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Computer Network Time Synchronization
Ben Rothke writes "For most people, having their clocks accurate to within a few millionths of a second is excessive. Yet there are plenty of reasons to ensure that clocks on networks and production systems are that accurate. In fact, the need for synchronized time is a practical business and technology decision that is an integral part of an effective network and security architecture. The reality is that an organizations network and security infrastructure is highly dependent on accurate, synchronized time." Read the rest of Ben's review.
Computer Network Time Synchronization
author
David L. Mills
pages
304
publisher
CRC
rating
10
reviewer
Ben Rothke
ISBN
0849358051
summary
Definitive reference on how to deploy and use NTP
From a practical perspective, nearly every activity requires synchronized time to operate at peak levels, from plane departures and sporting events, to industrial processes, IP telephony, GPS and much more. Within information technology, technologies from directory services, collaboration, to authentication, SIM and VoIP all require accurate and synchronized time to work effectively.
Computer Network Time Synchronization: The Network Time Protocol is a valuable book for those that are serious about network time synchronization. David Mills, the author of the book, is one of the pillars of the network time synchronization community, and an original developer of the IETF-based network time protocol (NTP). The book is the summation of his decades of experience and a detailed look at how to use NTP to achieve highly accurate time on your network.
While network time synchronization is indeed crucial to corporate networks, this is only the second book on the topic. Last year saw Expert Network Time Protocol: An Experience in Time with NTP, which is a most capable title. But this book is clearly the indisputable reference on the subject, given its extraordinary depth and breadth. While Expert Network Time Protocol gets into the metaphysics of time, Mills's book takes a much more rationalist and pragmatic approach, which explains the myriad mathematical equations.
Mills is an electrical engineer by training and a significant part of the books 15 chapters involve advanced mathematics. But even for those who can't manage such equations, there is enough relevant material to make the book most rewarding.
Chapters 1 and 2 provide an excellent overview of the basics of network timekeeping and an overview of how NTP works. We often take for granted that network computers have the capabilities to set their internal clock. But while the capabilities are there, the reality is that these clocks are rarely accurate and subjected to many externalities that affect their ability to provide accurate time. The book shows how highly accurate time is easily achievable; often without the need for additional hardware. The goal of book is to show the reader how they can use NTP to synchronize the time on their network hosts to within a few milliseconds.
Chapters 3 - 11 detail the internals of NTP and time synchronization. Topics such as clock discipline algorithms, clock drivers and more are detailed. For many readers, the information may be overkill, but remember that this is not a For Dummies book.
Chapters 13 - 15 ease up on the abstract mathematics and are much more readable to newbie to the world of time synchronization. Chapter 13 is quite readable and details the metrology and chronometry of how NTP measures time as opposed to other time scales.
One of the key differences is the notion of absolute vs. relative time. Relative or astronomic time is based on the earth's rotation. Since the earth's rotation is not absolute, leap seconds are added to keep UTC (Universal Coordinated Time) synchronized with the astronomical timescale.
So what exactly is this legendary thing called the second? In 1967, the 13th General Conference on Weights and Measures defined the International System unit of time, the second, in terms of atomic time rather than the motion of the Earth. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.
Since the 17th century, time has for the most part been measured astronomically via the solar day. But in the 1940s, it was established that the earth's rotation is not constant, as the earth is spinning slower than it did years ago.
Part of what NTP provides is coordination to UTC. UTC provides operating systems and applications with a common index to synchronize events and prove that events happened when timestamps state they did. UTC is a 24-hour clock system and that any given moment, UTC is the same no matter where you are located.
For the purist, UTC really stands for Coordinated Universal Time, but both terms are used. Mills somewhat humorously notes that we follow the politically correct convention of expressing international terms in English, and their abbreviations in French.
Chapter 15 concludes the book with a fascinating look at the technical history of NTP. As of mid-2006, NTP has been in use for over 25 years and remains one of the longest, if not longest running, continuously operating application protocols in use on the Internet. Currently in version 4.2.1, NTP is a well-developed, stable protocol.
For those that are simply interested in how time synchronization works, or are responsible for time synchronization in their organization, Computer Network Time Synchronization: The Network Time Protocol is the most comprehensive guide available to using NTP.
For those that need an exhaustive tome on all of the minutiae related to NTP and synchronization, this is the source. Short of a vendor and product analysis, the book covers every detail within NTP and is the definitive title on the subject.
