Subject: Re: even one reason why provably unique SL is needed?

From: "Steven M. Bellovin" <[email-masked]>
Date: Mon, 25 Nov 2002 14:05:56 -0500

In message <[email-masked]>, Pekka Savola w
rites:
>Hello,
>
>People seem to be dodging this, so let's take it into another thread.
>
>There seems to be an assumption that provably unique SL addresses would be 
>needed.
>
>Can you tell me at least one reason (note: don't say globally routable)
>where this would be required instead of "nearly enough" unique identifiers
>(chance of collision about 1/2^38)?

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.

		--Steve Bellovin, http://www.research.att.com/~smb (me)
		http://www.wilyhacker.com ("Firewalls" book)

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From: Pekka Savola <[email-masked]>
Date: Mon, 25 Nov 2002 21:18:39 +0200 (EET)

> turn depends on the average connectivity.  On that I have insufficient 

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_).

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords