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By Anna DeakSubmitted On October 20, 2009

The clock is one of the most important devices of all civilization. It is simply a gadget that we use to tell the exact time of day. Its name was borrowed from a Greek term that meant the ringing of a bell. Now since clocks would make some sort of sound after every hour, it was dubbed with this name.

We are living in a civilized time that has put such a heavy importance on time. Everything is dependent on it. That is how important therefore that the clock is. Due to this importance there are clocks all around us. In a developed area you cannot walk for five minutes before you see a clock somewhere.

The clocks we have in our society today all have a common ancestor. Their common ancestor is the sun dial. It would cast the sun's shadow around its fixed centre to show us the time. The sun dial had a huge disadvantage that today's clocks do not have. It only worked during the day.

The water clock came soon after the sun dial. As a matter of fact it is not known exactly when both these clocks were first invented. Let us just say that it was a long time ago. The sun dial was used to set the water clock which would then tell fairly accurate time. Yes, even through the night.

The pendulum clock brought a fresh sense of accurate timing in the world of clocks. It would rarely lose time like the water clock. It therefore became a huge success. I am sure we have all at one time or the other seen one. Many of us just know it as the grandfather clock.

The next evolution in clocks came with the electrical era. The clocks would have electric motors wound electromagnetically that would run for days. There are many forms of electrical clocks still in use today. One does not have to worry about winding them every six hours.

With the introduction of electricity into the world of clock manufacture, came even further advancements. There was the invention of the batteries. The two together led the inventors to digital clocks. These were clocks that did not need mechanically moving components. They are now the most common clocks in the outdoors of most urban places.

The alarm clock is another important development of the simple clock. You set the time that you want to wake up and it will go off at that time. There are very old models that were highly mechanical but today we have even digital ones that will wake you up to your favorite tune.

We may categorize the alarm clock with the auditory clock. A lot of people call the auditory clock the talking clock. You just press a button and it says the time out loud for you. This is very good for example when you are asleep and do not want to get up and switch on the lights.

This last one is not literally a clock. It is symbolically a clock. It is called the doomsday clock. It counts the threats to human existence in the number of minutes before the clock strikes twelve. These threats are for example the nuclear weapons we build. There will be total human annihilation when the doomsday clock strikes midnight.

Annie is an expert furniture and interior design writer. Her current area of specialism is computer desk, curtain poles, and decorating ideas.

Article Source: https://EzineArticles.com/expert/Anna_Deak/440217

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This section will try to explain how NTP will construct andmaintain a working time synchronization network.

5.1. Basic Concepts

To help understanding the details of planning, configuring, andmaintaining NTP, some basic concepts are presented here. The focus in thissection is on theory.

1. Time References

5.1.1.1. What is a reference clock?

5.1.1.2. How will NTP use a reference clock?

5.1.1.3. How will NTP know about Time Sources?

5.1.1.4. What happens if the Reference Time changes?

5.1.1.5. What is a stratum 1 Server?

2. Time Exchange

5.1.2.1. How is Time synchronized?

5.1.2.2. Which Network Protocols are used by NTP?

5.1.2.3. How is Time encoded in NTP?

5.1.2.4. When are the Servers polled?

3. Performance

5.1.3.1. How accurate will my Clock be?

5.1.3.2. How frequently will the System Clock be updated?

5.1.3.3. How frequently are Correction Values updated?

5.1.3.4. How reliable are those Error-Estimates?

5.1.3.5. What is the Limit for the Number of Clients?

4. Robustness

5.1.4.1. What is the stratum?

5.1.4.2. How are Synchronization Loops avoided?

5. Tuning

5.1.5.1. What is the allowed range for minpolland maxpoll?

5.1.5.2. What is the best polling Interval?

6. Operating System Clock Interface

5.1.6.1. How will NTP discipline my Clock?

1. Time References

5.1.1.1. What is a reference clock?Time zones 2.1.

A reference clock is some device or machinery that spits out thecurrent time. The special thing about these things isaccuracy: Reference clocks must be accurately following sometime standard.

Typical candidates for reference clocks are (veryexpensive) cesium clocks. Cheaper (and thus more popular) ones are receiversfor some time signals broadcasted by national standard agencies. A typicalexample would be a GPS (Global Positioning System) receiverthat gets the time from satellites. These satellites in turn have a cesiumclock that is periodically corrected to provide maximum accuracy.

Less expensive (and accurate) reference clocks use one of theterrestrial broadcasts known as DCF77, MSF, and WWV.

In NTP these time references are also namedstratum 0, the highest possible quality. (Each system that hasits time synchronized to some reference clock can also be a time reference for othersystems, but the stratum will increase for each synchronization.)