Two new books on the subject in a year demonstrate the importance of time synchronization. While this is not likely indicative of a flood of new books on time synchronization, this book should be considered the last word on the topic."
You can purchase Computer Network Time Synchronization from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
From a practical perspective, nearly every activity requires synchronized time to operate at peak levels, from plane departures and sporting events, to industrial processes, IP telephony, GPS and much more. Within information technology, technologies from directory services, collaboration, to authentication, SIM and VoIP all require accurate and synchronized time to work effectively.
Computer Network Time Synchronization: The Network Time Protocol is a valuable book for those that are serious about network time synchronization. David Mills, the author of the book, is one of the pillars of the network time synchronization community, and an original developer of the IETF-based network time protocol (NTP). The book is the summation of his decades of experience and a detailed look at how to use NTP to achieve highly accurate time on your network.
While network time synchronization is indeed crucial to corporate networks, this is only the second book on the topic. Last year saw Expert Network Time Protocol: An Experience in Time with NTP, which is a most capable title. But this book is clearly the indisputable reference on the subject, given its extraordinary depth and breadth. While Expert Network Time Protocol gets into the metaphysics of time, Mills's book takes a much more rationalist and pragmatic approach, which explains the myriad mathematical equations.
Mills is an electrical engineer by training and a significant part of the books 15 chapters involve advanced mathematics. But even for those who can't manage such equations, there is enough relevant material to make the book most rewarding.
Chapters 1 and 2 provide an excellent overview of the basics of network timekeeping and an overview of how NTP works. We often take for granted that network computers have the capabilities to set their internal clock. But while the capabilities are there, the reality is that these clocks are rarely accurate and subjected to many externalities that affect their ability to provide accurate time. The book shows how highly accurate time is easily achievable; often without the need for additional hardware. The goal of book is to show the reader how they can use NTP to synchronize the time on their network hosts to within a few milliseconds.
Chapters 3 - 11 detail the internals of NTP and time synchronization. Topics such as clock discipline algorithms, clock drivers and more are detailed. For many readers, the information may be overkill, but remember that this is not a For Dummies book.
Chapters 13 - 15 ease up on the abstract mathematics and are much more readable to newbie to the world of time synchronization. Chapter 13 is quite readable and details the metrology and chronometry of how NTP measures time as opposed to other time scales.
One of the key differences is the notion of absolute vs. relative time. Relative or astronomic time is based on the earth's rotation. Since the earth's rotation is not absolute, leap seconds are added to keep UTC (Universal Coordinated Time) synchronized with the astronomical timescale.
So what exactly is this legendary thing called the second? In 1967, the 13th General Conference on Weights and Measures defined the International System unit of time, the second, in terms of atomic time rather than the motion of the Earth. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.
Since the 17th century, time has for the most part been measured astronomically via the solar day. But in the 1940s, it was established that the earth's rotation is not constant, as the earth is spinning slower than it did years ago.
Part of what NTP provides is coordination to UTC. UTC provides operating systems and applications with a common index to synchronize events and prove that events happened when timestamps state they did. UTC is a 24-hour clock system and that any given moment, UTC is the same no matter where you are located.
For the purist, UTC really stands for Coordinated Universal Time, but both terms are used. Mills somewhat humorously notes that we follow the politically correct convention of expressing international terms in English, and their abbreviations in French.
Chapter 15 concludes the book with a fascinating look at the technical history of NTP. As of mid-2006, NTP has been in use for over 25 years and remains one of the longest, if not longest running, continuously operating application protocols in use on the Internet. Currently in version 4.2.1, NTP is a well-developed, stable protocol.
For those that are simply interested in how time synchronization works, or are responsible for time synchronization in their organization, Computer Network Time Synchronization: The Network Time Protocol is the most comprehensive guide available to using NTP.
For those that need an exhaustive tome on all of the minutiae related to NTP and synchronization, this is the source. Short of a vendor and product analysis, the book covers every detail within NTP and is the definitive title on the subject.
Two new books on the subject in a year demonstrate the importance of time synchronization. While this is not likely indicative of a flood of new books on time synchronization, this book should be considered the last word on the topic."
You can purchase Computer Network Time Synchronization from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
It's about time!
The difference between stupidity and genius is that genius has its limits.
Seriously... about how many people out there actually need to know NTP to this degree? Anyone have a rough estimate? I can't imagine any one organization would have to dedicate an individual to this sort of thing or would they?
