Related link: http://www.pool.ntp.org
Someone stops you on the street and asks “Hey, do you know what time
it is?” You tell him and continue on your way. But what if it was
hundreds of thousands of people every day, because they didn’t know
who else to ask? You might decide to not answer any more. That’s the
situation that some important Internet time servers are in, and some
simple changes in your computer’s configuration can help ease the strain.
NTP, the Network Time Protocol,
is a standard for net-connected computers to find out the correct time
from other computers. Your computer’s clock can easily lose accuracy
over time for many different reasons, and setting it to regularly check
a server via NTP will keep its clock always in sync with the correct time.
What’s the correct time? There are a number of servers attached to highly
accurate atomic clocks, or to clocks synced to GPS satellites, called
stratum 1.
These are basically the master clocks for the Internet. Servers that
sync to the stratum 1 servers are in
stratum 2.
Because of the time it takes to communicate between servers, there is
always a little bit of a drift from accuracy when connecting between
servers, so stratum 2 is slightly less accurate than stratum 1, and
stratum 3 slightly less accurate than stratum 2, and so on.
For servers that must be highly accurate, such as for extremely
precise scientific applications, only connections to stratum 1 servers
will do. For the rest of us who can tolerate being off by a few tenths
of a second, stratum 2 and below will do fine.
Unfortunately, as the number of computers on the Internet has exploded,
so has the number of computers syncing their clocks to stratum 1 servers.
These servers usually don’t need the accuracy of a stratum 1 server, such
as nist.time.gov,
but because they are so well known as a time server, people often use
them by default. The big popular servers were beginning to buckle, and
some stratum 1 servers have had to move to a permission-based system,
and some have withdrawn completely. The load of tens or hundreds of
thousands of users with hourly cron jobs for rdate -s time.nist.gov
to set the time has just proved to be too much.
To fight this problem of unnecessary load on stratum 1, and to
make it easier for people to make their systems well-behaved, in
2003 Adrian von Bidder created the all-volunteer NTP Pool project
(http://www.pool.ntp.org).
The pool is a set of freely usable time servers. When a client machine
tries to sync to pool.ntp.org, that machine is referred to one of the
pool servers, round-robin style. This helps distribute the loads to
different servers in the pool.
How you can help
You can help relieve the load on the strained servers by switching your
computer’s settings to use pool.ntp.org. Your system will start
using one of the pool servers. You can even choose a geographically
close pool by using us.pool.ntp.org for United States, europe.pool.ntp.org
for Europe, and so on. See
http://www.pool.ntp.org/use.html
for more details, including how to modify your system’s settings.
You may not need to change your settings. For example, Mac OS X comes
set to check Apple’s own time server at time.apple.com, and the
Ubuntu Linux distribution is set to check ntp.ubuntulinux.org.
Your ISP may also have a time server available. My ISP, Speakeasy,
runs a time server which is only four hops away. On the other hand,
if you’re checking clock.isc.org or time.nist.gov,
please change to pool.ntp.org immediately.
The pool.ntp.org project can use more volunteers, too. At this
writing, there are 600 servers in the pool, with the number growing
every week.
Ask Bjørn Hansen
of
Develooper.com has taken over the
administration of the project and is always seeking more servers to add
to the pool. All it takes to run an NTP server is a little tech savvy,
a static IP, and a continuous connection to the net. For more information
on how to join, visit
http://www.pool.ntp.org/join.html.
Have you checked what time server you’re using? Did you need to change it?
why do computer clocks have so much time drift?
Could someone explain in relatively simple terms why it is that my wristwatch and my little wall clock keep time just fine for months, but the sophisticated electronics in my PC slowly but surely drift off the right time?
why do computer clocks have so much time drift?
My understanding is that it's based on the heating/cooling cycle of your PC. The heat affects the accuracy of the clock.
GPS receivers
There have been problems (particularly around the leap second) with many of the pool servers, so some people may be reluctant to use them due to past bad experiences. Some of the bad experiences may have been due to using older ntp clients that didn't deal properly with misbehaving servers. Using another reliable time source, e.g., GPS, should be SOP for all but the smallest sites.
It should be noted that GPS receivers (_NOT_ USB) providing PPS signals suitable for decoding by most PC serial port hardware are now available for very reasonable prices (US$120).
Why don't ISP's make time server configuration part of their standard setup? Most ISP's probably have time servers and there are benefits for analyzing logs, etc. if their clients agree about time, and are in a position to block client access to external NTP servers.
The problems around the leap second where not particular to the pool servers. The 4 timeservers at my ISP misbehaved just as well. This was caused by the long time that has passed since the previous leap second, so that lots of clocks and NTP implementations had been developed that had never experienced one.
Sure a radio clock receiver is good to have. My system is synced to 2 different GPS receivers and a longwave receiver (just for fun).
However, take into account that receiving GPS or longwave is not always feasible in an office building or serverroom environment. NTP is a good solution for that problem.