[ntp:questions] Re: ntpd, boot time, and hot plugging
brad at stop.mail-abuse.org
Fri Feb 4 01:12:33 UTC 2005
At 2:23 PM -0600 2005-02-03, Kenneth Porter wrote:
> What about mobile clients? In the mobile environment, what does ntpd do
> when you sever the network connection (ie. undock)?
The current version of ntpd is not well-suited for use with
mobile clients. It assumes that your IP address does not change. It
assumes that your local network latency is pretty constant, and any
variation in network latency is largely due to WAN issues.
It assumes that there is just one absolute "right" canonical
time, and that all servers are closer or farther away from that, and
that it's job is to try to figure out which server is currently the
closest (using long-term statistical data) and then to make that one
It assumes a whole host of things that are not suitable to a
> Suppose I undock
> (taking down eth1) and plug in down the hall with the built-in NIC
> (bringing up eth0).
You may no longer be anywhere "close" to the upstream time
servers you had previously configured, and may have to tear down all
your server associations and put up all new ones.
Any time you switch interfaces, get a new IP address, or any of
the other things that are typical for mobile environments, you're
basically looking at a complete stop and restart, if not a complete
stop, re-configure (presumably with totally different servers), and
> What must one do to make ntpd tolerant of that?
I'm not convinced that is possible. At least, not in the way
you're thinking of.
> Or must mobile apps give up quality time because their network
> interfaces are transient?
I think you have to assume that a mobile client would have to be
a lot more dependant on the local network services that are provided
wherever they are, and the DHCP server to tell you what the
appropriate time servers are for you to use, etc.... Then you stop
ntpd, throw away everything you previously had, completely
re-configure with the new information, and restart ntpd.
> Would one need to script a complete
> stop/start of ntpd whenever interfaces come and go?
Brad Knowles, <brad at stop.mail-abuse.org>
"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
-- Benjamin Franklin (1706-1790), reply of the Pennsylvania
Assembly to the Governor, November 11, 1755
SAGE member since 1995. See <http://www.sage.org/> for more info.
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