5.1.1.2. How will NTP use a reference clock?

A reference clock will provide the current time, that's for sure.NTP will compute some additional statistical values that describe the qualityof time it sees. Among these values are: offset (orphase), jitter (or dispersion), frequency error, andstability (See also Section 3.3).Thus each NTP server will maintain an estimate of the quality of itsreference clocks and of itself.

5.1.1.3. How will NTP know about Time Sources?

There are serveral ways how a NTP client will know aboutNTP servers to use:

Servers to be polled can be configuredmanuallyServers can send the time directly to apeerServers may send out the time using multicast orbroadcast addresses

5.1.1.4. What happens if the Reference Time changes?

Ideally the reference time is the same everywhere in theworld. Once synchronized, there should not be any unexpected changes betweenthe clock of the operating system and the reference clock. Therefore, NTP has nospecial methods to handle the situation.

Instead, ntpd's reaction will depend on the offsetbetween the local clock and the reference time. For a tiny offset ntpd willadjust the local clock as usual; for small and larger offsets, ntpd willreject the reference time for a while. In the latter case the operationsystem's clock will continue with the last corrections effective while thenew reference time is being rejected. Aftersome time, small offsets (significantly less than a second) will beslewed (adjusted slowly), while larger offsets will cause theclock to be stepped (set anew). Huge offsets are rejected,and ntpd will terminate itself, believing something very strange must havehappened.

Naturally, the algorithm is also applied when ntpd isstarted for the first time or after reboot.

5.1.1.5. What is a stratum 1 Server?

A server operating at stratum 1 belongs tothe class of best NTP servers available, because it has a reference clock (See What is a reference clock?) attached to it. As accurate reference clocksare expensive, only rather few of these servers are publicallyavailable.

A stratum 1 server should not only have aprecise and well-maintained and calibrated reference clock, but also should be highlyavailable as other systems may rely on its time service. Maybe that's thereason why not every NTP server with a reference clock is publicallyavailable.

2. Time Exchange

5.1.2.1. How is Time synchronized?

Time can be passed from one time source to another,typically starting from a reference clock connected to a stratum 1server. Servers synchronized to a stratum 1 server will bestratum 2. Generally the stratum of a server will be one morethan the stratum of its reference (See also Q: 5.1.4.1.).

Synchronizing a client to a network server consists ofseveral packet exchanges where each exchange is a pair of request and reply.When sending out a request, the client stores its own time (originatetimestamp) into the packet being sent. When a server receives such apacket, it will in turn store its own time (receive timestamp)into the packet, and the packet will be returned after putting atransmit timestamp into the packet. When receiving the reply,the receiver will once more log its own receipt time to estimate the travellingtime of the packet. The travelling time (delay) is estimatedto be half of 'the total delay minus remote processing time',assuming symmetrical delays.

Those time differences can be used to estimate the timeoffset between both machines, as well as the dispersion(maximum offset error). The shorter and more symmetric the round-trip time,the more accurate the estimate of the current time.

Time is not believed until several packet exchanges havetaken place, each passing a set of sanity checks. Only if the replies from aserver satisfy the conditions defined in the protocol specification, the serveris considered valid. Time cannot be synchronized from a server that isconsidered invalid by the protocol. Some essential values are put intomulti-stage filters for statistical purposes to improve and estimate thequality of the samples from each server. All used servers are evaluated for aconsistent time. In case of disagreements, the largest set of agreeing servers(truechimers) is used to produce a combined referencetime, thereby declaring other servers as invalid(falsetickers).

Usually it takes about five minutes (five good samples)until a NTP server is accepted as synchronization source. Interestingly,this is also true for local reference clocks that have no delay at all bydefinition.

After initial synchronization, the quality estimate of theclient usually improves over time. As a client becomes more accurate, one ormore potential servers may be considered invalid after some time.

5.1.2.2. Which Network Protocols are used by NTP?

NTP uses UDP/IP packets for data transfer because ofthe fast connection setup and response times. The official port number for theNTP (that ntpd and ntpdate listen and talk to) is123.

5.1.2.3. How is Time encoded in NTP?

There was a nice answer from Don Payette innews://comp.protocols.time.ntp, slightly adapted:

The NTP timestamp is a 64 bit binary value with animplied fraction point between the two 32 bit halves. If you take all the bitsas a 64 bit unsigned integer, stick it in a floating point variable with atleast 64 bits of mantissa (usually double) and do a floating point divide by2^32, you'll get the right answer.

As an example the 64 bit binary value:equals a decimal 1.5. The multipliers to the right of the point are 1/2, 1/4,1/8, 1/16, etc.

