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Network Working Group D.L. Mills Request for Comments: 957 M/A-COM Linkabit

                                                        September 1985
            Experiments in Network Clock Synchronization

Status of this Memo

 This RFC discusses some experiments in clock synchronization in the
 ARPA-Internet community, and requests discussion and suggestions for
 improvements.  Distribution of this memo is unlimited.

Table of Contents

 1.      Introduction
 2.      Design of the Synchronization Algorithm
 2.1.    The Logical Clock
 2.2.    Linear Phase Adjustments
 2.3.    Nonlinear Phase Adjustments
 3.      Synchronizing Network Clocks
 3.1.    Reference Clocks and Reference Hosts
 3.2.    Distribution of Timing Information
 4.      Experimental Validation of the Design
 4.1.    Experiment Design
 4.2.    Experiment Execution
 4.3.    Discussion of Results
 4.3.1.  On Power-Grid Clocks
 4.3.2.  On Clocks Synchronized via Network Links
 4.3.3.  On the Accuracy of Radio Clocks The Spectracom 8170 WWVB Radio Clock The True Time 468-DC GOES Radio Clock The Heath GC-1000 WWV Radio Clock
 4.3.4.  On Handling Disruptions
 4.4.    Additional Experiments
 5.      Summary and Conclusions
 6.      References

List of Figures

 Figure 1. Clock Registers
 Figure 2. Network Configuration

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RFC 957 September 1985 Experiments in Network Clock Synchronization

List of Tables

 Table 1. Experiment Hosts
 Table 2. Link Measurements
 Table 3. First Derivative of Delay
 Table 4. GOES Radio Clock Offsets
 Table 5. WWV Radio Clock Offsets
 Table 6. ISI-MCON-GW Clock Statistics
 Table 7. LL-GW Clock Statistics
 Table 8. LL-GW Clock Statistics

1. Introduction

 One of the services frequently neglected in computer network design
 is a high-quality, time-of-day clock capable of generating accurate
 timestamps with small residual errors compared to intrinsic one-way
 network delays.  Such a service would be useful for tracing the
 progress of complex transactions, synchronizing cached data bases,
 monitoring network performance and isolating problems.
 Several mechanisms have been specified in the Internet protocol suite
 to record and transmit the time at which an event takes place,
 including the ICMP Timestamp message [6], Time Protocol [7], Daytime
 protocol [8] and IP Timestamp option [9].  A new Network Time
 Protocol [12] has been proposed as well.  Additional information on
 network time synchronization can be found in the References at the
 end of this document.  Synchronization protocols are described in [3]
 and [12] and synchronization algorithms in [2], [5], [10] and [11].
 Experimental results on measured roundtrip delays in the Internet are
 discussed in [4].  A comprehensive mathematical treatment of clock
 synchronization can be found in [1].
 Several mechanisms have been specified in the Internet protocol suite
 to record and transmit the time at which an event takes place,
 including the ICMP Timestamp message [6], Time protocol [7], Daytime
 protocol [8] and IP Timestamp option [9].  Issues on time
 synchronization are discussed in [4] and synchronization algorithms
 in [2] and [5].  Experimental results on measured roundtrip delays in
 the Internet are discussed in [2].  A comprehensive mathematical
 treatment of the subject can be found in [1], while an interesting
 discussion on mutual-synchonization techniques can be found in [10].
 There are several ways accurate timestamps can be generated.  One is
 to provide at every service point an accurate, machine-readable clock
 synchronized to a central reference, such as the National Bureau of
 Standards (NBS).  Such clocks are readily available in several models
 ranging in accuracies of a few hundred milliseconds to less than a

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RFC 957 September 1985 Experiments in Network Clock Synchronization

 millisecond and are typically synchronized to special ground-based or
 satellite-based radio broadcasts.  While the expense of the clocks
 themselves, currently in the range $300 to $3000, can often be
 justified, all require carefully sited antennas well away from
 computer-generated electromagnetic noise, as well as shielded
 connections to the clocks.  In addition, these clocks can require a
 lengthy synchonization period upon power-up, so that a battery-backup
 power supply is required for reliable service in the event of power
 If the propagation delays in the network are stable or can be
 predicted accurately, timestamps can be generated by a central
 server, equipped with a clock such as described above, in response to
 requests from remote service points.  However, there are many
 instances where the trans-network delay to obtain a timestamp would
 be intolerable, such as when timestamping a message before
 transmission.  In addition, propagation delays are usually not
 predictable with precisions in the order required, due to
 probabilistic queuing and channel-contention delays.
 In principle, a clock of sufficient accuracy can be provided at each
 service point using a stable, crystal-controlled clock which is
 corrected from time to time by messages from a central server.
 Suitable inexpensive, crystal-controlled clock interfaces are
 available for virtually any computer.  The interesting problem
 remaining is the design of the synchronization algorithm and protocol
 used to transmit the corrections.  In this document one such design
 will be described and its performance assessed.  This design has been
 incorprated as an integral part of the network routing and control
 protocols of the Distributed Computer Network (DCnet) architecture
 [5], clones of which have been established at several sites in the US
 and Europe.  These protocols have been in use since 1979 and been
 continuously tested and refined since then.

