[ntp:questions] Re: ntpd, boot time, and hot plugging

Brad Knowles 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 
the syspeer.

	It assumes a whole host of things that are not suitable to a 
mobile environment.

>                                                         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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