Internet-Draft UUID April 2023
Leach, et al. Expires 14 October 2023 [Page]
4122 (if approved)
Intended Status:
Standards Track
P. Leach
M. Mealling
VeriSign, Inc.
B. G. Peabody
K. R. Davis
Cisco Systems

Universally Unique IDentifiers (UUID)


This specification defines a Uniform Resource Name namespace for UUIDs (Universally Unique IDentifiers), also known as GUIDs (Globally Unique IDentifiers). A UUID is 128 bits long, and is intended to guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System and later in the Open Software Foundation's (OSF) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms.

This specification is derived from the DCE specification with the kind permission of the OSF (now known as The Open Group). Information from earlier versions of the DCE specification have been incorporated into this document. This document obsoletes RFC4122.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 14 October 2023.

Table of Contents

1. Introduction

This specification defines a Uniform Resource Name namespace for UUIDs (Universally Unique IDentifiers), also known as GUIDs (Globally Unique IDentifiers). A UUID is 128 bits long, and requires no central registration process.

The use of UUIDs is extremely pervasive in computing. They comprise the core identifier infrastructure for many operating systems such as Microsoft Windows and applications such as the Mozilla Web browser and in many cases, become exposed in many non-standard ways.

This specification attempts to standardize that practice as openly as possible and in a way that attempts to benefit the entire Internet. The information here is meant to be a concise guide for those wishing to implement services using UUIDs, as URNs [RFC8141], or otherwise.

There is an ITU-T Recommendation and an ISO/IEC Standard [X667] that are derived from [RFC4122]. Both sets of specifications have been aligned and are fully technically compatible. In addition, a global registration function is being provided by the Telecommunications Standardization Bureau of ITU-T; for details see Nothing in this document should be construed to override the DCE standards that defined UUIDs.

2. Motivation

One of the main reasons for using UUIDs is that no centralized authority is required to administer them (although one format uses IEEE 802 node identifiers, others do not). As a result, generation on demand can be completely automated, and used for a variety of purposes. The UUID generation algorithm described here supports very high allocation rates of 10 million per second per machine or more if necessary, so that they could even be used as transaction IDs.

UUIDs are of a fixed size (128 bits), which is reasonably small compared to other alternatives. This lends itself well to sorting, ordering, and hashing of all sorts, storing in databases, simple allocation, and ease of programming in general.

Since UUIDs are unique and persistent, they make excellent Uniform Resource Names. The unique ability to generate a new UUID without a registration process allows for UUIDs to be one of the URNs with the lowest minting cost.

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 as 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 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 negative performance effects of which 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 Version 1 UUIDs. Exposed MAC addresses can be used as an attack surface to locate machines 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 generation of a UUID versus an application that simply stores one, which 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 lead 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 UUIDs to address these issues.

3. Terminology

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:


Universally Unique Identifier


Uniform Resource Names


Augmented Backus-Naur Form


Cryptographically Secure Pseudo-Random Number Generator


Media Access Control


Most Significant Bit


Database Management System


Institute of Electrical and Electronics Engineers, Inc.


International Telecommunication Union


Message Digest 5


Secure Hash Algorithm


Secure Hash Algorithm 1 with message digest of 160 bits


Secure Hash Algorithm with message digest size of 224 bits


Secure Hash Algorithm with message digest size of 256 bits


Secure Hash Algorithm with message digest size of 512 bits


Secure Hash Algorithm 3


Secure Hash Algorithm 3 based on KECCAK algorithm


Coordinated Universal Time

3.3. changelog

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


  • 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


  • 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


  • 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


  • Merge RFC4122 with
  • 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 | 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. UUID Format

The UUID format is 16 octets (128 bits); the variant bits in conjunction with the version bits described in the next sections in determine finer structure. While discussing UUID formats and layout, bit definitions start at 0 and end at 127 while octet definitions start at 0 and end at 15.

UUIDs MAY be represented as binary data or integers. When in use with URNs or as text in applications, any given UUID SHOULD be represented by the "hex-and-dash" string format consisting of multiple groups of upper or lowercase alphanumeric hex characters separated by single dashes/hyphens. When used with databases please refer to Section 6.12.

The formal definition of the UUID string representation is provided by the following (ABNF) [RFC5234].

