    RFC 778

                    DCNET Internet Clock Service
                  D.L. Mills, COMSAT Laboratories
                           18 April 1981

    Introduction

         Following  is  a  description  of  the  Internet  Clock
    Service  (ICS)  provided  by  all DCNET hosts.  The service,
    intended primarily for  clock  synchronization  and  one-way
    delay  measurements  with  cooperating  internet  hosts,  is
    provided using the Timestamp and Timestamp Reply messages of
    the  proposed  Internet Control Message Protocol (ICMP).  In
    addition, in order to maintain  compatability  with  present
    systems,  this  service  will be provided for a limited time
    using  the   Echo   and   Echo   Reply   messages   of   the
    Gateway-Gateway Protocol (GGP).
         It should be understood that ICMP and GGP datagrams are
    normally  considered  tightly bound to the Internet Protocol
    (IP) itself and not directly accessable to  the  user  on  a
    TOPS-20  system,  for  example.  These datagrams are treated
    somewhat differently from user  datagrams  in  gateways  and
    DCNET hosts in that certain internal queueing mechanisms are
    bypassed.  Thus, they can be a useful tool in providing  the
    most   accurate   and  stable  time  reference.   The  prime
    motivation for this note is to promote  the  development  of
    this  service  in  other internet hosts and gateways so that
    the feasibility for its use thoughout the community  can  be
    assessed.

    ICS Datagrams and Timestamps

         At present, the ICS is provided using  either  ICMP  or
    GGP  datagrams.   The  only difference between these is that
    ICMP uses protocol number 1 and GGP uses protocol number  3.
    In the following these will be referred to interchangably as
    ICS datagrams.  ICS datagrams  include  an  internet  header
    followed by an ICS header in the following format:

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     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Type     |     Code      |            Sequence           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Originate Timestamp                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Receive Timestamp                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Transmit Timestamp                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           ICS Datagram Format

         The originator fills in all three timestamp fields just
    before  the datagram is forwarded to the net.  Each of these
    fields contain the local time at origination.  Although  the
    last   two   are   redundant,  they  allow  roundtrip  delay
    measurements  to  be  made  using   remote   hosts   without
    timestamping  facilities.   The "Type" field can be either 8
    (GGP Echo) or 13 (ICMP Timestamp).  The "Code" field  should
    be zero.  The "Sequence" field can contain either zero or an
    optional sequence number provided by the user.   The  length
    of  the datagram is thus 36 octets inclusive of the 20-octet
    internet header and exclusive of the local-network leader.

         The host or gateway receiving an ICS datagram fills  in
    the  "Receive  Timestamp"  field  just  as  the  datagram is
    received from the net and the "Transmit Timestamp"  just  as
    it is forwarded back to the sender.  It also sets the "Type"
    field to 0 (GGP Echo Reply), if the original value was 8, or
    14  (ICMP  Timestamp  Reply),  if  it was 13.  The remaining
    fields are unchanged.

         The timestamp values are in milliseconds from  midnight
    UT and are stored right-justified in the 32-bit fields shown
    above.  Ordinarily,  all  time  calculations  are  performed
    modulo-24 hours in milliseconds.  This provides a convenient
    match to those operating systems  which  maintain  a  system
    clock  in ticks past midnight.  The specified timestamp unit
    of milliseconds is consistent with the accuracy of  existing
    radio  clocks  and  the  errors expected in the timestamping
    process itself.

    Delay Measurements

         Delay measurements can be made with any DCNET  host  by
    simply sending an ICS datagram in the above format to it and
    processing the reply.  Let t1, t2 and t3 represent the three
    timestamp  fields  of  the reply in order and t4 the time of
    arrival at the original sender.  Then the delays,  exclusive
    of  internal  processing  within  the DCNET host, are simply
    (t2 - t1) to the DCNET host, (t4 - t3) for  the  return  and

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    (t2 - t1) + (t4 - t3)  for the roundtrip.  Note that, in the
    case of the roundtrip, the clock offsets between the sending
    host and DCNET host cancel.