2. Design of the Synchronization Algorithm

 The synchronization algorithm is distributed in nature, with protocol
 peers maintained in every host on the network.  Peers communicate
 with each other on a pairwise basis using special control messages,
 called Hello messages, exchanged periodically over the ordinary data
 links between them.  The Hello messages contain information necessary
 for each host to calculate the delay and offset between the local
 clock of the host and the clock of every other host on the network
 and are also used to drive the routing algorithm.
 The synchronization algorithm includes several features to improve
 the accuracy and stability of the local clock in the case of host or

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RFC 957 September 1985 Experiments in Network Clock Synchronization

 link failures.  In following sections the design of the algorithm is
 summarized.  Full design details are given in [5] along with a formal
 description of the Hello protocol.

2.1. The Logical Clock

 In the DCnet model each service point, or host, is equipped with a
 hardware clock, usually in the form of an off-the-shelf interface.
 Using this and software registers, a logical clock is constructed
 including a 48-bit Clock Register, which increments at a 1000 Hz
 rate, a 32-bit Clock-Adjust Register, which is used to slew the Clock
 Register in response to raw corrections received over the net, and a
 Counter Register, which is used in some interface designs as an
 auxilliary counter.  The configuration and decimal point of these
 registers are shown in Figure 1.
         Clock Register                                   
         0               16               32              
         |               |               |               |
                                   decimal point          
         Clock-Adjust Register                            
                         0               16               
                         |               |               |
                                   decimal point          
         Counter Register                                 
                         0              16                
                         |               |                
                                   decimal point          
                     Figure 1. Clock Registers
 The Clock Register and Clock-Adjust Register are implemented in
 memory.  In typical clock interface designs such as the DEC KMV11-A

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RFC 957 September 1985 Experiments in Network Clock Synchronization

 the Counter Register is implemented in the interface as a buffered
 counter driven by a crystal oscillator.  A counter overflow is
 signalled by an interrupt, which results in an increment of the Clock
 Register at bit 15 and the propagation of carries as required.  The
 time of day is determined by reading the Counter Register, which does
 not disturb its counting process, and adding its value to that of the
 Clock Register with decimal points aligned.
 In other interface designs such as the simple LSI-11 event-line
 mechanism, each tick of the clock is signalled by an interrupt at
 intervals of 10, 16-2/3 or 20 ms, depending on interface and clock
 source.  When this occurs the appropriate number of milliseconds,
 expressed to 32 bits in precision, is added to the Clock Register
 with decimal points aligned.
 It should be noted at this point that great care in operating system
 design is necessary in order to preserve the full accuracy of
 timestamps with respect to the application program, which must be
 protected from pre-emption, excessive device latencies and so forth.
 In addition, the execution times of all sequences operating with the
 interrupt system disabled must be strictly limited.  Since the PDP11
 operating system most often used in the DCnet (the "Fuzzball"
 operating system) has been constructed with these considerations
 foremost in mind, it has been especially useful for detailed network
 performance testing and evaluation.  Other systems, in particular the
 various Unix systems, have not been found sufficiently accurate for
 this purpose.
 Left uncorrected, the host logical clock runs at the rate of its
 intrinsic oscillator, whether derived from a crystal or the power
 frequency.  The correction mechanism uses the Clock-Adjust Register,
 which is updated from time to time as raw corrections are received.
 The corrections are computed using roundtrip delays and offsets
 derived from the routing algorithm, described later in this document,
 which are relatively noisy compared to the precision of the logical
 clock.  A carefully designed smoothing mechansim insures stability,
 as well as isolation from large transients that occur due to link
 retransmissions, host reboots and similar disruptions.

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RFC 957 September 1985 Experiments in Network Clock Synchronization

2.2. Linear Phase Adjustments

 The correction is introduced as a signed 32-bit integer in
 milliseconds.  If the magnitude of the correction is less than 128
 ms, the low-order 16 bits replaces bits 0-15 in the Clock-Adjust
 register. At suitable intervals, depending on the jitter of the
 intrinsic oscillator, the value of this register is divided by a
 fixed value, forming a quotient which is first added to the Clock
 Register, then subtracted from the Clock-Adjust Register.  This
 technique has several advantages:
    1.  The clock never runs backwards;  that is, successive
        timestamps always increase monotonically.
    2.  In the event of loss of correction information, the clock
        slews to the last correction received.
    3.  The rate of slew is proportional to the magnitude of the last
        correction.  This allows rapid settling in case of large
        corrections, but provides high stability in case of small
    4.  The sequence of computations preserves the highest precision
        and minimizes the propagation of round-off errors.
 Experience has indicated the choice of 256 as appropriate for the
 dividend above, which yields a maximum slew-rate magnitude less than
 0.5 ms per adjustment interval and a granularity of about 2.0
 microseconds, which is of the same order as the intrinsic tolerance
 of the crystal oscillators used in typical clock interfaces.  In the
 case of crystal-derived clocks, an adjustment interval of four
 seconds has worked well, which yields a maximum slew-rate magnitude
 of 125 microseconds per second.  In the case of power-frequency
 clocks or especially noisy links, the greatly increased jitter
 requires shorter adjustment intervals in the range of 0.5 second,
 which yields a maximum slew-rate magnitude of 1.0 ms per second.
 In most cases, independent corrections are generated over each link
 at intervals of 30 seconds or less.  Using the above choices a single
 sample error of 128 ms causes an error at the next sample interval no
 greater than about 7.5 ms with the longer adjustment interval and 30
 ms with the shorter.  The number of adjustment intervals to reduce
 the residual error by half is about 177, or about 12 minutes with the
 longer interval and about 1.5 minutes with the shorter.  This
 completely characterizes the linear dynamics of the mechanism.

