Universally Unique IDentifiers (UUID)

2.1. Update Motivation

Many things have changed in the time since UUIDs were originally created. Modern applications have a need to create and utilize UUIDs as the primary identifier for a variety of different items in complex computational systems, including but not limited to database keys, file names, machine or system names, and identifiers for event-driven transactions.¶

One area in which UUIDs have gained popularity is database keys. This stems from the increasingly distributed nature of modern applications. In such cases, "auto increment" schemes often used by databases do not work well, as the effort required to coordinate sequential numeric identifiers across a network can easily become a burden. The fact that UUIDs can be used to create unique, reasonably short values in distributed systems without requiring coordination makes them a good alternative, but UUID versions 1-5, which were originally defined by [RFC4122], lack certain other desirable characteristics:¶

1. Non-time-ordered UUID versions such as UUIDv4 (described in Section 5.4) have poor database index locality. This means that new values created in succession are not close to each other in the index and thus require inserts to be performed at random locations. The resulting negative performance effects on common structures used for this (B-tree and its variants) can be dramatic.¶

2. The 100-nanosecond Gregorian epoch used in UUIDv1 (described in Section 5.1) timestamps is uncommon and difficult to represent accurately using a standard number format such as [IEEE754].¶

3. Introspection/parsing is required to order by time sequence, as opposed to being able to perform a simple byte-by-byte comparison.¶

4. Privacy and network security issues arise from using a MAC address in the node field of UUID version 1. Exposed MAC addresses can be used as an attack surface to locate network interfaces and reveal various other information about such machines (minimally manufacturer, potentially other details). Additionally, with the advent of virtual machines and containers, MAC address uniqueness is no longer guaranteed.¶

5. Many of the implementation details specified in [RFC4122] involved trade offs that are neither possible to specify for all applications nor necessary to produce interoperable implementations.¶

6. [RFC4122] did not distinguish between the requirements for generating a UUID and those for simply storing one, although they are often different.¶

Due to the aforementioned issues, many widely distributed database applications and large application vendors have sought to solve the problem of creating a better time-based, sortable unique identifier for use as a database key. This has led to numerous implementations over the past 10+ years solving the same problem in slightly different ways.¶

While preparing this specification, the following 16 different implementations were analyzed for trends in total ID length, bit layout, lexical formatting/encoding, timestamp type, timestamp format, timestamp accuracy, node format/components, collision handling, and multi-timestamp tick generation sequencing:¶

1. [ULID] by A. Feerasta¶

2. [LexicalUUID] by Twitter¶

3. [Snowflake] by Twitter¶

4. [Flake] by Boundary¶

5. [ShardingID] by Instagram¶

6. [KSUID] by Segment¶

7. [Elasticflake] by P. Pearcy¶

8. [FlakeID] by T. Pawlak¶

9. [Sonyflake] by Sony¶

10. [orderedUuid] by IT. Cabrera¶

11. [COMBGUID] by R. Tallent¶

12. [SID] by A. Chilton¶

13. [pushID] by Google¶

14. [XID] by O. Poitrey¶

15. [ObjectID] by MongoDB¶

16. [CUID] by E. Elliott¶

An inspection of these implementations and the issues described above has led to this document which intends to adapt new UUIDs to address these issues.¶

Further, [RFC4122] itself was in need an overhaul to address a number of topics such as but not limited to the following:¶

1. Miscellaneous erratas. Mostly around bit layout clarifications which lead to inconsistent implementations.¶

2. Decouple other UUID versions from UUIDv1 bit layout so that fields like "time_hi_and_version" do not need to be referenced within a non-time-based UUID while also providing "UUIDv1 like" definition sections for UUIDv3, UUIDv4, and UUIDv5.¶

3. Provide implementation best practices around many real-world scenarios and corner cases observed by existing and prototype implementations.¶

4. Update the document to address security best practices and considerations for the modern age as it pertains MAC addresses, hashing algorithms, secure randomness, and other topics.¶

5. Provide implementations a standard-based option for implementation specific and/or experimental UUID designs.¶

6. Provide more test vectors that illustrate real UUIDs created as per the specification.¶

3.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3.2. Abbreviations

The following abbreviations are used in this document:¶

UUID

Universally Unique Identifier¶

UUIDv1

Universally Unique Identifier Version 1¶

UUIDv2

Universally Unique Identifier Version 2¶

UUIDv3

Universally Unique Identifier Version 3¶

UUIDv4

Universally Unique Identifier Version 4¶

UUIDv5

Universally Unique Identifier Version 5¶

UUIDv6

Universally Unique Identifier Version 6¶

UUIDv7

Universally Unique Identifier Version 7¶

UUIDv8

Universally Unique Identifier Version 8¶

URN

Uniform Resource Names¶

ABNF

Augmented Backus-Naur Form¶

CSPRNG

Cryptographically Secure Pseudo-Random Number Generator¶

MAC

Media Access Control¶

MSB

Most Significant Bit¶

DBMS

Database Management System¶

IEEE

Institute of Electrical and Electronics Engineers, Inc.¶

ITU

International Telecommunication Union¶

MD5

Message Digest 5¶

SHA

Secure Hash Algorithm¶

SHA-1

Secure Hash Algorithm 1 with message digest of 160 bits¶

SHA-224

Secure Hash Algorithm 2 with message digest size of 224 bits¶

SHA-256

Secure Hash Algorithm 2 with message digest size of 256 bits¶

SHA-512

Secure Hash Algorithm 2 with message digest size of 512 bits¶

SHA-3

Secure Hash Algorithm 3¶

SHAKE

Secure Hash Algorithm 3 based on KECCAK algorithm¶

UTC

Coordinated Universal Time¶

OID

Object Identifier¶

3.3. Changelog

This section is to be removed before publishing as an RFC.¶

draft-14¶

• AD Review #2: IANA Subtype Modifications #170¶

• AD Review #2: Specify Values for Variant/Subtype Column #171¶

• AD Review #2: Grammar change at the end of 5.1 #172¶

• SHA and Integer Verbiage clarifications #174¶

• Disclaimer in nil/max that these do not fall into IETF variants #175¶

• Fix v1/v6 final UUID in test vector #176¶

draft-13¶

• Request IANA Registry #144¶

• Describe allocation logic of Namespace ID #161¶

• Move citation of Namesapce ID up to first instance in v3/v5 #167¶

• Further normalize Namespace verbiage #166¶

• Fix Timestamp of Time-Based UUIDv8 Example #164¶

• Change RFC8937 and RFC4086 to Informative References #163¶

• Discuss why v3/v5 are bad for Database Usage #155¶

draft-12¶

• Typos #148 #156¶

• SECDIR Review #141¶

• SECDIR Review 2 #142¶

• OPSDIR Review #145¶

• INDIR Review 2 #140¶

• IESG Grammar #146¶

• Revise 16-bit MAC Node Usage #149¶

• Add MSB3 to Variant Table #153¶

• Additional Update Motivations #157¶

• Expand v8 Time-based Example to larger timestamp #159¶

• Fix Randomized Node value's mcast bit in Appendix #151¶

• Clarify "Name-Based" is the same as "Hash-Based" #154¶

• Move UUIDv8 Examples out of Test Vectors #150¶

• Simplify UUIDv8 Hash-based Example #147¶

draft-11¶

• Normalize "name space" to "namespace" everywhere #137¶

• IANA Review: Verbiage to update RFC4122 references #134¶

• DNSDIR re-review: Better Define "a canonical sequence of octets" #136¶

• Crosspost: Typo in Approximate UUID timestamp calculations #135¶

• INTDIR Review #139¶

draft-10¶

• ARTART Review and Feedback #130¶

• Clarify Hash Space IDs listed are not the only options #132¶

• Add example to timestamp fuzzing #133¶

draft-09¶

• Late addition of IETF reference for CSPRNG guidance #123¶

• DNSDIR Review: Typos! #122¶

• DNSDIR Review: DNS Considerations Update #121¶

• Error in UUIDv8 Name-based Test Vector #129¶

• Improve consistency of layout field definitions #128¶

draft-08¶

• Fix typos #113¶

• Fix errata 6225 (again) #117 #118¶

• AD Review: BCP 14 - SHOULD #114¶

• AD Review: Add proper references to v1 and v6 #116¶

• AD Review: Remove SHOULD in section 4 #120¶

• Discuss "front-loaded rollover counter" for 32-bit epoch with Padding method #115¶

draft-07¶

• Even more grammar tweaks! #109¶

• Remove unnecessary "32 bit" in UUIDv7 example #108¶

• Change "fixed millisecond" -> "millisecond by default" relating to v7 #110¶

• Revert Max UUID Naming #107¶

• Author Changes¶

draft-06¶

• More Grammar edits! #102¶

• Tweak v7 description to de-emphasize optional components #103¶

• Better Clarify Case in ABNF #104¶

• Verbiage change in 6.2 #105¶

draft-05¶

• Changed Max UUID to Max UUID to better complement Latin Nil UUID verbiage. #95¶

