Re: [ntp:questions] ntp pool servers disappear - more data
On 2021-06-24, Jim Pennino wrote: > Jim Pennino wrote: >> I was checking the stability of a new USB GPS refclock on a server which >> is configured to use the GPS, servers from the ntp pool, and another server >> of mine that has a PPS GPS receiver. >> >> I noticed that almost all the pool servers had disappeared. >> >> I then checked other machines that use my "good" server and the ntp >> pool; most all of the pool servers had also disappeared on those >> machines. >> >> This is a mix of PC linux, rasberry pi linux, rasberry pi buster, and >> Windows 10 machines with Meinberg, all with the latest ntp from their >> distros. >> >> Long story short: I realized I had had a network outage and tested the >> theory that was the cause. It was. >> >> It seems that any server in ntp.conf that is specified as a name, as >> the pool servers are, will after a sufficiently long DNS outage just >> disappear and not come back after the outage without restarting ntp. >> >> It would seem to me that ntp should only need to do a DNS lookup on >> startup and from then on continue to use the address found. >> >> But that is not how ntp works. >> >> Anyway, the bottom line is that if the pool is your only source of time >> and if there is a DNS failure for a sufficiently long time, you will >> lilely not have any source of time afterwards. >> >> As for the USB GPS I was testing, it is called a VK-162 G-Mouse >> available from Amazon for $14, uses the Windows 10 native driver so it >> works with Meinberg ntp, and keeps the time within single digit >> milliseconds without any other servers. > > Further testing shows the following: > > I took a machine and ran watch -p -n 10 ntpq -pn to monitor ntp status. > > I then pulled the network connection from the machine. > > After about 7 minutes the pool servers started to disappear. > > If the machine was reconnected to the network within about 15 minutes, > the pool servers would reappear. > > If the machine was off the network for more than about 15 minutes, the > pool servers do NOT reappear until ntp is restarted. > I suspect it is the number of times that ntpd tries to contact the server and fails rather than the time that is important. You could try putting the server offline and then online again (I use chrony so do not remember if ntpd has that option). > ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
Jim Pennino wrote: > I was checking the stability of a new USB GPS refclock on a server which > is configured to use the GPS, servers from the ntp pool, and another server > of mine that has a PPS GPS receiver. > > I noticed that almost all the pool servers had disappeared. > > I then checked other machines that use my "good" server and the ntp > pool; most all of the pool servers had also disappeared on those > machines. > > This is a mix of PC linux, rasberry pi linux, rasberry pi buster, and > Windows 10 machines with Meinberg, all with the latest ntp from their distros. > > Long story short: I realized I had had a network outage and tested the > theory that was the cause. It was. > > It seems that any server in ntp.conf that is specified as a name, as > the pool servers are, will after a sufficiently long DNS outage just > disappear and not come back after the outage without restarting ntp. > > It would seem to me that ntp should only need to do a DNS lookup on > startup and from then on continue to use the address found. > > But that is not how ntp works. > > Anyway, the bottom line is that if the pool is your only source of time > and if there is a DNS failure for a sufficiently long time, you will > lilely not have any source of time afterwards. > > As for the USB GPS I was testing, it is called a VK-162 G-Mouse > available from Amazon for $14, uses the Windows 10 native driver so it > works with Meinberg ntp, and keeps the time within single digit > milliseconds without any other servers. Further testing shows the following: I took a machine and ran watch -p -n 10 ntpq -pn to monitor ntp status. I then pulled the network connection from the machine. After about 7 minutes the pool servers started to disappear. If the machine was reconnected to the network within about 15 minutes, the pool servers would reappear. If the machine was off the network for more than about 15 minutes, the pool servers do NOT reappear until ntp is restarted. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
William Unruh wrote: > I suspect it is the number of times that ntpd tries to contact the > server and fails rather than the time that is important. You could try > putting the server offline and then online again (I use chrony so do not > remember if ntpd has that option). No, it doesn't. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
