james woodyatt wrote:
On Jun 25, 2007, at 09:33, Templin, Fred L wrote:
I already gave my use-case in:
http://www1.ietf.org/mail-archive/web/ipv6/current/msg07806.html
The use-case I am most interested in is Mobile Ad-Hoc Networks
(MANETs) in which two or more MANETs can merge (e.g., due to
mobility). If each MANET used ULA-C's, then they could inject each
others' prefixes into their IGPs with no opportunity for collision.
If each MANET instead used RFC4193 ULAs, then they could *probably*
still inject each others' prefixes. But, however small the risk of
collision, RFC4193 ULAs still have one drawback that ULA-C's do not -
uncertainty.
So perhaps another question is whether it is too much to ask for more
certainty (ULA-C) and less mystery (RFC4193 ULA)?
The "no opportunity for collision" thing is really the question here, as
Brian E. Carpenter said in
<http://www1.ietf.org/mail-archive/web/ipv6/current/msg07834.html> and
others have said elsewhere. Most of the pushback is directed at this
claim that central registration has some hope for mitigating an already
*epsilon* risk of collision. I don't think simply reiterating the claim
is an appropriate response to the legitimate rebuttal it's received.
Depends on what level of "collision risk" you are happy with, and this
depends on the scenario where you are assessing that risk.
- you pick a number from a pool, I pick a number from a pool, then we
compare numbers - chance of collision = 1 / pool size
- a set of us pick numbers from a pool, and we compare numbers. The
probability that two or us have picked the same number is the case where
a random draw function exceeds 0.5 after 1.24 million random draws.
The general solution of the probability of a collision after d draws
from n possible values is given by:
P = 1 - ((n!) / ((n**d)((n-d)!)))
Given that the value for n here is 2.199,023,255,552, then the objective
is to find the lowest value of d for which P is greater than or equal
to 0.5. In this case the value for d is some 1.24 million.
i.e. if we all pick numbers and stuff them into the DNS, then by the
time the 1,240,000 selection had taken place the probability that a
collision has occurred exceeds 0.5
regards,
Geoff
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