randomizing lease renewal?

Scott Helms khelms at zcorum.com
Fri Mar 30 19:26:03 UTC 2007

> To put this in perspective, we've had requests on this list in the 
> past for 2 hours (that I can remember), and downloads lasting 2 hours 
> are quite feasible - not only that, but you have to consider that the 
> user doesn't wait until just after an address change to hit download 
> so you can affect downloads after any subset of the max lease time.

I'm sure people have asked because they'd give people a new IP every X
time period, I never said that everyone who wanted this feature wanted
it for good reasons.  However, I believe that simply because it can be
used in a poor way isn't a reason to not include it as a tool.  Like
anything else you do to a network, you had better know what you're

> As to download managers, taking your previous comments, I think you 
> have to assume that the average level of 'cluelessness' is going to 
> be far worse in your user base as a whole than amongst those with 
> enough ability to run a server - and hence I think you can safely 
> assume that few of your users will be running anything but the 
> default software, and few will think to use any resume function 
> instead of just clicking the 'download' link again.

True to a degree, but I'd be willing to wager large sums of beer that
more end users know about download managers than know about DDNS and
some of the download sites include the functionality via embedded apps
and applets to conserve their bandwidth.

> >   Again, I ask the
> >question, how is forcing a client to acquire a new IP address any more
> >harmful than having a cable, DSL, or wireless connection drop offline?
> How badly engineered is your cable network ? I would suggest that if 
> a cable modem can't keep it's link up for at least days at a time 
> then there's something wrong. That's a lot different to deliberately 
> engineering a broken network - like forcing all cable modems to 
> disconnect every few hours.

Again, I said explicitly that simply using a timer to make everyone
change IP's for no good reason is dumb.  I'm not disagreeing, but having
a tool that forces a new IP address upon lease renew doesn't mean you
have to use it that way.  As for cable modems and their up times, the
number of re associations that occur on them would startle most people
who (wrongly) assume that their connection truly is rock solid.  CMTS
and other broadband networks are not like a LAN where you get to control
how the world works.  Temperature variations, in home wiring, noisy
(from an RF pov) appliances, radar, and a myriad of other outside
sources of problems have a big impact.

For me, it was easier to code some tools that exist around ISC than
trying to write something that was convincing enough to get things
added.  My real reason for responding in the first place was to gently
say that this "feature" does have legitimate uses.

Scott Helms
Vice President of Technology
ISP Alliance, Inc. DBA ZCorum
(678) 507-5000

More information about the dhcp-users mailing list