Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Thu, 28 Mar 2013 21:21:55 GMT, Harlan Stenn wrote: On Thu, Mar 28, 2013 at 5:57 AM, Rob nom...@example.com wrote: so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. Sending at a rate of more than 1 packet every 2 seconds or more than 8 packets per 64 seconds will trigger KOD responses. Use sufficiently many servers so the rate to each is below radar. Keep list of latest servers and their roundtip time. Sort list by roundtrip time. Use them in descending order(Use worst to kick link up in speed) Hope there are some good measurements in the last measurements. Update list. /hjj ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Robert Scott no-one@notreal.invalid wrote: On Thu, 28 Mar 2013 21:21:55 GMT, Harlan Stenn st...@ntp.org wrote: On Thu, Mar 28, 2013 at 5:57 AM, Rob nom...@example.com wrote: so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. Sending at a rate of more than 1 packet every 2 seconds or more than 8 packets per 64 seconds will trigger KOD responses. Perhaps these wake-up packets could be some other UDP packet, like a Ping, and not to some time server, but to some other internet address, like www.microsoft.com (not really!). If I understand Rob's suggestion, all that is needed is some kind of activity to wake up the radio/internet channel, not necessarily NTP activity. That is right, you can just use ping to an address that you know responds to pings (the NTP server might block pings), or you could fetch the homepage of your website via TCP, whatever action that causes a quick packet exchange of more than typically 4 packets will do it. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
In the US, the stratum 2 time servers are much less used than the stratum 1 servers, and they dispense fairly accurate time. The stratum 1 time servers frequently have erratic and lengthy processing delays. You can find time servers here http://support.ntp.org/bin/view/Servers/WebHome and finding ones near you is well worth the effort. Nothing will help though if your neighbor decides to download a movie or two. Charles Elliott -Original Message- From: questions-bounces+elliott.ch=verizon@lists.ntp.org [mailto:questions-bounces+elliott.ch=verizon@lists.ntp.org] On Behalf Of Magnus Danielson Sent: Thursday, March 28, 2013 8:32 AM To: questions@lists.ntp.org Subject: Re: [ntp:questions] NIST vs. pool.ntp.org ? Hi again Robert, On 03/28/2013 04:22 AM, Robert Scott wrote: On Thu, 28 Mar 2013 02:50:17 GMT, unruhun...@invalid.ca wrote: You really should read my posts before responding. No, I do not intend to hard-code NIST or any other server. I never said I wanted to. No, the app is not intended for all musicians. It is intended for professional piano tuners only. I sell about one per day. And I never said the pool would not be good enough for my needs. I only asked about the relative benefits of the pool vs. NIST, which E-mail sent...Blacklists answered very nicely. There is no real benefit in using either, rather you should use the mix of servers which gives you good confidence in removing false-tickers as well as good precision due to use of short distances. Look at the NTP code and book, as many of the filtering steps aims at removing noise which polute the time and frequency errors. Do the to- way time-transfer. Cheers, Magnus ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 3/28/2013 8:22 AM, Magnus Danielson wrote: On 03/27/2013 10:45 PM, David Woolley wrote: Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular The pool system has no provision for enforcing registration. It wouldn't make sense to hand out a random server address if most of them then refused to serve you because you hadn't registered. users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. I would hope all the pool servers ultimately reference their national equivalent of NIST and therefore what becomes, after the fact, UTC. I think you will find that Navstar (GPS) and WWV times are traceable to NIST. Yes and no. GPS is traceable to USNO. USNO and NIST have traceability between each other within the BIPM framework. MSF times are traceable to NPL. NPL is traceable to both USNO and NIST within the BIPM framework. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? The open NIST servers are heavily overloaded, so probably don't serve the highest quality time, but they are likely to be around for a long time. I would setup a local server under your control. It will help both from debugging and noise perspective. Cheers, Magnus If you really want the best time, consider a GPS Timing Receiver. The GPS Timing Receiver should give you a tick that's +- 50 nanoseconds. If you need better, you will need your own atomic clock! It also gives you the value of that tick; e.g. 4:03:32. . . . . Many of us here do not actually need +/- 50 nanoseconds. What I DID want was to have all the clocks in all the clocks in the house in agreement! It's not perfect but it's close enough. . . . ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Il 03/27/2013 10:24 PM, unruh ha scritto: You do NOT want to hard code anything into your program. That is extremely bad form, unless that address is one controlled by you. Indeed. Robert, please see: http://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse and in particular: http://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse#D-Link_and_Poul-Henning_Kamp a notable case among already notable cases... Ciao -- bronto ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Robert Scott no-one@notreal.invalid wrote: To achieve 11 ppm accuracy in frequency I need to have a calibration time interval that is about 90,000 times as long as the timestamp uncertainty. If the timestamp uncertainty is, say, 100 msec., the calibration time period needs to be at least 2.5 hours. That's where my figure of 3 hours comes from. I don't think it will difficult for a user of my app to perform this calibration. All he has to do is to ensure Internet connectivity is turned on (it could be cell or wi-fi), hit the calibrate button in my app, and leave the phone on charge and go to bed. To achieve an uncertainty below 100ms over connections like that you will need to do some clever programming where you get multiple time stamps, measure the roundtriptime on each of them, and discard outliers before you calculate an average. The kind of thing that ntpd already does. Radio protocols often use polling to grant the permission to the mobiles to send, and adaptive polling intervals depending on the recent traffic. (or they drop clients from the polling list when there is no traffic, offering some method for them to join again) The result is that when you start traffic on an otherwise idle client, you will observe roundtrip intervals like this: time=531 ms time=500 ms time=540 ms time=874 ms time=99.7 ms time=101 ms time=96.3 ms time=97.3 ms time=101 ms time=96.9 ms time=102 ms time=102 ms time=97.4 ms time=102 ms time=103 ms in this example, the first four exchanges were with the device still in idle mode and only then the cell network (UMTS in this case) decided to step down the polling interval on that client, and from the fifth one the roundtriptime is pretty consistent. you will need to discard those first 4 samples and then calculate an average over a couple more. however, that 4 is not a constant! it depends on parameters and circumstances beyond your control. so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. even then, I have seen links (e.g. over WiFi) where a stable state is never reached and varying roundtriptimes between nearly zero and about 200ms are seen no matter how often and how frequently you ping over them. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 03/27/2013 10:45 PM, David Woolley wrote: Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular The pool system has no provision for enforcing registration. It wouldn't make sense to hand out a random server address if most of them then refused to serve you because you hadn't registered. users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. I would hope all the pool servers ultimately reference their national equivalent of NIST and therefore what becomes, after the fact, UTC. I think you will find that Navstar (GPS) and WWV times are traceable to NIST. Yes and no. GPS is traceable to USNO. USNO and NIST have traceability between each other within the BIPM framework. MSF times are traceable to NPL. NPL is traceable to both USNO and NIST within the BIPM framework. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? The open NIST servers are heavily overloaded, so probably don't serve the highest quality time, but they are likely to be around for a long time. I would setup a local server under your control. It will help both from debugging and noise perspective. Cheers, Magnus ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Hi again Robert, On 03/28/2013 04:22 AM, Robert Scott wrote: On Thu, 28 Mar 2013 02:50:17 GMT, unruhun...@invalid.ca wrote: You really should read my posts before responding. No, I do not intend to hard-code NIST or any other server. I never said I wanted to. No, the app is not intended for all musicians. It is intended for professional piano tuners only. I sell about one per day. And I never said the pool would not be good enough for my needs. I only asked about the relative benefits of the pool vs. NIST, which E-mail sent...Blacklists answered very nicely. There is no real benefit in using either, rather you should use the mix of servers which gives you good confidence in removing false-tickers as well as good precision due to use of short distances. Look at the NTP code and book, as many of the filtering steps aims at removing noise which polute the time and frequency errors. Do the to-way time-transfer. Cheers, Magnus ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 28 Mar 2013 09:57:37 GMT, Rob nom...@example.com wrote: To achieve an uncertainty below 100ms over connections like that you will need to do some clever programming where you get multiple time stamps, measure the roundtriptime on each of them, and discard outliers before you calculate an average. The kind of thing that ntpd already does. If I could use a packaged implementation of NTP I would. But I don't have that option. If I understand correctly, packaged implementations of NTP are designed specifically for setting the system time. I cannot impose that effect on users of my app. My app must be stand-alone and not attempt to affect the system time. So I will have to take the algorithms and basic ideas of NTP and re-implement them in a stand-along fashion. In any case the language of implementation will have to depend on the target: Java for Android devices, Objective-C for iPhones. Radio protocols often use polling to grant the permission to