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Ntp Time Maximum Error

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NTP "maximum error" on local network Messages sorted by: [ date ] [ thread ] [ subject ] [ author ] ajit.warrier at gmail.com wrote: > I

What Is Ntp Offset

am currently involved in a research project requiring tight time > synchronization ntp protocol port between nodes on an ethernet LAN. We require pairwise > time offsets between nodes to be accurate by ntp stratum levels less than 1ms. As a simple > test scenario, we set up a Linux machine with 2.6.16 kernel as our ntp > server and use the local clock as

Ntp Stratum 16

a reference: > > ------- /etc/ntp.conf at server ------------------- > restrict 127.0.0.1 nomodify > driftfile /var/lib/ntp/ntp.drift > server 127.127.1.1 prefer > fudge 127.127.1.1 stratum 0 refid NIST > > Two clients connect to this server through a 10Mbps hub, and get > synced. > > ------- on one of the client ------- > # ntptime > ntp_gettime() returns code

Ntp Polling Interval

0 (OK) > time c8164ed5.d3251000 Wed, May 17 2006 21:39:33.824, (.824784), > maximum error 41499 us, estimated error 2 us > ntp_adjtime() returns code 0 (OK) > modes 0x0 (), > offset -1.000 us, frequency 212.242 ppm, interval 4 s, > maximum error 41499 us, estimated error 2 us, > status 0x1 (PLL), > time constant 0, precision 1.000 us, tolerance 512 ppm, > pps frequency 0.000 ppm, stability 512.000 ppm, jitter 200.000 us, > intervals 0, jitter exceeded 0, stability exceeded 0, errors 0 > > > Clearly, I am able to achieve an "offset" of 1us and an "estimated > error" of 2us, which looks extremely good. But strangely, the "maximum > error" field gives an error of about 42ms. When I compare time stamps > between the server and client (at the MAC layer, to cancel out effects > of latency at higher layers), I see a gap of almost 60ms. Does anybody > have any idea of this kind of problem ? > > http://groups.google.com/group/comp.protocols.time.ntp/browse_thread/thread/3343cc5fec1b6597/effe74a33bb03cf2?q=accuracy&rnum=1#effe74a33bb03cf2 > > The above thread seems

>Next

This is the NTP home page that some people like to see here
5.2. The Kernel Discipline

In addition to the NTP protocol specification there also exists a description for a kernel clock model ([RFC

Ntp Jitter

1589]) that is discussed here.

1. Basic Functionality5.2.1.1. What is special about the Kernel ntp accuracy Clock?5.2.1.2. Does my Operating System have the Kernel Discipline?5.2.1.3. How can I verify the Kernel Discipline?2. Alternatives5.2.2.1. Should the Kernel Discipline be used?http://lists.ntp.org/pipermail/questions/2006-May/010306.html >3. Monitoring5.2.3.1. What are the individual monitoring values about?4. PPS Processing5.2.4.1. What is PPS Processing?5.2.4.2. How is PPS Processing related to the Kernel Discipline?5.2.4.3. What does hardpps() do?1. Basic Functionality5.2.1.1. What is special about the Kernel Clock?

http://www.ntp.org/ntpfaq/NTP-s-algo-kernel.htm >NTP keeps precision time by applying small adjustments to to system clock periodically (See also Q: 5.1.6.1.). However, some clock implementations do not allow small corrections to be applied to the system clock, and there is no standard interface to monitor the system clock's quality.

Therefore a new clock model is suggested that has the following features (See also [RFC 1589]):

Two new system calls to query and control the clock: ntp_gettime() and ntp_adjtime()

The clock keeps time with a precision of one microsecond.[1] In real life operating systems there are clocks that are much worse.

Time can be corrected in quantities of one microsecond, and repetitive corrections accumulate.[1] The UNIX system call adjtime() does not accumulate successive corrections.

The clock model maintains additional parameters that can be queried or controlled. Among these are:

A clock synchronization status that shows the state of the clock machinery (e.g. TIME_OK).Several clock control and status bits that control and show the state of the machinery (e.g. STA_PLL). This includes automatic handling of leap seconds (when announced).Correction values f

How does it work?

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

https://www.eecis.udel.edu/~ntp/ntpfaq/NTP-s-algo.htm >5.1. Basic Concepts

To help understanding the details of planning, configuring, and maintaining NTP, some basic concepts are presented here. The focus in this section is on theory.

http://networking.ringofsaturn.com/Protocols/ntp.php >

1. Time References5.1.1.1. What is a reference clock?5.1.1.2. How will NTP use a reference clock?5.1.1.3. How will ntp offset 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 Exchange5.1.2.1. 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. Performance5.1.3.1. 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. Robustness5.1.4.1. What is the stratum?5.1.4.2. How are Synchronization Loops avoided?5. Tuning5.1.5.1. What is the allowed range for minpoll and maxpoll?5.1.5.2. What is the best polling Interval?6. Operating System Clock Interface5.1.6.1. How will NTP discipline my Clock?1. Time References5.1.1.1. What is a reference clock?

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

Typical candidates for reference clocks are (very expensive) cesium clocks. Cheaper (and thus more popular) ones are receivers f

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