UUID     = 4hexOctet "-"
           2hexOctet "-"
           2hexOctet "-"
           2hexOctet "-"
DIGIT    = %x30-39
HEXDIG   = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

An example UUID using this textual representation from the above ABNF is shown in Figure 1. Note that in this example the alphabetic characters may be all uppercase, all lowercase or mixed case as per [RFC5234], Section 2.3

Figure 1: Example String UUID format

The same UUID from Figure 1 is represented in Binary Figure 2, Integer Figure 3 and as a URN Figure 4 defined by [RFC8141].

Figure 2: Example Binary UUID
Figure 3: Example Integer UUID (shown as a decimal number)
Figure 4: Example URN UUID

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.

Table 1: UUID Variants
Msb0 Msb1 Msb2 Description
0 x x Reserved, NCS backward compatibility and includes Nil UUID as per Section 5.9.
1 0 x The variant specified in this document.
1 1 0 Reserved, Microsoft Corporation backward compatibility.
1 1 1 Reserved for future definition and includes Max UUID as per Section 5.10.

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 specified in this document.

Table 2: UUID variant 10x versions defined by this specification
Msb0 Msb1 Msb2 Msb3 Version Description
0 0 0 0 0 Unused
0 0 0 1 1 The Gregorian time-based UUID specified in this document.
0 0 1 0 2 Reserved for DCE Security version, with embedded POSIX UUIDs.
0 0 1 1 3 The name-based version specified in this document that uses MD5 hashing.
0 1 0 0 4 The randomly or pseudo-randomly generated version specified in this document.
0 1 0 1 5 The name-based version specified in this document that uses SHA-1 hashing.
0 1 1 0 6 Reordered Gregorian time-based UUID specified in this document.
0 1 1 1 7 Unix Epoch time-based UUID specified in this document.
1 0 0 0 8 Reserved for custom UUID formats specified in this document.
1 0 0 1 9 Reserved for future definition.
1 0 1 0 10 Reserved for future definition.
1 0 1 1 11 Reserved for future definition.
1 1 0 0 12 Reserved for future definition.
1 1 0 1 13 Reserved for future definition.
1 1 1 0 14 Reserved for future definition.
1 1 1 1 15 Reserved for future definition.

An example version/variant layout for UUIDv4 follows the table where M represents the version placement for the hex representation of 4 (0100) and the N represents the variant placement for one of the four possible hex representation of variant 10x: 8 (1000), 9 (1001), A (1010), B (1011)

Figure 5: UUIDv4 Variant Examples

5. UUID Layouts

To minimize confusion about bit assignments within octets and among differing versions, the UUID record definition is provided as a a grouping of fields within bit layout consisting four octets to a row. The fields are presented with the most significant one first.

In the absence of explicit application or presentation protocol specification to the contrary, each field is encoded with the Most Significant Byte first (known as network byte order).

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).

UUID Version 1 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. For systems with multiple IEEE 802 addresses, any available one MAY be used. The lowest addressed octet (octet number 10) contains the global/local bit and the unicast/multicast bit, and is the first octet of the address transmitted on an 802.3 LAN.

 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                             |
Figure 6: UUIDv1 Field and Bit Layout

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


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


The 4 bit version field as defined by Section 4.2 set to 0001. Occupies bits 48 through 51 of octet 6.


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


The 2 bit variant field as defined by Section 4.1 set to 10. Occupies bits 64 and 65 of octet 8.


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


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 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.

For systems with no IEEE address, a randomly or pseudo-randomly generated value may be used; see Section 6.8 and Section 6.9.

5.2. UUID Version 2

UUID Version 2 is known as DCE Security UUIDs [C309] and [C311]. As such the definition of these UUIDs are 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 "name space" as per Section 6.5.

UUIDv3 values are created by computing an MD5 [RFC1321] hash over a given name space value concatenated with the desired name value after both have been converted to a canonical sequence of octets 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.

Some common name space values have been defined via Appendix A.

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                            |
Figure 7: UUIDv3 Field and Bit Layout

The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed MD5 value.


The 4 bit version field as defined by Section 4.2 set to 0011


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.


The 2 bit variant field as defined by Section 4.1 set to 10


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.

5.4. UUID Version 4

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

An implementation may generate 128 bits of random 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) then concatenate the version and variant in the required position.

For guidelines on random data generation see Section 6.8.

 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                            |
Figure 8: UUIDv4 Field and Bit Layout

The first 48 bits of the layout that can be filled with random data as specified in Section 6.8


The 4 bit version field as defined by Section 4.2 set to 0100


12 more bits of the layout that can be filled random data as per Section 6.8


The 2 bit variant field as defined by Section 4.1 set to 10


The final 62 bits of the layout immediately following the var field to be filled with random data as per Section 6.8

5.5. UUID Version 5

UUID Version 5 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "name space" as per Section 6.5.