the mobiles to send, and adaptive polling intervals depending on the recent traffic. (or they drop clients from the polling list when there is no traffic, offering some method for them to join again) The result is that when you start traffic on an otherwise idle client, you will observe roundtrip intervals like this: time=531 ms time=500 ms time=540 ms time=874 ms time=99.7 ms time=101 ms time=96.3 ms time=97.3 ms time=101 ms time=96.9 ms time=102 ms time=102 ms time=97.4 ms time=102 ms time=103 ms Are these exchanges all with the same Time Server? I thought good citizen use of these Time Servers required at least 4 seconds inbetween queries. Or does the start-up polling time penalty only once even if every query is to a different Time Server? in this example, the first four exchanges were with the device still in idle mode and only then the cell network (UMTS in this case) decided to step down the polling interval on that client, and from the fifth one the roundtriptime is pretty consistent. you will need to discard those first 4 samples and then calculate an average over a couple more. however, that 4 is not a constant! it depends on parameters and circumstances beyond your control. so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. even then, I have seen links (e.g. over WiFi) where a stable state is never reached and varying roundtriptimes between nearly zero and about 200ms are seen no matter how often and how frequently you ping over them. In those cases how asymmetric is the polling delay likely to be if I just take the midpoint of the polling interval, just like NTP? I realize that it is possible to be extremely assymetric, but in practice? Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Robert Scott no-one@notreal.invalid wrote: On 28 Mar 2013 09:57:37 GMT, Rob nom...@example.com wrote: To achieve an uncertainty below 100ms over connections like that you will need to do some clever programming where you get multiple time stamps, measure the roundtriptime on each of them, and discard outliers before you calculate an average. The kind of thing that ntpd already does. If I could use a packaged implementation of NTP I would. But I don't have that option. I understand that, I only want you to know that you need to implement a clever filter just like in ntpd or else you will have a problem. Are these exchanges all with the same Time Server? I thought good citizen use of these Time Servers required at least 4 seconds inbetween queries. Or does the start-up polling time penalty only once even if every query is to a different Time Server? That is a complicating factor. You need to cause some traffic to kick the link into faster action but you don't want to cause trouble with the Time Server admin (who may have set an access rule that blocks you when you send a burst of requests). You can instead ping something that you know responds to pings, e.g. send 10 requests at .25 second intervals, and then immediately request the time from a timeserver. The link does not look at what system you communicate with, it only checks if there is a certain number of packets per second to see if you are active. even then, I have seen links (e.g. over WiFi) where a stable state is never reached and varying roundtriptimes between nearly zero and about 200ms are seen no matter how often and how frequently you ping over them. In those cases how asymmetric is the polling delay likely to be if I just take the midpoint of the polling interval, just like NTP? I realize that it is possible to be extremely assymetric, but in practice? Very asymmetric. The access point can always transmit when it likes, the clients have to wait for the access point to tell them that they can have a go. It varies widely between implementations. It can be like the behaviour shown for UMTS, or it can be that the access point polls at some fixed interval (that you aren't synchronous to) so you will see variation between 0 and that interval, only in the direction client-server. Maybe for your application it would be wise to check the roundtriptime only to see if it has already fallen from some initial high value, but not to use it in the calculation of real time (i.e. don't take a midpoint between request and reply). That is because excessive and variable delays are more likely to occur on the uplink than on the downlink. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 28 Mar 2013 13:37:50 GMT, Rob nom...@example.com wrote: Maybe for your application it would be wise to check the roundtriptime only to see if it has already fallen from some initial high value, but not to use it in the calculation of real time (i.e. don't take a midpoint between request and reply). That is because excessive and variable delays are more likely to occur on the uplink than on the downlink. I think what I may do is make the most conservative assumption about the assymetry, i.e. that it is as bad as possible. Then calculate the precision based on the round-trip time. If the round-trip time is small enough then I don't care about the asymmetry. Since the overall calibration period is under my control, and is only limited by the time the user leaves his phone on charge overnight, I could potentially wait for 7 or 8 hours before taking my final time-stamp reading. That will relax the time-stamp precision to something more like 280 msec. Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Thu, Mar 28, 2013 at 5:57 AM, Rob nom...