• Align Method 3 text with the 12 bits limitation #96¶

• Make Version/version casing consistent across 5. UUID Layouts #97¶

• Cite MS COM GUID as little-endian #95¶

draft-04¶

• Remove extra words #82, #88, and #93¶

• Punctuation and minor style fixes #84¶

• Change rounding mode of Method 4 Section 6.2 #90 (from #86)¶

• Add verbal description of v7 generation to 5.7. UUID Version 7 #91¶

• Remove Re-randomize Until Monotonic (Method 3) from Monotonicity and Counters #92¶

• Fix ambiguous text around UUIDv6 clock sequence #89¶

• Move endianness statement from layout to format section #85¶

• Further modified abstract to separate URN topic from UUID definition #83¶

• Provided three more UUID format examples #83¶

• Added text further clarifying version construct is for the variant in this doc #83¶

• Provided further clarification for local/global bit vs multicast bit #83¶

draft-03¶

• Revised IANA Considerations #71¶

• Fix "integral numbers of octets" verbiage #67¶

• Transpose UUID Namespaces to match UUID Hashspaces #70¶

• Reference all Hash Algorithms. #69¶

• Normalize SHA abbreviation formats #66¶

• Add other Hash Abbreviations #65¶

• Remove URN from title #73¶

• Move Community Considerations to Introduction #68¶

• Move some Normative Reference to Informative #74¶

• Misc formatting changes to address IDNITS feedback¶

• Downgrade MUST NOT to SHOULD NOT for guessability of UUIDs #75¶

• Misc. text formatting, typo fixes #78¶

• Misc. text clarifications #79¶

• Misc. SHOULD/MUST adjustments #80¶

• Method 3 and 4 added to monotonic section #81¶

draft-02¶

• Change md5_high in SHA-1 section to sha1_mid #59¶

• Describe Nil/Max UUID in variant table #16¶

• Further Clarify that non-descript node IDs are the preferred method in distributed UUID Generation #49¶

• Appendix B, consistent naming #55¶

• Remove duplicate ABNF from IANA considerations #56¶

• Monotonic Error Checking missing newline #57¶

• More Security Considerations Randomness #26¶

• SHA-256 UUID Generation #50¶

• Expand multiplexed fields within v1 and v6 bit definitions #43¶

• Clean up text in UUIDs that Do Not Identify the Host #61¶

• Revise UUID Generator States section #47¶

• Expand upon why unix epoch rollover is not a problem #44¶

• Delete Sample Code Appendix #62¶

draft-01¶

• Mixed Case Spelling error #18¶

• Add "UUIDs that Do Not Identify the Host as well" reference to security considerations #19¶

• Out of Place Distributed node text #20¶

• v6 clock_seq and node usage ambiguity #21¶

• Figure 2 and 3 Fix Title #22¶

• Move Namespace Registration Template to IANA Considerations #23¶

• Verify ABNF formatting against RFC5234 #24¶

• Bump ABNF reference to RFC 5234 #25¶

• Modify v8 SHOULD NOT to MUST NOT #27¶

• Remove "time-based" constraint from version 8 UUID #29¶

• Further clarify v7 field description #125 #30¶

• Typo: Section 4.2, Version Field, "UUID from in this" #33¶

• Create better ABNF to represent Hex Digit #39¶

• Break Binary form of UUID into two lines. #40¶

• Move octet text from section 4 to section 5 #41¶

• Add forward reference to UUIDv1 and UUIDv4 in Section 2 #42¶

• Erroneous reference to v1 in monotonicity #45¶

• Add Label for "Monotonic Error Checking" paragraph to frame the topic #46¶

• Remove IEEE paragraph from "uuids that do not identify the host" #48¶

• Grammar Review #52¶

draft-00¶

• Merge RFC4122 with draft-peabody-dispatch-new-uuid-format-04.md¶

• Change: Reference RFC1321 to RFC6151¶

• Change: Reference RFC2141 to RFC8141¶

• Change: Reference RFC2234 to RFC5234¶

• Change: Reference FIPS 180-1 to FIPS 180-4 for SHA-1¶

• Change: Converted UUIDv1 to match UUIDv6 section from Draft 04¶

• Change: Trimmed down the ABNF representation¶

• Change: http websites to https equivalent¶

• Errata: Bad Reference to RFC1750 | 3641 #4¶

• Errata: Change MD5 website to example.com | 3476 #6 (Also Fixes Errata: Fix uuid_create_md5_from_name() | 1352 #2)¶

• Errata: Typo in code comment | 6665 #11¶

• Errata: Fix BAD OID acronym | 6225 #9¶

• Errata: Incorrect Parenthesis usage Section 4.3 | 184 #5¶

• Errata: Lexicographically Sorting Paragraph Fix | 1428 #3¶

• Errata: Fix 4.1.3 reference to the correct bits | 1957 #13¶

• Errata: Fix reference to variant in octet 8 | 4975 #7¶

• Errata: Further clarify 3rd/last bit of Variant for spec | 5560 #8¶

• Errata: Fix clock_seq_hi_and_reserved most-significant bit verbiage | 4976 #10¶

• Errata: Better Clarify network byte order when referencing most significant bits | 3546 #12¶

• Draft 05: B.2. Example of a UUIDv7 Value two "var" in table #120¶

• Draft 05: MUST verbiage in Reliability of 6.1 #121¶

• Draft 05: Further discourage centralized registry for distributed UUID Generation.¶

• New: Further Clarity of exact octet and bit of var/ver in this spec¶

• New: Block diagram, bit layout, test vectors for UUIDv4¶

• New: Block diagram, bit layout, test vectors for UUIDv3¶

• New: Block diagram, bit layout, test vectors for UUIDv5¶

• New: Add MD5 Security Considerations reference, RFC6151¶

• New: Add SHA-1 Security Considerations reference, RFC6194¶

4.1. Variant Field

The variant field determines the layout of the UUID. That is, the interpretation of all other bits in the UUID depends on the setting of the bits in the variant field. As such, it could more accurately be called a type field; we retain the original term for compatibility. The variant field consists of a variable number of the most significant bits of octet 8 of the UUID.¶

Table 1 lists the contents of the variant field, where the letter "x" indicates a "don't-care" value.¶

Interoperability, in any form, with variants other than the one defined here is not guaranteed but is not likely to be an issue in practice.¶

Specifically for UUIDs in this document, bits 64 and 65 of the UUID (bits 0 and 1 of octet 8) MUST be set to 1 and 0 as specified in row 2 of Table 1. Accordingly, all bit and field layouts avoid the use of these bits.¶

4.2. Version Field

The version number is in the most significant 4 bits of octet 6 (bits 48 through 51 of the UUID).¶

Table 2 lists all of the versions for this UUID variant 10xx specified in this document.¶

An example version/variant layout for UUIDv4 follows the table where M represents the version placement for the hexadecimal representation of 0x4 (0b0100) and the N represents the variant placement for one of the four possible hexadecimal representation of variant 10xx: 0x8 (0b1000), 0x9 (0b1001), 0xA (0b1010), 0xB (0b1011)¶