UUIDv5 values are created by computing an SHA-1 [FIPS180-4] hash over a given name space value concatenated with the desired name value after both have been converted to a canonical sequence of octets in network byte order. This SHA-1 value is then used to populate all 128 bits of the UUID layout. Excess bits beyond 128 are discarded. The UUID version and variant then replace the respective bits as defined by Section 4.2 and Section 4.1

Some common name space values have been defined via Appendix A.

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. For implementation guidance around utilizing UUIDv8 for name-based UUIDs refer to the sub-section of Section 6.5.

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                            |
Figure 9: UUIDv5 Field and Bit Layout

The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed SHA-1 value.


The 4 bit version field as defined by Section 4.2


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.


The 2 bit variant field as defined by Section 4.1.


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.

5.6. UUID Version 6

UUID version 6 is a field-compatible version of UUIDv1, reordered for improved DB locality. It is expected that UUIDv6 will primarily be used in contexts where there are existing v1 UUIDs. Systems that do not involve legacy UUIDv1 SHOULD use UUIDv7 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 bits remain unchanged from their usage and 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 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

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


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


The 4 bit version field as defined by Section 4.2 set to 0110. Occupies bits 48 through 51 of Octet 6.


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


The 2 bit variant field as defined by Section 4.1 set to 10. Occupies bits 64 and 65 of octet 8.


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


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 seconds since midnight 1 Jan 1970 UTC, leap seconds excluded. UUID version 7 also has improved entropy characteristics over versions 1 or 6.

Implementations SHOULD utilize UUID version 7 instead of UUID version 1 and 6 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                             |
Figure 11: UUIDv7 Field and Bit Layout

48 bit big-endian unsigned number of Unix epoch timestamp in milliseconds as per Section 6.1.


4 bit UUIDv7 version set as per Section 4.2


12 bits pseudo-random data to provide uniqueness as per Section 6.8 and/or an optional counter to guarantee additional monotonicity as per Section 6.2.


The 2 bit variant defined by Section 4.1.


The final 62 bits of pseudo-random data to provide uniqueness as per Section 6.8 and/or an optional counter to guarantee additional monotonicity as per Section 6.2.

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 the Version and Variant leaving 122 bits for implementation specific UUIDs. To be clear: UUIDv8 is not a replacement for UUIDv4 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.
 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                            |
Figure 12: UUIDv8 Field and Bit Layout

The first 48 bits of the layout that can be filled as an implementation sees fit.


The 4 bit version field as defined by Section 4.2


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


The 2 bit variant field as defined by Section 4.1.


The final 62 bits of the layout immediately following the var field to be filled as an implementation sees fit.

5.9. Nil UUID

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

Figure 13: Nil UUID Format

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.

5.10. Max UUID

The Max UUID is 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.

Figure 14: Max UUID Format

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.

6. UUID Best Practices

The minimum requirements for generating UUIDs are described in this document for each version. Everything else is an implementation detail and up to the implementer to decide what is appropriate for a given implementation. Various relevant factors are covered below to help guide an implementer through the different trade-offs among differing UUID implementations.

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.


Implementations SHOULD use the current timestamp from a reliable source to provide values that are time-ordered and continually increasing. Care SHOULD 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.)


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 fixed millisecond level of precision within the Unix epoch that does not exceed the granularity capable in most modern systems. For other levels of precision UUIDv8 SHOULD be utilized. 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 will range between zero and the number of 100-nanosecond intervals per system time interval.


The length of a given timestamp directly impacts how long a given UUID will be valid. That is, how many timestamp ticks can be contained in a UUID before the maximum value for the timestamp field is reached. Care SHOULD be given 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, or to handle leap seconds. 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.


When timestamp padding is required, implementations MUST pad the most significant bits (left-most) bits with zeros. An example is padding the most significant, left-most bits of a 32 bit Unix timestamp with zeros to fill out the 48 bit timestamp in UUIDv7.


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 SHOULD either return an error, or stall the UUID generator until the system clock catches up, and MUST NOT return knowingly duplicate values. 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.

Care SHOULD be taken 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 SHOULD increment 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 choose one method 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, SHOULD 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 of UUIDv7 SHOULD be used as fixed-length dedicated counter bits that are incremented by one 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 SHOULD be 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 one when the number of UUIDs generated in a particular period of time is important and guessability is not an issue. However, it SHOULD NOT be used by implementations that favor unguessablity, as the resulting values are easily guessable.

