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NAME    [Toc]    [Back]

       ntp_intro  -  Network  Time  Protocol  (NTP)  introductory

DESCRIPTION    [Toc]    [Back]

       The Tru64 UNIX software supports two time  services:  Network
  Time  Protocol  (NTP)  Time Synchronization Protocol

       Because it can be traced to clocks of high absolute  accuracy,
  NTP provides a more accurate time service than TSP.
       By contrast, TSP synchronizes time to the average  of  the
       network  host times.  TSP is an acceptable time service if
       your system is not on  the  Internet  and  does  not  have
       access to a highly accurate time server; otherwise, NTP is

   Network Time Protocol    [Toc]    [Back]
       The Network Time Protocol (NTP) provides accurate, dependable,
  and  synchronized  time for hosts on both wide area
       networks (like the Internet) and local area  networks.  In
       particular,  NTP  provides  synchronization  traceable  to
       clocks of high absolute accuracy, and avoids  synchronization
 to clocks keeping bad time.

       Hosts running NTP periodically exchange datagrams querying
       each other about  their  current  estimate  of  the  time.
       Using  the round-trip time of the packet, a host can estimate
 the one-way delay to the other host.  (The assumption
       is that the delay is roughly equal in both directions.) By
       measuring the one-way delay and examining  the  timestamps
       that are returned with the NTP packet, a host computes the
       difference between its clock time and that of the host  it

       A  host  queries a remote host several times over a period
       and feeds the results from the multiple samples to a digital-filtering
  algorithm.   The  algorithm provides a more
       accurate estimate of the delay, clock  offset,  and  clock
       stability than could be obtained with a single sample.

       NTP  messages  also contain information about the accuracy
       and reliability of the time sources.   An  NTP  host  connected
  directly to a highly accurate time source, such as
       a radio receiver tuned to a time code signal broadcast  by
       a  government agency, is called a stratum 1 server.  Every
       other NTP host adopts a stratum number that is one  higher
       than  the host from which it sets its own time.  For example,
 a host synchronized to a stratum 1 server  becomes  a
       stratum  2  host.  Stratum determination is done automatically,
 and the stratum of a host can vary as  its  connectivity

       A  host running NTP combines various information to decide
       which of  the  hosts  it  queried  provides  the  time  it
       believes  to  be  the  most  accurate.   This  information
       includes the output of the digital-filtering algorithm and
       the  stratum numbers of the hosts it queried.  By communicating
 with several other hosts, an NTP host  can  usually
       detect  those hosts that are keeping bad time, and is able
       to stay synchronized even  if  some  of  the  other  hosts
       become unavailable for long periods.

       In practice, NTP is able to synchronize clocks to within a
       few milliseconds even over  wide  area  networks  spanning
       thousands  of  miles. To obtain even greater accuracy, use
       the NTP_TIME kernel option.  See System Administration for
       more information.

       You  can  optionally use a high-resolution clock, enabling
       the time returned  by  the  clock_gettime  routine  to  be
       extrapolated  between the clock ticks.  The granularity of
       the time returned will be  in  microseconds.  To  use  the
       high-resolution clock, use the MICRO_TIME kernel option.

       For detailed information on NTP, see Network Time Protocol
       (Version 3) (RFC 1305).

   Time Synchronization Protocol    [Toc]    [Back]
       The Time Synchronization Protocol (TSP)  is  the  protocol
       used  by  the  /usr/sbin/timed  daemon.   In  its simplest
       application, the TSP servers on a broadcast  network  (for
       example,  an Ethernet) periodically broadcast TSP packets.
       The hosts on the network elect one of  the  hosts  on  the
       network running TSP as a master.  The master then controls
       the further operation of the protocol until it fails and a
       new  master  is  elected.  The master collects time values
       from the other hosts and computes the average of  all  the
       times  reported.  It then sets its own clock to this average,
 and tells the other hosts to synchronize their clocks
       with it.

       TSP  quickly  synchronizes  all participating hosts.  However,
 because TSP does not trace time back to  sources  of
       known  accuracy,  it  is  unable to correct for systematic
       errors.  If a clock drifts significantly, or if a  mistake
       is  made  in setting the time on a participating host, the
       average time calculated and distributed by the master  can
       be affected significantly.

       For  information  on setting up the network time services,
       refer to Network Administration: Services.

SEE ALSO    [Toc]    [Back]

       Commands:ntp(1), ntpdate(8),  ntptrace(8),  timedsetup(8),

       Functions: ctime(3)

       Files: ntp.conf(4), ntp.drift(4), ntp.keys(4)

       Daemons: timed(8), xntpd(8)

       Network Time Protocol (Version 3) (RFC 1305)

       Network Administration: Services System Administration

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