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RFC 957 September 1985 Experiments in Network Clock Synchronization

2.3. Nonlinear Phase Adjustments

 When the magnitude of the correction exceeds 128 ms, the possiblity
 exists that the clock is so far out of synchronization with the
 reference host that the best action is an immediate and wholesale
 replacement of Clock Register contents, rather than a graduated
 slewing as described above.  In practice the necessity to do this is
 rare and occurs when the local host or reference host is rebooted,
 for example. This is fortunate, since step changes in the clock can
 result in the clock apparently running backward, as well as incorrect
 delay and offset measurements of the synchronization mechanism
 However, it sometimes happens that, due to link retransmissions or
 occasional host glitches, a single correction sample will be computed
 with magnitude exceeding 128 ms.  In practice this happens often
 enough that a special procedure has been incorporated into the
 design.  If a sample exceeding the limit is received, its value is
 saved temporarily and does not affect the Clock-Adjust Register.  In
 addition, a timer is initialized, if not already running, to count
 down to zero in a specified time, currently 30 seconds.
 If the timer is already running when a new correction sample with
 magnitude exceeeding 128 ms arrives, its value and the saved sample
 value are averaged with equal weights to form a new saved sample
 value. If a new correction sample arrives with magnitude less than
 128 ms, the timer is stopped and the saved sample value discarded.
 If the timer counts down to zero, the saved sample value replaces the
 contents of the Clock Register and the Clock-Adjust Register is set
 to zero.  This procedure has the effect that occasional spikes in
 correction values are discarded, but legitimate step changes are
 prefiltered and then used to reset the clock after no more than a
 30-second delay.

3. Synchronizing Network Clocks

 The algorithms described in the previous section are designed to
 achieve a high degree of accuracy and stability of the logical clocks
 in each participating host.  In this section algorithms will be
 described which synchronize these logical clocks to each other and to
 standard time derived from NBS broadcasts.  These algorithms are
 designed to minimize the cumulative errors using linear synchronizing
 techniques. See [10] for a discussion of algorithms using nonlinear

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RFC 957 September 1985 Experiments in Network Clock Synchronization

3.1. Reference Clocks and Reference Hosts

 The accuracy of the entire network of logical clocks depends on the
 accuracy of the device used as the reference clock.  In the DCnet
 clones the reference clock takes the form of a precision crystal
 oscillator which is synchronized via radio or satellite with the NBS
 standard clocks in Boulder, CO.  The date and time derived from the
 oscillator can be sent continuously or read upon command via a serial
 asynchronous line.  Stand-alone units containing the oscillator,
 synchronizing receiver and controlling microprocessor are available
 from a number of manufacturers.
 The device driver responsible for the reference clock uses its
 serial-line protocol to derive both an "on-time" timestamp relative
 to the logical clock of the reference host and an absolute time
 encoded in messages sent by the clock.  About once every 30 seconds
 the difference between these two quantities is calculated and used to
 correct the logical clock according to the mechanisms described
 previously.  The corrected logical clock is then used to correct all
 other logical clocks in the network.  Note the different
 nomenclature:  The term "reference clock" applies to the physical
 clock itself, while the term "reference host" applies to the logical
 clock of the host to which it is connected. Each has an individual
 address, delay and offset in synchronizing messages.
 There are three different commercial units used as reference clocks
 in DCnet clones.  One of these uses the LF radio broadcasts on 60 KHz
 from NBS radio WWVB, another the HF radio broadcasts on 5, 10 and 15
 MHz from NBS radio WWV or WWVH and the third the UHF broadcasts from
 a GOES satellite.  The WWVB and GOES clocks claim accuracies in the
 one-millisecond range.  The WWV clock claims accuracies in the 100-ms
 range, although actual accuracies have been measured somewhat better
 than that.
 All three clocks include some kind of quality indication in their
 messages, although of widely varying detail.  At present this
 indication is used only to establish whether the clock is
 synchronized to the NBS clocks and convey this information to the
 network routing algorithm as described later.  All clocks include
 some provision to set the local-time offset and propagation delay,
 although for DCnet use all clocks are set at zero offset relative to
 Universal Time (UT).  Due to various uncertaincies in propagation
 delay, serial-line speed and interrupt latencies, the absolute
 accuracy of timestamps derived from a reference host equipped with a
 WWVB or GOES reference clock is probably no better than a couple of

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RFC 957 September 1985 Experiments in Network Clock Synchronization

