TUUID

Class Description

                                                                      
 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.