         Although ICS datagrams are returned by all DCNET  hosts
    regardless  of  other connections that may be in use by that
    host at any given time, the most useful host  will  probably
    be   the   COMSAT-WWV   virtual  host  at  internet  address
    [29,0,9,2], which is also the  internet  echo  virtual  host
    formerly  called  COMSAT-ECH.  This virtual host is resident
    in  the  COMSAT-GAT  physical  host  at   internet   address
    [29,0,1,2], which is connected to the ARPANET via the COMSAT
    Gateway, Clarksburg SIMP and a 4800-bps line to  IMP  71  at
    BBN.    The  roundtrip  delay  via  this  path  between  the
    COMSAT-GAT  host  and  the  BBN  Gateway  is  typically  550
    milliseconds as the ICS datagram flies.

         As in the case of all DCNET hosts,  if  the  COMSAT-WWV
    virtual  host  is  down  (in  this case possible only if the
    Spectracom radio clock is down or misbehaving) a  "host  not
    reachable"   GGP   datagram   is   returned.    In   unusual
    circumstances a "net not reachable" or "source  quench"  GGP
    datagram  could  be  returned.   Note that the references to
    "GGP" here will be read "ICMP" at  some  appropriate  future
    time.

    Local Offset Corrections

         All DCNET timestamps are  referenced  to  a  designated
    virtual  host  called  COMSAT-WWV (what else?) with internet
    address [29,0,9,2].  This host is equipped with a Spectracom
    radio  clock  which  normally provides WWVB time and date to
    within a millisecond.  The clock  synchronization  mechanism
    provides  offset  and  drift  corrections  for  other  hosts
    relative to this host; however, offsets up to an appreciable
    fraction  of  a second routinely occur due to the difficulty
    of tracking with power-line  clocks  in  some  machines.   A
    table  of  the  current  offsets  can  be obtained using the
    following procedure.

    1.  Connect  to  COMSAT-GAT   host   at   internet   address
        [29,0,1,2] using TELNET and local echo.

    2.  Send the command SET HOST HOST.  A table with  one  line
        per DCNET host should be returned.  Note the entry under
        the "Offset" column for the WWV host.  This contains the
        offset  in  milliseconds  that  should  be  added to all
        timestamps  generated  by  either  the   COMSAT-GAT   or
        COMSAT-WWV  hosts to yield the correct time as broadcast
        by WWVB.

    3.  Send the command SET WWV SHOW.  A  summary  of  datagram
        traffic  is  returned  along with an entry labelled "NBS

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        time." The string  following  this  is  the  last  reply
        received from the Spectracom unit in the format:

                      <code>  DDD HH:MM:SS  TZ=00

        where <code> is normally <SP> in case the WWVB signal is
        being  received  correctly  or ? in case it is not.  The
        DDD represents the day of the year and HH:MM:SS the time
        past   UT   midnight.   The  two  digits  following  TZ=
        represent the time zone, here 00 for UT.

    4.  Close the connection (please!).

    REFERENCES

    [1]  ICMP

       Postel, J., "Internet Control Message Protocol", RFC 777,
       USC/Information Sciences Institute, April 1981.

    [2]  GGP

       Strazisar, V., "How to Build a Gateway", IEN 109, Bolt
       Beranek and Newman, August 1979.

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    Following is a specification of  the  ICS  header  in  PDP11
    code:

    ;
    ; GGP/ICMP Header
    ;
     .       =       0
    GH.TYP:  .BLKB   1               ;Message type
    GC.RPY   =       0               ;Echo reply
    GC.UPD   =       1               ;Routing update
    GC.ACK   =       2               ;Positive acknowledgment
    GC.DNR   =       3               ;Destination unreachable
    GC.SQN   =       4               ;Source quench
    GC.RDR   =       5               ;Redirect
    GC.ECH   =       10              ;Echo
    GC.STA   =       11              ;Net interface status
    GC.NAK   =       12              ;Negative acknowledgment
    GC.TIM   =       15              ;Timestamp
    GC.TRP   =       16              ;Timestamp Reply
    GH.COD:  .BLKB   1               ;Message code
    GH.SEQ:  .BLKW   1               ;Sequence number
    GH.HDR   =       .               ;Beginning of original
                                     ;internet header
    GH.ORG:  .BLKW   2               ;Originating timestamp
    GH.REC:  .BLKW   2               ;Received timestamp
    GH.XMT:  .BLKW   2               ;Transmitted timestamp
    GH.LEN   =       .               ;End of timestamp header

         Note that all  PDP11  word  fields  (.BLKW  above)  are
    "byte-swapped,"  that  is, the order of byte transmission is
    the high-order byte followed by the low-order  byte  of  the
    PDP11 word.