processing of
   many megabytes of data collected over a ten-day period that included
   the insertion of a leap second in the standard NBS time scale.  Among
   the lessons learned were the following:

      1.  The algorithms and protocols operate as designed, yielding
          accuracies throughout the experimental net in the order of a
          few milliseconds to a few tens of milliseconds, depending on
          the topology and link type.

      2.  Glitches due to congestion, rebooted hosts and link failures
          are acceptably low, even in the face of massive congestion
          resulting from inappropriate host implementations elsewhere in
          the Internet.

      3.  A synchronization scenario where the clocks in all hosts are
          locked to the line frequency and corrections are broadcast
          from a central time standard will work only if all hosts are
          on the same power grid, which is unlikely in the present
          Internet configuration, but may be appropriate for some
          applications.

      4.  In spite of the eastern power grid wandering over as much as
          six seconds in a day, it is possible to achieve accuracies in
          the 30-ms range using line-frequency interface clocks and
          corrections broadcast on the local net.

      5.  Radio clocks can vary widely in accuracy depending on signal
          reception conditions.  Absolute time