MirOS Manual: 11.timedop(SMM)


           Timed Installation and Operation Guide

      Riccardo Gusella, Stefano Zatti, James M. Bloom

              Computer Systems Research Group
                 Computer Science Division
 Department of Electrical Engineering and Computer Science
             University of California, Berkeley
                     Berkeley, CA 94720

                         Kirk Smith

                Engineering Computer Network
            Department of Electrical Engineering
                     Purdue University
                  West Lafayette, IN 47906

Introduction

     The clock synchronization service for the  UNIX  4.3BSD
operating system is composed of a collection of time daemons
(timed) running on the machines in a local area network. The
algorithms  implemented by the service is based on a master-
slave scheme. The time daemons communicate with  each  other
using the Time Synchronization Protocol (TSP) which is built
on the DARPA UDP protocol and described in detail in [4].

     A time daemon has a twofold function.  First,  it  sup-
ports the synchronization of the clocks of the various hosts
in a local area network. Second, it starts  (or  takes  part
in)  the election that occurs among slave time daemons when,
for any reason, the master disappears.  The  synchronization
mechanism and the election procedure employed by the program
_________________________
This  work  was  sponsored  by  the  Defense   Advanced
Research  Projects Agency (DoD), monitored by the Naval
Electronics Systems Command under contract No.  N00039-
84-C-0089,  and  by the CSELT Corporation of Italy. The
views and conclusions contained in  this  document  are
those  of  the authors and should not be interpreted as
representing official policies, either expressed or im-
plied,  of the Defense Research Projects Agency, of the
US Government, or of CSELT.

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SMM:11-2                    Timed Installation and Operation

timed are described in other  documents  [1,2,3].  The  next
paragraphs  are  a  brief  overview  of  how the time daemon
works. This document is mainly concerned with  the  adminis-
trative  and technical issues of running timed at a particu-
lar site.

     A master time  daemon  measures  the  time  differences
between  the clock of the machine on which it is running and
those of all other machines. The master computes the network
time  as  the  average  of  the  times provided by nonfaulty
clocks.[1] It then sends  to  each  slave  time  daemon  the
correction  that  should  be  performed  on the clock of its
machine. This process is repeated  periodically.  Since  the
correction  is expressed as a time difference rather than an
absolute time, transmission delays do not interfere with the
accuracy of the synchronization. When a machine comes up and
joins the network, it starts a slave time daemon which  will
ask  the  master  for  the  correct  time and will reset the
machine's clock before any user activity can begin. The time
daemons  are able to maintain a single network time in spite
of the drift of clocks away from  each  other.  The  present
implementation keeps processor clocks synchronized within 20
milliseconds.

     To ensure that the service provided is  continuous  and
reliable, it is necessary to implement an election algorithm
to elect a new master should the machine running the current
master  crash, the master terminate (for example, because of
a run-time error), or the network be partitioned. Under  our
algorithm,  slaves  are  able to realize when the master has
stopped functioning and to elect a  new  master  from  among
themselves.  It is important to note that, since the failure
of the master results only in a gradual divergence of  clock
values, the election need not occur immediately.

     The machines that are gateways between  distinct  local
area networks require particular care. A time daemon on such
machines may act as a submaster. This  artifact  depends  on
the current inability of transmission protocols to broadcast
a message on a network other  than  the  one  to  which  the
broadcasting  machine is connected. The submaster appears as
a slave on one network, and as a master on one  or  more  of
the other networks to which it is connected.

     A submaster classifies each network  as  one  of  three
types.  A  slave network is a network on which the submaster
acts as a slave. There can only be one slave network. A mas-
ter  network  is  a network on which the submaster acts as a
master. An  ignored  network  is  any  other  network  which
_________________________
  [1] A clock is considered to be faulty when its value
is  more than a small specified interval apart from the
majority of the clocks of the other machines [1,2].

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Timed Installation and Operation                    SMM:11-3

already has a valid master. The submaster tries periodically
to become master on an ignored network, but gives up immedi-
ately if a master already exists.

Guidelines

     While the synchronization algorithm is  quite  general,
the election one, requiring a broadcast mechanism, puts con-
straints on the kind of network on which  time  daemons  can
run.  The time daemon will only work on networks with broad-
cast  capability  augmented   with   point-to-point   links.
Machines  that  are  only  connected to point-to-point, non-
broadcast networks may not use the time daemon.