Funnypics
2. Get D-Link to use you as the non-configurable time source for a line of disposable networking gear.
3. Profit!
Congrats to PHK for finding the elusive middle step!
Dewey, what part of this looks like authorities should be involved?
From the intro:
In case anyone's interested, one of the reasons that the abbreviation is UTC is because there are a series of Universal Time time references: UT0, UT1, etc. Despite being officially "Coordinated Universal Time", it's abbreviated as UTC partly to continue the UTx notation.
Don't synchronize with a time server in Denmark, unless of course you are in Denmark.
It's been proven that the Earth is rotating slower than it used to be, and the definition of a second was changed so that the length of a second remains constant. The day, however, remains the same as it always has been: one full rotation of the Earth. Eventually there will be conflict between the two. If the rotation of the Earth continues to slow, there will be more seconds (and, in turn, more minutes, and then more hours) in a given day. To that end, I've always wondered what would be more disruptive to the human populace: longer days or longer seconds?
Mills is a prof in my department and was my advisor back when I was an undergrad. He is a very smart guy (A bit of trivia about him - he was asked to consult for the Chinese government on the Great Firewall and turned down the offer for ethical reasons). He also prides himself on the fact that NTP has never had a serious (any?) security issue despite being around damn-near forever. One very neat observation he described during a seminar on NTP was that high CPU load increases CPU heat, and CPU heat increases clock drift. Thus, NTP can, in effect, be used to measure CPU loads remotely. Another thing is, assuming CPU load is constant, it can be used as a thermometer, and in practice he has used it to detect fan failures.
To make laws that man cannot, and will not obey, serves to bring all law into contempt.
--E.C. Stanton
a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.
Well of course, I mean, what took them so long? Seriously though it's things like this that make me ask, what on earth lead them to define it like that? Its not 9 million cycles, not 9.5 million, not an obvious number of cycles at all. How did 9,192,631,770 cycles become it, not 9,192,631,771, thats too long, not 9,192,631,769 thats too short. Only 9,192,631,770 was good enough.
"I use a Mac because I'm just better than you are."
Accurate time is very useful in computer security work. For one, it's needed to accurately correlate log file entries from one computer to another in case of a breach, to identify means of access and creating an accurate picture of what happened and when.
SecureThe.Net - Practical Resources for Securing Systems
I run the network and phone system in a college, and whilst I appreciate NTP is great, it does have drawbacks.
The biggest problem is keeping computer systems synched to 'real life' systems, such as analogue clocks and college bells. These systems have a mind of their own, and are seemingly set to random times.
A prime example; my computer at work synchs from the web, as do the servers, which in turn means all the Cisco VoIP phones are synched as well. The bells however, are never quite spot on, nor are the many analogue clocks in offices and classrooms.
Does anyone have a method of keeping everything in synch, because centralised and synchronised systems fall apart when dealing with 'real life' systems that are out of my hands.
"A man with one clock knows what time it is. A man with two clocks is never quite sure."
An interesting anagram of "BANACH TARSKI" is "BANACH TARSKI BANACH TARSKI"
For various reasons, I'm trying to synchronize a clock to millisecond accuracy among ~50 Microsoft Windows stations, and it's nearly impossible -- No NTP client for Windows (including AboutTime, 2000's internal client, XP's internal client, and a port of the standard NTP client) appears to be able to keep time reasonably synchronized.
Part of the problem is the Windows Kernel counting time in 10ms or 15ms (depending on whether or not you use an SMP kernel), which automatically says you can't get more than ~30ms precision. But it seems so much worse, with every machine drifting up to ~1 second daily unless they are syncrhonized very frequently -- I get somewhat reasonable results synchronizing them every minute.
On Linux and FreeBSD, this is so trivial it's not even funny; My linux machines manage to keep synchronization to ~0.5 ms over months. Please wake me up when Windows is ready for the enterprise. And, yes, the "enterprise" I work in does need millisecond precision time-of-day synchronization among machine, as does any place that seriously tries to correlate network events (especially those related to security) collected at different points in the network.
Just jackrabbit your starts to the west, and let the engine brake you going east. Do your part to stop global slowing!
If you really care about time why not use a GPS card in your PC.
eg http://www.visualgps.net/NMEATime/
For my computer I am testing an old Heath Most Accurate Clock II* with its RS232 attachment that goes to the serial port on my HP Pavilion. The only problem is the brick sized power transformer gets very hot because its supplying two amp heavy circuits. Use ThinkGeek's KillAWatt to measure power consumption. AWK the transformer is hungry. I guess for real use eventually I will peek at time once a day or so.