00000000-0000-4000-8000-000000000000
00000000-0000-4000-9000-000000000000
00000000-0000-4000-A000-000000000000
00000000-0000-4000-B000-000000000000
xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx

It should be noted that the other remaining UUID variants found in Table 1 leverage different sub-typing/versioning mechanisms. The recording and definition of the remaining UUID variant and sub-typing combinations are outside of the scope of this document.¶

5.1. UUID Version 1

UUID version 1 is a time-based UUID featuring a 60 bit timestamp represented by Coordinated Universal Time (UTC) as a count of 100- nanosecond intervals since 00:00:00.00, 15 October 1582 (the date of Gregorian reform to the Christian calendar).¶

UUIDv1 also features a clock sequence field which is used to help avoid duplicates that could arise when the clock is set backwards in time or if the node ID changes.¶

The node field consists of an IEEE 802 MAC address, usually the host address or a randomly derived value per Section 6.9 and Section 6.10.¶

 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_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           time_mid            |  ver  |       time_high       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|         clock_seq         |             node              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              node                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

time_low:

The least 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 0b0001 (1). Occupies bits 48 through 51 of octet 6.¶

time_high:

12 bits that will contain the most 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).¶

For systems that do not have UTC available, but do have the local time, they may use that instead of UTC, as long as they do so consistently throughout the system. However, this is not recommended since generating the UTC from local time only needs a time zone offset.¶

If the clock is set backwards, or might have been set backwards (e.g., while the system was powered off), and the UUID generator can not be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence MUST be changed. If the previous value of the clock sequence is known, it MAY be incremented; otherwise it SHOULD be set to a random or high-quality pseudo-random value.¶

Similarly, if the node ID changes (e.g., because a network card has been moved between machines), setting the clock sequence to a random number minimizes the probability of a duplicate due to slight differences in the clock settings of the machines. If the value of the clock sequence associated with the changed node ID were known, then the clock sequence MAY be incremented, but that is unlikely.¶

The clock sequence MUST be originally (i.e., once in the lifetime of a system) initialized to a random number to minimize the correlation across systems. This provides maximum protection against node identifiers that may move or switch from system to system rapidly. The initial value MUST NOT be correlated to the node identifier.¶

Notes about IEEE 802 derived nodes:¶

• On systems with multiple IEEE 802 addresses, any available one MAY be used.¶

• On systems with no IEEE address, a randomly or pseudo-randomly generated value MUST be used; see Section 6.9 and Section 6.10.¶

• On systems utilizing a 64 bit MAC address the least significant, right-most 48 bits MAY be used.¶

• On systems utilizing an IEEE 802.15.4 16 bit address SHOULD instead utilize their 64 bit MAC address where least significant, right-most 48 bits MAY be used. An alternative is to generate 32 bits of random data and postfix at the end of the 16 bit MAC address to create a 48 bit value.¶

5.2. UUID Version 2

UUID version 2 is known as DCE Security UUIDs [C309] and [C311]. As such, the definition of these UUIDs is outside the scope of this specification.¶

5.3. UUID Version 3

UUID version 3 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "namespace" as per Section 6.5.¶

UUIDv3 values are created by computing an MD5 [RFC1321] hash over a given namespace ID value (Section 6.6) concatenated with the desired name value after both have been converted to a canonical sequence of octets, as defined by the standards or conventions of its namespace, in network byte order. This MD5 value is then used to populate all 128 bits of the UUID layout. The UUID version and variant then replace the respective bits as defined by Section 4.2 and Section 4.1. An example of this bit substitution can be found in Appendix A.2.¶

Information around selecting a desired name's canonical format within a given namespace can be found in Section 6.5, "A note on names".¶

Where possible UUIDv5 SHOULD be used in lieu of UUIDv3. For more information on MD5 security considerations see [RFC6151].¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            md5_high                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          md5_high             |  ver  |       md5_mid         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                        md5_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            md5_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

md5_high:

The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed MD5 value. Occupies bits 0 through 47 (octets 0-5).¶

ver:

The 4 bit version field as defined by Section 4.2, set to 0b0011 (3). Occupies bits 48 through 51 of octet 6.¶

md5_mid:

12 more bits of the layout consisting of the least significant, right-most 12 bits of 16 bits immediately following md5_high from the computed MD5 value. 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.¶

md5_low:

The final 62 bits of the layout immediately following the var field to be filled with the least-significant, right-most bits of the final 64 bits from the computed MD5 value. Occupies bits 66 through 127 (octets 8-15)¶

5.4. UUID Version 4

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

An implementation may generate 128 bits of random data which is used to fill out the UUID fields in Figure 8. The UUID version and variant then replace the respective bits as defined by Section 4.2 and Section 4.1.¶

Alternatively, an implementation MAY choose to randomly generate the exact required number of bits for random_a, random_b, and random_c (122 bits total), and then concatenate the version and variant in the required position.¶

For guidelines on random data generation see Section 6.9.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           random_a                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          random_a             |  ver  |       random_b        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                       random_c                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           random_c                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

random_a:

The first 48 bits of the layout that can be filled with random data as specified in Section 6.9. Occupies bits 0 through 47 (octets 0-5).¶

ver:

The 4 bit version field as defined by Section 4.2, set to 0b0100 (4). Occupies bits 48 through 51 of octet 6.¶

random_b:

12 more bits of the layout that can be filled random data as per Section 6.9. 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.¶

random_c:

The final 62 bits of the layout immediately following the var field to be filled with random data as per Section 6.9. Occupies bits 66 through 127 (octets 8-15).¶

5.5. UUID Version 5

UUID version 5 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "namespace" as per Section 6.5.¶

UUIDv5 values are created by computing an SHA-1 [FIPS180-4] hash over a given namespace ID value (Section 6.6) concatenated with the desired name value after both have been converted to a canonical sequence of octets, as defined by the standards or conventions of its namespace, in network byte order. The most significant, left-most 128 bits of the SHA-1 value is then used to populate all 128 bits of the UUID layout and the remaining 32 least significant, right-most bits of SHA-1 output are discarded. The UUID version and variant then replace the respective bits as defined by Section 4.2 and Section 4.1. An example of this bit substitution and discarding excess bits can be found in Appendix A.4.¶

Information around selecting a desired name's canonical format within a given namespace can be found in Section 6.5, "A note on names".¶

There may be scenarios, usually depending on organizational security policies, where SHA-1 libraries may not be available or deemed unsafe for use. As such, it may be desirable to generate name-based UUIDs derived from SHA-256 or newer SHA methods. These name-based UUIDs MUST NOT utilize UUIDv5 and MUST be within the UUIDv8 space defined by Section 5.8. An illustrative example of UUIDv8 for SHA-256 name-based UUIDs is provided in the appendix Appendix B.2.¶

For more information on SHA-1 security considerations see [RFC6194].¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           sha1_high                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|         sha1_high             |  ver  |      sha1_mid         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                       sha1_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           sha1_low                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

sha1_high:

The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed SHA-1 value. Occupies bits 0 through 47 (octets 0-5).¶

ver:

The 4 bit version field as defined by Section 4.2, set to 0b0101 (5). Occupies bits 48 through 51 of octet 6.¶

sha1_mid:

12 more bits of the layout consisting of the least significant, right-most 12 bits of 16 bits immediately following sha1_high from the computed SHA-1 value. 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.¶

sha1_low:

The final 62 bits of the layout immediately following the var field to be filled by skipping the 2 most significant, left-most bits of the remaining SHA-1 hash and then using the next 62 most significant, left-most bits. Any leftover SHA-1 bits are discarded and unused. Occupies bits 66 through 127 (octets 8-15).¶

5.6. UUID Version 6

UUID version 6 is a field-compatible version of UUIDv1 Section 5.1, reordered for improved DB locality. It is expected that UUIDv6 will primarily be used 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 pseudo-random 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                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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.¶