To get the 200 picoseconds, take a one and divide it by2^32 (4294967296), you get0.00000000023283064365386962890625 or about233E-12 seconds. A picosecond is1E-12 seconds. Start restaurant dallas.

In addition one should know that the epoch for NTP startsin year 1900 while the epoch in UNIX starts in1970. Therefore the following values both correspond to2000-08-31_18:52:30.735861

5.1.2.4. When are the Servers polled?

When polling servers, a similar algorithm as described inQ: 5.1.3.3. is used. Basically the jitter(white phase noise) should not exceed the wander (random walkfrequency noise). The polling interval tries to be close to the point wherethe total noise is minimal, known as Allan intercept, and theinterval is always a power of two. The minimum and maximum allowable exponentscan be specified using minpoll andmaxpoll respectively (See Q: 5.1.5.1.).If a local reference clock with low jitter is selected to synchronize the systemclock, remote servers may be polled more frequently than without a localreference clock in recent version of ntpd. The intended purpose is to detect afaulty reference clock in time.[1]

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3. Performance

5.1.3.1. How accurate will my Clock be?

For a general discussion see Section 3.Also keep in mind that corrections are applied gradually, so it may take up tothree hours until the frequency error is compensated (see Figure 3).

Figure 3. Initial Run of NTP

Of course the final achievable accuracy depends on the timesource being used. Basically, no client can be more accurate than its server.In addition the quality of network connection also influences the finalaccuracy. Slow and non predictable networks with varying delays are very badfor good time synchronization.

A time difference of less than 128ms between server andclient is required to maintain NTP synchronization. The typical accuracy onthe Internet ranges from about 5ms to 100ms, possibly varying with network delays.A recent survey[2] suggests that 90% ofthe NTP servers have network delays below 100ms, and about 99% aresynchronized within one second to the synchronizationpeer.

With PPS synchronization an accuracy of 50µs and astability below 0.1 PPM is achievable on a Pentium PC (running Linux forexample). However, there are some hardware facts to consider. Judah Levinewrote:

In addition, the FreeBSD system I have been playingwith has a clock oscillator with a temperature coefficient of about 2 PPM perdegree C. This results in time dispersions on the order of lots ofmicroseconds per hour (or lots of nanoseconds per second) due solely to thecycling of the room heating/cooling system. This is pretty good by PCstandards. I have seen a lot worse.

Terje Mathisen wrote in reply to a question aboutthe actual offsets achievable: 'I found that 400 of theservers had offsets below 2ms, (..)'

David Dalton wrote about the same subject:

The true answer is: It All Depends...

Mostly, it depends on your networking. Sure, you canget your machines within a few milliseconds of each other if they are connectedto each other with normal 10-Base-T Ethernet connections and not too manyrouters hops in between. If all the machines are on the same quiet subnet,NTP can easily keep them within one millisecond all the time. But whathappens if your network get congested? What happens if you have a broadcaststorm (say 1,000 broadcast packets per second) that causes your CPU to go over100% load average just examining and discarding the broadcast packets? Whathappens if one of your routers loses its mind? Your local system time coulddrift well outside the 'few milliseconds' window in situations likethese.

5.1.3.2. How frequently will the System Clock be updated?

As time should be a continuous and steady stream, ntpdupdates the clock in small quantities. However, to keep up with clock errors,such corrections have to be applied frequently. Ifadjtime() is used, ntpd will update the system clockevery second. If ntp_adjtime() is available, theoperating system can compensate clock errors automatically, requiring onlyinfrequent updates. See also Section 5.2 and Q: 5.1.6.1.

5.1.3.3. How frequently are Correction Values updated?

NTP maintains an internal clock quality indicator. Ifthe clock seems stable, updates to the correction parameters happen lessfrequent. If the clock seems instable, more frequent updates are scheduled.Sometimes the update interval is also termed stiffness of thePLL, because only small changes are possible for long updateintervals.

There's a decision value named polladjust that can be queried with ntpdc's loopinfocommand. A value of -30 means to decrease the pollinginterval, while a value of 30 means to increase it (withinthe bounds of minpoll and maxpoll). Thevalue of watchdog timer is the time since the lastupdate.

Recent versions of ntpd (like 4.1.0)seem to update the correction values less frequently, possibly causingproblems. Even if the reference time sources are polled more frequently, thelocal system clock is adjusted less often.

5.1.3.4. How reliable are those Error-Estimates?

While in theory estimates of the clock error are maintained,there were practically some software bugs that made these numbers questionable.For example the new kernel clock model dealing with nanosecond resolution(nanokernel) produced overly optimistic estimates regarding the clockoffset. The bug has been fixed in August 2000, but also different versions ofthe NTP daemon may produce different estimates for the same hardware.

