Messages in this thread | | | Date | Sun, 21 Oct 2007 07:32:25 -0400 | From | Theodore Tso <> | Subject | Re: [PATCH 1/2] UUID: set multicast bit in pseudo-random MAC address |
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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.
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