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[lwip-devel] [bug #27049] DHCP IP address assignment with multiple lwIP

From: Jakob Stoklund Olesen
Subject: [lwip-devel] [bug #27049] DHCP IP address assignment with multiple lwIP devices fails
Date: Tue, 21 Jul 2009 20:41:15 +0000
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Follow-up Comment #20, bug #27049 (project lwip):

The problem with increasing the timeout is that it doesn't solve your
problem. It looks like your router can make about one offer per second. This
caused problems with an initial timeout of one second. If you increase the
timeout to 4 seconds, you will get into trouble when you have 5 devices or
more; 10 seconds, 11 devices. And so on.

On the other hand, if lwip reuses XIDs in retransmissions, it can accept an
offer that has been arbitrarily delayed. You will be able to boot 100 devices
in two minutes.

Of course your fallback to a static IP after a fixed timeout will limit the
number of devices that can be turned on at once. Note that AutoIP fallback
keeps the DHCP client process running. Yes, it will choose an AutoIP address
after N tries, but as soon as it receives a DHCP offer+ACK, it goes back to
the DHCP address. That means it can handle very long DHCP delays and still be
quick to boot with AutoIP.

Sure, retransmissions is not a good thing, but you have to admit that booting
5 devices simultaneously off a consumer grade router is not a normal

Thanks for explaining the 5-device pcap. Notice that all requests have been
acked after 7 seconds, and in fact the last 4 devices are handled (discover to
ack) in 4 seconds. That limit is imposed by the router, not by lwip


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