using "reserved" IPv6 space

Fri Jul 20 00:30:25 UTC 2012

On 7/19/12, Mark Andrews <marka at isc.org> wrote:
> Actually you can't.
> 	fdaa:aaaa:aaaa has 20/20 0/1 bits but is entirely non random.
> 	fdf0:f0f0:f0f0 has 20/20 0/1 bits but is entirely non random.
[snip]
> The ratio of the number of bits doesn't tell you anything about whether
> the number was random or not.
[snip]

Sure it does.   A ratio of 1s to 0s  of a sufficient deviation, is a
sufficient but not a necessarily condition, for establishing that a
sequence of binary numbers shown almost certainly was not chosen
randomly.

As for whether "fdf0:f0f0:f0f0"  is a random number or not,  I cannot
say, not without a valid test for randomness on the sequence of bits
that were chosen,  and there are  multiple appropriate tests
available;  use any reasonable test you like,  they do exist,  and 40
random bits is an amply large sample size.

Despite that it is also definitely possible to manually construct
strings that are not produced randomly, which nevertheless by design
pass any specific test for randomness;   intentional 'malice' cannot
really be eliminated.

However, there _are_  many  non-random strings  that exist which  a
'lazy' or broken ULA ID generator might pick,  that can be very easily
detected as non-random  with sufficient confidence,  to  tell the user
"Hey, sorry, you can't use that.   Please generate a new ULA ID".

> 	improbable != impossible

Improbable with a sufficiently small probability is equal to
impossible intents and purposes.
The probability of generating any specific decimal number you pick a
priori, constructed out of 40 bits,  is essentially zero,  no matter
what number you pick;  there are _a very large number_ of  possible
ULA IDs  you can exclude,   before you have excluded enough that it
actually matters..

Rejecting ULA IDs on equipment that have less than a 10^-11  chance of
being a random sequence of bits;    is less likely  to reject a valid
ID,  than there is to be a collision on a ULA ID,  and it would have a
high probability of preventing future collisions  caused by accident,
misconfiguration, etc.

Which means that it may be a large improvement on the "honor system"
for picking ULA IDs with no verification.

"The collision doesn't happen"   is a better scenario than  "I know
who to blame....  the guy before me who just picked zero..  and some
former employee in the other company that just picked a ULA ID of
zero."

--
-JH