Computer network time synchronization : the Network Time Protocol on Earth and in space

Carefully coordinated, reliable, and accurate time synchronization is vital to a wide spectrum of fields--from air and ground traffic control, to buying and selling goods and services, to TV network programming. Ill-gotten time could even lead to the unimaginable and cause DNS caches to expire, leaving the entire Internet to implode on the root servers.

Written by the original developer of the Network Time Protocol (NTP), Computer Network Time Synchronization: The Network Time Protocol on Earth and in Space, Second Edition addresses the technological infrastructure of time dissemination, distribution, and synchronization--specifically the architecture, protocols, and algorithms of the NTP. This system has been active in one form or another for almost three decades on the Internet and numerous private networks on the nether side of firewalls. Just about everything today that can be connected to a network wire has support for NTP.

This book:

Describes the principal components of an NTP client and how it works with redundant servers and diverse network paths Provides an in-depth description of cryptographic and other critical algorithms Presents an overview of the engineering principles guiding network configuration

Evaluating historic events that have taken place since computer network timekeeping started almost three decades ago, the author details a number of systems and drivers for current radio, satellites, and telephone modem dissemination and explains how we reckon the time, according to the stars and atoms. The original 16 chapters of the first edition have been rewritten, updated, and enhanced with new material. Four new chapters cover new algorithms and previously uncovered concepts, including timekeeping in space missions.

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Praise for the first edition:

"... For those that need an exhaustive tome on all of the minutiae related to NTP and synchronization, this is the source. ... definitive ... this book should be considered the last word on the topic."
--Ben Rothke on Slashdot.org

"... the bible of the subject... contains enough information to take you just as far as you want to go....Dr. Mills is the original developer of NTP."
--Books On-Line

Author Notes

Dr. David L. Mills is professor emeritus of electrical and computer engineering and computer and information sciences at the University of Delaware. He has been an active contributor for many years to the field of Internet technology, particularly computer network time synchronization. He is the original developer of the Network Time Protocol and has authored over 30 articles and technical reports on the subject, including the current operative standards documents. His doctoral degree in computer science was conferred by the University of Michigan in 1971. He is a member of the National Academy of Engineering and a Fellow in both the Association for Computing Machinery and the Institute of Electrical and Electronics Engineers.

