280 lines
12 KiB
Plaintext
280 lines
12 KiB
Plaintext
|
.\" Copyright (c) 1986, 1993
|
||
|
|
.\" The Regents of the University of California. All rights reserved.
|
||
|
|
.\"
|
||
|
|
.\" Redistribution and use in source and binary forms, with or without
|
||
|
|
.\" modification, are permitted provided that the following conditions
|
||
|
|
.\" are met:
|
||
|
|
.\" 1. Redistributions of source code must retain the above copyright
|
||
|
|
.\" notice, this list of conditions and the following disclaimer.
|
||
|
|
.\" 2. Redistributions in binary form must reproduce the above copyright
|
||
|
|
.\" notice, this list of conditions and the following disclaimer in the
|
||
|
|
.\" documentation and/or other materials provided with the distribution.
|
||
|
|
.\" 3. All advertising materials mentioning features or use of this software
|
||
|
|
.\" must display the following acknowledgement:
|
||
|
|
.\" This product includes software developed by the University of
|
||
|
|
.\" California, Berkeley and its contributors.
|
||
|
|
.\" 4. Neither the name of the University nor the names of its contributors
|
||
|
|
.\" may be used to endorse or promote products derived from this software
|
||
|
|
.\" without specific prior written permission.
|
||
|
|
.\"
|
||
|
|
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
|
||
|
|
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
|
||
|
|
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
|
||
|
|
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
|
||
|
|
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
|
||
|
|
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
|
||
|
|
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
|
||
|
|
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
|
||
|
|
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
|
||
|
|
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
|
||
|
|
.\" SUCH DAMAGE.
|
||
|
|
.\"
|
||
|
|
.\" @(#)timed.ms 8.1 (Berkeley) 6/8/93
|
||
|
|
.\"
|
||
|
|
.TL
|
||
|
|
Timed Installation and Operation Guide
|
||
|
|
.AU
|
||
|
|
Riccardo Gusella, Stefano Zatti, James M. Bloom
|
||
|
|
.AI
|
||
|
|
Computer Systems Research Group
|
||
|
|
Computer Science Division
|
||
|
|
Department of Electrical Engineering and Computer Science
|
||
|
|
University of California, Berkeley
|
||
|
|
Berkeley, CA 94720
|
||
|
|
.AU
|
||
|
|
Kirk Smith
|
||
|
|
.AI
|
||
|
|
Engineering Computer Network
|
||
|
|
Department of Electrical Engineering
|
||
|
|
Purdue University
|
||
|
|
West Lafayette, IN 47906
|
||
|
|
.FS
|
||
|
|
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 implied, of the Defense Research Projects Agency,
|
||
|
|
of the US Government, or of CSELT.
|
||
|
|
.FE
|
||
|
|
.LP
|
||
|
|
.EH 'SMM:11-%''Timed Installation and Operation'
|
||
|
|
.OH 'Timed Installation and Operation''SMM:11-%'
|
||
|
|
.SH
|
||
|
|
Introduction
|
||
|
|
.PP
|
||
|
|
The clock synchronization service for
|
||
|
|
the UNIX 4.3BSD operating system is composed of a collection of
|
||
|
|
time daemons (\fItimed\fP) 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
|
||
|
|
\fITime Synchronization Protocol\fP (TSP) which
|
||
|
|
is built on the DARPA UDP protocol and described in detail in [4].
|
||
|
|
.PP
|
||
|
|
A time daemon has a twofold function.
|
||
|
|
First, it supports 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 \fItimed\fP 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 administrative and technical
|
||
|
|
issues of running \fItimed\fP at a particular site.
|
||
|
|
.PP
|
||
|
|
A \fImaster time daemon\fP 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 \fInetwork time\fP as the average of the
|
||
|
|
times provided by nonfaulty clocks.\**
|
||
|
|
.FS
|
||
|
|
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].
|
||
|
|
.FE
|
||
|
|
It then sends to each \fIslave time daemon\fP 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.
|
||
|
|
.PP
|
||
|
|
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.
|
||
|
|
.PP
|
||
|
|
The machines that are gateways between distinct local area
|
||
|
|
networks require particular care.
|
||
|
|
A time daemon on such machines may act as a \fIsubmaster\fP.
|
||
|
|
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.
|
||
|
|
.PP
|
||
|
|
A submaster classifies each network as one of three types.
|
||
|
|
A \fIslave network\fP is a network on which the submaster acts as a slave.
|
||
|
|
There can only be one slave network.
|
||
|
|
A \fImaster network\fP is a network on which the submaster acts as a master.
|
||
|
|
An \fIignored network\fP is any other network which already has a valid master.
|
||
|
|
The submaster tries periodically to become master on an ignored
|
||
|
|
network, but gives up immediately if a master already exists.