@example.com wrote: so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. If he sends requests too fast when the latency is so high, isn't there a chance they'll just get combined due to Nagle which will throw off his calculations even more? ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Trevor Woerner twoer...@gmail.com wrote: On Thu, Mar 28, 2013 at 5:57 AM, Rob nom...@example.com wrote: so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. If he sends requests too fast when the latency is so high, isn't there a chance they'll just get combined due to Nagle which will throw off his calculations even more? This is UDP, not TCP. Of course there is not much point in shotgunning requests when there is no reply. First attempt could be to send a new request only after the first reply has been received, but I am not sure this will always guarantee the radio link goes to active mode. Maybe send 3 requests at .25 or .5 second intervals ant then wait until at least one reply comes back. With GPRS it sometimes takes several seconds for the first replies to come back. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
A report on UTC(NIST) - UTC(USNO) can be found at http://tf.boulder.nist.gov/general/pdf/2663.pdf On Thu, Mar 28, 2013 at 7:22 AM, Magnus Danielson mag...@rubidium.dyndns.org wrote: On 03/27/2013 10:45 PM, David Woolley wrote: Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular The pool system has no provision for enforcing registration. It wouldn't make sense to hand out a random server address if most of them then refused to serve you because you hadn't registered. users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. I would hope all the pool servers ultimately reference their national equivalent of NIST and therefore what becomes, after the fact, UTC. I think you will find that Navstar (GPS) and WWV times are traceable to NIST. Yes and no. GPS is traceable to USNO. USNO and NIST have traceability between each other within the BIPM framework. MSF times are traceable to NPL. NPL is traceable to both USNO and NIST within the BIPM framework. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? The open NIST servers are heavily overloaded, so probably don't serve the highest quality time, but they are likely to be around for a long time. I would setup a local server under your control. It will help both from debugging and noise perspective. Cheers, Magnus __**_ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/**questionshttp://lists.ntp.org/listinfo/questions ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Thu, Mar 28, 2013 at 5:57 AM, Rob nom...@example.com wrote: so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. Sending at a rate of more than 1 packet every 2 seconds or more than 8 packets per 64 seconds will trigger KOD responses. H ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Thu, 28 Mar 2013 21:21:55 GMT, Harlan Stenn st...@ntp.org wrote: On Thu, Mar 28, 2013 at 5:57 AM, Rob nom...@example.com wrote: so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. Sending at a rate of more than 1 packet every 2 seconds or more than 8 packets per 64 seconds will trigger KOD responses. Perhaps these wake-up packets could be some other UDP packet, like a Ping, and not to some time server, but to some other internet address, like www.microsoft.com (not really!). If I understand Rob's suggestion, all that is needed is some kind of activity to wake up the radio/internet channel, not necessarily NTP activity. Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Robert Scott wrote: If I could use a packaged implementation of NTP I would. But I don't have that option. If I understand correctly, packaged implementations of NTP are designed specifically for setting the system time. I The interface to the OS clock is fairly well abstracted for portability reasons. If you forego the kernel time discipline, you should be able to modify an offset from system time, rather than the system time itself, although the fact that ntpd really controls frequency may be a bit of a problem, as you can only really fake offset. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 2013-03-28, Robert Scott no-one@notreal.invalid wrote: On 28 Mar 2013 09:57:37 GMT, Rob nom...@example.com wrote: To achieve an uncertainty below 100ms over connections like that you will need to do some clever programming where you get multiple time stamps, measure the roundtriptime on each of them, and discard outliers before you calculate an average. The kind of thing that ntpd already does. If I could use a packaged implementation of NTP I would. But I don't have that option. If I understand correctly, packaged implementations of NTP are designed specifically for setting the system time. I cannot impose that effect on users of my app. My app must be stand-alone and not attempt to affect the system time. So I will have to take the algorithms and basic ideas of NTP and re-implement them in a stand-along fashion. In any case the language of implementation will have to depend on the target: Java for Android devices, Objective-C for iPhones. Well, you could always tell ntpd not to use the outside sources to set the time. Radio protocols often use polling to grant the permission to the mobiles to send, and adaptive polling intervals depending on the recent traffic. (or they drop clients from the polling list when