








           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 mas-
ter-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
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.
-----------
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 poli-
cies, either expressed or implied, of the  Defense
Research Projects Agency, of the US Government, or
of CSELT.









SMM:11-2                    Timed Installation and Operation


     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 net-
work time as the average of the times provided by  nonfaulty
clocks.1 It then sends to each slave time daemon the correc-
tion  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 implemen-
tation  keeps  processor  clocks synchronized within 20 mil-
liseconds.

     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 broad-
cast  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
master network is a network on which the submaster acts as a
master.  An ignored  network  is  any  other  network  which
already  has  a  valid master.  The submaster tries periodi-
cally to become master on an ignored network, but  gives  up
immediately if a master already exists.



-----------
  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].









Timed Installation and Operation                    SMM:11-3


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 dae-
mon 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 adminis-
trative 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/etc/timed/globals.h  limits the maximum
number of machines that may be directly  controlled  by  one
master time daemon.  The current maximum is 29 (NHOSTS - 1).
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  possible  for  submaster  time daemon A to be a slave on
network 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









SMM:11-4                    Timed Installation and Operation


properly  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 the local daemons
section in the file /etc/rc.local:


          if [ -f /etc/timed ]; then
               /etc/timed flags & echo -n ' timed' >/dev/console
          fi


In any case, they must appear after the network  is  config-
ured via ifconfig(8).

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


          timed          525/udp        timeserver


The flags are:

-n network   to consider the named network.

-i network   to ignore the named network.

-t           to     place     tracing     information     in
             /usr/adm/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.

Daily Operation

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

o    measure the differences between machines' clocks,

o    find the location where the master timed is running,

o    cause  election timers on several machines to expire at
     the same time,









Timed Installation and Operation                    SMM:11-5


o    enable or  disable  tracing  of  messages  received  by
     timed.

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).






















































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.









































