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SubjectRe: [PATCH 1/2] UUID: set multicast bit in pseudo-random MAC address
On Sat, Oct 20, 2007 at 09:58:40PM +0200, Helge Deller wrote:
> Fix a bug in the current random UUID generator:
>
> Section 4.1.6 of RFC 4122 states regarding the "NodeID" of a UUID:
> : For systems with no IEEE address, a randomly or pseudo-randomly
> : generated value may be used; see Section 4.5. The multicast bit must
> : be set in such addresses, in order that they will never conflict with
> : addresses obtained from network cards.
>
> So up to now it was just pure ("random") luck if this bit was set or not.
> This tiny patch sets the bit explicitely.

NACK. Your patch degrades the random UUID by a bit, for no good reason.

The part of Section 4.1.6 which you quoted only applies to version 1
UUID's --- i.e., MAC and time based UUID's. Random uuids are version
4 UUID's, and are already distinguished from version 1 UUID's by the
high 4 bits of the time_hi_and_version field, in octets 6-7 of the
URL. Hence, there is no danger of conflict. If you had looked 3
paragraphs later section 4.1.6:

For UUID version 4, the node field is a randomly or pseudo-randomly
generated 48-bit value as described in Section 4.4.

And the summary can be found in Section 4.4 of RFRC 4122:

4.4. Algorithms for Creating a UUID from Truly Random or
Pseudo-Random Numbers

The version 4 UUID is meant for generating UUIDs from truly-random or
pseudo-random numbers.

The algorithm is as follows:

o Set the two most significant bits (bits 6 and 7) of the
clock_seq_hi_and_reserved to zero and one, respectively.

o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the 4-bit version number from
Section 4.1.3.

o Set all the other bits to randomly (or pseudo-randomly) chosen
values.

See Section 4.5 for a discussion on random numbers.

- Ted
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