Re-randomize Until Monotonic (Method 3):

If the performance impact is acceptable (depending on the UUID version, granularity of the system clock, and expected throughput of values generated per clock value), an implementation can, in the case of multiple UUID values using the same clock tick, simply regenerate a new random value as many times as needed in order to produce a UUID that is ordered after the last one. The primary benefit of this approach is simplicity of implementation. One downside to this approach is the fact that generation will become increasingly slower for a given timestamp value as more attempts are required to produce monotonic output, and the fact that subsequent values will have less entropy and be more easily guessable.

Replace Left-Most Random Bits with Increased Clock Precision (Method 4):

For UUIDv7, which has millisecond timestamp precision, it is possible to use any additional clock precision available on the system to substitute for 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 maximum value that can be represented in the available bit space, 4095 for the UUIDv7 rand_a field. Using floating point math, multiply this fraction of a millisecond value by 4095 and round to an integer result to arrive at a number between 0 and the maximum allowed for the indicated bits which is 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 maximum value that fits in those bits (4095, or 2 to the 12th power minus 1) and multiply it by our millisecond fraction value of 0.4567 and round the result to an integer, which gives an integer value of 1870. Expressed as hex it is 0x74E, or the binary bits 011101001110. 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 and desired 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 SHOULD randomly initialize the counter with each new timestamp tick. However, when the timestamp has not incremented, the counter SHOULD be frozen and incremented via 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 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 bits are 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 SHOULD also be given 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 the total length of a possible increment in the least significant, right most position is less than the total length of the random being incremented. As such the most significant, left-most, bits can be incremented as rollover guarding.

Counter Rollover Handling:

Counter rollovers SHOULD 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 SHOULD 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.

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 6.6 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 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

The concept of name and name space should be broadly construed, and not limited to textual names. For example, some name spaces are the domain name system, URLs, Object Identifiers (OIDs), X.500 Distinguished Names (DNs), and reserved words in a programming language. The mechanisms or conventions used for allocating names and ensuring their uniqueness within their name spaces are beyond the scope of this specification.

The requirements for name-based UUIDs are as follows:

  • UUIDs generated at different times from the same name in the same namespace MUST be equal.
  • UUIDs generated from two different names in the same namespace should be different (with very high probability).
  • UUIDs generated from the same name in two different namespaces should be different (with very high probability).
  • If two UUIDs that were generated from names are equal, then they were generated from the same name in the same namespace (with very high probability).
A note on namespaces:

While Appendix A details a few interesting namespaces; implementations SHOULD provide the ability input a custom namespace. For example, any other UUID MAY be generated and uses as the desired namespace input for a given application context to ensure all names created are unique within the newly created namespace.

Name-based UUIDs using version 8:

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 protocols such as SHA-256 or SHA-512 defined by [FIPS180-4], SHA-3 or SHAKE defined by [FIPS202], or even protocols that have not been defined yet. To ensure UUIDv8 Name-Based UUID values of different hashing protocols can exist in the same bit space; this document defines various "hashspaces" in Appendix B. Creation of name-based version 8 UUIDs follow the same logic defined in Section 5.5 but the hashspace should be used to as the starting point with the desired namespace and name concatenated to the end of the hashspace. Then an implementation may apply the desired hashing algorithm to the entire value after all have been converted to a canonical sequence of octets in network byte order. Ensure the version and variant and variant bits are modified as per Section 5.8 bit layout and finally trim any excess bits beyond 128. An important note for secure hashing algorithms that produce variable rate outputs, such as those found in SHAKE, the output hash MUST be 128 bits or larger. See Appendix C.8 for a SHA-256 UUIDv8 example test vector.

Advertising the Hash Algorithm:

Name-based UUIDs utilizing UUIDv8 do not allocate any available bits to identifying the hashing algorithm. As such where common knowledge about the hashing algorithm for a given UUIDv8 name-space UUID is required, sharing the Hash Space ID proves useful for identifying a the algorithm. That is, to detail SHA-256 was used to create a given UUIDv8 name-based UUID an implementation may also share the "3fb32780-953c-4464-9cfd-e85dbbe9843d" hash space which uniquely identifies the SHA-256 hashing algorithm for the purpose of UUIDv8. Mind you that this need not be the only method of sharing the hashing algorithm; this is one example of how two systems could share knowledge. The protocol of choice, communication channels and actual method of sharing this data between systems it outside the scope of this specification.

6.6. 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.