there is no traffic, offering some method for them to join again) The result is that when you start traffic on an otherwise idle client, you will observe roundtrip intervals like this: time=531 ms time=500 ms time=540 ms time=874 ms time=99.7 ms time=101 ms time=96.3 ms time=97.3 ms time=101 ms time=96.9 ms time=102 ms time=102 ms time=97.4 ms time=102 ms time=103 ms Are these exchanges all with the same Time Server? I thought good citizen use of these Time Servers required at least 4 seconds inbetween queries. Or does the start-up polling time penalty only once even if every query is to a different Time Server? in this example, the first four exchanges were with the device still in idle mode and only then the cell network (UMTS in this case) decided to step down the polling interval on that client, and from the fifth one the roundtriptime is pretty consistent. you will need to discard those first 4 samples and then calculate an average over a couple more. however, that 4 is not a constant! it depends on parameters and circumstances beyond your control. so you will need to write an adaptive algorithm that recognizes what is happening here, and send the queries quickly enough (I would say at least two per second, maybe 4) to force the active user algorithm to kick in. even then, I have seen links (e.g. over WiFi) where a stable state is never reached and varying roundtriptimes between nearly zero and about 200ms are seen no matter how often and how frequently you ping over them. In those cases how asymmetric is the polling delay likely to be if I just take the midpoint of the polling interval, just like NTP? I realize that it is possible to be extremely assymetric, but in practice? Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
[ntp:questions] NIST vs. pool.ntp.org ?
I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? I posted here several months ago about my specific application. I want to use SNTP, not to synchronize a system clock but to make a frequency measurement on the audio sampling rate in a smartphone. My use of time servers will be very occasional - typically a user of my app would only use the time servers once when the app is first installed to do a calibration of the audio sampling rate in that phone. And my app will have very limited circulation. So I won't be hitting the servers very often at all. Today I found the last bug that prevented me from getting response from the time servers in my socket code. I had neglected to set the port number (123) using htons(123) to put the port number in network byte order. When I fixed that I finally started getting responses. I have gotten responses from pool.ntp.org as well as several of the NIST time servers listed on their website. However the round-robin address of time.nist.gov does not seem to return anything. The recv() function just times out. And I sure don't want to hard-code any of the other NIST server URLs in my app. I was going to go with pool.ntp.org until I saw the NIST servers that did not seem to be part of the pool. So I guess my main question is which servers should I use for my very limited application? Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 3/27/2013 3:20 PM, Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? I posted here several months ago about my specific application. I want to use SNTP, not to synchronize a system clock but to make a frequency measurement on the audio sampling rate in a smartphone. My use of time servers will be very occasional - typically a user of my app would only use the time servers once when the app is first installed to do a calibration of the audio sampling rate in that phone. And my app will have very limited circulation. So I won't be hitting the servers very often at all. Today I found the last bug that prevented me from getting response from the time servers in my socket code. I had neglected to set the port number (123) using htons(123) to put the port number in network byte order. When I fixed that I finally started getting responses. I have gotten responses from pool.ntp.org as well as several of the NIST time servers listed on their website. However the round-robin address of time.nist.gov does not seem to return anything. The recv() function just times out. And I sure don't want to hard-code any of the other NIST server URLs in my app. I was going to go with pool.ntp.org until I saw the NIST servers that did not seem to be part of the pool. So I guess my main question is which servers should I use for my very limited application? Robert Scott Hopkins, MN I think you need to start by defining the quality of time you need. Is 4:13 PM Eastern Timegood enough? Some people just want to Get me to the church on time! OR the bus, or the train or plane. A radio astronomer is almost certainly going to insist on time to the nearest nano second. Most of the range above is probably going to be overkill for most people. Please try to define your requirements a little more closely! Nano-seconds, get me to the church/train/plane . . . . on time! or something else. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Robert Scott wrote:
I am confused about the proper usage of pool.ntp.org and NIST.
NIST isn't of much value outside the continental US,
other regional equivalent national time and frequency
standards closer to the point of usage should have much
greater value for those that run Stratum One NTP Servers.