5.7. UUID Version 7

UUID version 7 features a time-ordered value field derived from the widely implemented and well known Unix Epoch timestamp source, the number of milliseconds since midnight 1 Jan 1970 UTC, leap seconds excluded. UUIDv7 generally has improved entropy characteristics over UUIDv1 Section 5.1 or UUIDv6 Section 5.6.¶

UUIDv7 values are created by allocating a Unix timestamp in milliseconds in the most significant 48 bits and filling the remaining 74 bits, excluding the required version and variant bits, with random bits for each new UUIDv7 generated to provide uniqueness as per Section 6.9. Alternatively, implementations MAY fill the 74 bits, jointly, with a combination of the following subfields, in this order from the most significant bits to the least, to guarantee additional monotonicity within a millisecond:¶

1. An OPTIONAL sub-millisecond timestamp fraction (12 bits at maximum) as per Section 6.2 (Method 3).¶

2. An OPTIONAL carefully seeded counter as per Section 6.2 (Method 1 or 2).¶

3. Random data for each new UUIDv7 generated for any remaining space.¶

Implementations SHOULD utilize UUIDv7 instead of UUIDv1 and UUIDv6 if possible.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           unix_ts_ms                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          unix_ts_ms           |  ver  |       rand_a          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                        rand_b                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                            rand_b                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

unix_ts_ms:

48 bit big-endian unsigned number of Unix epoch timestamp in milliseconds as per Section 6.1. Occupies bits 0 through 47 (octets 0-5).¶

ver:

The 4 bit version field as defined by Section 4.2, set to 0b0111 (7). Occupies bits 48 through 51 of octet 6.¶

rand_a:

12 bits pseudo-random data to provide uniqueness as per Section 6.9 and/or optional constructs to guarantee additional monotonicity as per Section 6.2. 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.¶

rand_b:

The final 62 bits of pseudo-random data to provide uniqueness as per Section 6.9 and/or an optional counter to guarantee additional monotonicity as per Section 6.2. Occupies bits 66 through 127 (octets 8-15).¶

5.8. UUID Version 8

UUID version 8 provides an RFC-compatible format for experimental or vendor-specific use cases. The only requirement is that the variant and version bits MUST be set as defined in Section 4.1 and Section 4.2. UUIDv8's uniqueness will be implementation-specific and MUST NOT be assumed.¶

The only explicitly defined bits are those of the version and variant fields, leaving 122 bits for implementation specific UUIDs. To be clear: UUIDv8 is not a replacement for UUIDv4 Section 5.4 where all 122 extra bits are filled with random data.¶

Some example situations in which UUIDv8 usage could occur:¶

• An implementation would like to embed extra information within the UUID other than what is defined in this document.¶

• An implementation has other application/language restrictions which inhibit the use of one of the current UUIDs.¶

The appendix, Appendix B, provides two illustrative examples of custom UUIDv8 algorithms to address two example scenarios.¶

 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           custom_a                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          custom_a             |  ver  |       custom_b        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var|                       custom_c                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           custom_c                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

custom_a:

The first 48 bits of the layout that can be filled as an implementation sees fit. Occupies bits 0 through 47 (octets 0-5).¶

ver:

The 4 bit version field as defined by Section 4.2, set to 0b1000 (8). Occupies bits 48 through 51 of octet 6.¶

custom_b:

12 more bits of the layout that can be filled as an implementation sees fit. 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.¶

custom_c:

The final 62 bits of the layout immediately following the var field to be filled as an implementation sees fit. Occupies bits 66 through 127 (octets 8-15).¶

5.9. Nil UUID

The nil UUID is special form of UUID that is specified to have all 128 bits set to zero.¶

00000000-0000-0000-0000-000000000000

A Nil UUID value can be useful to communicate the absence of any other UUID value in situations that otherwise require or use a 128 bit UUID. A Nil UUID can express the concept "no such value here". Thus it is reserved for such use as needed for implementation-specific situations.¶

Note that the Nil UUID value falls within the range of the Apollo NCS variant as per the first row of Table 1 rather than the variant defined by this document.¶

5.10. Max UUID

The Max UUID is a special form of UUID that is specified to have all 128 bits set to 1. This UUID can be thought of as the inverse of Nil UUID defined in Section 5.9.¶

FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF

A Max UUID value can be used as a sentinel value in situations where a 128 bit UUID is required but a concept such as "end of UUID list" needs to be expressed, and is reserved for such use as needed for implementation-specific situations.¶

Note that the Max UUID value falls within the range of the "yet to be defined" future UUID variant as per the last row of Table 1 rather than the variant defined by this document.¶

6.1. Timestamp Considerations

UUID timestamp source, precision, and length was the topic of great debate while creating UUIDv7 for this specification. Choosing the right timestamp for your application is a very important topic. This section will detail some of the most common points on this topic.¶

Reliability:

Implementations acquire the current timestamp from a reliable source to provide values that are time-ordered and continually increasing. Care must be taken to ensure that timestamp changes from the environment or operating system are handled in a way that is consistent with implementation requirements. For example, if it is possible for the system clock to move backward due to either manual adjustment or corrections from a time synchronization protocol, implementations need to determine how to handle such cases. (See Altering, Fuzzing, or Smearing below.)¶

Source:

UUID version 1 and 6 both utilize a Gregorian epoch timestamp while UUIDv7 utilizes a Unix Epoch timestamp. If other timestamp sources or a custom timestamp epoch are required, UUIDv8 MUST be used.¶

Sub-second Precision and Accuracy:

Many levels of precision exist for timestamps: milliseconds, microseconds, nanoseconds, and beyond. Additionally fractional representations of sub-second precision may be desired to mix various levels of precision in a time-ordered manner. Furthermore, system clocks themselves have an underlying granularity and it is frequently less than the precision offered by the operating system. With UUID version 1 and 6, 100-nanoseconds of precision are present while UUIDv7 features millisecond level of precision by default within the Unix epoch that does not exceed the granularity capable in most modern systems. For other levels of precision UUIDv8 is available. Similar to Section 6.2, with UUIDv1 or UUIDv6, a high resolution timestamp can be simulated by keeping a count of the number of UUIDs that have been generated with the same value of the system time, and using it to construct the low order bits of the timestamp. The count of the high resolution timestamp will range between zero and the number of 100-nanosecond intervals per system time interval.¶

Length:

The length of a given timestamp directly impacts how many timestamp ticks can be contained in a UUID before the maximum value for the timestamp field is reached. Take care to ensure that the proper length is selected for a given timestamp. UUID version 1 and 6 utilize a 60 bit timestamp valid until 5623 AD and UUIDv7 features a 48 bit timestamp valid until the year 10889 AD.¶

Altering, Fuzzing, or Smearing:

Implementations MAY alter the actual timestamp. Some examples include security considerations around providing a real clock value within a UUID, to correct inaccurate clocks, to handle leap seconds, or instead of dividing a number of microseconds by 1000 to obtain a millisecond value; dividing by 1024 (or some other value) for performance reasons. This specification makes no requirement or guarantee about how close the clock value needs to be to the actual time. If UUIDs do not need to be frequently generated, the UUIDv1 or UUIDv6 timestamp can simply be the system time multiplied by the number of 100-nanosecond intervals per system time interval.¶

Padding:

When timestamp padding is required, implementations MUST pad the most significant bits (left-most) bits with data. An example for this padding data is to fill the most significant, left-most bits of a Unix timestamp with zeroes to complete the 48 bit timestamp in UUIDv7. An alternative approach for padding data is to fill the most significant, left-most bits with the number of 32 bit Unix timestamp roll-overs after 2038-01-19.¶

Truncating:

When timestamps need to be truncated, the lower, least significant bits MUST be used. An example would be truncating a 64 bit Unix timestamp to the least significant, right-most 48 bits for UUIDv7.¶

Error Handling:

If a system overruns the generator by requesting too many UUIDs within a single system time interval, the UUID service can return an error, or stall the UUID generator until the system clock catches up, and MUST NOT return knowingly duplicate values due to a counter rollover. Note that if the processors overrun the UUID generation frequently, additional node identifiers can be allocated to the system, which will permit higher speed allocation by making multiple UUIDs potentially available for each time stamp value. Similar techniques are discussed in Section 6.4.¶

6.2. Monotonicity and Counters

Monotonicity (each subsequent value being greater than the last) is the backbone of time-based sortable UUIDs. Normally, time-based UUIDs from this document will be monotonic due to an embedded timestamp; however, implementations can guarantee additional monotonicity via the concepts covered in this section.¶

Take care to ensure UUIDs generated in batches are also monotonic. That is, if one thousand UUIDs are generated for the same timestamp, there should be sufficient logic for organizing the creation order of those one thousand UUIDs. Batch UUID creation implementations MAY utilize a monotonic counter that increments for each UUID created during a given timestamp.¶

For single-node UUID implementations that do not need to create batches of UUIDs, the embedded timestamp within UUID version 6 and 7 can provide sufficient monotonicity guarantees by simply ensuring that timestamp increments before creating a new UUID. Distributed nodes are discussed in Section 6.4.¶

Implementations SHOULD employ the following methods for single-node UUID implementations that require batch UUID creation, or are otherwise concerned about monotonicity with high frequency UUID generation.¶

Fixed-Length Dedicated Counter Bits (Method 1):

Some implementations allocate a specific number of bits in the UUID layout to the sole purpose of tallying the total number of UUIDs created during a given UUID timestamp tick. A fixed bit-length counter, if present, MUST be positioned immediately after the embedded timestamp. This promotes sortability and allows random data generation for each counter increment. With this method, the rand_a section (or a subset of its left-most bits) of UUIDv7 is used as fixed-length dedicated counter bits that are incremented for every UUID generation. The trailing random bits generated for each new UUID in rand_b can help produce unguessable UUIDs. In the event more counter bits are required, the most significant (left-most) bits of rand_b MAY be used as additional counter bits.¶

Monotonic Random (Method 2):

With this method, the random data is extended to also function as a counter. This monotonic value can be thought of as a "randomly seeded counter" which MUST be incremented in the least significant position for each UUID created on a given timestamp tick. UUIDv7's rand_b section SHOULD be utilized with this method to handle batch UUID generation during a single timestamp tick. The increment value for every UUID generation is a random integer of any desired length larger than zero. It ensures the UUIDs retain the required level of unguessability provided by the underlying entropy. The increment value MAY be 1 when the number of UUIDs generated in a particular period of time is important and guessability is not an issue. However, incrementing the counter by 1 SHOULD NOT be used by implementations that favor unguessability, as the resulting values are easily guessable.¶

Replace Left-Most Random Bits with Increased Clock Precision (Method 3):

For UUIDv7, which has millisecond timestamp precision, it is possible to use additional clock precision available on the system to substitute for up to 12 random bits immediately following the timestamp. This can provide values that are time-ordered with sub-millisecond precision, using however many bits are appropriate in the implementation environment. With this method, the additional time precision bits MUST follow the timestamp as the next available bit, in the rand_a field for UUIDv7.¶

To calculate this value, start with the portion of the timestamp expressed as a fraction of clock's tick value (fraction of a millisecond for UUIDv7). Compute the count of possible values that can be represented in the available bit space, 4096 for the UUIDv7 rand_a field. Using floating point or scaled integer arithmetic, multiply this fraction of a millisecond value by 4096 and round down (toward zero) to an integer result to arrive at a number between 0 and the maximum allowed for the indicated bits which sorts monotonically based on time. Each increasing fractional value will result in an increasing bit field value, to the precision available with these bits.¶

For example, let's assume a system timestamp of 1 Jan 2023 12:34:56.1234567. Taking the precision greater than 1ms gives us a value of 0.4567, as a fraction of a millisecond. If we wish to encode this as 12 bits, we can take the count of possible values that fit in those bits (4096, or 2 to the 12th power) and multiply it by our millisecond fraction value of 0.4567 and truncate the result to an integer, which gives an integer value of 1870. Expressed as hexadecimal it is 0x74E, or the binary bits 0b011101001110. One can then use those 12 bits as the most significant (left-most) portion of the random section of the UUID (e.g., the rand_a field in UUIDv7). This works for any desired bit length that fits into a UUID, and applications can decide the appropriate length based on available clock precision, but for UUIDv7, it is limited to 12 bits at maximum to reserve sufficient space for random bits.¶

The main benefit to encoding additional timestamp precision is that it utilizes additional time precision already available in the system clock to provide values that are more likely to be unique, and thus may simplify certain implementations. This technique can also be used in conjunction with one of the other methods, where this additional time precision would immediately follow the timestamp, and then if any bits are to be used as clock sequence they would follow next.¶

The following sub-topics cover topics related solely with creating reliable fixed-length dedicated counters:¶

Fixed-Length Dedicated Counter Seeding:

Implementations utilizing the fixed-length counter method randomly initialize the counter with each new timestamp tick. However, when the timestamp has not increased, the counter is instead incremented by the desired increment logic. When utilizing a randomly seeded counter alongside Method 1, the random value MAY be regenerated with each counter increment without impacting sortability. The downside is that Method 1 is prone to overflows if a counter of adequate length is not selected or the random data generated leaves little room for the required number of increments. Implementations utilizing fixed-length counter method MAY also choose to randomly initialize a portion of the counter rather than the entire counter. For example, a 24 bit counter could have the 23 bits in least-significant, right-most, position randomly initialized. The remaining most significant, left-most counter bit is initialized as zero for the sole purpose of guarding against counter rollovers.¶

Fixed-Length Dedicated Counter Length:

Select a counter bit-length that can properly handle the level of timestamp precision in use. For example, millisecond precision generally requires a larger counter than a timestamp with nanosecond precision. General guidance is that the counter SHOULD be at least 12 bits but no longer than 42 bits. Care must be taken to ensure that the counter length selected leaves room for sufficient entropy in the random portion of the UUID after the counter. This entropy helps improve the unguessability characteristics of UUIDs created within the batch.¶

The following sub-topics cover rollover handling with either type of counter method:¶

Counter Rollover Guards:

The technique from Fixed-Length Dedicated Counter Seeding that describes allocating a segment of the fixed-length counter as a rollover guard is also helpful to mitigate counter rollover issues. This same technique can be used with monotonic random counter methods by ensuring that the total length of a possible increment in the least significant, right most position is less than the total length of the random value being incremented. As such, the most significant, left-most, bits can be incremented as rollover guarding.¶

Counter Rollover Handling:

Counter rollovers MUST be handled by the application to avoid sorting issues. The general guidance is that applications that care about absolute monotonicity and sortability should freeze the counter and wait for the timestamp to advance which ensures monotonicity is not broken. Alternatively, implementations MAY increment the timestamp ahead of the actual time and reinitialize the counter.¶

Implementations MAY use the following logic to ensure UUIDs featuring embedded counters are monotonic in nature:¶

1. Compare the current timestamp against the previously stored timestamp.¶

2. If the current timestamp is equal to the previous timestamp, increment the counter according to the desired method.¶

3. If the current timestamp is greater than the previous timestamp, re-initialize the desired counter method to the new timestamp and generate new random bytes (if the bytes were frozen or being used as the seed for a monotonic counter).¶

Monotonic Error Checking:

Implementations SHOULD check if the currently generated UUID is greater than the previously generated UUID. If this is not the case then any number of things could have occurred, such as clock rollbacks, leap second handling, and counter rollovers. Applications SHOULD embed sufficient logic to catch these scenarios and correct the problem to ensure that the next UUID generated is greater than the previous, or at least report an appropriate error. To handle this scenario, the general guidance is that application MAY reuse the previous timestamp and increment the previous counter method.¶

6.3. UUID Generator States

The (optional) UUID generator state only needs to be read from stable storage once at boot time, if it is read into a system-wide shared volatile store (and updated whenever the stable store is updated).¶

This stable storage MAY be used to record various portions of the UUID generation which prove useful for batch UUID generation purposes and monotonic error checking with UUIDv6 and UUIDv7. These stored values include but are not limited to last known timestamp, clock sequence, counters, and random data.¶

If an implementation does not have any stable store available, then it MAY proceed with UUID generation as if this was the first UUID created within a batch. This is the least desirable implementation because it will increase the frequency of creation of values such as clock sequence, counters, or random data, which increases the probability of duplicates. Further, frequent generation of random numbers also puts more stress on any entropy source and or entropy pool being used as the basis for such random numbers.¶

An implementation MAY also return an application error in the event that collision resistance is of the utmost concern. The semantics of this error are up to the application and implementation. See Section 6.7 for more information on weighting collision tolerance in applications.¶

For UUIDv1 and UUIDv6, if the node ID can never change (e.g., the network interface card from which the node ID is derived is inseparable from the system), or if any change also re-initializes the clock sequence to a random value, then instead of keeping it in stable store, the current node ID may be returned.¶

For UUIDv1 and UUIDv6, the state does not always need to be written to stable store every time a UUID is generated. The timestamp in the stable store can be periodically set to a value larger than any yet used in a UUID. As long as the generated UUIDs have timestamps less than that value, and the clock sequence and node ID remain unchanged, only the shared volatile copy of the state needs to be updated. Furthermore, if the timestamp value in stable store is in the future by less than the typical time it takes the system to reboot, a crash will not cause a re-initialization of the clock sequence.¶

If it is too expensive to access shared state each time a UUID is generated, then the system-wide generator can be implemented to allocate a block of time stamps each time it is called; a per- process generator can allocate from that block until it is exhausted.¶

6.4. Distributed UUID Generation

Some implementations MAY desire to utilize multi-node, clustered, applications which involve two or more nodes independently generating UUIDs that will be stored in a common location. While UUIDs already feature sufficient entropy to ensure that the chances of collision are low, as the total number of UUID generating nodes increase; so does the likelihood of a collision.¶

This section will detail the two additional collision resistance approaches that have been observed by multi-node UUID implementations in distributed environments.¶

It should be noted that, although this section details two methods for the sake of completeness, implementations should utilize the pseudo-random Node ID option if additional collision resistance for distributed UUID generation is a requirement. Likewise, utilization of either method is not required for implementing UUID generation in distributed environments.¶

Node IDs:

With this method, a pseudo-random Node ID value is placed within the UUID layout. This identifier helps ensure the bit-space for a given node is unique, resulting in UUIDs that do not conflict with any other UUID created by another node with a different node id. Implementations that choose to leverage an embedded node id SHOULD utilize UUIDv8. The node id SHOULD NOT be an IEEE 802 MAC address as per Section 8. The location and bit length are left to implementations and are outside the scope of this specification. Furthermore, the creation and negotiation of unique node ids among nodes is also out of scope for this specification.¶

Centralized Registry:

With this method all nodes tasked with creating UUIDs consult a central registry and confirm the generated value is unique. As applications scale, the communication with the central registry could become a bottleneck and impact UUID generation in a negative way. Shared knowledge schemes with central/global registries are outside the scope of this specification and is NOT RECOMMENDED.¶

Distributed applications generating UUIDs at a variety of hosts MUST be willing to rely on the random number source at all hosts.¶

6.5. Name-Based UUID Generation

Although some prefer to use the word "hash-based" to describe UUIDs featuring hashing algorithms (MD5 or SHA-1), this document retains the usage of the adjective "name-based" in order to maintain consistency with historical documents and existing implementations.¶

The requirements for name-based UUIDs are as follows:¶

• UUIDs generated at different times from the same name (using the same canonical format) in the same namespace MUST be equal.¶

• UUIDs generated from two different names (same or differing canonical format) in the same namespace should be different (with very high probability).¶

• UUIDs generated from the same name (same or differing canonical format) in two different namespaces should be different (with very high probability).¶

• If two UUIDs that were generated from names (using the same canonical format) are equal, then they were generated from the same name in the same namespace (with very high probability).¶

A note on names:

The concept of name (and namespace) should be broadly construed and not limited to textual names. A canonical sequence of octets is one that conforms to the specification for that name form's canonical representation. A name can have many usual forms, only one of which can be canonical. An implementer of new namespaces for UUIDs needs to reference the specification for the canonical form of names in that space, or define such a canonical for the namespace if it does not exist. For example, at the time of this specification, [RFC8499] domain name system (DNS) has three conveyance formats: common (www.example.com), presentation (www.example.com.) and wire format (3www7example3com0). Looking at [X500] distinguished names (DNs), the previous version of this specification allowed either text based or binary distinguished encoding rules (DER) based names as inputs. For [RFC1738] uniform resource locators (URLs), one could provide a fully-qualified domain-name (FQDN) with or without the protocol identifier (www.example.com) or (https://www.example.com). When it comes to [X660] object identifiers (OIDs) one could choose dot-notation without the leading dot (2.999), choose to include the leading dot (.2.999) or select one of the many formats from [X680] such as OID Internationalized Resource Identifier (OID-IRI) (/Joint-ISO-ITU-T/Example). While most users may default to the common format for DNS, FQDN format for a URL, text format for X.500 and dot-notation without a leading dot for OID; name-based UUID implementations generally SHOULD allow arbitrary input which will compute name-based UUIDs for any of the aforementioned example names and others not defined here. Each name format within a namespace will output different UUIDs. As such, the mechanisms or conventions used for allocating names and ensuring their uniqueness within their namespaces are beyond the scope of this specification.¶

6.6. Namespace ID Usage and Allocation

This section and table, Table 3, details the namespace IDs for some potentially interesting namespaces such those for [RFC8499] domain name system (DNS), [RFC1738] uniform resource locators (URLs), [X660] object identifiers (OIDs), and [X500] distinguished names (DNs).¶

Further, this section also details allocation, IANA registration and other details pertinent to Namespace IDs. IANA may use the table Table 3 as-is replacing "This Document" replaced as required.¶

Items may be added to this table using "Specification Required" policy as per [RFC8126].¶

For designated experts, generally speaking, Namespace IDs are allocated as follows:¶

• The first namespace ID value, for DNS, was calculated from a time-based UUIDv1 and "6ba7b810-9dad-11d1-80b4-00c04fd430c8" used as a starting point.¶

• Subsequent namespace ID values increment the least-significant, right-most bit of time_low "6ba7b810" while freezing the rest of the UUID to "9dad-11d1-80b4-00c04fd430c8".¶

• New namespace ID values MUST use this same logic and MUST NOT use a previously used Namespace ID value.¶

• Thus, "6ba7b815" is the next available time_low for a new Namespace ID value with the full ID being "6ba7b815-9dad-11d1-80b4-00c04fd430c8".¶

• The upper bound for time_low in this special use, namespace ID values, is "ffffffff" or "ffffffff-9dad-11d1-80b4-00c04fd430c8" which should be sufficient space for future namespace ID values.¶

Note that the namespace ID value "6ba7b813-9dad-11d1-80b4-00c04fd430c8" and its usage is not defined by this document or by [RFC4122], as such it SHOULD NOT be used as a Namespace ID value.¶

New Namespace ID values MUST be documented as per Section 7 if they are to be globally available and fully interoperable. Implementations MAY continue to use vendor-specific, application-specific, and deployment-specific Namespace ID values but know that interoperability is not guaranteed. These custom Namespace ID values MUST NOT use the logic above and instead are RECOMMENDED to generate a UUIDv4 or UUIDv7 Namespace ID value. If collision probability (Section 6.7) and uniqueness (Section 6.8) of the final name-based UUID are not a problem; an implementation MAY also leverage UUIDv8 instead to create a custom, application-specific Namespace ID value.¶

Implementations SHOULD provide the ability to input a custom namespace to account for newly registered IANA Namespace ID values outside of those listed in this section or custom, application specific Namespace ID values.¶

6.7. Collision Resistance

Implementations should weigh the consequences of UUID collisions within their application and when deciding between UUID versions that use entropy (randomness) versus the other components such as those in Section 6.1 and Section 6.2. This is especially true for distributed node collision resistance as defined by Section 6.4.¶

There are two example scenarios below which help illustrate the varying seriousness of a collision within an application.¶

Low Impact:

A UUID collision generated a duplicate log entry which results in incorrect statistics derived from the data. Implementations that are not negatively affected by collisions may continue with the entropy and uniqueness provided by the traditional UUID format.¶

High Impact:

A duplicate key causes an airplane to receive the wrong course which puts people's lives at risk. In this scenario there is no margin for error. Collisions must be avoided and failure is unacceptable. Applications dealing with this type of scenario must employ as much collision resistance as possible within the given application context.¶

6.8. Global and Local Uniqueness

UUIDs created by this specification MAY be used to provide local uniqueness guarantees. For example, ensuring UUIDs created within a local application context are unique within a database MAY be sufficient for some implementations where global uniqueness outside of the application context, in other applications, or around the world is not required.¶

Although true global uniqueness is impossible to guarantee without a shared knowledge scheme, a shared knowledge scheme is not required by UUID to provide uniqueness for practical implementation purposes. Implementations MAY implement a shared knowledge scheme introduced in Section 6.4 as they see fit to extend the uniqueness guaranteed by this specification.¶

6.9. Unguessability

Implementations SHOULD utilize a cryptographically secure pseudo-random number generator (CSPRNG) to provide values that are both difficult to predict ("unguessable") and have a low likelihood of collision ("unique"). The exception is when a suitable CSPRNG is unavailable in the execution environment. Take care to ensure the CSPRNG state is properly reseeded upon state changes, such as process forks, to ensure proper CSPRNG operation. CSPRNG ensures the best of Section 6.7 and Section 8 are present in modern UUIDs.¶

Further advice on generating cryptographic-quality random numbers can be found in [RFC4086], [RFC8937] and in [RANDOM].¶

6.10. UUIDs That Do Not Identify the Host

This section describes how to generate a UUIDv1 or UUIDv6 value if an IEEE 802 address is not available, or its use is not desired.¶

Implementations MAY leverage MAC address randomization techniques (IEEE 802.11bh) as an alternative to the pseudo-random logic provided in this section.¶

Alternatively, implementations MAY elect to obtain a 48 bit cryptographic-quality random number as per Section 6.9 to use it as the node ID. After generating the 48 bit fully randomized node value, implementations MUST set the least significant bit of the first octet of the node ID set to 1. This bit is the unicast/multicast bit, which will never be set in IEEE 802 addresses obtained from network cards. Hence, there can never be a conflict between UUIDs generated by machines with and without network cards. An example of generating a randomized 48 bit node value and the subsequent bit modification is detailed in the Appendix A appendix. For more information about IEEE 802 address and the unicast/multicast or local/global bits please review [RFC7042].¶

For compatibility with earlier specifications, note that this document uses the unicast/multicast bit, instead of the arguably more correct local/global bit because MAC addresses with the local/global bit set or not are both possible in a network. This is not the case with the unicast/multicast bit. One node cannot have a MAC address that multicasts to multiple nodes.¶

In addition, items such as the computer's name and the name of the operating system, while not strictly speaking random, will help differentiate the results from those obtained by other systems.¶

The exact algorithm to generate a node ID using these data is system specific, because both the data available and the functions to obtain them are often very system specific. A generic approach, however, is to accumulate as many sources as possible into a buffer, use a message digest (such as SHA-256 or SHA-512 defined by [FIPS180-4]), take an arbitrary 6 bytes from the hash value, and set the multicast bit as described above.¶

6.11. Sorting

UUIDv6 and UUIDv7 are designed so that implementations that require sorting (e.g., database indexes) sort as opaque raw bytes, without need for parsing or introspection.¶

Time ordered monotonic UUIDs benefit from greater database index locality because the new values are near each other in the index. As a result objects are more easily clustered together for better performance. The real-world differences in this approach of index locality vs random data inserts can be one order of magnitude or more.¶

UUID formats created by this specification are intended to be lexicographically sortable while in the textual representation.¶

UUIDs created by this specification are crafted with big-endian byte order (network byte order) in mind. If little-endian style is required, UUIDv8 is available for custom UUID formats.¶

6.12. Opacity

As general guidance, it is recommend to avoid parsing UUID values unnecessarily, and instead treating UUIDs as opaquely as possible. Although application-specific concerns could of course require some degree of introspection (e.g., to examine the Section 4.1, Section 4.2 or perhaps the timestamp of a UUID), the advice here is to avoid this or other parsing unless absolutely necessary. Applications typically tend to be simpler, more interoperable, and perform better, when this advice is followed.¶

6.13. DBMS and Database Considerations

For many applications, such as databases, storing UUIDs as text is unnecessarily verbose, requiring 288 bits to represent 128 bit UUID values. Thus, where feasible, UUIDs SHOULD be stored within database applications as the underlying 128 bit binary value.¶

For other systems, UUIDs MAY be stored in binary form or as text, as appropriate. The trade-offs to both approaches are:¶

• Storing as binary requires less space and may result in faster data access.¶

• Storing as text requires more space but may require less translation if the resulting text form is to be used after retrieval, which thus may make it simpler to implement.¶

DBMS vendors are encouraged to provide functionality to generate and store UUID formats defined by this specification for use as identifiers or left parts of identifiers such as, but not limited to, primary keys, surrogate keys for temporal databases, foreign keys included in polymorphic relationships, and keys for key-value pairs in JSON columns and key-value databases. Applications using a monolithic database may find using database-generated UUIDs (as opposed to client-generate UUIDs) provides the best UUID monotonicity. In addition to UUIDs, additional identifiers MAY be used to ensure integrity and feedback.¶

Designers of database schema are cautioned against using name-based UUIDs (Section 5.3/Section 5.5) as primary keys in tables. A common issue observed in database schema design is the assumption that a particular value will never change, which then later turns out to be an incorrect assumption. Postal codes, license or other identification numbers, and numerous other such identifiers seem unique and unchanging at a given point time; only to later turn out to have edge cases where they need to change. The subsequent change of the identifier, used as a "name" input for name-based UUIDs, can invalidate a given database structure. In such scenarios it is observed that using any non-name-based UUID version would have resulted in the field in question being placed somewhere that would have been easier to adapt to such changes (primary key excluded from this statement). The general advice is to avoid name-based UUID natural keys and instead utilize time-based UUID surrogate keys based on the aforementioned problems detailed in this section.¶

7.1. IANA UUID Subtype Registry and Registration

This specification defines the "UUID Subtype" registry for common, widely used UUID standards.¶

Table 4 should be used as-is for this registry with "This document" replaced as required.¶

This table may be extended by the "Standards Action" policy, per [RFC8126].¶

For designated experts:¶

• The minimum and maximum "ID" value for the subtype "version" within the "OSF DCE / IETF" variant is 0 through 15. The versions, within Table 1, described as "Reserved for future definition" or "unused" are omitted from this IANA registry until properly defined.¶

• The "Subtype" column is free-form text however at the time of this publication only "version" and "family" are known UUID subtypes. The "family" subtype is part of the "Apollo NCS" variant space (Both are outside the scope of this specification). The Microsoft variant may have subtyping mechanisms defined however they are unknown and outside of the scope of this specification. Similarly, the final "Reserved for future definition" variant may introduce new subtyping logic at a future date. Subtype IDs are permitted to overlap, that is, an ID of "1" may exist in multiple variant spaces.¶

• The "Variant" column is free-form text however it is likely one of four values will be included. The first three are "OSF DCE / IETF", "Apollo NCS", "Microsoft". The final variant value belongs to the "Reserved for future definition" variant and may introduce a new name at a future date.¶

7.2. IANA UUID Namespace ID Registry and Registration

This specification defines the "UUID Namespace ID" registry for common, widely used Namespace ID values.¶

