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Re: clock problems
On Fri, Mar 23, 2001, Tzafrir Cohen wrote about "Re: clock problems":
> Well, this address was given to me by BezeqInt support people, after
> asking them what ntp server should I use when connecting through them (not
> with my Technion connection).
>
> This may be off-topic, but which server should I use? (or: in which FM it
> is written? as my ISP's people seem to give the wrong answer)
The egotist answer is - "just use any ntp server which accepts your
connection".
The net-friendly way, however, is to build a heirarchy of NTP servers: stratum
1 servers have access to a local time source (atomic clock, GPS reciever, or
something like that). But there are relatively few of those (as far as I
know, only one in Israel) and if everyone connected to those they would be
overwhelmed. Instead, only a relatively small number of stratum 2 should
connect to the stratum 1 server - say one stratum 2 server per ISP. If your
organization connects, say, a 100 comuters to that one ISP's stratum 2 server,
it is more polite to create a local stratum 3 server in your company and have
all these computers use this server.
So to be friendly when choosing an NTP server to your computer, you better
avoid using stratum 1 servers altogether, and use a stratum 2 (like relay.huji,
ntp.netvision.net.il, etc.) or stratum 3 if you are in an organization that
provides one (like the Technion's ntp.technion.ac.il). Watch out that
relay.huji is probably the busiest ntp server in Israel - does anybody know
of more stratum 2 server in Israel? Bezekint SHOULD create a stratum 2
server too - perhaps using the same stratum-1 source (ntp.ilan.net.il) that
the others (relay.huji and ntp.netvision) are using.
In http://www.eecis.udel.edu/~mills/ntp/servers.html you can find a list
of stratum 1 and 2 servers around the world. Note that the accuracy of NTP
depends on a connection with very low latency variability - so using a
server outside Israel will probably mean that you shouldn't expect accuracy
of more than 300ms or so. If you want better synchronization between several
local hosts and still want to use a forien server, start a local
higher-stratum ntp server and have all the local hosts use it.
You can also use several sources, and ntp will use a source with the lowest
available stratum, and when several (3 or more) are available with the same
stratum it will attempt to find if any of them have bad time. Your home PC
should probably have only one source, but if you run a server that others
would use, you should probably use 3 sources of the same stratum (if possible).
P.S. If you don't use the full ntp protocol, but rather use "ntpdate" for
a one-time update of your time when you log in, your impact on the network
will be even lower (normally, the full protocol contacts all the time sources
every 64 to 1024 seconds) - this method is especially useful on a dialup
home computer, where you're anyway not connected to the network most of the
time.
--
Nadav Har'El | Friday, Mar 23 2001, 29 Adar 5761
nyh@math.technion.ac.il |-----------------------------------------
Phone: +972-53-245868, ICQ 13349191 |This space is for sale - inquire inside.
http://nadav.harel.org.il |
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