Jim, can you please subscribe to the mailing list, so that I don't have to approve manually every single post you send? Thanks in advance Ciao -- bronto Il giorno ven 25 giu 2021 alle ore 08:58 Jim Pennino ha scritto: > William Unruh wrote: > > > > > I suspect it is the number of times that ntpd tries to contact the > > server and fails rather than the time that is important. You could try > > putting the server offline and then online again (I use chrony so do not > > remember if ntpd has that option). > > No, it doesn't. > > ___ > 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] ntp pool servers disappear - more data
On 2021-06-25, Jim Pennino wrote: > chris wrote: >> On 06/25/21 04:08, Jim Pennino wrote: >>> William Unruh wrote: >>> >>> >>> I suspect it is the number of times that ntpd tries to contact the server and fails rather than the time that is important. You could try putting the server offline and then online again (I use chrony so do not remember if ntpd has that option). >>> >>> No, it doesn't. >>> >> >> You could use a one line cron script to restart every day, week, >> whenever... > > Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and a > public server that does not request use of DNS. You could try specifying the server by IP rather than by name, so DNS is not needed. Of course this rule out using pool, unless you put them in by IP. DNS is just used to translate names to IP, so if you use IP, then DNS is not needed. > > ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
On 6/25/21 12:10 PM, William Unruh wrote: You could try specifying the server by IP rather than by name, so DNS is not needed. Of course this rule out using pool, unless you put them in by IP. DNS is just used to translate names to IP, so if you use IP, then DNS is not needed. We strongly discourage that unless you own the IP address. Owners of the servers have the right to change the system to use for NTP as well as remove it from service altogether. We've had IP addresses bombarded at high rates for years when a formerly active server has been taken out of service. Danny ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
On 06/25/21 04:08, Jim Pennino wrote: William Unruh wrote: I suspect it is the number of times that ntpd tries to contact the server and fails rather than the time that is important. You could try putting the server offline and then online again (I use chrony so do not remember if ntpd has that option). No, it doesn't. You could use a one line cron script to restart every day, week, whenever... ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
On 06/25/21 17:28, Jim Pennino wrote: William Unruh wrote: On 2021-06-25, Jim Pennino wrote: chris wrote: On 06/25/21 04:08, Jim Pennino wrote: William Unruh wrote: I suspect it is the number of times that ntpd tries to contact the server and fails rather than the time that is important. You could try putting the server offline and then online again (I use chrony so do not remember if ntpd has that option). No, it doesn't. You could use a one line cron script to restart every day, week, whenever... Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and a public server that does not request use of DNS. You could try specifying the server by IP rather than by name, so DNS is not needed. Of course this rule out using pool, unless you put them in by IP. DNS is just used to translate names to IP, so if you use IP, then DNS is not needed. Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A public server that does NOT request use of DNS which yields 3 sources of time without using a pool or DNS lookups. Or for $28/machine I could use 2 USB GPS receivers and my machine with PPS GPS, which also provides 3 sources of time without any network access at all. Your choice, but when I registered the ntp server here with the pool, I just used the fixed ip address. That's what they ask for and it does bypass dns altogether. The less translation the better, unless you really need it... ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
chris wrote: > On 06/25/21 04:08, Jim Pennino wrote: >> William Unruh wrote: >> >> >> >>> I suspect it is the number of times that ntpd tries to contact the >>> server and fails rather than the time that is important. You could try >>> putting the server offline and then online again (I use chrony so do not >>> remember if ntpd has that option). >> >> No, it doesn't. >> > > You could use a one line cron script to restart every day, week, > whenever... Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and a public server that does not request use of DNS. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
