Geoff Huston wrote:
I see that the local use address draft has been revised and published as a WG
document.
In section 3.2.2 of the draft, it notes that, in reference to Locally Assigned Global IDs that "the likelihood of conflict is small. "
I had noted in draft-huston-ipv6-local-use-comments-00.txt that the likelihood of conflict is given by the formula:
P = 1 - ((n!) / ((n**d)(n-d)!))
where n = 2**40 and d is the number of random 'draws' from the pool.
The likelihood of conflict exceeds 0.5 after only 1.24 million draws. I'd
contend that this is definitely not "small" as described in the draft.
My disagreement with this calculation is two-fold.
The first issue I have is that these addresses are not meant to be routed globally, so there is an additional variable that needs to be considered when calculating P for collisions. That is, the P above needs to take into account the probability that two networks trying to use the same prefix are connected.
My second issue is with the formula itself. RFC 1889 uses an MD5 hash for calculating its SSRC Identifier (a 32-bit number). That document states:
the probability that two sources independently pick the same value
can be approximated for large N [20] as 1 - exp(-N**2 / 2**(L+1)).
For N=1000, the probability is roughly 10**-4.N represents the number sources (number of entities making a random selection)
and L is the length of the identifier. I see this formula being more representative
of the probability of collision.
Regards, Brian H.
-------------------------------------------------------------------- IETF IPng Working Group Mailing List IPng Home Page: http://playground.sun.com/ipng FTP archive: ftp://playground.sun.com/pub/ipng Direct all administrative requests to [EMAIL PROTECTED] --------------------------------------------------------------------
