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1.1 root 1: .\" Copyright (c) 1986 The Regents of the University of California.
2: .\" All rights reserved.
3: .\"
4: .\" Redistribution and use in source and binary forms are permitted
5: .\" provided that the above copyright notice and this paragraph are
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7: .\" advertising materials, and other materials related to such
8: .\" distribution and use acknowledge that the software was developed
9: .\" by the University of California, Berkeley. The name of the
10: .\" University may not be used to endorse or promote products derived
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12: .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
13: .\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
14: .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
15: .\"
16: .\" @(#)timed.ms 6.2 (Berkeley) 3/7/89
17: .\"
18: .TL
19: Timed Installation and Operation Guide
20: .AU
21: Riccardo Gusella, Stefano Zatti, James M. Bloom
22: .AI
23: Computer Systems Research Group
24: Computer Science Division
25: Department of Electrical Engineering and Computer Science
26: University of California, Berkeley
27: Berkeley, CA 94720
28: .AU
29: Kirk Smith
30: .AI
31: Engineering Computer Network
32: Department of Electrical Engineering
33: Purdue University
34: West Lafayette, IN 47906
35: .FS
36: This work was sponsored by the Defense Advanced Research Projects Agency
37: (DoD), monitored by the Naval Electronics Systems
38: Command under contract No. N00039-84-C-0089, and by the CSELT
39: Corporation of Italy.
40: The views and conclusions contained in this document are those of the
41: authors and should not be interpreted as representing official policies,
42: either expressed or implied, of the Defense Research Projects Agency,
43: of the US Government, or of CSELT.
44: .FE
45: .LP
46: .EH 'SMM:8-%''Timed Installation and Operation'
47: .OH 'Timed Installation and Operation''SMM:8-%'
48: .SH
49: Introduction
50: .PP
51: The clock synchronization service for
52: the UNIX 4.3BSD operating system is composed of a collection of
53: time daemons (\fItimed\fP) running on the machines in a local
54: area network.
55: The algorithms implemented by the service is based on a master-slave scheme.
56: The time daemons communicate with each other using the
57: \fITime Synchronization Protocol\fP (TSP) which
58: is built on the DARPA UDP protocol and described in detail in [4].
59: .PP
60: A time daemon has a twofold function.
61: First, it supports the synchronization of the clocks
62: of the various hosts in a local area network.
63: Second, it starts (or takes part in) the election that occurs
64: among slave time daemons when, for any reason, the master disappears.
65: The synchronization mechanism and the election procedure
66: employed by the program \fItimed\fP are described
67: in other documents [1,2,3].
68: The next paragraphs are a brief overview of how the time daemon works.
69: This document is mainly concerned with the administrative and technical
70: issues of running \fItimed\fP at a particular site.
71: .PP
72: A \fImaster time daemon\fP measures the time
73: differences between the clock of the machine on which it
74: is running and those of all other machines.
75: The master computes the \fInetwork time\fP as the average of the
76: times provided by nonfaulty clocks.\**
77: .FS
78: A clock is considered to be faulty when its value
79: is more than a small specified
80: interval apart from the majority of the clocks
81: of the other machines [1,2].
82: .FE
83: It then sends to each \fIslave time daemon\fP the
84: correction that should be performed on the clock of its machine.
85: This process is repeated periodically.
86: Since the correction is expressed as a time difference rather than an
87: absolute time, transmission delays do not interfere with
88: the accuracy of the synchronization.
89: When a machine comes up and joins the network,
90: it starts a slave time daemon which
91: will ask the master for the correct time and will reset the machine's clock
92: before any user activity can begin.
93: The time daemons are able to maintain a single network time in spite of
94: the drift of clocks away from each other.
95: The present implementation keeps processor clocks synchronized
96: within 20 milliseconds.
97: .PP
98: To ensure that the service provided is continuous and reliable,
99: it is necessary to implement an election algorithm to elect a
100: new master should the machine running the current master crash, the master
101: terminate (for example, because of a run-time error), or
102: the network be partitioned.
103: Under our algorithm, slaves are able to realize when the master has
104: stopped functioning and to elect a new master from among themselves.
105: It is important to note that, since the failure of the master results
106: only in a gradual divergence of clock values, the election
107: need not occur immediately.
108: .PP
109: The machines that are gateways between distinct local area
110: networks require particular care.
111: A time daemon on such machines may act as a \fIsubmaster\fP.
112: This artifact depends on the current inability of
113: transmission protocols to broadcast a message on a network
114: other than the one to which the broadcasting machine is connected.
115: The submaster appears as a slave on one network, and as a master
116: on one or more of the other networks to which it is connected.
117: .PP
118: A submaster classifies each network as one of three types.
119: A \fIslave network\fP is a network on which the submaster acts as a slave.
120: There can only be one slave network.
121: A \fImaster network\fP is a network on which the submaster acts as a master.
122: An \fIignored network\fP is any other network which already has a valid master.
123: The submaster tries periodically to become master on an ignored
124: network, but gives up immediately if a master already exists.
125: .SH
126: Guidelines
127: .PP
128: While the synchronization algorithm is quite general, the election
129: one, requiring a broadcast mechanism, puts constraints on
130: the kind of network on which time daemons can run.
131: The time daemon will only work on networks with broadcast capability
132: augmented with point-to-point links.