chris wrote: > On 06/25/21 17:28, Jim Pennino wrote: >> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A >> public server that does NOT request use of DNS which yields 3 sources of >> time without using a pool or DNS lookups. >> >> Or for $28/machine I could use 2 USB GPS receivers and my machine with >> PPS GPS, which also provides 3 sources of time without any network >> access at all. >> >> > > Your choice, but when I registered the ntp server here with the pool, I > just used the fixed ip address. That's what they ask for and it does > bypass dns altogether. The less translation the better, unless > you really need it... Actually what I plan to do is to put a $14 USB GPS on the machine that already has a PPS GPS attached and do away with ALL external machines. If there are two GPS receivers attached to the machine I have a backup if one receiver fails. As GPS receivers are highly unlikely to fail in some wonky mode, e.g. time being off by some large amount, but to fail completely, there is no need for any other reference source while I replace the failed receiver. Now if there is a Carrington-class coronal mass ejection or WWIII breaks out, I will lose all time references but I will have lots of other things to worry about that are much more important than the computer clock and it is likely that all internet access will also be down. Then on two other machines I attach two $14 USB GPS receivers and no external references. These three machines then provide time for all other machines on my network. The three machines will provide the redundancy needed for when one of those machines gets rebooted for updates/upgrades. Done. The only foreseeable change to that I might ever make is if and when USB 3.0 GPS receivers with PPS become cheap and available, I might swap out the USB receivers with one of those just to see how well they work. Yes, this scheme only gets my machines to within 10s of milliseconds to the actual time, but that is good enough for me. If I needed better, I would buy one of the $685 GPS GNSS Disciplined Rubidium clocks off ebay and get time to the nanosecond. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
William Unruh wrote: > On 2021-06-25, Jim Pennino wrote: >> chris wrote: >>> On 06/25/21 04:08, Jim Pennino wrote: William Unruh wrote: > I suspect it is the number of times that ntpd tries to contact the > server and fails rather than the time that is important. You could try > putting the server offline and then online again (I use chrony so do not > remember if ntpd has that option). No, it doesn't. >>> >>> You could use a one line cron script to restart every day, week, >>> whenever... >> >> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and a >> public server that does not request use of DNS. > > You could try specifying the server by IP rather than by name, so DNS is > not needed. Of course this rule out using pool, unless you put them in > by IP. DNS is just used to translate names to IP, so if you use IP, then > DNS is not needed. Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A public server that does NOT request use of DNS which yields 3 sources of time without using a pool or DNS lookups. Or for $28/machine I could use 2 USB GPS receivers and my machine with PPS GPS, which also provides 3 sources of time without any network access at all. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
On 2021-06-25, Jim Pennino wrote: > William Unruh wrote: >> On 2021-06-25, Jim Pennino wrote: >>> chris wrote: On 06/25/21 04:08, Jim Pennino wrote: > William Unruh wrote: > > > >> I suspect it is the number of times that ntpd tries to contact the >> server and fails rather than the time that is important. You could try >> putting the server offline and then online again (I use chrony so do not >> remember if ntpd has that option). > > No, it doesn't. > You could use a one line cron script to restart every day, week, whenever... >>> >>> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and a >>> public server that does not request use of DNS. >> >> You could try specifying the server by IP rather than by name, so DNS is >> not needed. Of course this rule out using pool, unless you put them in >> by IP. DNS is just used to translate names to IP, so if you use IP, then >> DNS is not needed. > > > Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A > public server that does NOT request use of DNS which yields 3 sources of > time without using a pool or DNS lookups. Not at all sure what you are suggesting. DNS is a way of translating names to IP addresses, which your machine MUST use to talk to a remote machine not on your network. The remote machine has nothing to do with this. Now some remote machines will as for the name associated with the IP address of machines sending the remote machine a query, to try to see if someone is spoofing the IP address, but as far as I know ntpd does not do that. Takes too much time and would make the time responses really bad. > > Or for $28/machine I could use 2 USB GPS receivers and my machine with > PPS GPS, which also provides 3 sources of time without any network >e access at all. Sure. The problem of course is that that $28 onlybuys you a pretty bad time source (pretty bad meaning milliseconds rather than microseconds or nanoseconds), which for most of man's history on this earth is absolutely astonishingly, and inconceivably good. Note that hanging all three off of one machine can lead to conflict between them as to interrupt processing, leading to degraded time performance. But again that is at the microsecond level, not milli or second level. Of course if you machine is at the bottom of a mineshaft in mountains, gps receivers are pretty useless. Or in the basement of a highrise without windows. > > ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