3.2. Distribution of Timing Information

 The timekeeping accuracy at the various hosts in the network depends
 critically on the precision whith which corrections can be
 distributed throughout the network.  In the DCnet design a
 distributed routing algorithm is used to determine minimum-delay
 routes between any two hosts in the net.  The algorithms used, which
 are described in detail in [5] and only in outline form here, provide
 reachability and delay information, as well as clock offsets, between
 neighboring hosts by means of periodic exchanges of routing packets
 called Hello messages. This information is then incorporated into a
 set of routing tables maintained by each host and spread throughout
 the network by means of the Hello messages.
 The detailed mechanisms to accomplish these functions have been
 carefully designed to minimize timing uncertaincies.  For instance,
 all timestamping is done in the drivers themselves, which are given
 the highest priority for resource access.  The mechanism to measure
 roundtrip delays on the individual links is insensitive to the delays
 inherent in the processing of the Hello message itself, as well as
 the intervals between transmissions.  Finally, care is taken to
 isolate and discard transient timing errors that occur when a host is
 rebooted or a link is restarted.
 The routing algorithm uses a table called the Host Table, which
 contains for each host in the network the computed roundtrip delay
 and clock offset, in milliseconds.  In order to separately identify
 each reference clock, if there is more than one in the network, a
 separate entry is used for each clock, as well as each host.  The
 delay and offset fields of the host itself are set to zero, as is the
 delay field of each attached reference clock.  The offset field of
 each attached reference clock is recomputed periodically as described
 Hello messages containing a copy of the Host Table are sent
 periodically to each neighbor host via the individual links
 connecting them.  In the case of broadcast networks the Hello message
 is broadcast to all hosts sharing the same cable.  The Hello message
 also contains a timestamp inserted at the time of transmission, as
 well as information used to accurately compute the roundtrip delay on
 point-to-point links.
 A host receiving a Hello message processes the message for each host
 in turn, including those corresponding to the reference clocks.  It
 adds the delay field in the message to the previously determined
 roundtrip link delay and compares this with the entry already in its
 Host Table.  If the sum is greater than the delay field in the Host

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RFC 957 September 1985 Experiments in Network Clock Synchronization

 Table, nothing further is done.  If the sum is less, an update
 procedure is executed.  The update procedure, described in detail in
 [5], causes the new delay to replace the old and the routing to be
 amended accordingly.
 The update procedure also causes a new correction sample to be
 computed as the difference between the timestamp in the Hello message
 and the local clock, which is used to correct the local clock as
 described above.  In addition, the sum of this correction sample plus
 the offset field in the Hello message replaces the offset field in
 the Hello Table.  The effect of these procedures is that the local
 clock is corrected relative to the neighbor clock only if the
 neighbor is on the path of least delay relative to the selected
 reference clock.  If there is no route to the reference clock, as
 determined by the routing algorithm, no corrections are computable
 and the local clock free-runs relative to the last received
 As the result of the operation of the routing algorithm, all local
 clocks in the network will eventually stabilize at a constant offset
 relative to the reference clock, depending upon the drift rates of
 the individual oscillators.  For most applications the offset is
 small and can be neglected.  For the most precise measurements the
 computed offset in the Host Table entry associated with any host,
 including the reference clock, can be used to adjust the apparent
 time relative to the local clock of that host.  However, since the
 computed offsets are relatively noisy, it is necessary to smooth them
 using some algorithm depending upon application.  For this reason,
 the computed offsets are provided separately from the local time.

4. Experimental Validation of the Design

 The original DCnet was established as a "port expander" net connected
 to an ARPAnet IMP in 1978.  It was and is intended as an experimental
 testbed for the development of protocols and measurement of network
 performance.  Since then the DCnet network-layer protocols have
 evolved to include multi-level routing, logical addressing,
 multicasting and time synchronization.  Several DCnet clones have
 been established in the US and Europe and connected to the DARPA
 Internet system.  The experiments described below were performed
 using the DCnet clone at Linkabit East, near Washington, DC, and
 another at Ford Motor Division, near Detroit, MI.  Other clones at
 Ford Aerospace and the Universities of Maryland and Michigan were
 used for calibration and test, while clones at various sites in
 Norway and Germany were used for occasional tests.  Additional

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RFC 957 September 1985 Experiments in Network Clock Synchronization

 ARPANET gateways of the WIDEBAND/EISN satellite system were also
 included in the experiments in order to determine the feasability of
 synchronizing clocks across the ARPANET.
 There were four principal issues of interest in the experiments:
    1.  What are the factors affecting accuracy of a network of clocks
        using the power grid as the basic timing source, together with
        corrections broadcast from a central point?
    2.  What are the factors affecting accuracy of a network of clocks
        synchronized via links used also to carry ordinary data.
    3.  How does the accuracy of the various radio clocks - WWVB, GOES
        and WWV compare?
    4.  What is the best way to handle disruptions, such as a leap
 These issues will be discussed in turn after presentation of the
 experiment design and execution.

4.1. Experiment Design

 Figure 2 shows the configuration used in a series of tests conducted
 during late June and early July of 1985.  The tests involved six
 hosts, three reference clocks and several types of communication
 links.  The tests were designed to coincide with the insertion of a
 leap second in the standard time broadcast by NBS, providing an
 interesting test of system stability in the face of such disruptions.
 The test was also designed to test the feasability of using the power
 grid as a reference clock, with corrections broadcast as described
 above, but not used to adjust the local clock.