133: Machines that are only connected to point-to-point,
134: non-broadcast networks may not use the time daemon.
135: .PP
136: If we exclude submasters, there will normally be, at most, one master time
137: daemon in a local area internetwork.
138: During an election, only one of the slave time daemons
139: will become the new master.
140: However, because of the characteristics of its machine,
141: a slave can be prevented from becoming the master.
142: Therefore, a subset of machines must be designated as potential
143: master time daemons.
144: A master time daemon will require CPU resources
145: proportional to the number of slaves, in general, more than
146: a slave time daemon, so it may be advisable to limit master time
147: daemons to machines with more powerful processors or lighter loads.
148: Also, machines with inaccurate clocks should not be used as masters.
149: This is a purely administrative decision: an organization may
150: well allow all of its machines to run master time daemons.
151: .PP
152: At the administrative level, a time daemon on a machine
153: with multiple network interfaces, may be told to ignore all
154: but one network or to ignore one network.
155: This is done with the \fI\-n network\fP and \fI\-i network\fP
156: options respectively at start-up time.
157: Typically, the time daemon would be instructed to ignore all but
158: the networks belonging to the local administrative control.
159: .PP
160: There are some limitations to the current
161: implementation of the time daemon.
162: It is expected that these limitations will be removed in future releases.
163: The constant NHOSTS in /usr/src/etc/timed/globals.h limits the
164: maximum number of machines that may be directly controlled by one
165: master time daemon.
166: The current maximum is 29 (NHOSTS \- 1).
167: The constant must be changed and the program recompiled if a site wishes to
168: run \fItimed\fP on a larger (inter)network.
169: .PP
170: In addition, there is a \fIpathological situation\fP to
171: be avoided at all costs, that might occur when
172: time daemons run on multiply-connected local area networks.
173: In this case, as we have seen, time daemons running on gateway machines
174: will be submasters and they will act on some of those
175: networks as master time daemons.
176: Consider machines A and B that are both gateways between
177: networks X and Y.
178: If time daemons were started on both A and B without constraints, it would be
179: possible for submaster time daemon A to be a slave on network X
180: and the master on network Y, while submaster time daemon B is a slave on
181: network Y and the master on network X.
182: This \fIloop\fP of master time daemons will not function properly
183: or guarantee a unique time on both networks, and will cause
184: the submasters to use large amounts of system resources in the form
185: of network bandwidth and CPU time.
186: In fact, this kind of \fIloop\fP can also be generated with more
187: than two master time daemons,
188: when several local area networks are interconnected.
189: .SH
190: Installation
191: .PP
192: In order to start the time daemon on a given machine,
193: the following lines should be
194: added to the \fIlocal daemons\fP section in the file \fI/etc/rc.local\fP:
195: .sp 2
196: .in 1i
197: .nf
198: if [ -f /etc/timed ]; then
199: /etc/timed \fIflags\fP & echo -n ' timed' >/dev/console
200: fi
201: .fi
202: .in -1i
203: .sp
204: .LP
205: In any case, they must appear after the network
206: is configured via ifconfig(8).
207: .PP
208: Also, the file \fI/etc/services\fP should contain the following
209: line:
210: .sp 2
211: .ti 1i
212: timed 525/udp timeserver
213: .sp
214: .LP
215: The \fIflags\fP are:
216: .IP "-n network" 13
217: to consider the named network.
218: .IP "-i network"
219: to ignore the named network.
220: .IP -t
221: to place tracing information in \fI/usr/adm/timed.log\fP.
222: .IP -M
223: to allow this time daemon to become a master.
224: A time daemon run without this option will be forced in the state of
225: slave during an election.
226: .SH
227: Daily Operation
228: .PP
229: \fITimedc(8)\fP is used to control the operation of the time daemon.
230: It may be used to:
231: .IP \(bu
232: measure the differences between machines' clocks,
233: .IP \(bu
234: find the location where the master \fItimed\fP is running,
235: .IP \(bu
236: cause election timers on several machines to expire at the same time,
237: .IP \(bu
238: enable or disable tracing of messages received by \fItimed\fP.
239: .LP
240: See the manual page on \fItimed\fP\|(8) and \fItimedc\fP\|(8)
241: for more detailed information.
242: .PP
243: The \fIdate(1)\fP command can be used to set the network date.
244: In order to set the time on a single machine, the \fI-n\fP flag
245: can be given to date(1).
246: .bp
247: .SH
248: References
249: .IP 1.
250: R. Gusella and S. Zatti,
251: \fITEMPO: A Network Time Controller for Distributed Berkeley UNIX System\fP,
252: USENIX Summer Conference Proceedings, Salt Lake City, June 1984.
253: .IP 2.
254: R. Gusella and S. Zatti, \fIClock Synchronization in a Local Area Network\fP,
255: University of California, Berkeley, Technical Report, \fIto appear\fP.
256: .IP 3.
257: R. Gusella and S. Zatti,
258: \fIAn Election Algorithm for a Distributed Clock Synchronization Program\fP,
259: University of California, Berkeley, CS Technical Report #275, Dec. 1985.
260: .IP 4.
261: R. Gusella and S. Zatti,
262: \fIThe Berkeley UNIX 4.3BSD Time Synchronization Protocol\fP,
263: UNIX Programmer's Manual, 4.3 Berkeley Software Distribution, Volume 2c.
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