Re: [STDS-802-Privacy] using only 24 bits of random MAC
Hi all,
Even if in the case of 46 random bits, can the eventuallity of a
collision be ruled out ? The current reccord is 30,0000+ devices and
it already gives a 1:156000 probability.
Pushing the numbers to 60.000 and 300.000 devices, we obtain:
p(60000, 2^46) = 0.000025
p(300000, 2^46) = 0.00064
I agree that 300.000 of device is high (unrealistic ?) and that the
corresponding probabilities are small, but can we consider that
collisions will never happen even with 46 bits ?
Mathieu
On 10/01/2014 05:51 PM, Dan Harkins wrote:
>
> Hello,
>
> As Mathieu reported today, when randomizing only 24 bits of the MAC
> a collision is basically assured. We can calculate the probability of a
> collision out of a pool of c when there are n people by:
>
> p(n; c) = 1 - ((c-1)/c)^(n*(n-1)/2)
>
> If we are only randomizing 24 bits of MAC we end up with numbers
> like this:
>
> p(500, 2^24) = 0.0074
> p(1000, 2^24) = 0.029
> p(5000, 2^24) = 0.525 <-- worse than a coin flip
>
> Even with only 500 people it's basically assured that there will be a
> collision after a while. Whereas if we randomize 46 bits of MAC we end
> up with numbers like this:
>
> p(500, 2^46) = 0.0000000018
> p(1000, 2^46) = 0.0000000071
> p(5000, 2^46) = 0.0000001776
> p(10000, 2^46) = 0.0000007105
> p(30000, 2^46) = 0.00000639
>
> The record for most simultaneous associations in a wi-fi network is
> 30,0000+ and even in that situation, assuming everyone is randomizing
> MAC addresses it's still around 1:156000. Never say never but we can say
> "highly unlikely."
>
> Whereas if we only randomize 24 bits we can safely say "definitely assured".
> I do not think this study group wants to come up with recommendations on
> how to most assuredly screw up a network.
>
> The critical issue is not whether someone temporarily sets his MAC address
> to that "owned" by someone somewhere (and I am still not convinced that
> purchasers of OUIs get them with the "local" bit set), the critical issue is
> whether there will be collisions on the switched network. We do not need to
> assure that a randomly chosen MAC address is unique in the world, we just
> need to make it as unlikely as possible that that address is already used on the
> same switched network. We can't make that assurance with only 24 bits
> but we can with 46.
>
> Please, let's abandon the idea of following 802.1's recommendations for
> their small, local LAN randomized MAC scheme. It won't work in wireless
> in the real world.
>
> regards,
>
> Dan.
>
>