bss_max_count needed in real-world scenario

[Ben Greear]

On 07/11/2011 10:48 AM, Jouni Malinen wrote:
> On Mon, Jul 11, 2011 at 10:18:28AM -0700, Ben Greear wrote:
>> We had a user testing our wifi station emulator in a big, new
>> trade-show facility, and there were so many APs there that
>> supplicant would not associate.
>>
>> Turns out, we needed to set the bss_max_count to something
>> larger (we chose 2000).  Then it worked like a charm.
>
> How did you configure wpa_supplicant to connect to the network? With a
> configuration file that had a single network block with the SSID
> specified?
>
>> However, I suspect that many folks are not aware of this
>> setting and will be disappointed when they fire up their
>> Linux laptop in this building.  It is quite difficult
>> to debug what is going on...we mainly got lucky with a
>> google search to find this bss_max_count option.
>>
>> I'd suggest either making the bss-count larger by default,
>> able to grow dynamically, or at the very least, big loud
>> warning messages with suggested config changes in the
>> logs when there are more BSS's than can fit in the
>> current buffers.
>
> The BSS table itself is allocated one BSS at a time, so it can grow in
> size dynamically. The main reason for the string limit on the number of
> entries is to avoid unbounded memory use that would be quite unfortunate
> on many embedded designs and well, even on other devices since it would
> be trivial for an attacker to generate thousands of APs to fill memory..
> As such, I'm not sure that I would really like to increase the default
> value.
>
> I think that a better approach would be to make wpa_bss_add() more
> clever on which old entry to remove when adding a new one. If you have
> already configured a network (or multiple networks), it would be better
> to prefer to remove BSS entries that do not have a matching SSID instead
> of the one that was updated longest time ago. This should address the
> most realistic use cases without increasing memory use.

As a follow-up, it appears they have around 650 APs in this facility,
though you probably can't hear them all at once.

Not sure how many are on the same ESSID, but perhaps all of them (with multiple
ESSIDs per AP).

Maybe default the value large, and let users on small boxes decrease it
to save memory?

Thanks,
Ben

>


-- 
Ben Greear <greearb at candelatech.com>
Candela Technologies Inc  http://www.candelatech.com