*Heath Most Accurate Clock II, synchronizes with WWV at 10 meters.
I think that the network, with all its erratic latency, is not really the best source to use as a timing transport.
Some people have occasionally picked up old cesium clocks from ebay to set the PC's time. Most are from labs and after purchase, probably gather dust in the garage.
http://tycho.usno.navy.mil/cesium.html
For my wrist, myself and lots of us geeks, use a Casio G-Shock (GW-700a) that updates its time from WWV three times a night. Its more accurate than the clocks at our local public DART train station. They are always four seconds slow.
I also have a great little Nixie clock kit that gets its info, not from WWV via radio, but from satellite GPS time. Its the dinky one at the bottom of the page. Looks fantastic though.
http://www.amug.org/~jthomas/clockpage.html
Even shorter still!
emerge ntp
A rather confident 007 walks into a bar and takes a seat next to a very attractive woman. He gives her a quick glance, then casually looks at his watch for a moment. The woman notices this and asks, "Is your date running late?" "No", he replies, "I am here alone. Q has just given me this state-of-the-art watch and I was just testing it." The intrigued woman says, "A state-of-the-art watch? What's so special about it?" "It uses alpha waves to telepathically talk to me," he explains. "What's it telling you now?" "Well, it says you're not wearing any panties..." The woman giggles and replies, "Well it must be broken because I am wearing panties!" 007 taps his watch, ...and says "Bloody thing must be an hour fast..."
Actually, having set up the NTP servers in our network, I have to say that the Windows version of NTP draws very substantial vacuum. It's not nearly as easy to configure. It can't be queried about what it thinks of the configured time standards, and I'm not exactly sure how they expect you to manage keys.
As long as you don't give a damn about sub-second accuracy (in our SCADA system, we like to stay in sync within 7 milliseconds or less) and as long as you don't care about traceability, then I guess it's better than nothing. However, the NT version of Mills' NTP is free, it is very stable on all versions I've tested it on from NT through 2003 server, and the configuration is exactly the same as most POSIX systems.
Having been there and tried it, I have to say that Microsoft did a piss poor job with their version of NTP. Get the GNU version. It Just Works Better.
Nearly fifty percent of all graduates come from the bottom half of the class!
On production systems it's much more important that the servers are all close to each other, not so much that they are close to NIST time. So, don't care so much that your servers are stratum 2 or 3, set up a couple of sources and then sync the rest of your boxes to them. I'd rather have all my machines be one second off but the same one second off, than have them all be closer to real time with larger differences between them.
Also, one thing about the time on earth changing that I didn't realize before. Damming water is one of the few activities that has changed the rotation speed of the earth, I've been told. Because it collects large masses of water further from the equater.
And if you don't want to buy a GPS, the guy responsible for the NIST time standard at NIST Boulder says that syncing your clock once a day via phone from one of their services is good enough to be considered stratum 1.
One final time note... We used to hold our LUG meetings at NIST. One time during a meeting, their official digital clocks stopped for the better part of a minute, and then ran quickly to catch up.
Sean
Actually, synchronization is less important than it used to be, because more stuff is buffered. All three US television networks used to be locked together in frame sync to a master clock in New York, so that video sources could be switched without all the TV receivers rolling for a few frames. Now everything goes through frame buffers, so that's not an issue.
Similarly, US telephony used to be locked to a master clock in New Jersey, so that all the T1 lines ran in sync and bit for bit transfer worked. That's not as important as it used to be, with so many different transmission media, some synchronous and some packetized.
First, my credentials: I've been working with NTP for more than 10 years, my personal web server, which you can find via http://www.ntp.org/ (I won't link directly to try to avoid the /. effect.) have hosted windows binaries of the official NTP distribution for some years now.
Since the original article didn't mention this, I would like to warn NTP users against ever configuring two servers! The reason is that NTP by design requires a plurality of all sources to agree on what the time is, before it will believe any of them.
This means that if you have two sources that disagree slightly, you can relatively easily get into a situation where your local machine decides to distrust both and simply start drifting away. I have actually seen this happen multiple times.
This means that you need to configure either a single or at least three servers, and if you want fault tolerance you actually need four, since that will leave three even when one of them fails.
Terje
"almost all programming can be viewed as an exercise in caching"