6.7. 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 this specification.

6.8. 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"). Care SHOULD be taken 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 6.6 and Section 8 are present in modern UUIDs.

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

6.9. 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 SHOULD obtain a 47-bit cryptographic-quality random number as per Section 6.8 and use it as the low 47 bits of the node ID.

Implementations MUST set the least significant bit of the first octet of the node ID set to one to create a 48-bit node id. 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.

For compatibility with earlier specifications, note that this document uses the unicast/multicast bit, instead of the arguably more correct local/global bit.

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 MD5 [RFC1321] or SHA-1 [FIPS180-4], take an arbitrary 6 bytes from the hash value, and set the multicast bit as described above.

6.10. Sorting

UUIDv6 and UUIDv7 are designed so that implementations that require sorting (e.g. database indexes) SHOULD 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 quite large.

UUIDs formats created by this specification SHOULD 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 a custom UUID format SHOULD be created using UUIDv8.

6.11. Opacity

UUIDs SHOULD be treated as opaque values and implementations SHOULD NOT examine the bits in a UUID. However, inspectors MAY refer to Section 4.1 and Section 4.2 when required to determine UUID version and variant.

As general guidance, we recommend not parsing UUID values unnecessarily, and instead treating them as opaquely as possible. Although application-specific concerns could of course require some degree of introspection (e.g. to examine the variant, version 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.

6.12. 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 and thus maybe 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.

7. IANA Considerations

There is no update required to the IANA URN namespace registration [URNNamespaces] for UUID filed in [RFC4122]. Further, at this time the authors and working group have concluded that IANA is not required to track UUIDs used for identifying items such as versions, variants, namespaces or hashspaces.

8. Security Considerations

Implementations SHOULD NOT assume that UUIDs are hard to guess. Foe example, they MUST NOT be used as security capabilities (identifiers whose mere possession grants access). Discovery of predictability in a random number source will result in a vulnerability.

Implementations MUST NOT assume that it is easy to determine if a UUID has been slightly transposed in order to redirect a reference to another object. Humans do not have the ability to easily check the integrity of a UUID by simply glancing at it.

MAC addresses pose inherent security risks and SHOULD NOT be used within a UUID. Instead CSPRNG data SHOULD be selected from a source with sufficient entropy to ensure guaranteed uniqueness among UUID generation. See Section 6.8 and Section 6.9 for more information.

Timestamps embedded in the UUID do pose a very small attack surface. The timestamp in conjunction with an embedded counter does signal the order of creation for a given UUID and its corresponding data but does not define anything about the data itself or the application as a whole. If UUIDs are required for use with any security operation within an application context in any shape or form then UUIDv4, Section 5.4 SHOULD be utilized.

See [RFC6151] for MD5 Security Considerations and [RFC6194] for SHA-1 security considerations.

9. Acknowledgements

The authors gratefully acknowledge the contributions of Rich Salz, Ben Campbell, Ben Ramsey, Fabio Lima, Gonzalo Salgueiro, Martin Thomson, Murray S. Kucherawy, Rick van Rein, Rob Wilton, Sean Leonard, Theodore Y. Ts'o., Robert Kieffer, Sergey Prokhorenko, LiosK

As well as all of those in the IETF community and on GitHub to who contributed to the discussions which resulted in this document.

This document draws heavily on the OSF DCE specification for UUIDs. Ted Ts'o provided helpful comments, especially on the byte ordering section which we mostly plagiarized from a proposed wording he supplied (all errors in that section are our responsibility, however).

We are also grateful to the careful reading and bit-twiddling of Ralf S. Engelschall, John Larmouth, and Paul Thorpe. Professor Larmouth was also invaluable in achieving coordination with ISO/IEC.

10. References

10.1. Normative References

"DCE: Remote Procedure Call", Open Group CAE Specification C309, ISBN 1-85912-041-5, , <>.
"DCE 1.1: Authentication and Security Services", Open Group CAE Specification C311, , <>.
National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-4, , <>.
National Institute of Standards and Technology, "SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions", FIPS PUB 202, , <>.
Occil, P., "Random Number Generator Recommendations for Applications", , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, , <>.
Saint-Andre, P. and J. Klensin, "Uniform Resource Names (URNs)", RFC 8141, DOI 10.17487/RFC8141, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
"Information Technology, "Procedures for the operation of OSI Registration Authorities: Generation and registration of Universally Unique Identifiers (UUIDs) and their use as ASN.1 Object Identifier components"", ISO/IEC 9834-8:2004, ITU-T Rec. X.667, .

