Re: draft-bourbaki-6man-classless-ipv6-00

[Warren Kumari <[e-mail]>]

On Thu, Jun 15, 2017 at 5:20 AM, Philip Homburg
<[e-mail]> wrote:
>>SLAAC doesn't work unless the network provides enough address space to make
>>collisions vanishingly unlikely.
>
> Indeed. So any RFC that makes smaller pseudo random IIDs possible has to
> have at least a section that analyses collision probabilities.

So, a couple of years ago I was doing some work with Juan Carlos and
Dan Harkins in the IEEE on MAC Randomization (basically randomizing
your (WiFi) MAC address to improve privacy / thwart pervasive
monitors).

There was some discussion on how many bits you would need to randomize
to make it unlikely that multiple machines will independently choose
the same address. I argued that if you randomize 24 bits, and have a
stadium of ~2000 people, you will basically never get a collision.
Dan disagreed, and so, in a fit of pique, I wrote an App Engine app to
prove him wrong -- and achieved exactly the opposite. This is a
birthday paradox problem, and you will get a collision once every ~9
times.

I've just updated my little app and put it here:
https://ipv6-collision-probability.appspot.com/

It's interesting to play with the numbers and see how likely a
collision is, given a certain subnet length and number of hosts.

>  And set
> a lower limit that is generally safe.
>
> Just leaving it to an operator to figure out that a /112 prefix will
> sometimes lead to an IoT device failing due to an address collision is
> extremely poor protocol design.

Example: With a /112, and 100 hosts, you will have a collision every
~14 times...

W

>
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-- 
I don't think the execution is relevant when it was obviously a bad
idea in the first place.
This is like putting rabid weasels in your pants, and later expressing
regret at having chosen those particular rabid weasels and that pair
of pants.
   ---maf