5.6. UUID Version 6

UUIDv6 is a field-compatible version of UUIDv1 (Section 5.1), reordered for improved DB locality. It is expected that UUIDv6 will primarily be implemented in contexts where UUIDv1 is used. Systems that do not involve legacy UUIDv1 SHOULD use UUIDv7 (Section 5.7) instead.

Instead of splitting the timestamp into the low, mid, and high sections from UUIDv1, UUIDv6 changes this sequence so timestamp bytes are stored from most to least significant. That is, given a 60-bit timestamp value as specified for UUIDv1 in Section 5.1, for UUIDv6 the first 48 most significant bits are stored first, followed by the 4-bit version (same position), followed by the remaining 12 bits of the original 60-bit timestamp.

The clock sequence and node bits remain unchanged from their position in Section 5.1.

The clock sequence and node bits SHOULD be reset to a pseudorandom value for each new UUIDv6 generated; however, implementations MAY choose to retain the old clock sequence and MAC address behavior from Section 5.1. For more information on MAC address usage within UUIDs, see the Section 8.

The format for the 16-byte, 128-bit UUIDv6 is shown in Figure 10.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           time_high                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           time_mid            |  ver  |       time_low        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|         clock_seq         |             node              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              node                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 10: UUIDv6 Field and Bit Layout

Field Definitions:

time_high: The most significant 32 bits of the 60-bit starting timestamp. Occupies bits 0 through 31 (octets 0-3).

time_mid: The middle 16 bits of the 60-bit starting timestamp. Occupies bits 32 through 47 (octets 4-5).

ver: The 4-bit version field as defined by Section 4.2, set to 0b0110 (6). Occupies bits 48 through 51 of octet 6.

time_low: 12 bits that will contain the least significant 12 bits from the 60-bit starting timestamp. Occupies bits 52 through 63 (octets 6-7).

var: The 2-bit variant field as defined by Section 4.1, set to 0b10. Occupies bits 64 and 65 of octet 8.

clock_seq: The 14 bits containing the clock sequence. Occupies bits 66 through 79 (octets 8-9).

node: 48-bit spatially unique identifier. Occupies bits 80 through 127 (octets 10-15).

With UUIDv6, the steps for splitting the timestamp into time_high and time_mid are OPTIONAL since the 48 bits of time_high and time_mid will remain in the same order. An extra step of splitting the first 48 bits of the timestamp into the most significant 32 bits and least significant 16 bits proves useful when reusing an existing UUIDv1 implementation.

5.6. UUID Version 6​

5.6. UUID Version 6