Regional / National / Country time frequency standard services
typically have cesium beam atomic clocks, hydrogen maser atomic clocks,
caesium fountain atomic clocks, rubidium gas cells, ...
e.g. Places like these http://en.wikipedia.org/wiki/Radio_clock
CSIRO National Measurement Laboratory, Melbourne Sydney, Australia
Canadian Meteorological Centre, Dorval, Québec, Canada
Integrated Systems Laboratory, Swiss Fed. Inst. of Technology, Zurich,
Switzerland
CSTV of National Research Council, Torino, Italy
New Delhi, National Physical Laboratory of India
National Institute of Information and Communications Technology, Tokyo, Japan
SP Swedish National Testing and Research Institute, Boras, Sweden
NPL Time and Frequency Services National Physical Laboratory Middlesex UK
NIST National Institute of Standards and Technology US
...
NIST (and other national equivilants) are usually used by Stratum One Servers
{that have Local Stratum Zero Reference clocks, e.g. GPS}
that provide NTP service to many (hundreds ?) of clients
e.g. Corporate NTP servers providing service to their LAN clients,
College NTP servers providing service to classroom and dorm clients,
...
pool.ntp.org seems to be a collection of private sector
time servers offered for all to use, but with registration
expected for regular users.
The Vendor Pool? www.pool.ntp.org/en/vendors.html
That is due to the several vendors (NetGear, SMC, DLink, Tardis)
who (unintentionally) had poor implementations / bugs in their
ntp code that hammered ntp server farms to death.
e.g. http://en.wikipedia.org/wiki/NTP_server_misuse_and_abuse
It allow them to more easily identify (and hopefully contact)
a source of abuse, as well as being able to shift or spread
the source of abuse to prevent it from DDOS taking down
NTP servers, Networks, ISPs, ...
the round-robin address of time.nist.gov does not seem to return anything.
Sound like you are not dealing with CNames correctly?
Question:
Name=time.nist.gov, QTYPE=A, QCLASS=1
Answer Section:
- Name=time.nist.gov
Type=CNAME, Class=1, TTL=1145 (19 Minutes 5 Seconds), RDLENGTH=10
CNAME=ntp.glb.nist.gov
- Name=ntp.glb.nist.gov
Type=A, Class=1, TTL=101 (1 Minute 41 Seconds), RDLENGTH=4
IP Address=216.171.120.36
Question:
Name=time.nist.gov, QTYPE=A, QCLASS=1
Answer Section:
- Name=time.nist.gov
Type=CNAME, Class=1, TTL=777 (12 Minutes 57 Seconds), RDLENGTH=10
CNAME=ntp.glb.nist.gov
- Name=ntp.glb.nist.gov
Type=A, Class=1, TTL=518 (8 Minutes 38 Seconds), RDLENGTH=4
IP Address=64.236.96.53
Question:
Name=time.nist.gov, QTYPE=A, QCLASS=1
Answer Section:
- Name=time.nist.gov
Type=CNAME, Class=1, TTL=746 (12 Minutes 26 Seconds), RDLENGTH=10
CNAME=ntp.glb.nist.gov
- Name=ntp.glb.nist.gov
Type=A, Class=1, TTL=598 (9 Minutes 58 Seconds), RDLENGTH=4
IP Address=128.138.141.172
...
...I saw the NIST servers that did not seem to be part of the pool.
Many (but not all) are. {I see them in there all the time.}
For a brief test, set your floor to 1,
and it will be easier for you to notice them.
--
E-Mail Sent to this address blackl...@anitech-systems.com
will be added to the BlackLists.
