In message <[EMAIL PROTECTED]>, Pekka Savola
writes:
>On Mon, 25 Nov 2002, Steven M. Bellovin wrote:
>> You run into the birthday paradox here. In a space of 2^38 address
>> blocks, with 2^19 "allocations" there's a 50% chance of at least one
>> collision. Given how many home networks will be using this stuff,
>> we'll have far more than 2^19.
>>
>> But that's not the interesting question. The interesting question is
>> what the odds are of two users of the space "colliding", and that in
>> turn depends on the average connectivity. On that I have insufficient
>> data.
>
>The requirement for birthday paradox to be valid is when those networks
>are totally interconnected; for that, the number is like 2^19.
>
>I fail to see valid scenarios where even 2^5 site-local networks would be
>interconnected.
>
>I'm still assuming the site-locals, are, well, site locals.
>
>So valid interconnections would be:
> 1) sites connecting (e.g. two physical locations of one organization)
> 2) site-local address info leaking though some ways outside of sites
>where there is site-local connectivity
>
>1) is which seems to be critical, but I still fail to see a huge need for
>interconnection.
>
>2) should not be relevant in the case that collisions are extremely rare,
>as the connections will fail anyway (the question is only _how_).
>
Don't forget mergers and private interconnects. The latter are *very*
common, even without counting telecommuters. One shouldn't use
site-local there, but it's a path that often bypasses firewalls and
other official demarcation points.
If interconnections never occur, we don't need to worry about the
problems that can happen. My fear is that they occur all too often.
(What percentage of queries to the root name servers come from 1918
addresses?)
--Steve Bellovin, http://www.research.att.com/~smb (me)
http://www.wilyhacker.com ("Firewalls" book)
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