List of Illustrations	p. xv
List of Tables	p. xxi
Preface	p. xxiii
About the Author	p. xxvii
1 Basic Concepts	p. 1
1.1 Time Synchronization	p. 1
1.2 Time Synchronization Protocols	p. 3
1.3 Computer Clocks	p. 5
1.4 Processing Time Values	p. 7
1.5 Correctness and Accuracy Expectations	p. 8
1.6 Security	p. 10
1.7 NTP in the Internet	p. 12
1.8 Parting Shots	p. 13
References	p. 14
Further Reading	p. 14
2 How NTP Works	p. 15
2.1 General Infrastructure Requirements	p. 16
2.2 How NTP Represents the Time	p. 17
2.3 How NTP Reckons the Time	p. 19
2.4 How NTP Disciplines the Time	p. 21
2.5 How NTP Manages Associations	p. 22
2.6 How NTP Discovers Servers	p. 25
2.7 How NTP Deals with Stale Time Values	p. 26
2.8 How NTP Manages Network Resources	p. 27
2.9 How NTP Avoids Errors	p. 28
2.10 How NTP Performance Is Determined	p. 30
2.11 How NTP Controls Access	p. 31
2.12 How NTP Watches for Terrorists	p. 32
2.13 How NTP Clocks Are Watched	p. 33
2.14 Parting Shots	p. 34
References	p. 35
Further Reading	p. 35
3 In the Belly of the Beast	p. 37
3.1 Related Technology	p. 37
3.2 Terms and Notation	p. 40
3.3 Process Flow	p. 41
3.4 Peer Process	p. 43
3.5 Poll Process	p. 44
3.6 On-Wire Protocol	p. 46
3.7 Clock Filter Algorithm	p. 48
3.8 Select Algorithm	p. 51
3.9 Cluster Algorithm	p. 56
3.10 Combine Algorithm	p. 59
3.11 Mitigation Rules and the Prefer Peer	p. 60
3.11.1 The Prefer Peer	p. 61
3.11.2 Peer Classification	p. 61
3.11.3 Mitigation Rules	p. 62
3.11.4 The minsane Option	p. 63
3.12 Huff-'n-Puff Filter	p. 64
3.13 Orphan Mode	p. 65
3.14 Rate Control and the Kiss-o'-Death Packet	p. 67
3.14.1 Average Headway and Guard Time	p. 68
3.14.2 The Kiss-o'-Death Packet	p. 69
3.15 Parting Shots	p. 70
References	p. 70
Further Reading	p. 72
4 Clock Discipline Algorithm	p. 73
4.1 Feedback Control Systems	p. 74
4.2 Phase and Frequency Discipline	p. 76
4.3 Time and Frequency Response	p. 78
4.4 Poll Interval Control	p. 81
4.5 Popcorn and Step Control	p. 82
4.6 Clock State Machine	p. 84
4.7 Parting Shots	p. 86
References	p. 87
Further Reading	p. 87
5 NTP System Engineering and Configuration	p. 89
5.1 Core Engineering Principles	p. 90
5.2 Engineering Large Corporate and Campus NTP Systems	p. 92
5.3 Examples of Large Corporate NTP Systems	p. 93
5.4 Engineering Home Office and Small Business NTP Systems	p. 96
5.5 Network Considerations	p. 97
5.6 General Configuration Considerations	p. 98
5.7 Manual Server Discovery	p. 100
5.8 Automatic Server Discovery	p. 101
5.8.1 Broadcast/Multicast Discovery	p. 102
5.8.2 NTP Manycast Discovery	p. 103
5.8.3 NTP Pool Discovery	p. 103
5.9 Configuration Commands and Options	p. 105
5.9.1 Association Configuration Options (server, peer, and the Like)	p. 105
5.9.2 Type of Service Options (tos)	p. 107
5.9.3 Tinker Options (tinker)	p. 109
5.10 Parting Shots	p. 110
Further Reading	p. 110
6 NTP Performance in the Internet	p. 111
6.1 Performance Measurement Tools	p. 112
6.2 System Clock Latency Characteristics	p. 113
6.3 Characteristics of a Primary Server and Reference Clock	p. 114
6.4 Characteristics between Primary Servers on the Internet	p. 117
6.5 Characteristics of a Client and a Primary Server on a Fast Ethernet	p. 123
6.6 Results from an Internet Survey	p. 125
6.7 Server and Network Resource Requirements	p. 128
6.8 Parting Shots	p. 130
References	p. 130
7 Primary Servers and Reference Clocks	p. 131
7.1 Driver Structure and Interface	p. 132
7.2 Serial Timecode Drivers	p. 135
7.3 Generic Modem Driver	p. 137
7.4 Local Clock Driver	p. 138
7.5 PPS Interface and Driver	p. 139
7.6 Audio Drivers	p. 140
7.6.1 IRIG Driver	p. 142
7.6.2 WWV/H Driver	p. 142
7.6.3 CHU Driver	p. 145
References	p. 147
8 Kernel Timekeeping Support	p. 149
8.1 System Clock Reading Algorithm	p. 151
8.2 Kernel Discipline Algorithms	p. 153
8.3 Kernel PLL/FLL Discipline	p. 155
8.4 Kernel PPS Discipline	p. 156
8.5 Clock Adjust Algorithm	p. 158
8.6 Proof of Performance	p. 159
8.7 Kernel PLL/FLL Discipline Performance	p. 160
8.8 Kernel PPS Discipline	p. 165
8.9 Parting Shots	p. 168
References	p. 168
Further Reading	p. 168
9 Cryptographic Authentication	p. 169
9.1 NTP Security Model	p. 170
9.1.1 On the Provenance of Filestamps	p. 172
9.1.2 On the Naming of Things	p. 173
9.1.3 On Threats and Countermeasures	p. 173
9.2 NTP Secure Groups	p. 175
9.3 Autokey Security Protocol	p. 178
9.3.1 Session Key Operations	p. 179
9.3.2 X509 Certificates	p. 181
9.3.3 Protocol Operations	p. 182
9.4 Parting Shots	p. 183
References	p. 183
Further Reading	p. 184
10 Identity Schemes	p. 185
10.1 Identity Schemes	p. 186
10.1.1 Private Certificate Identity Scheme	p. 187
10.1.2 Trusted Certificate Identity Scheme	p. 188
10.1.3 Schnorr Identity Scheme	p. 188
10.1.4 Guillou-Quisquater Identity Scheme	p. 190
10.1.5 Mu-Varadharajan Identity Scheme	p. 192
10.2 Cryptotypes	p. 195
10.3 Parting Shots	p. 198
