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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.
I'm n'sync here!
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
These prisoners were hanging out in the cell when the new guy asks, "Anybody know what time it is?", and one of the older inmates says, "Oh, it's about 2006".
try { do() || do_not(); } catch (JediException err) { yoda(err); }
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."
Our General raised hell over the fact that our wall clock (which is a set of LED clocks of local time, zulu time, Baghdad and Kabul) in one conference room was two minutes faster than the wall clock in another conference room. I'm not really sure why so much vitrol was spent over a clock discrepency (the clocks aren't used to conduct operations with, just to give rough situational awareness of what time it is in different parts of the world) but that day our systems guys learned the importance of synchronized clocks. Although I think their solution wasn't anything elaborate (like syncing to a central database), just adjusting the slower clock two minutes forward.
I propose a new standard:
Concurrent
Unified
Network
Time
Synchronization
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.
.. C*NTS?
My client recently purchased 40K Apple iBook G4 Laptops. Once deployed, the machines were curiously setting themselves to 1969. Found out that there was a bug in the systems' BIOS, (not sure if that is the traditional term for the MAC firmware) that would loose its time setting. Similar to a PC when the BIOS battery goes south.
So from a 'time sync' security standpoint, this bug may potentially need patched?
-- I like the above 'naked' sig...gonna steal it next forum...
"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"
AHHH!!! What have we done!? I blame Bush and the American obsession with fast cars. Each time we peel out a light we slow the Earth just a little bit more.
screw this! bring on metric time!
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.
You can get "Computer Network Time Synchronization" cheapest at eCampus, see here:
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http://www.alienegg.com/lowest-price-finder.php?u
Big LED displays aren't cheap. Usually they have serial data input, so you can scroll random stuff on them.
Anyway, I used to work in support at <company that used to build really fast, big, expensive supercomputers>. Just for the hell of it, a user wanted to hook up their $30 Epson dot matrix printer up to their new supercomputer, and we didn't really have a decent cheapo Epson printer driver.
"I just paid $15 million for this damn computer and what do you mean the serial port doesn't work?"
We fixed the serial port driver...
Give a man a fish and you have fed him for today. Teach a man to fish, and he'll say "WHERE'S MY FISH, YOU IDIOT?"
apt-get install ntp ntp-server ntp-doc ntpdate
I have discovered a truly marvelous proof of killer sig, which this margin is too narrow to contain.
If you really care about time why not use a GPS card in your PC.
eg http://www.visualgps.net/NMEATime/
You don't by chance happen to be a nudist?
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
Was he a member of "the pool"? TFA I saw didn't really say,
These are the [0-3].north-america.pool.ntp.org lines you put in your conf files these days, unless you are D-Link.
The 4 IPs are set up to round robin to a big bunch of volunteer servers.
Give a man a fish and you have fed him for today. Teach a man to fish, and he'll say "WHERE'S MY FISH, YOU IDIOT?"
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!
When I connect to our site's DC from my Windows workstation here are work, the times are indentical to the second. I also have my OpenBSD machines get their time from our Windows DC.
But i realize, "to the second" doesn't mean "to the milisecond". How do you even find out how far off your clock is from another machine in Windows? `net time` sure as hell doesn't do it.
I don't always use unix-like operating systems; but when I do, I prefer FreeBSD.
that despite having been around for 25 years there are A TON of devices that don't use the NIST time server to get an accurate time signal, especially considering the ICs for picking up the signal have gotten ridiculously cheap (pennies)... Wake me up when my microwave, VCR, and alarm clock (from Walgreens) sync up their time automatically after a power outage.
Rishi Chopra
www.rishichopra.org
Best dept. reference EVER!
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
The cost of setting up a very accurate NTP set-up is far more than the cost of setting up a GPS with a timing output. The GPS would be far more accurate too (accurate to tens of nanoseconds).
Engineering is the art of compromise.
Actually not exactly, there's no way you can put it in french so it fits the UTC abbreviation, but as the wikipedia says, it's a compromise between the english CUT and the french TUC, which is quite unusual.
You just got troll'd!
I have an application at a research lab where we have 100 networked computer/RF systems distributed over a distance of approximately 1 kilometer. We need to maintain the system clocks such that the time drift between any of the clocks is maintained at less than 250 fs. We do this (or attempt to) with a distributed RF system but if anyone knows how to do this over the computer network it would be quite interesting.
What can you do? Tell folks they can either put the other clocks and clock-driven systems on some sort of regularly updated, highly accurate, time synchronization system or come to grips with the fact that the $20 clocks on the classroom walls aren't perfectly accurate. My suggestion is to hand them a catalog with some better clocks in it and ask what parts of their budgets / paychecks they'd be willing to contribute towards rectifying this obviously intolerable situation...
I don't read ACs: If a post isn't worth so much as a nom de plume to its author then I wont bother either.
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.
The reference ntpd implimentation (xntpd) has had numerous security issues, including at least one remote root exploit. This would not suprise anyone who has look at the code. OpenNTPd exists for a reason you know.
If the invasion had gone to plan, the invading troops would have hit the bunkers before the Turkish troops and the invasion would have gone completely differently.
In Soviet Russia the insensitive clod is YOU!
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"
It's so important that a computer be millisecond accurate, so they can choose *exactly* the right moment to tell the control tower guy to radio the pilot, and tell him to start moving? Doesn't the human factor /way/ overshadow any errors from computers being even an entire second off?
I mod down anyone who says "I will be modded down for this", regardless of the rest of their comment
If UTC is a French term shouldnt the lunchhour does not contain 9,000 seconds?
Old COBOL programmers never die. They just code in C.
There is also an OpenNTPD implementation, from the OpenBSD project. While early versions were considered harmful by the NTP community, I believe it is now accepted.
For Windows clients, I use Tardis.
It is. But depending on the application, there are several different time scales. A few off the top of my head:
Most people who measure things time-related need a time scale that runs at a constant rate. UTC, derived from vibrating atoms, is the way to go. Time was originally derived from Earth's rotation, but since it wobbles around so much, we use more accurate time sources now.
People who do things like navigation need time that is synchronized to Earth's rotation. This is the application of stuff like UT1, which is derived from UTC, but is measured and adjusted to track Earth's rotation very precisely. The double ticks at the top of the minute on WWV are the current UTC-UT1 offset, in 100 millisecond increments.
People who play with things in space need a time scale that runs at a constant rate, but is aware of relativistic effects. This is Dynamical Time, currently about 65 seconds off UTC. Instead of leap seconds these space folks just keep track of the offset, called delta T. Among other things, predictions of eclipses and planet positions work in Dynamical Time.
...laura
...Oprah's weight. Didn't you get the memo?
Apparently everyone except you understands the shorthand "IP" to refer to "IP Address" in this context. So think for a sec before flaming.
That's just plain retarded. Why kind of security issue is the protocol going to have? And the guy wrote xntpd, not the ntp protocol. It would make no sense for him to be proud that an incredibly simple protocol that he had nothing to do with has had no security problems. Unfortunately, it also makes no sense for him to be proud of the security record of the ntp software he wrote, since it is complete and utter shit.
Leap second.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
Because NTP is free?
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
Does it explain why some servers, both claiming or seeming to use authoritative time sources, are off by as much as two minutes?
Sounds like something Google and Sony could have used for the Da Vinci code Quest! I'd say there are thousands of people out there all comparing "times" they finished the last puzzle. Really what matters is the time stamp given in Google world, and who knows if they all use the same watch. ;)