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RFC 957 September 1985 Experiments in Network Clock Synchronization

    ARPAnet                              |                      
    - - - - - - - - - - - - - - - - - -  |  - - - - - - - - - - 
                                     56K |                      
    +---------+     +---------+     +----+----+ 1.2 +---------+ 
    |   WWV   | 1.2 |         | 4.8 |         +-----+  WWVB   | 
    |  radio  +-----+  DCN6   +-----+  DCN1   |async|  radio  | 
    |  clock  |async|         |DDCMP|         +--+  |  clock  | 
    +---------+     +---------+     +----+----+  |  +---------+ 
                     Ethernet            |       |              
    DCnet     ===o===============o=======o===    | 1822/DH      
                 |               |               |              
            +----+----+     +----+----+     +----+----+         
    power   |         |     |         |     |         |    power
    freq <--+  DCN3   |     |  DCN5   |     |  DCN7   +--> freq 
    60 Hz   |         |     |         |     |         |    60 Hz
            +---------+     +----+----+     +---------+         
                             9.6 |                              
    - - - - - - - - - - - - - -  |  - - - - - - - - - - - - - - 
                                 | DDCMP                        
                            +----+----+     +---------+         
                            |         | 1.2 |  GOES   |         
    FORDnet                 |  FORD1  +-----+satellite|         
                            |         |async|  clock  |         
                            +---------+     +---------+         
                  Figure 2. Network Configuration
 Only those hosts and links directly participating in the tests are
 shown in Figure 2.  All hosts shown operate using the DCnet protocols
 and timekeeping algorithms summarized in this document and detailed
 in [5].  The DCnet hosts operate as one self-contained net of the
 Internet systems, while the FORDnet hosts operate as another with
 distinct net numbers.  The gateway functions connecting the two nets
 are distributed in the DCN5 and FORD1 hosts and the link connecting
 them.  This means that, although the clock offsets of individual
 DCnet hosts are visible to other DCnet hosts and the clock offsets of
 individual FORDnet hosts are visible to other FORDnet hosts, only the
 clock offset of the gateway host on one net is visible to hosts on
 the other net.
 In Figure 2 the links are labelled with both the intrinsic speed, in
 kilobits per second, as well as the link protocol type.  The DDCMP
 links use microprocessor-based DMA interfaces that retransmit in case
 of message failure.  The 1822/DH link connecting DCN1 and DCN7
 operates at DMA speeds over a short cable.  The Ethernet link uses

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RFC 957 September 1985 Experiments in Network Clock Synchronization

 DMA interfaces that retransmit only in case of collisions.  The
 asynchronous links are used only to connect the reference clocks to
 the hosts over a short cable.
 While all hosts and links were carrying normal traffic throughout the
 test period, the incidence of retransmissions was very low, perhaps
 no more than a few times per day on any link.  However, the DDCMP
 link protocol includes the use of short control messages exhanged
 between the microprocessors about once per second in the absence of
 link traffic. These messages, together with retransmissions when they
 occur, cause small uncertaincies in Hello message delays that
 contribute to the total measurement error.  An additional uncertaincy
 (less than 0.5 per-cent on average) in Hello message length can be
 introduced when the link protocol makes use of character-stuffing or
 bit-stuffing techniques to achieve code transparency, such as with
 the LAPB link-level protocol of X.25.  However, the particular links
 used in the tests use a count field in the header, so that no
 stuffing is required.
 Although the timekeeping algorithms have been carefully designed to
 be insensitive to traffic levels, it sometimes happens that an
 intense burst of traffic results in a shortage of memory buffers in
 the various hosts.  In the case of the Ethernet interfaces, which
 have internal buffers, this can result in additional delays while the
 message is held in the interface pending delivery to the host.
 Conditions where these delays become significant occur perhaps once
 or twice a day in the present system and were observed occasionally
 during the tests.  As described above, the correction-sample
 processing incorporates a filtering procedure that discards the vast
 majority of glitches due to this and other causes.

4.2. Experiment Execution

 The series of experiments conducted in late June and early July of
 1985 involved collecting data on the delays and offsets of the six
 hosts and three reference clocks shown in Figure 2.  In order to
 accomplish this, a special program was installed in a Unix 4.2bsd
 system connected to the Ethernet link but not shown in Figure 2.  The
 program collected each 128-octet Hello message broadcast from DCN1
 every 16 seconds and appended it bit-for-bit to the data base.  The
 total volume of raw data collected amounted to almost 0.7 megabyte
 per day.
 The raw Hello-message data were processed to extract only the
 timestamp and measured clock offsets for the hosts shown in Table 1
 and then reformatted as an ASCII file, one line per Hello message.

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RFC 957 September 1985 Experiments in Network Clock Synchronization

