Riassunto:
What started with the sundial has, thus far, been refined to a level of precision based on atomic resonance: Time. Our obsession with time is evident in this continued scaling down to nanosecond resolution and beyond. But this obsession is not without warrant. Precision and time synchronization are critical in many applications, such as air traffic control and stock trading, and pose complex and important challenges in modern information networks. Penned by David L. Mills, the original developer of the Network Time Protocol (NTP), Computer Network Time Synchronization: The Network Time Protocol takes a broad look at the architecture, protocols, and algorithms involved in time dissemination, distribution, and synchronization. Focused on the fourth generation of NTP, Mills' magnum opus gives an in-depth configuration and performance analysis of typical deployed NTP networks as well as a detailed error budget showing the influence of network and computer delay variations on accuracy. Mills demonstrates the algorithms used to select measurements accessed via diverse pathways to achieve the best accuracy, addresses security issues in depth, and describes the engineering principles underlying NTP architecture and timestamping exchange procedures. Opening the door to highly precise time synchronization in both public and private networks, Computer Network Time Synchronization offers in-depth analysis, theory, and practical concerns in a unified and authoritative reference.
Contenuti:
BASIC CONCEPTS
Time Synchronization
Time Synchronization Protocols
Computer Clocks
Processing Time Values
Correctness and Accuracy Expectations
Security
NTP in the Internet
Parting Shots
References
HOW NTP WORKS
General Infrastructure Requirements
How NTP Represents the Time
How NTP Reckons the Time
How NTP Disciplines the Time
How NTP Clients and Servers Associate
How NTP Discovers Servers
How NTP Manages Network Resources
How NTP Avoids Errors
How NTP Performance Is Determined
How NTP Controls Access
How NTP Watches for Terrorists
How NTP Clocks Are Watched
Parting Shots
References
Further Reading
IN THE BELLY OF THE BEAST
Related Technology
Terms and Notation
Process Flow
Packet Processing
Clock Filter Algorithm
Selection Algorithm
Clustering Algorithm
Combining Algorithm
Huff-'n-Puff Filter
Mitigation Rules and the Prefer Peer
Poll Process
Parting Shots
References
Further Reading
CLOCK DISCIPLINE ALGORITHM
Feedback Control Systems
Phase and Frequency Discipline
Weight Factors
Poll Interval Control
Popcorn and Step Control
Clock State Machine
Parting Shots
References
Further Reading
NTP SUBNET CONFIGURATION
Automatic Server Discovery
Manual Server Discovery and Configuration
Evaluating the Sources
Selecting the Stratum
Selecting the Number of Configured Servers
Engineering Campus and Corporate Networks
Engineering Home Office and Small Business Networks
Hardware and Network Considerations
Parting Shots
References
Further Reading
NTP PERFORMANCE IN THE INTERNET
Performance Measurement Tools
System Clock Latency Characteristics
Characteristics of a Primary Server and Reference Clock
Characteristics between Primary Servers on the Internet
Characteristics of a Client and a Primary Server on a Fast Ethernet
Results from an Internet Survey
Server and Network Resource Requirements
Parting Shots
References
PRIMARY SERVERS AND REFERENCE CLOCKS
Driver Structure and Interface
Reference Clock Drivers
Further Reading
KERNEL TIMEKEEPING SUPPORT
System Clock Reading Algorithm
Clock Discipline Algorithms
Kernel PLL/FLL Discipline
Kernel PPS Discipline
Clock Adjust Algorithm
Proof of Performance
Kernel PLL/FLL Discipline Performance
Kernel PPS Discipline
Parting Shots
References
Further Reading
CRYPTOGRAPHIC AUTHENTICATION
NTP Security Model
NTP Secure Groups
Autokey Security Protocol
Parting Shots
References
Further Reading
IDENTITY SCHEMES
X509 Certificates
Private Certificate (PC) Identity Scheme
Trusted Certificate (TC) Identity Scheme
Schnorr (IFF) Identity Scheme
Guillou-Quisquater (GQ) Identity Scheme
Mu-Varadharajan (MV) Identity Scheme
Parting Shots
References
Further Reading
ANALYSIS OF ERRORS
Clock Reading Errors
Timestamp Errors
Sawtooth Errors
Maximum Error Budget
Expected Error Budget
Parting Shots
References
MODELING AND ANALYSIS OF COMPUTER CLOCKS
Computer Clock Concepts
Mathematical Model of the Generic Feedback Loop
Synthetic Timescales and Clock Wranglers
Parting Shots
References
Further Reading
METROLOGY AND CHRONOMETRY OF THE NTP TIMESCALE
Scientific Timescales Based on Astronomy and Atomic Physics
Civil Timescales Based on Earth Rotation
How NTP Reckons with UTC Leap Seconds
On Numbering the Calendars and Days
On the Julian Day Number System
On Timescales, Leap Events, and the Age of Eras
The NTP Era and Buddy Epoch
Comparison with Other Computer Timescales
Primary Frequency and Time Standards
Time and Frequency Dissemination
Parting Shots
References
Further Reading
NTP REFERENCE IMPLEMENTATION
NTP Packet Header
Control Flow
Main Program and Common Routines
Peer Process
System Process
Clock Discipline Process
Clock Adjust Process
Poll Process
Parting Shots
Reference
Further Reading
TECHNICAL HISTORY OF NTP
On the Antiquity of NTP
On the Proliferation of NTP around the Globe
Autonomous Authentication
Autonomous Configuration
Radios, We Have Radios
Hunting the Nanoseconds
Experimental Studies
Theory and Algorithms
Growing Pains
As Time Goes By
Parting Shots
References
Further Reading
BIBLIOGRAPHY
INDEX
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