Question on 802.11r

[Nicolas Cavallari]

On 12/11/2013 11:19, Krishna Chaitanya wrote:
> On Tue, Nov 12, 2013 at 3:46 PM, Nicolas Cavallari
> <Nicolas.Cavallari at green-communications.fr> wrote:
>> On 12/11/2013 11:08, Krishna Chaitanya wrote:
>>> On Tue, Nov 12, 2013 at 3:35 PM, Nicolas Cavallari
>>> <Nicolas.Cavallari at green-communications.fr> wrote:
>>>> On 12/11/2013 10:19, Antonio Quartulli wrote:
>>>>> On Tue, Nov 12, 2013 at 08:54:55AM +0000, michael-dev wrote:
>>>>>> Hi,
>>>>>>
>>>>>> Am 12.11.2013 05:36, schrieb Ben Greear:
>>>>>>> Now, I have a somewhat related question:  Should the station
>>>>>>> re-acquire DHCP lease after successfully roaming? ...
>>>>>>> So, it would be nice to skip
>>>>>>> DHCP, but I'm not sure if that is how real-world networks
>>>>>>> are configured?
>>>>>>
>>>>>> even without 802.11r when devices are roaming within the same SSID, some
>>>>>> don't refresh DHCP after changing the BSSID. I believe a recent version
>>>>>> of MAC OS was doing so and maybe others, though I'm not sure which HW/SW
>>>>>> exactly was used on client side. So that is why I built DHCP-Snooping /
>>>>>> IP+ARP-Filter on my APs in a way that those stations don't loose network
>>>>>> connectivity.
>>>>
>>>> When devices roams, they expect that the layer2 environment is the same.
>>>> They are not required to refresh any DHCP lease or anything.
>>>
>>> How does the STA knows whether it had done a L2/L3 roaming?
>>
>> In 802.11r, the STA initiates the roaming, so it knows.
> 
> STA has no idea of the topology of the network.

A STA scan the nearby AP, and chooses the best one with the same SSID and security
setting.  This nearby AP *should* belong to the same L2 network (of course, it's not
guaranteed, especially with access point with no security).

> 
>>> Only way it can know is if it tries to renew the IP and gets a NACK from
>>> server, then it things subnet is changed (from NACK) and tries to do
>>> a full DHCP handshake.
>>
>> 802.11 Roaming is always layer 2.  If you have two AP with the same SSID and security
>> parameters, it mean they belong to the same ESS, so to the same layer 2 network.
>>
>> If you want to do a layer 3 roaming, just disconnect and reconnect.  Or use different SSIDs.
> 
> That's now true, there is no rule that entire ESS must be in the
> single L2.

See 802.11-2012 4.3.4.2.  Roaming must be transparent to the LLC layer.