References	p. 198
Further Reading	p. 198
11 Analysis of Errors	p. 199
11.1 Clock Reading Errors	p. 200
11.2 Timestamp Errors	p. 201
11.3 Sawtooth Errors	p. 203
11.4 Peer Error Budget	p. 204
11.5 System Error Budget	p. 206
11.6 Parting Shots	p. 208
References	p. 208
12 Modeling and Analysis of Computer Clocks	p. 209
12.1 Computer Clock Concepts	p. 210
12.2 Mathematical Model of the Generic Feedback Loop	p. 214
12.2.1 Type IFLL Feedback Control Loop	p. 216
12.2.2 Type II Feedback Control Loop	p. 217
12.3 Synthetic Timescales and Clock Wranglers	p. 219
12.4 Parting Shots	p. 222
References	p. 222
Further Reading	p. 223
13 Metrology and Chronometry of the NTP Timescale	p. 225
13.1 Scientific Timescales Based on Astronomy and Atomic Physics	p. 227
13.2 UTC and the Leap Second	p. 229
13.3 The Leap Second Controversy	p. 230
13.4 How NTP Reckons with UTC Leap Seconds	p. 233
13.5 On Numbering the Calendars and Days	p. 235
13.6 On the Julian Day Number System	p. 237
13.7 On Timescales and the Age of Eras	p. 238
13.8 On NTP Era and Timestamp Calculations	p. 240
13.9 Comparison with Other Computer Timescales	p. 241
13.10 Primary Frequency and Time Standards	p. 243
13.11 Time and Frequency Coordination	p. 244
13.12 Time and Frequency Dissemination	p. 245
13.12.1 Shortwave Radio Services	p. 246
13.12.2 Long-Wave Radio Services	p. 246
13.12.3 Geosynchronous Operational Environmental Satellite Service	p. 247
13.12.4 Telephone Modem Services	p. 248
13.12.5 Global Positioning System	p. 248
13.12.6 LORAN-C Radio Navigation System	p. 249
13.13 Parting Shots	p. 252
References	p. 253
Further Reading	p. 254
14 NTP Reference Implementation	p. 255
14.1 NTP Packet Header	p. 256
14.2 Control Flow	p. 259
14.3 Main Program and Common Routines	p. 263
14.4 Peer Process	p. 264
14.5 System Process	p. 268
14.6 Clock Discipline Process	p. 272
14.7 Clock Adjust Process	p. 275
14.8 Poll Process	p. 276
14.9 Parting Shots	p. 277
References	p. 279
Further Reading	p. 279
15 Precision System Clock Architecture	p. 281
15.1 Limitations of the Art	p. 281
15.2 Precision System Clock	p. 282
15.2.1 Timer Oscillator	p. 283
15.2.2 Timestamp Counter	p. 283
15.2.3 Real-Time Clock	p. 284
15.2.4 Precision System Clock Implementation	p. 285
15.2.5 Precision System Clock Operations	p. 287
15.3 Principles of Precision Timestamping	p. 289
15.3.1 Timestamp Transposition	p. 291
15.3.2 Error Analysis	p. 293
15.3.2.1 Reciprocity Errors	p. 294
15.3.2.2 Transposition Errors	p. 295
15.3.2.3 Interworking Errors	p. 295
15.3.2.4 Store-and-Forward Errors	p. 296
15.3.2.5 Nonreciprocal Rate Errors	p. 297
15.4 IEEE 1588 Precision Time Protocol	p. 298
15.4.1 Timestamp Capture	p. 298
15.4.2 PTP Clock Architecture	p. 300
15.4.3 PTP Messages	p. 301
15.4.4 Best Master Clock Algorithm	p. 302
15.4.5 Data Set Comparison Algorithm	p. 303
15.4.6 PTP Time Transfer	p. 304
15.4.7 PTP and NTP Compared	p. 305
15.5 Have Quick, STANAG 4330, and Precise Time and Time Interval Have Quick Interfaces	p. 307
15.6 Parting Shots	p. 309
References	p. 309
16 NTP Interleaved Modes	p. 311
16.1 Basic/Interleaved Protocol State Machines and Flowcharts	p. 312
16.2 Basic Symmetric Mode	p. 315
16.3 Interleaved Symmetric Mode	p. 317
16.4 Interleaved Broadcast Mode	p. 319
16.5 Error Detection and Recovery	p. 322
16.6 Measured Performance with the Interleaved Modes	p. 323
16.7 Parting Shots	p. 327
17 Time Transfer for Space Data Links	p. 329
17.1 Orbit Mechanics	p. 330
17.2 Clock Comparisons and the Effects of General Relativity	p. 333
17.3 Time Transfer from a Planet to the Solar System Barycenter	p. 338
17.4 Time Comparisons between Clocks in Space	p. 339
17.5 Spacecraft Electronics	p. 342
17.6 Proximity-1 Protocol	p. 345
17.7 Proximity-1 Time Service	p. 346
17.8 Time Transfer Using the Electra Transceiver	p. 347
17.9 Parting Shots	p. 349
References	p. 351
Further Reading	p. 352
18 Time Transfer for Deep-Space Missions	p. 353
18.1 Time Transfer between Earth Stations	p. 354
18.2 Time Transfer to Earth Satellites	p. 357
18.3 Time Transfer to the Moon and Satellites of Other Planets	p. 359
18.4 Time Transfer for a Planetary Space Fleet	p. 361
18.5 Time Transfer for Deep-Space Missions	p. 367
18.6 Parting Shots	p. 372
References	p. 375
19 Technical History of NTP	p. 377
19.1 On the Antiquity of NTP	p. 378
19.2 On the Proliferation of NTP around the Globe	p. 381
19.3 Autonomous Authentication	p. 381
19.4 Autonomous Configuration	p. 383
19.5 Radios, We Have Radios	p. 384
19.6 Hunting the Nanoseconds	p. 385
19.7 Experimental Studies	p. 388
19.8 Theory and Algorithms	p. 389
19.9 Growing Pains	p. 391
19.10 As Time Goes By	p. 391
19.11 Parting Shots	p. 392
References	p. 393
Further Reading	p. 396
Bibliography	p. 397
Acronyms	p. 407
Index	p. 411

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Table of Contents