The full details of this registration, including information for designated experts, can be found in Section 6.6.¶

A.1. Example of a UUIDv1 Value

-------------------------------------------
field      bits value
-------------------------------------------
time_low   32   0xC232AB00
time_mid   16   0x9414
ver         4   0x1
time_high  12   0x1EC
var         2   0b10
clock_seq  14   0b11, 0x3C8
node       48   0x9F6BDECED846
-------------------------------------------
total      128
-------------------------------------------
final: C232AB00-9414-11EC-B3C8-9F6BDECED846

A.2. Example of a UUIDv3 Value

The MD5 computation from is detailed in Figure 17 using the DNS Namespace ID value and the Name "www.example.com". while the field mapping and all values are illustrated in Figure 18. Finally to further illustrate the bit swapping for version and variant see Figure 19.¶

Namespace (DNS):  6ba7b810-9dad-11d1-80b4-00c04fd430c8
Name:             www.example.com
------------------------------------------------------
MD5:              5df418813aed051548a72f4a814cf09e

-------------------------------------------
field     bits value
-------------------------------------------
md5_high  48   0x5df418813aed
ver        4   0x3
md5_mid   12   0x515
var        2   0b10
md5_low   62   0b00, 0x8a72f4a814cf09e
-------------------------------------------
total     128
-------------------------------------------
final: 5df41881-3aed-3515-88a7-2f4a814cf09e

MD5 hex and dash:      5df41881-3aed-0515-48a7-2f4a814cf09e
Ver and Var Overwrite: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
Final:                 5df41881-3aed-3515-88a7-2f4a814cf09e

A.3. Example of a UUIDv4 Value

This UUIDv4 example was created by generating 16 bytes of random data resulting in the hexadecimal value of 919108F752D133205BACF847DB4148A8. This is then used to fill out the fields as shown in Figure 20.¶

Finally to further illustrate the bit swapping for version and variant see Figure 21.¶

-------------------------------------------
field     bits value
-------------------------------------------
random_a  48   0x919108f752d1
ver        4   0x4
random_b  12   0x320
var        2   0b10
random_c  62   0b01, 0xbacf847db4148a8
-------------------------------------------
total     128
-------------------------------------------
final: 919108f7-52d1-4320-9bac-f847db4148a8

Random hex:            919108f752d133205bacf847db4148a8
Random hex and dash:   919108f7-52d1-3320-5bac-f847db4148a8
Ver and Var Overwrite: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
Final:                 919108f7-52d1-4320-9bac-f847db4148a8

A.4. Example of a UUIDv5 Value

The SHA-1 computation from is detailed in Figure 22 using the DNS Namespace ID value and the Name "www.example.com". while the field mapping and all values are illustrated in Figure 23. Finally to further illustrate the bit swapping for version and variant and the unused/discarded part of the SHA-1 value see Figure 24.¶

Namespace (DNS):  6ba7b810-9dad-11d1-80b4-00c04fd430c8
Name:             www.example.com
----------------------------------------------------------
SHA-1:            2ed6657de927468b55e12665a8aea6a22dee3e35

-------------------------------------------
field      bits value
-------------------------------------------
sha1_high  48   0x2ed6657de927
ver         4   0x5
sha1_mid   12   0x68b
var         2   0b10
sha1_low   62   0b01, 0x5e12665a8aea6a2
-------------------------------------------
total      128
-------------------------------------------
final: 2ed6657d-e927-568b-95e1-2665a8aea6a2

SHA-1 hex and dash:    2ed6657d-e927-468b-55e1-2665a8aea6a2-2dee3e35
Ver and Var Overwrite: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
Final:                 2ed6657d-e927-568b-95e1-2665a8aea6a2
Discarded:                                                 -2dee3e35

A.5. Example of a UUIDv6 Value

-------------------------------------------
field       bits value
-------------------------------------------
time_high   32   0x1EC9414C
time_mid    16   0x232A
ver          4   0x6
time_high   12   0xB00
var          2   0b10
clock_seq   14   0b11, 0x3C8
node        48   0x9F6BDECED846
-------------------------------------------
total       128
-------------------------------------------
final: 1EC9414C-232A-6B00-B3C8-9F6BDECED846

A.6. Example of a UUIDv7 Value

This example UUIDv7 test vector utilizes a well-known Unix epoch timestamp with millisecond precision to fill the first 48 bits.¶

rand_a and rand_b are filled with random data.¶

The timestamp is Tuesday, February 22, 2022 2:22:22.00 PM GMT-05:00 represented as 0x017F22E279B0 or 1645557742000¶

-------------------------------------------
field       bits value
-------------------------------------------
unix_ts_ms  48   0x017F22E279B0
ver          4   0x7
rand_a      12   0xCC3
var          2   0b10
rand_b      62   0b01, 0x8C4DC0C0C07398F
-------------------------------------------
total       128
-------------------------------------------
final: 017F22E2-79B0-7CC3-98C4-DC0C0C07398F

B.1. Example of a UUIDv8 Value (time-based)

This example UUIDv8 test vector utilizes a well-known 64 bit Unix epoch timestamp with 10ns precision, truncated to the least-significant, right-most, bits to fill the first 60 bits of custom_a and custom_b while setting the version bits between these two segments to the version value of 8.¶

The variant bits are set and the final segment, custom_c, is filled with random data.¶

Timestamp is Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00 represented as 0x2489E9AD2EE2E00 or 164555774200000000 (10ns-steps).¶

-------------------------------------------
field     bits value
-------------------------------------------
custom_a  48   0x2489E9AD2EE2
ver        4   0x8
custom_b  12   0xE00
var        2   0b10
custom_c  62   0b00, 0xEC932D5F69181C0
-------------------------------------------
total     128
-------------------------------------------
final: 2489E9AD-2EE2-8E00-8EC9-32D5F69181C0

B.2. Example of a UUIDv8 Value (name-based)

As per Section 5.5 name-based UUIDs that desire to use modern hashing algorithms MUST be created within the UUIDv8 space. These MAY leverage newer hashing algorithms such as SHA-256 or SHA-512 defined by [FIPS180-4], SHA-3 or SHAKE defined by [FIPS202], or even algorithms that have not been defined yet.¶

A SHA-256 version of Appendix A.4 is detailed in Figure 28 as an illustrative example detailing how this can be achieved. The creation of the name-based UUIDv8 value in this section follows the same logic defined in Section 5.5 with the difference being SHA-256 in place of SHA-1.¶

The field mapping and all values are illustrated in Figure 29. Finally to further illustrate the bit swapping for version and variant and the unused/discarded part of the SHA-256 value see Figure 30. An important note for secure hashing algorithms that produce outputs of an arbitrary size, such as those found in SHAKE, the output hash MUST be 128 bits or larger.¶

Namespace (DNS):       6ba7b810-9dad-11d1-80b4-00c04fd430c8
Name:                  www.example.com
----------------------------------------------------------------
SHA-256:
5c146b143c524afd938a375d0df1fbf6fe12a66b645f72f6158759387e51f3c8

-------------------------------------------
field     bits value
-------------------------------------------
custom_a  48   0x5c146b143c52
ver        4   0x8
custom_b  12   0xafd
var        2   0b10
custom_c  62   0b00, 0x38a375d0df1fbf6
-------------------------------------------
total     128
-------------------------------------------
final: 5c146b14-3c52-8afd-938a-375d0df1fbf6

A: 5c146b14-3c52-4afd-938a-375d0df1fbf6-fe12a66b645f72f6158759387e51f3c8
B: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
C: 5c146b14-3c52-8afd-938a-375d0df1fbf6
D:                                     -fe12a66b645f72f6158759387e51f3c8

Examining Figure 30:¶

• Line A details the full SHA-256 as a hexadecimal value with the dashes inserted.¶

• Line B details the version and variant hexadecimal positions which must be overwritten.¶

• Line C details the final value after the ver/var have been overwritten.¶

• Line D details the discarded, leftover values from the original SHA-256 computation.¶