TUUID
This class defines a UUID (Universally Unique IDentifier), also
known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits
long, and if generated according to this algorithm, is either
guaranteed to be different from all other UUIDs/GUIDs generated
until 3400 A.D. or extremely likely to be different. UUIDs were
originally used in the Network Computing System (NCS) and
later in the Open Software Foundation's (OSF) Distributed Computing
Environment (DCE).
Structure of universal unique IDs (UUIDs).
Depending on the network data representation, the multi-
octet unsigned integer fields are subject to byte swapping
when communicated between dissimilar endian machines.
| low 32 bits of time | 0-3 .fTimeLow
+-------------------------------+----
| mid 16 bits of time | 4-5 .fTimeMid
+
| vers. | hi 12 bits of time | 6-7 .fTimeHiAndVersion
|Res | clkSeqHi | 8 .fClockSeqHiAndReserved
| clkSeqLow | 9 .fClockSeqLow
+
| node ID | 10-15 .fNode
The adjusted time stamp is split into three fields, and the
clockSeq is split into two fields.
The timestamp is a 60-bit value. For UUID version 1, this
is represented by Coordinated Universal Time (UTC/GMT) as
a count of 100-nanosecond intervals since 00:00:00.00,
15 October 1582 (the date of Gregorian reform to the
Christian calendar).
The version number is multiplexed in the 4 most significant
bits of the 'fTimeHiAndVersion' field. There are two defined
versions:
MSB <---
Version 4-Bit Code Description
| 1 0 0 0 1 DCE version, as specified herein.
| 2 0 0 1 0 DCE Security version, with
| embedded POSIX UIDs.
| 3 0 0 1 1 node id is a random value
Clock Sequence
The clock sequence value must be changed whenever:
The UUID generator detects that the local value of UTC
has gone backward; this may be due to re-syncing of the system
clock.
While a node is operational, the UUID service always saves
the last UTC used to create a UUID. Each time a new UUID
is created, the current UTC is compared to the saved value
and if either the current value is less or the saved value
was lost, then the clock sequence is incremented modulo
16,384, thus avoiding production of duplicted UUIDs.
The clock sequence must be initialized to a random number
to minimize the correlation across system. This provides
maximum protection against node identifiers that may move
or switch from system to system rapidly.
Clock Adjustment
UUIDs may be created at a rate greater than the system clock
resolution. Therefore, the system must also maintain an
adjustment value to be added to the lower-order bits of the
time. Logically, each time the system clock ticks, the
adjustment value is cleared. Every time a UUID is generated,
the current adjustment value is read and incremented, and
then added to the UTC time field of the UUID.
Clock Overrun
The 100-nanosecond granularity of time should prove sufficient
even for bursts of UUID production in the next generation of
high-performance multiprocessors. If a system overruns the
clock adjustment by requesting too many UUIDs within a single
system clock tick, the UUID generator will stall until the
system clock catches up.