      Host    Clock   Drift   Experiment Use                 
      Name    ID      (ppm)                                  
      DCN1    WWVB    -2.5    WWVB reference host            
      DCN3    -       60-Hz   power-grid (unlocked)          
      DCN5    DCN1    6.8     Ethernet host                  
      DCN6    DCN1    -1.7    DDCMP host, WWV reference host 
      DCN7    DCN1    60-Hz   power-grid (locked)            
      FORD1   GOES    17.9    GOES reference host            
      WWV     -       -       WWV reference clock            
      WWVB    -       -       WWVB reference clock           
                     Table 1. Experiment Hosts
 In Table 1 the Clock ID column shows the reference host selected as
 the master clock for each host shown.  In this particular
 configuration host DCN1 was locked to host WWVB, while hosts DCN5,
 DCN6 and DCN7 were locked to DCN1.  Although the offset of GOES can
 not be directly determined from the Hello messages exchanged between
 DCnet and FORDnet hosts, the offset of FORD1 relative to GOES was
 determined by observation to be in the order of a millisecond, so for
 all practical purposes the offset of FORD1 represents the offset of
 GOES.  In addition, since the WWVB clock was considered by experience
 the most accurate and reliable and the offset of DCN1 relative to
 WWVB was negligible, DCN1 was considered the reference clock with
 offset zero relative to the NBS clocks.
 During the setup phase of the experiments the intrinsic drift rates
 of the crystal oscillators in the four hosts DCN1, DCN5, DCN6 and
 FORD1 equipped with them was measured as shown in the "Drift" column
 in Table 1.  The two hosts DCN3 and DCN7 are equipped with
 line-frequency clocks. For experimental purposes DCN3 was unlocked
 and allowed to free-run at the line-frequency rate, while DCN7
 remained locked.
 An ASCII file consisting of about 0.2 megabyte of reformatted data,
 was collected for each Universal-Time (UT) day of observation
 beginning on 28 June and continuing through 8 July.  Each file was
 processed by a program that produces an eight-color display of
 measured offsets as a function of time of observation.  Since the
 display technique uses a bit-map display and each observation
 overwrites the bit-map in an inclusive-OR fashion, the sample
 dispersion is immediately apparent. Over eight samples per pixel on
 the time axis are available in a 24-hour collection period.  On the
 other hand, the fine granularity of almost four samples per minute
 allows zooming the display to focus on interesting short-term
 fluctuations, such as in the case of the WWV clock.

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RFC 957 September 1985 Experiments in Network Clock Synchronization

4.3. Discussion of Results

 Each of the four previously mentioned issues of interest will be
 discussed in following subsections.

4.3.1. On Power-Grid Clocks

 Telephone interviews with operators and supervisors of the Potomac
 Electric Power Company (PEPCO), the electric utility serving the
 Washington, DC, area, indicate that there are three major operating
 regions or grids, one east of the Rockies, a second west of the
 Rockies and a third in parts of Texas.  The member electric utilities
 in each grid operate on a synchronous basis, so that clocks anywhere
 within the grid should keep identical time.  However, in the rare
 case when a utility drops off the grid, no attempt is made to
 re-establish correct time upon rejoining the grrd.  In the much more
 common case when areas within the grid are isolated due to local
 thunderstorms, for example, clock synchronization is also disrupted.
 The experiments provided an opportunity to measure with exquisite
 precision the offset between a clock connected to the eastern grid
 (DCN3) and the NBS clocks.  The results, confirmed by the telephone
 interviews, show a gradual gain in time of between four and six
 seconds during the interval from about 1700 local time to 0800 the
 next morning, followed by a more rapid loss in time between 0800 and
 1700.  If the time was slewed uniformly throughout these extremes,
 the rate would be about 100 ppm.
 The actual slewing rates depend on the demand, which itself is a
 function of weather, day of the week and season of the year.  Similar
 effects occur in the western and Texas grids, with more extreme
 variations in the Texas grid due to the smaller inertia of the
 system, and less extreme variations in the western grid, due to
 smaller extremes in temperature, less total industrial demand and a
 larger fraction of hydro-electric generation.
 The uilities consider timekeeping a non-tariffed service provided as
 a convenience to the customer.  In the eastern grid a control station
 in Ohio manually establishes the baseline system output, which
 indirectly affects the clock offset and slewing rate.  The local time
 is determined at the control station with respect to a WWVB radio
 clock. The maximum slewing rate is specified as .025 Hz (about 400
 ppm), which is consistent with the maximum rates observed.  In the
 western grid the baseline system output is adjusted automatically
 using a servomechanism driven by measured offsets from the NBS

Mills [Page 15]

RFC 957 September 1985 Experiments in Network Clock Synchronization

 Given the considerations above, it would seem feasable for hosts to
 synchronize logical clocks to a particular power grid, but only if
 corrections were transmitted often enough to maintain the required
 accuracy and these corrections were delivered to the hosts
 essentially at the same time.  Assuming a worst-case 400-ppm slewing
 rate and one minute between correction broadcasts, for example, it
 would in principle be possible to achieve accuracies in the 20-ms
 range.  There are a number of prediction and smoothing techniques
 that could be used to inhance accuracy and reduce the overhead of the
 Host DCN3, which uses a line-frequency clock interface, was unlocked
 during the experiment period so that the offset between the PEPCO
 clock, which is locked to the eastern power grid, could be measured
 with respect to the reference host DCN1.  Host DCN7, which uses the
 same interface, remained locked to DCN1.  In spite of the previously
 noted instability of the power grid, DCN7 remained typically within
 30 ms of DCN1 and only infrequently exceeded 100 ms in the vicinity
 of large changes in system load that occured near 0800 and 1700 local
 time. Over the seven-day period from 2 July through 8 July the mean
 offset was less than a millisecond with standard deviation about 24
 ms, while the maximum was 79 ms and minimum -116 ms.
 Experiments were also carried out using ICMP Timestamp messages with
 hosts known to use line-frequency clock interfaces in California,
 Norway and Germany.  The results indicated that the western power
 grid is rather more stable than the eastern grid and that the
 overseas grids are rather less stable.  In the Oslo, Munich and
 Stuttgart areas, for example, the diurnal variation was observed to
 exceed ten seconds.

