DCNET Internet Clock Service
RFC 778

Document Type RFC - Historic (April 1981; No errata)
Last updated 2013-03-02
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RFC 778

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


     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

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:

DCNET Internet Clock Service                        PAGE   2

 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

DCNET Internet Clock Service                        PAGE   3

(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
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