On 2021-06-25, Jim Pennino wrote: > chris wrote: >> On 06/25/21 17:28, Jim Pennino wrote: > ... > > Actually what I plan to do is to put a $14 USB GPS on the machine that > already has a PPS GPS attached and do away with ALL external machines. > > If there are two GPS receivers attached to the machine I have a backup > if one receiver fails. Two is in general bad, because your machine has no idea which the better one is and is likely to pick the GPS ratehr than the PPS. > > As GPS receivers are highly unlikely to fail in some wonky mode, e.g. time > being off by some large amount, but to fail completely, there is no need > for any other reference source while I replace the failed receiver. Since both are attached to the same machine, the probability of common mode errors become high. The cleaner unpluggin the line which feeds both receivers, etc. > > Now if there is a Carrington-class coronal mass ejection or WWIII > breaks out, I will lose all time references but I will have lots of > other things to worry about that are much more important than the > computer clock and it is likely that all internet access will also be > down. That of course is a very very general common mode error, and is extremely hard to counteract. More likely are those in your office, on your floor, or in your building. > > Then on two other machines I attach two $14 USB GPS receivers and no > external references. Remember pps is a factor of about 1 more accurate than than NMEA GPS. > > These three machines then provide time for all other machines on my > network. The three machines will provide the redundancy needed for when > one of those machines gets rebooted for updates/upgrades. Again, make sure they are all on separate electrical circuits, prefereably also in separate buildings. > > Done. > > The only foreseeable change to that I might ever make is if and when USB > 3.0 GPS receivers with PPS become cheap and available, I might swap out > the USB receivers with one of those just to see how well they work. The usb level is irrelevant. It is the PPS that is important. And pps receivers are also coming down. In fact that UBLOCK probably has a PPS output, which the manufacturer never bothered to hook upon the puck. It is hard to feed ppd over usb with any accuracy. However a separate pps line which you can attach to some irq line on the computer is probably possible even for that cheap puck. > > Yes, this scheme only gets my machines to within 10s of milliseconds to > the actual time, but that is good enough for me. > > If I needed better, I would buy one of the $685 GPS GNSS Disciplined > Rubidium clocks off ebay and get time to the nanosecond. There is still a wide gap between namosecond and 10s of milliseconds. "If walking is too slow, I can always buy a X15 to get there." Actually the difference there is far less than the difference between ns and msec. > > ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
William Unruh wrote: > On 2021-06-25, Jim Pennino wrote:A >> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, and A >> public server that does NOT request use of DNS which yields 3 sources of >> time without using a pool or DNS lookups. > > Not at all sure what you are suggesting. DNS is a way of translating > names to IP addresses, which your machine MUST use to talk to a remote > machine not on your network. The remote machine has nothing to do with > this. Now some remote machines will as for the name associated with the > IP address of machines sending the remote machine a query, to try to see > if someone is spoofing the IP address, but as far as I know ntpd does > not do that. Takes too much time and would make the time responses > really bad. This is not quite correct. If a program has an IP address, as in put the IP address in ntp.conf, then the program already has the IP address and does NOT need to do a DNS query ever. Using a IP address for ntp pools is a bad idea as someone else has said. However, there are lists of publicly available ntp servers which list the owners preference for DNS usage. Some servers want you to use the fully qualified domain name and some servers don't care if you use the IP address. >> Or for $28/machine I could use 2 USB GPS receivers and my machine with >> PPS GPS, which also provides 3 sources of time without any network >>e access at all. > > Sure. The problem of course is that that $28 onlybuys you a pretty bad > time source (pretty bad meaning milliseconds rather than microseconds or > nanoseconds), which for most of man's history on this earth is > absolutely astonishingly, and inconceivably good. Except you are forgetting a few things: 1. I have a ntp server with a real PPS GPS attached which is good to microseconds. 