|
||
|
|
.SH
|
||
|
|
Guidelines
|
||
|
|
.PP
|
||
|
|
While the synchronization algorithm is quite general, the election
|
||
|
|
one, requiring a broadcast mechanism, puts constraints on
|
||
|
|
the kind of network on which time daemons can run.
|
||
|
|
The time daemon will only work on networks with broadcast 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.
|
||
|
|
.PP
|
||
|
|
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 characteristics of its machine,
|
||
|
|
a slave can be prevented from becoming the master.
|
||
|
|
Therefore, a subset of machines must be designated 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.
|
||
|
|
.PP
|
||
|
|
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 \fI\-n network\fP and \fI\-i network\fP
|
||
|
|
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.
|
||
|
|
.PP
|
||
|
|
There are some limitations to the current
|
||
|
|
implementation of the time daemon.
|
||
|
|
It is expected that these limitations 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 \fItimed\fP on a larger (inter)network.
|
||
|
|
.PP
|
||
|
|
In addition, there is a \fIpathological situation\fP 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 \fIloop\fP of master time daemons will not function 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 \fIloop\fP can also be generated with more
|
||
|
|
than two master time daemons,
|
||
|
|
when several local area networks are interconnected.
|
||
|
|
.SH
|
||
|
|
Installation
|
||
|
|
.PP
|
||
|
|
In order to start the time daemon on a given machine,
|
||
|
|
the following lines should be
|
||
|
|
added to the \fIlocal daemons\fP section in the file \fI/etc/rc.local\fP:
|
||
|
|
.sp 2
|
||
|
|
.in 1i
|
||
|
|
.nf
|
||
|
|
if [ -f /etc/timed ]; then
|
||
|
|
/etc/timed \fIflags\fP & echo -n ' timed' >/dev/console
|
||
|
|
fi
|
||
|
|
.fi
|
||
|
|
.in -1i
|
||
|
|
.sp
|
||
|
|
.LP
|
||
|
|
In any case, they must appear after the network
|
||
|
|
is configured via ifconfig(8).
|
||
|
|
.PP
|
||
|
|
Also, the file \fI/etc/services\fP should contain the following
|
||
|
|
line:
|
||
|
|
.sp 2
|
||
|
|
.ti 1i
|
||
|
|
timed 525/udp timeserver
|
||
|
|
.sp
|
||
|
|
.LP
|
||
|
|
The \fIflags\fP are:
|
||
|
|
.IP "-n network" 13
|
||
|
|
to consider the named network.
|
||
|
|
.IP "-i network"
|
||
|
|
to ignore the named network.
|
||
|
|
.IP -t
|
||
|
|
to place tracing information in \fI/usr/adm/timed.log\fP.
|
||
|
|
.IP -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 election.
|
||
|
|
.SH
|
||
|
|
Daily Operation
|
||
|
|
.PP
|
||
|
|
\fITimedc(8)\fP is used to control the operation of the time daemon.
|
||
|
|
It may be used to:
|
||
|
|
.IP \(bu
|
||
|
|
measure the differences between machines' clocks,
|
||
|
|
.IP \(bu
|
||
|
|
find the location where the master \fItimed\fP is running,
|
||
|
|
.IP \(bu
|
||
|
|
cause election timers on several machines to expire at the same time,
|
||
|
|
.IP \(bu
|
||
|
|
enable or disable tracing of messages received by \fItimed\fP.
|
||
|
|
.LP
|
||
|
|
See the manual page on \fItimed\fP\|(8) and \fItimedc\fP\|(8)
|
||
|
|
for more detailed information.
|
||
|
|
.PP
|
||
|
|
The \fIdate(1)\fP command can be used to set the network date.
|
||
|
|
In order to set the time on a single machine, the \fI-n\fP flag
|
||
|
|
can be given to date(1).
|
||
|
|
.bp
|
||
|
|
.SH
|
||
|
|
References
|
||
|
|
.IP 1.
|
||
|
|
R. Gusella and S. Zatti,
|
||
|
|
\fITEMPO: A Network Time Controller for Distributed Berkeley UNIX System\fP,
|
||
|
|
USENIX Summer Conference Proceedings, Salt Lake City, June 1984.
|
||
|
|
.IP 2.
|
||
|
|
R. Gusella and S. Zatti, \fIClock Synchronization in a Local Area Network\fP,
|
||
|
|
University of California, Berkeley, Technical Report, \fIto appear\fP.
|
||
|
|
.IP 3.
|
||
|
|
R. Gusella and S. Zatti,
|
||
|
|
\fIAn Election Algorithm for a Distributed Clock Synchronization Program\fP,
|
||
|
|
University of California, Berkeley, CS Technical Report #275, Dec. 1985.
|
||
|
|
.IP 4.
|
||
|
|
R. Gusella and S. Zatti,
|
||
|
|
\fIThe Berkeley UNIX 4.3BSD Time Synchronization Protocol\fP,
|
||
|
|
UNIX Programmer's Manual, 4.3 Berkeley Software Distribution, Volume 2c.
|