Fwd: Rapid DHCP

[Nathaniel McCallum]

Dan, that works on wireless networks. On wired networks the ARP technique
determines *which* of the valid leases you should attempt to restore. So on
a wired network you:
1. ARP the known DHCP server IPs to discover the subnet.
2. ARP the IP from the valid lease on that subnet to avoid collision.
3. Restore the ifconfig from the still valid lease.
4. Renew the lease.

This should be pretty sane and gives large speedups to resuming on wired
(which people with docks do a lot).

Nathaniel
On Jul 30, 2011 6:45 PM, "Dan Williams" <dcbw at redhat.com> wrote:
> On Sat, 2011-07-30 at 11:46 -0400, Genes MailLists wrote:
>> On 07/30/2011 10:37 AM, Lennart Poettering wrote:
>> > On Sat, 30.07.11 10:31, Genes MailLists (lists at sapience.com) wrote:
>> >
>> >>>> http://cafbit.com/entry/rapid_dhcp_or_how_do
>> >>>
>>
>> >
>> > IIRC connman (i.e. NM's competition) can do the ARP magic, too.
>> >
>> > Lennart
>> >
>>
>>
>> Seems like a pretty reasonable thing to do - surely better than just
>> waiting for a timeout to decide if the server is not there ... are you
>> aware of any gotcha's ?
>
> NM already keeps DHCP information around based on the network you're
> connecting to, so we don't need to ARP a bunch of servers just to
> determine whether the DHCP server we wanted is still there. dhclient is
> smart enough to attempt to reclaim the lease if it's not already
> expired. Note that the Mac attempts to ARP a number of different DHCP
> servers (192.168.2.1, 192.168.4.1, 192.168.1.1) which would be pointless
> with NetworkManager, because it's extremely unlikely that the DHCP
> server on your wifi network has changed; NM would simply know that the
> last DHCP server used *on that wifi network* was 192.168.1.1 and not
> bother to try talking to other ones like Mac OS X appears to do.
>
> NM could use the same method of ARPing multiple DHCP servers that Mac OS
> X does, but it wouldn't provide much additional benefit, if any, at
> least on WiFi networks. It could be used on wired networks to (a)
> determine which wired network you're connected to, and (b) do rapid
> DHCP. Again, NM already knows what DHCP server and what lease was last
> used on the specific wifi network you just connected to, and it won't
> bother doing a DISCOVER, it'll just jump to RENEW if your lease is still
> valid.
>
> What's unique about the method described there is that the Mac
> configures the interface with the same IP address it previously had if
> the lease is still valid, while NetworkManager waits for the DHCP server
> confirm the lease. So we could presumptuously configure the interface
> with the previous address from the lease and then only tear it down if
> the DHCP server fails or rejects the renewal.
>
> Of course, none of this helps if your DHCP leases are short, but it
> certainly helps if you put your laptop to sleep a lot and wake it up in
> the same location.
>
> Dan
>
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