2. My actual real time requirement is in the 10s of millisecond range. 3. Any accuracy past the requirements of number 2 is purely out of curiosity. > Note that hanging all three off of one machine can lead to conflict > between them as to interrupt processing, leading to degraded time > performance. But again that is at the microsecond level, not milli or > second level. As each will go into a separate plug, that is HIGHLY unlikely to happen. I never said anything about hanging three receivers on one machine, as two receivers are more than sufficient for normal, i.e. WWIII isn't happening, times. > Of course if you machine is at the bottom of a mineshaft in mountains, > gps receivers are pretty useless. Or in the basement of a highrise > without windows. At one place I worked at where the computer room was in the basement and they did care about accurate time, they bought a commercial ntp server black box that cost several thousands of dollars and ran a cable to the roof for the antenna. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
William Unruh wrote: > On 2021-06-25, Jim Pennino wrote: >> chris wrote: >>> On 06/25/21 17:28, Jim Pennino wrote: >> > ... >> >> Actually what I plan to do is to put a $14 USB GPS on the machine that >> already has a PPS GPS attached and do away with ALL external machines. >> >> If there are two GPS receivers attached to the machine I have a backup >> if one receiver fails. > > Two is in general bad, because your machine has no idea which the better > one is and is likely to pick the GPS ratehr than the PPS. It will pick the one with PPS as the jitter is much better. Tested and verified. >> As GPS receivers are highly unlikely to fail in some wonky mode, e.g. time >> being off by some large amount, but to fail completely, there is no need >> for any other reference source while I replace the failed receiver. > > Since both are attached to the same machine, the probability of common > mode errors become high. The cleaner unpluggin the line which feeds both > receivers, etc. Common mode errors from what? The GPS receivers connect to separte ports on different interal busses. >> Now if there is a Carrington-class coronal mass ejection or WWIII >> breaks out, I will lose all time references but I will have lots of >> other things to worry about that are much more important than the >> computer clock and it is likely that all internet access will also be >> down. > > That of course is a very very general common mode error, and is > extremely hard to counteract. More likely are those in your office, on > your floor, or in your building. The last time there was a Carrington-class coronal mass ejection that hit the Earth was 1859. >> >> Then on two other machines I attach two $14 USB GPS receivers and no >> external references. > > Remember pps is a factor of about 1 more accurate than than NMEA > GPS. Yeah, so? How many times do I have to say I DO HAVE A GPS WITH REAL PPS? >> These three machines then provide time for all other machines on my >> network. The three machines will provide the redundancy needed for when >> one of those machines gets rebooted for updates/upgrades. > > Again, make sure they are all on separate electrical circuits, > prefereably also in separate buildings. Why? This is a hobby, not the New York Stock exchange. >> Done. >> >> The only foreseeable change to that I might ever make is if and when USB >> 3.0 GPS receivers with PPS become cheap and available, I might swap out >> the USB receivers with one of those just to see how well they work. > > The usb level is irrelevant. It is the PPS that is important. And pps > receivers are also coming down. In fact that UBLOCK probably has a PPS > output, which the manufacturer never bothered to hook upon the puck. > It is hard to feed ppd over usb with any accuracy. However a separate > pps line which you can attach to some irq line on the computer is > probably possible even for that cheap puck. Sigh, the USB level is highly relevant. There is nowhere in USB 2 interface to "hook up" a PPS signal. As USB does not have any lines other than serial data, any PPS signal would have to be emulated in the interface as two virtual serial ports and basically you can not do that with USB 2. With USB 3 you CAN have multiple virtual serial ports. Also, USB 3 is orders of magnitude faster than USB 2, which means the latency and jitter of the signals is much better. >> Yes, this scheme only gets my machines to within 10s of milliseconds to >> the actual time, but that is good enough for me. >> >> If I needed better, I would buy one of the $685 GPS GNSS Disciplined >> Rubidium clocks off ebay and get time to the nanosecond. > > There is still a wide gap between namosecond and 10s of milliseconds. > "If walking is too slow, I can always buy a X15 to get there." Actually > the difference there is far less than the difference between ns and msec. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
