RE: draft-ietf-v6ops-addcon-03.txt ... ULAs of shorter-than-/48 and ULA multicast scope matching ...

[Christian Huitema <huitema@windows.microsoft.com>]

> On Friday, March 16, 2007 Tim Chown wrote:
> >
> > On Fri, Mar 16, 2007 at 03:04:17PM +0100, Brian E Carpenter wrote:
> > >
> > > On 2007-03-13 16:28, Tim Chown wrote:
> > >
> > > >Fair comment.   I guess we're also nibbling here at the 'what's a
> > site'
> > > >question.  I don't know where Thomas Narten's 3177-bis proposal
> got
> > to,
> > > >but if end sites were allocated a /56 you'd probably want a /56
> > ULA...
> > >
> > > A /48 ULA would do just fine. They aren't going to be in short
> > supply.
> >
> > I would assume doing that would affect the collision probabilities?
> 
> Basic maths: the probability of at least one collision is proportional
> to the number of allocations (networks) and inversely proportional to

.. oops, bad typo. It is proportional to the square of the number of
allocations! This is reflected in the computations below.

> the size of the address space. With a /48, the ULA address space is 40
> bits wide, 2^40. If you run the numbers, the probabilities of at least
> one collision for the various examples cited in the discussion are:
> 
> 	For 256 independent /48 prefixes: 6 E-8
> 	For 2 independent /40 prefixes:   9 E-10
> 	For 1,000,000 independent /48 prefixes: 0.9 (90%)
> 	For 1,000,000 independent /56 prefixes: 0.003 (0.3%)
> 	For 1,000,000 independent /64 prefixes: 1.4 E-5 (i.e. a success
> rate of 99.9986%, not quite five nines)
> 
> Of course, this can be mitigated in all sorts of ways, e.g. running a
> registry...
> 
> -- Christian Huitema
> 
> 
> 
>