     If we exclude submasters, there will  normally  be,  at
most,  one  master time daemon in a local area internetwork.
During an election, only one of the slave time daemons  will
become  the new master. However, because of the characteris-
tics of its machine, a slave can be prevented from  becoming
the  master.  Therefore, a subset of machines must be desig-
nated as potential master time daemons. A master time daemon
will  require  CPU  resources  proportional to the number of
slaves, in general, more than a slave time daemon, so it may
be  advisable  to limit master time daemons to machines with
more powerful processors or lighter  loads.  Also,  machines
with  inaccurate  clocks should not be used as masters. This
is a purely administrative  decision:  an  organization  may
well allow all of its machines to run master time daemons.

     At the administrative level, a time daemon on a machine
with  multiple network interfaces, may be told to ignore all
but one network or to ignore one network. This is done  with
the  -n  network  and  -i  network  options  respectively at
start-up  time.  Typically,  the  time   daemon   would   be
instructed  to  ignore all but the networks belonging to the
local administrative control.

     There are some limitations to the  current  implementa-
tion  of  the time daemon. It is expected that these limita-
tions will be  removed  in  future  releases.  The  constant
NHOSTS in /usr/src/usr.sbin/timed/timed/globals.h limits the
maximum number of machines that may be  directly  controlled
by  one master time daemon. The current maximum is 1013. The
constant  must be changed and the program  recompiled  if  a
site wishes to run timed on a larger (inter)network.

     In addition, there is a pathological  situation  to  be
avoided at all costs, that might occur when time daemons run
on multiply-connected local area networks. In this case,  as
we  have seen, time daemons running on gateway machines will
be submasters and they will act on some of those networks as
master time daemons. Consider machines A and B that are both
gateways between networks X and  Y.  If  time  daemons  were
started  on  both  A  and B without constraints, it would be

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SMM:11-4                    Timed Installation and Operation

possible for submaster time daemon A to be a slave  on  net-
work  X  and  the  master on network Y, while submaster time
daemon B is a slave on network Y and the master  on  network
X.  This loop of master time daemons will not function prop-
erly or guarantee a unique time on both networks,  and  will
cause   the  submasters  to  use  large  amounts  of  system
resources in the form of network bandwidth and CPU time.  In
fact, this kind of loop can also be generated with more than
two master time daemons, when several  local  area  networks
are interconnected.

Installation

     In order to start the time daemon on a  given  machine,
the following lines should be added to /etc/rc.conf.local:

          timed_flags=""

     Also, the file /etc/services should contain the follow-
ing line:

          timed           525/udp         timeserver

Some of the most commonly used flags are:

-n network   to add network to the list of allowed networks.

-i network   to ignore the named network.

-t           to     place     tracing     information     in
             /var/log/timed.log.

-M           to allow this time daemon to become a master. A
             time  daemon  run  without  this option will be
             forced in the state of slave  during  an  elec-
             tion.

See the timed(8) manual page for  more  information  on  the
various flags.

Daily Operation

     Timedc(8) is used to control the operation of the  time
daemon. It may be used to:

+    measure the differences between machines' clocks

+    find the location where the master timed is running

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Timed Installation and Operation                    SMM:11-5

+    enable or disable tracing of messages received by timed

+    perform various debugging actions

See the manual page  on  timed(8)  and  timedc(8)  for  more
detailed information.

     The date(1) command can be  used  to  set  the  network
date.  In  order to set the time on a single machine, the -n
flag can be given to date(1).

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SMM:11-6                    Timed Installation and Operation

References

1.   R. Gusella and S. Zatti, TEMPO:  A  Network  Time  Con-
     troller  for  Distributed  Berkeley UNIX System, USENIX
     Summer Conference Proceedings,  Salt  Lake  City,  June
     1984.

2.   R. Gusella and S. Zatti,  Clock  Synchronization  in  a
     Local Area Network, University of California, Berkeley,
     Technical Report, to appear.

3.   R. Gusella and S. Zatti, An Election  Algorithm  for  a
     Distributed  Clock  Synchronization Program, University
     of California, Berkeley, CS Technical Report #275, Dec.
     1985.

4.   R. Gusella and S. Zatti, The Berkeley UNIX 4.3BSD  Time
     Synchronization Protocol, UNIX Programmer's Manual, 4.3
     Berkeley Software Distribution, Volume 2c.

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