On 26/06/2021 00:12, William Unruh wrote: Not at all sure what you are suggesting. DNS is a way of translating names to IP addresses, which your machine MUST use to talk to a remote As already noted, there is no MUST about it. I'd put it as low as MAY, and it is definitely no more than SHOULD. machine not on your network. The remote machine has nothing to do with DNS can be used for local network machines, as well, and this is very common. this. Now some remote machines will as for the name associated with the IP address of machines sending the remote machine a query, to try to see if someone is spoofing the IP address, but as far as I know ntpd does not do that. Takes too much time and would make the time responses really bad. ntpd doesn't care about who is sending it a query, and, in any case reverse DNS lookups often provide bad results, which won't match the preferred forward lookup, in the real world. ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] ntp pool servers disappear - more data
Hello: Someone wrote on this list that their employer refused to allow them to use GPS is a time source for an (I believe) embedded system, saying that spoofing attacks and variations in signal strength made GPS unreliable. There's actually been quite a bit of discussion on the Internet and elsewhere about GPS insecurities. New generations of satellites are supposed to be more secure, but all that is quite expensive. This is an important issue now that so much public navigation depends on GPS. You write that there are $14 and $28 GPS devices available to provide time, but I would be willing to bet they don't have built-in spoofing detectors. U-Blox claims to have anti-spoofing detection on its devices, which I suppose could be verified. Also it's unknown to me how the user would determine that the signal might be spoofed from a U-Blox device. Just something to think about if accurate time is important to you. Charles Elliott -Original Message- From: questions [mailto:questions-bounces+elliott.ch=comcast@lists.ntp.org] On Behalf Of Jim Pennino Sent: Friday, June 25, 2021 2:07 PM To: questions@lists.ntp.org Subject: Re: [ntp:questions] ntp pool servers disappear - more data chris wrote: > On 06/25/21 17:28, Jim Pennino wrote: >> Or for $14/machine I could use a USB GPS, my machine with PPS GPS, >> and A public server that does NOT request use of DNS which yields 3 >> sources of time without using a pool or DNS lookups. >> >> Or for $28/machine I could use 2 USB GPS receivers and my machine >> with PPS GPS, which also provides 3 sources of time without any >> network access at all. >> >> > > Your choice, but when I registered the ntp server here with the pool, > I just used the fixed ip address. That's what they ask for and it does > bypass dns altogether. The less translation the better, unless you > really need it... Actually what I plan to do is to put a $14 USB GPS on the machine that already has a PPS GPS attached and do away with ALL external machines. If there are two GPS receivers attached to the machine I have a backup if one receiver fails. As GPS receivers are highly unlikely to fail in some wonky mode, e.g. time being off by some large amount, but to fail completely, there is no need for any other reference source while I replace the failed receiver. Now if there is a Carrington-class coronal mass ejection or WWIII breaks out, I will lose all time references but I will have lots of other things to worry about that are much more important than the computer clock and it is likely that all internet access will also be down. Then on two other machines I attach two $14 USB GPS receivers and no external references. These three machines then provide time for all other machines on my network. The three machines will provide the redundancy needed for when one of those machines gets rebooted for updates/upgrades. Done. The only foreseeable change to that I might ever make is if and when USB 3.0 GPS receivers with PPS become cheap and available, I might swap out the USB receivers with one of those just to see how well they work. Yes, this scheme only gets my machines to within 10s of milliseconds to the actual time, but that is good enough for me. If I needed better, I would buy one of the $685 GPS GNSS Disciplined Rubidium clocks off ebay and get time to the nanosecond. ___ 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