4.3.2. On Clocks Synchronized via Network Links

 As mentioned previously, all network links used to synchronize the
 clocks were carrying normal data traffic throughout the experiment
 period.  It would therefore be of interest to investigate how this
 affects the accuracy of the individual clocks.
 Table 2 summarizes the mean and standard deviation of the measured
 offsets between the WWVB radio clock and various hosts as shown in
 Figure 2.  Measurements were made over the 24-hour period for each of
 several days during the experimental period.  Each entry shown in
 Table 2 includes the mean of the statistic over these days, together
 with the maximum variation.

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RFC 957 September 1985 Experiments in Network Clock Synchronization

    Host  Mean          Deviation    Link Type and Speed        
    DCN1  .08/.02       0.53/.02     WWVB radio clock (1200 bps)
    DCN5  -13.61/.04    1.1/0.4      Ethernet (10 Mbps)         
    DCN6  0.27/.18      5.8/1.0      DDCMP (4800 bps)           
    FORD1 38.5/1.6      2.5/0.5      DDCMP (9600 bps)           
                     Table 2. Link Measurements
 The departure of the mean shown in Table 2 from zero is related to
 the drift of the crystal oscillator used in the hardware interface
 (see Table 1).  As described previously, FORD1 was synchonized to the
 GOES radio clock with neglible offset, so that the mean and standard
 deviation shown can be accurately interpreted to apply to the GOES
 radio clock as well.
 The results show that the uncertaincies inherent in the
 synchronization algorithm and protocols is in the same order as that
 of the reference clocks and is related to the speed of the links
 connected the reference hosts to the other hosts in the network.
 Further discussion on the FORD1/GOES statistics can be found in the
 next section.
 Further insight into the error process can be seen in Table 3, which
 shows the first derivative of delay.
               Host    Dev     Max     Min     Error 
               DCN3    2.3     12      -17     10    
               DCN5    1.5     45      -45     5     
               DCN6    9       94      -54     40    
               DCN7    1.4     6       -7      5     
               FORD1   3.4     68      -51     15    
                 Table 3. First Derivative of Delay
 The mean and standard deviation of delay were computed for all hosts
 on a typical day during the experimental period.  In all cases the
 magnitude of the mean was less than one.  The standard deviation,
 maximum and minimum for each link is summarized by host in Table 3.
 A common characteristic of the distribution in most cases was that
 only a handful of samples approached the maximum or minimum extrema,
 while the vast majority of samples were much less than this.  The
 "Error" colum in Table 3 indicates the magnitude of the estimated
 error when these extrema are discarded.

Mills [Page 17]

RFC 957 September 1985 Experiments in Network Clock Synchronization

 A very interesting feature of the observations was the unexpectedly
 low standard deviation of DCN3, which was locked to the power grid
 and thus would be expected to show wide variations.  Upon analysis,
 this turned out to be a natural consequence of the fact that the
 Hello messages are generated as the result of interrupts based on the
 line frequency when the local clock had just been incremented by
 1/60th of a second.
 The synchronizing protocol and implementation were carefully
 constructed to minimize the loss of accuracy due to sharing of the
 network links between data and control traffic, as long as sufficient
 resources (in particular, packet buffers) are available.  Since the
 various network links shown in Figure 2 operate over a wide range of
 rates, it is possible that undisciplined bursts of traffic can swamp
 a host or gateway and precipitate a condition of buffer starvation.
 While most hosts using paths through the experimental configuration
 were relatively well-disciplined in their packetization and
 retransmission policies, some Unix 4.2bsd systems were notorious
 exceptions.  On occasion these hosts were observed sending floods of
 packets, with only a small amount of data per packet, together with
 excessive retransmissions.  As expected, this caused massive
 congestion, unpredictable link delays and occasional clock
 synchronizing errors.
 The synchronizing algorithms described above successfully cope with
 almost all instances of congestion as described, since delay-induced
 errors tend to be isolated, while inherent anti-spike and smoothing
 properties of the synchronizing algorithm help to preserve accuracies
 in any case.  Only one case was found during the ten-day experiment
 period where a host was mistakenly synchronized outside the
 linear-tracking window due to congestion.  Even in this case the host
 was quickly resynchronized to the correct time when the congestion
 was cleared.

4.3.3. On the Accuracy of Radio Clocks

 One of the more potent motivations for the experiments was to assess
 the accuracy of the various radio clocks and to determine whether the
 WWV radio clock was an appropriate replacement for the expensive WWVB
 or GOES clocks.  A secondary consideration, discussed further in the
 next section, was how the various clocks handled disruptions due to
 power interruptions, leap seconds and so forth.

