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

[Tim Chown <tjc@ecs.soton.ac.uk>]

On Fri, Mar 16, 2007 at 01:06:44PM -0700, Christian Huitema wrote:
>
> > 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...

What did ever become of the centrally assigned ULA proposal?

The question here is what text should we put in the addon draft?  We
would like to have some text for three cases:

a) Site has multiple /48 prefixes (e.g. a /44) and wishes to use ULAs
b) Site has one /48 and wishes to use ULAs 
c) Site has a less-than-/48 prefix (e.g. a /56 or /64) and wishes to use ULAs

For a) the discussion was whether to pick 16 'random' /48 ULA prefixes or
to pick one ULA prefix using 36 random bits rather than 40.

For b) I think we have no issue :)

For c) does the site use a random 48-bit ULA prefix with 0's to the actual
network boundary, or does it minimise collision likelihood (in particular for
cases of network merger or overlap) by randomising the bits to the network 
boundary?

The simplest text is to use fixed /48 ULAs in all cases.   The text could
describe the above three cases and state that conclusion and note briefly
the tradeoffs.

-- 
Tim