___
questions mailing list
questions@lists.ntp.org
http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 2013-03-27, Robert Scott no-one@notreal.invalid wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? I posted here several months ago about my specific application. I want to use SNTP, not to synchronize a system clock but to make a frequency measurement on the audio sampling rate in a smartphone. My use of time servers will be very occasional - typically a user of my app would only use the time servers once when the app is first installed to do a calibration of the audio sampling rate in that phone. And my app will have very limited circulation. So I won't be hitting the servers very often at all. Today I found the last bug that prevented me from getting response from the time servers in my socket code. I had neglected to set the port number (123) using htons(123) to put the port number in network byte order. When I fixed that I finally started getting responses. I have gotten responses from pool.ntp.org as well as several of the NIST time servers listed on their website. However the round-robin address of time.nist.gov does not seem to return anything. The recv() function just times out. And I sure don't want to hard-code any of the other NIST server URLs in my app. I was going to go with pool.ntp.org until I saw the NIST servers that did not seem to be part of the pool. You do NOT want to hard code anything into your program. That is extremely bad form, unless that address is one controlled by you. (Ie, if you want, you can set up your own ntp server, controlled by a GPS say, and hard code that IP into your design), but you must not do that for anyone else. It is far far too easy to have a bug in your system (sounds likely since it has taken you so long to get your system running at all-- ie there were already bugs) which will then swamp the server you are using. If you swamp yourself, that is fine-- you suffer from your own errors. If you swamp NIST, you are destroying a public resource. So I guess my main question is which servers should I use for my very limited application? Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular The pool system has no provision for enforcing registration. It wouldn't make sense to hand out a random server address if most of them then refused to serve you because you hadn't registered. users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. I would hope all the pool servers ultimately reference their national equivalent of NIST and therefore what becomes, after the fact, UTC. I think you will find that Navstar (GPS) and WWV times are traceable to NIST. MSF times are traceable to NPL. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? The open NIST servers are heavily overloaded, so probably don't serve the highest quality time, but they are likely to be around for a long time. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Wed, 27 Mar 2013 13:40:28 -0700, E-Mail Sent to this address will be added to the BlackLists Null@BlackList.Anitech-Systems.invalid wrote: Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. NIST isn't of much value outside the continental US, Ah, good point. I do have international customers. So I will go with the pool. Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Wed, 27 Mar 2013 16:14:53 -0400, Richard B. Gilbert rgilber...@comcast.net wrote: I think you need to start by defining the quality of time you need. Is 4:13 PM Eastern Timegood enough? Some people just want to Get me to the church on time! OR the bus, or the train or plane. A radio astronomer is almost certainly going to insist on time to the nearest nano second. Most of the range above is probably going to be overkill for most people. Please try to define your requirements a little more closely! Nano-seconds, get me to the church/train/plane . . . . on time! or something else. I did post my requirements a month ago in a thread about calibrating a piano tuning app using timestamps, and it degenerated into a long discussion on the accuracy of quartz crystals and the accuracy needed for piano tuning, which eventually spilled over into comp.dsp. Please let's not open that discussion again. But for those of you that may have missed it, here are the main points. I have an app I have been providing to professional piano tuners for many years. My competition advertizes an accuracy of 0.02 cents (which is 11 ppm). So whether or not you think piano tuning standards need to be that good, I need to meet that standard to be competitive. For whatever reason, the manufacturers of smartphones (especially Android devices) do not hold the audio sample rate of their devices to that standard. So I need to provide a way for the user to perform an initial calibration after the app is installed. Currently I am doing that by having the user get a trusted audio frequency source and let the app listen to it. One of my competitors actually sends each user a calibrated tone source/metronome to use in calibrating the app. This is sometimes difficult for non-tech-savy piano tuners. I thought it would be great if I could provide a means to calibrate the app using network time servers. The plan is to start up an audio input stream on the smartphone and timestamp the blocks of data as they are received from the microphone. The rate of arrival of these blocks of data is tied to the audio sample rate I wish to determine. If I timestamp a block of audio data, then count data blocks for about 3 hours, then timestamp another block of data I will be able to calculate the audio sample rate. To achieve 11 ppm accuracy in frequency I need to have a calibration time interval that is about 90,000 times as long as the timestamp uncertainty. If the timestamp uncertainty is, say, 100 msec., the calibration time period needs to be at least 2.5 hours. That's where my figure of 3 hours comes from. I don't think it will difficult for a user of my app to perform this calibration. All he has to do is to ensure Internet connectivity is turned on (it could be cell or wi-fi), hit the calibrate button in my app, and leave the phone on charge and go to bed. The app will only hit the time servers a couple of times at the beginning of the calibration period and a couple of times at the end. In between time it will just be counting audio data blocks. Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 03/27/13 16:45, David Woolley wrote: Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. SNIP I would hope all the pool servers ultimately reference their national equivalent of NIST and therefore what becomes, after the fact, UTC. I think you will find that Navstar (GPS) and WWV times are traceable to NIST. GPS is traceable to US Naval Observatory, not NIST. I've not seen a reference that NO and NIST coordinate their time, other than that they both contribute to UTC. (Note, according to www.time.gov/about.html, USNO and NIST cooperatively provide the www.time.gov service.) MSF times are traceable to NPL. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? The open NIST servers are heavily overloaded, so probably don't serve the highest quality time, but they are likely to be around for a long time. Jason ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On 2013-03-27, Richard B. Gilbert rgilber...@comcast.net wrote: On 3/27/2013 3:20 PM, Robert Scott wrote: I am confused about the proper usage of pool.ntp.org and NIST. pool.ntp.org seems to be a collection of private sector time servers offered for all to use, but with registration expected for regular users. And NIST has a government run set of time servers. Neither group (NIST or pool.ntp.org) seems to include or referece the other. Are they in competition? Who normally uses the NIST servers and who uses pool.ntp.org? I posted here several months ago about my specific application. I want to use SNTP, not to synchronize a system clock but to make a frequency measurement on the audio sampling rate in a smartphone. My use of time servers will be very occasional - typically a user of my app would only use the time servers once when the app is first installed to do a calibration of the audio sampling rate in that phone. And my app will have very limited circulation. So I won't be hitting the servers very often at all. Today I found the last bug that prevented me from getting response from the time servers in my socket code. I had neglected to set the port number (123) using htons(123) to put the port number in network byte order. When I fixed that I finally started getting responses. I have gotten responses from pool.ntp.org as well as several of the NIST time servers listed on their website. However the round-robin address of time.nist.gov does not seem to return anything. The recv() function just times out. And I sure don't want to hard-code any of the other NIST server URLs in my app. I was going to go with pool.ntp.org until I saw the NIST servers that did not seem to be part of the pool. So I guess my main question is which servers should I use for my very limited application? Robert Scott Hopkins, MN I think you need to start by defining the quality of time you need. Is 4:13 PM Eastern Timegood enough? Some people just want to Get me to the church on time! OR the bus, or the train or plane. A radio astronomer is almost certainly going to insist on time to the nearest nano second. Most of the range above is probably going to be overkill for most people. Please try to define your requirements a little more closely! Nano-seconds, get me to the church/train/plane . . . . on time! or something else. He wants to calibrate the onboard clock on a cellphone/tablet to about 10PPM for an app he is writing to act as a tuner which is supposed to be better than 1 cent in its tuning ability. He is willing to spend a few hours on this (ie, the time accuracy needs to be about 10-100 ms), but then let the clock freewheel except perhaps an occasional check that the calibration is still good. At least that is how I understand it. This app is to be sold to every musician in the world, and he is wondering whether he should hard code the NIST IP into the phone to use as an ntp server. (NONONONONO) or perhaps use the pool. I think he is worried that since the pool does not apparently use NIST as the source for the time, that the time in the pool is not good enough. He does not really understand ntp and the pool, and GPS time and thus is worried that using the pool might not be good enough for his needs. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] NIST vs. pool.ntp.org ?
On Thu, 28 Mar 2013 02:50:17 GMT, unruh un...@invalid.ca wrote: He wants to calibrate the onboard clock on a cellphone/tablet to about 10PPM for an app he is writing to act as a tuner which is supposed to be better than 1 cent in its tuning ability. He is willing to spend a few hours on this (ie, the time accuracy needs to be about 10-100 ms), but then let the clock freewheel except perhaps an occasional check that the calibration is still good. At least that is how I understand it. This app is to be sold to every musician in the world, and he is wondering whether he should hard code the NIST IP into the phone to use as an ntp server. (NONONONONO) or perhaps use the pool. I think he is worried that since the pool does not apparently use NIST as the source for the time, that the time in the pool is not good enough. He does not really understand ntp and the pool, and GPS time and thus is worried that using the pool might not be good enough for his needs. You really should read my posts before responding. No, I do not intend to hard-code NIST or any other server. I never said I wanted to. No, the app is not intended for all musicians. It is intended for professional piano tuners only. I sell about one per day. And I never said the pool would not be good enough for my needs. I only asked about the relative benefits of the pool vs. NIST, which E-mail sent...Blacklists answered very nicely. Robert Scott Hopkins, MN ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