Mills [Page 18]

RFC 957 September 1985 Experiments in Network Clock Synchronization The Spectracom 8170 WWVB Radio Clock

 As the result of several years of experience with the WWVB radio
 clock, which is manufactured by Spectracom Corporation as Model 8170,
 it was chosen as the reference for comparison for the GOES and WWV
 radio clocks.  Washington, DC, is near the 100-microvolt/meter
 countour of the WWVB transmitter at Boulder, CO, well in excess of
 the 25-microvolt/meter sensitivity of the receiver.  The antenna is
 located in a favorable location on the roof of a four-storey building
 in an urban area.
 Using the data from the instruction manual, the propagation delay for
 the path from Boulder to Washington is about 8 ms, while the
 intrinsic receiver delay is about 17 ms.  The clock is read via a
 1200-bps asynchronous line, which introduces an additional delay of
 about 7 ms between the on-time transition of the first character and
 the interrupt at the middle of the first stop bit.  Thus, the WWVB
 radio clock indications should be late by 8 + 17 + 7 = 32 ms relative
 to NBS standard time.  While it is possible to include this delay
 directly in the clock indication, this was not done in the
 experiments.  In order to account for this, 32 ms should be
 subtracted from all indications derived from this clock.  The
 uncertaincy in the indication due to all causes is estimated to be a
 couple of milliseconds. The True Time 468-DC GOES Radio Clock

 The GOES radio clock is manufactured by True Time Division of
 Kinemetrics, Incorporated, as Model 468-DC.  It uses the
 Geosynchronous Orbiting Environmental Satellite (GOES), which
 includes an NBS-derived clock channel.  Early in the experiment
 period there was some ambiguity as to the exact longitude of the
 satellite and also whether the antenna was correctly positioned.
 This was reflected in the rather low quality-of-signal indications
 and occasional signal loss reported by the clock and also its
 apparent offset compared with the other radio clocks.
 Table 4 shows a summary of offset statistics for the GOES radio clock
 by day (all day numbers refer to July, 1985).

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RFC 957 September 1985 Experiments in Network Clock Synchronization

               Day     Mean    Dev     Max     Min   
               2       31.6    9.4     53      -76  
               3       19.8    22.1    53      -64  
               4       42.8    17.1    >150    19   
               5       39.3    2.2     54      -45  
               6       37.8    2.7     53      19   
               7       62.2    13.0    89      22   
               8       38.2    2.8     90      -7   
                 Table 4. GOES Radio Clock Offsets
  On all days except days 5, 6 and 8 long periods of poor-quality
 signal reception were evident.  Since the antenna and satellite
 configuration are known to be marginal, these conditions are not
 considered representative of the capabilities of the clock.  When the
 data from these days are discarded, the mean offset is 38.4 ms with
 standard deviation in the range 2.2 to 2.8.  The maximum offset is 90
 ms and the minimum is -45 ms;  however, only a very small number of
 samples are this large - most excursions are limited to 10 ms of the
 In order to compute the discrepancy between the GOES and WWVB clocks,
 it is necessary to subtract the WWVB clock delay from the mean
 offsets computed above.  Thus, the GOES clock indications are 38.4 -
 32 = 6.4 ms late with respect to the WWVB clock indications.  which
 is probably within the bounds of experiment error. The Heath GC-1000 WWV Radio Clock

 The WWV radio clock is manufactured by Heath Company as Model
 GC-1000.  It uses a three-channel scanning WWV/WWVH receiver on 5, 10
 and 15 MHz together with a microprocessor-based controller.  The
 receiver is connected to an 80-meter dipole up about 15 meters and
 located in a quiet suburban location.  Signal reception from the Fort
 Collins transmitters was average to poor during the experiment period
 due to low sunspot activity together with a moderate level of
 geomagnetic disturbances, but was best during periods of darkness
 over the path.  The clock locked at one of the frequencies for
 varying periods up to an hour from two to several times a day.
 The propagation delay on the path between Fort Collins and Washington
 is estimated at about 10 ms and can vary up to a couple of
 milliseconds over the day and night.  While it is possible to include
 this delay in the clock indications, which are already corrected for

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

Mills [Page 21]

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

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
    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 can be determined to
        within a couple of milliseconds using WWVB and GOES radio
        clocks, but only if they are calibrated using an independent

Mills [Page 25]

RFC 957 September 1985 Experiments in Network Clock Synchronization

        standard such as a portable clock.  The inexpensive WWV clocks
        perform surprisingly well most of the time, but can be in
        error up to a significant fraction of a second under some
    6.  Adjustments in the time scale due to leap seconds must be
        anticipated before they occur.  The synchronization protocol
        must include a mechanism to broadcast an adjustment in advance
        of its occurance, so that it can be incorporated in each host
        simultaneously.  There is a need to incorporate advance notice
        of leap seconds in the broadcast time code.
    7.  Time synchronization via ARPANET paths can yield accuracies in
        the order of a few milliseconds, but only if relatively large
        numbers of samples are available.  Further work is needed to
        develop efficient protocols capable of similar accuracies but
        using smaller numbers of samples.

6. References

 1.  Lindsay, W.C., and A.V.  Kantak.  Network Synchronization of
     Random Signals.  IEEE Trans.  Comm.  COM-28, 8 (August 1980),
 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.

Mills [Page 26]

RFC 957 September 1985 Experiments in Network Clock Synchronization

 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.  Algorithms for Synchronizing Network Clocks.  DARPA
     Network Working Group Report RFC-956, M/A-COM Linkabit, September
 12. Mills, D.L.  Network Time Protocol (NTP).  DARPA Network Working
     Group Report RFC-958, M/A-COM Linkabit, September 1985.

Mills [Page 27]

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