5.1.3.5. What is the Limit for the Number of Clients?

The limit actually depends on several factors, like speed ofthe main processor and network bandwidth, but the limit is quite high. Terje Mathisen once presented a calculation:

2 packets/256 seconds * 500 K machines-> 4 K packets/second (half in each direction).

Packet size is close to minimum, definitely less than128 bytes even with cryptographic authentication:

4 K * 128 -> 512 KB/s.

So, as long as you had a dedicated 100 Mbit/s fullduplex leg from the central switch for each server, it should see averagenetworks load of maximim 2-3%.

4. Robustness

5.1.4.1. What is the stratum?

The stratum is a measure forsynchronization distance. Opposed to jitter ordelay the stratum is a more static measure.Basically (and from the perspective from a client) it is the number of serversto a reference clock. So a reference clock itself appears at stratum 0, while the closestservers are at stratum 1. On the network there is no valid NTP message withstratum 0.

A server synchronized to a stratumn server will be running at stratumn + 1. The upper limit forstratum is 15. The purpose ofstratum is to avoid synchronization loops by preferringservers with a lower stratum.

5.1.4.2. How are Synchronization Loops avoided?

In a synchonization loop, the time derived from one sourcealong a specific path of servers is used as reference time again within such apath. This may cause an excessive accumulation of errors that is to beavoided. Therefore NTP uses different means to accomplish that:

• The Internet address of a time source is used asreference identifier to avoid duplicates. The referenceidentifier is limited to 32 bits however.

• The stratum as described in Q: 5.1.4.1. is used to form an acyclic synchronizationnetwork.

More precisely[2], the algorithm finds a shortest path spanningtree with metric based on synchronization distance dominated by hop count. Thereference identifier provides additional information to avoid neighbor loopsunder conditions where the topology is changing rapidly. This is a very wellknown problem with algorithms such as this. See any textbook on computernetwork routing algorithms. Computer Networks byBertsekas and Gallagher is a good one.

In IPv6 the reference ID field is a timestamp that canbe used for the same purpose.

5. Tuning

5.1.5.1. What is the allowed range for minpolland maxpoll?

The default polling value after restart of NTP is thevalue specified by minpoll. The default values forminpoll and maxpoll are6 (64 seconds) and 10 (1024 seconds)respectively.

For xntp3-5.93e the smallest andlargest allowable polling values are 4 (16 seconds) and14 (4.5 hours) respectively. For actual polling intervalslarger than 1024 seconds the kernel discipline is switched to FLLmode.

For ntp-4.0.99f the smallest andlargest allowable polling values are 4 (16 seconds) and17 (1.5 days) respectively. These values come from theinclude file ntp.h. The revised kernel disciplineautomatically switches to FLL mode if the update interval is longer than 2048seconds. Below 256 seconds PLL mode is used, and in between these limits themode can be selected using the STA_FLL bit.

5.1.5.2. What is the best polling Interval?

Actually there is none: Short polling intervals update theparameters frequently and are sensitive to jitter and random errors. Longintervals may require larger corrections with significant errors between theupdates. However there seems to be an optimum between those two. For commonoperating system clocks this value happens to be close to the default maximumpolling time, 1024s. See also Q: 5.1.3.1.

6. Operating System Clock Interface

5.1.6.1. How will NTP discipline my Clock?

In order to keep the right time, xntpd must makeadjustments to the system clock. Different operating systems providedifferent means, but the most popular ones are listed below.

Basically there are four mechanisms (systemcalls) an NTP implementation can use to discipline the system clock(For details see the different RFCs found in Table 4):

• settimeofday(2) tostep (set) the time. This method is used if the time if off bymore than 128ms.

• adjtime(2) toslew (gradually change) the time. Slewing thetime means to change the virtual frequency of the software clock to make theclock go faster or slower until the requested correction is achieved. Slewingthe clock for a larger amount of time may require some time, too. For examplestandard Linux adjusts the time with a rate of 0.5ms per second.

• ntp_adjtime(2) to controlseveral parameters of the software clock (also known as kerneldiscipline). Among these parameters are:

  • Adjust the offset of the software clock,possibly correcting the virtual frequency as well

  • Adjust the virtual frequency of the softwareclock directly

  • Enable or disable PPS eventprocessing

  • Control processing of leap seconds

  • Read and set some related characteristic values ofthe clock

• hardpps() is a function that isonly called from an interrupt service routine inside the operating system. Ifenabled, hardpps() will update the frequency and offsetcorrection of the kernel clock in response to an external signal (See alsoSection 6.2.4).

Notes[1]

This statement was derived from a mail message byProfessor David L. Mills in response to a suspected bug in version4.1.0.

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[2]

..says Professor David L. Mills..

This is the NTP home page that some people like to see here