10.2. Informative References

Tallent, R., "Creating sequential GUIDs in C# for MSSQL or PostgreSql", Commit 2759820, , <>.
Elliott, E., "Collision-resistant ids optimized for horizontal scaling and performance.", Commit 215b27b, , <>.
Pearcy, P., "Sequential UUID / Flake ID generator pulled out of elasticsearch common", Commit dd71c21, , <>.
Boundary, "Flake: A decentralized, k-ordered id generation service in Erlang", Commit 15c933a, , <>.
Pawlak, T., "Flake ID Generator", Commit fcd6a2f, , <>.
IEEE, "IEEE Standard for Floating-Point Arithmetic.", Series 754-2019, , <>.
Segment, "K-Sortable Globally Unique IDs", Commit bf376a7, , <>.
Twitter, "A Scala client for Cassandra", commit f6da4e0, , <>.
MongoDB, "ObjectId - MongoDB Manual", <>.
Cabrera, I. B., "Laravel: The mysterious "Ordered UUID"", , <>.
Google, "The 2^120 Ways to Ensure Unique Identifiers", , <>.
Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, DOI 10.17487/RFC1321, , <>.
Leach, P., Mealling, M., and R. Salz, "A Universally Unique IDentifier (UUID) URN Namespace", RFC 4122, DOI 10.17487/RFC4122, , <>.
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <>.
Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms", RFC 6151, DOI 10.17487/RFC6151, , <>.
Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, , <>.
Instagram Engineering, "Sharding & IDs at Instagram", , <>.
Chilton, A., "sid : generate sortable identifiers", Commit 660e947, , <>.
Twitter, "Snowflake is a network service for generating unique ID numbers at high scale with some simple guarantees.", Commit b3f6a3c, , <>.
Sony, "A distributed unique ID generator inspired by Twitter's Snowflake", Commit 848d664, , <>.
Feerasta, A., "Universally Unique Lexicographically Sortable Identifier", Commit d0c7170, , <>.
IANA, "Uniform Resource Names (URN) Namespaces", , <>.
Poitrey, O., "Globally Unique ID Generator", Commit efa678f, , <>.

Appendix A. Some Name Space IDs

This appendix lists the name space IDs for some potentially interesting name spaces such those for fully-qualified domain names (DNS), uniform resource locators (URLs), ISO OIDs, and X.500 distinguished names (DNs) in distinguished encoding rule (DER) or text format.

NameSpace_DNS  = "6ba7b810-9dad-11d1-80b4-00c04fd430c8"
NameSpace_URL  = "6ba7b811-9dad-11d1-80b4-00c04fd430c8"
NameSpace_OID  = "6ba7b812-9dad-11d1-80b4-00c04fd430c8"
NameSpace_X500 = "6ba7b814-9dad-11d1-80b4-00c04fd430c8"

Appendix B. Some Hash Space IDs

This appendix lists the hash space IDs for use with UUIDv8 name-based UUIDs.

SHA2_224     = "59031ca3-fbdb-47fb-9f6c-0f30e2e83145"
SHA2_256     = "3fb32780-953c-4464-9cfd-e85dbbe9843d"
SHA2_384     = "e6800581-f333-484b-8778-601ff2b58da8"
SHA2_512     = "0fde22f2-e7ba-4fd1-9753-9c2ea88fa3f9"
SHA2_512_224 = "003c2038-c4fe-4b95-a672-0c26c1b79542"
SHA2_512_256 = "9475ad00-3769-4c07-9642-5e7383732306"
SHA3_224     = "9768761f-ac5a-419e-a180-7ca239e8025a"
SHA3_256     = "2034d66b-4047-4553-8f80-70e593176877"
SHA3_384     = "872fb339-2636-4bdd-bda6-b6dc2a82b1b3"
SHA3_512     = "a4920a5d-a8a6-426c-8d14-a6cafbe64c7b"
SHAKE_128    = "7ea218f6-629a-425f-9f88-7439d63296bb"
SHAKE_256    = "2e7fc6a4-2919-4edc-b0ba-7d7062ce4f0a"

Appendix C. Test Vectors

Both UUIDv1 and UUIDv6 test vectors utilize the same 60 bit timestamp: 0x1EC9414C232AB00 (138648505420000000) Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00

Both UUIDv1 and UUIDv6 utilize the same values in clock_seq, and node. All of which have been generated with random data.

The pseudocode used for converting from a 64 bit Unix timestamp to a 100ns Gregorian timestamp value has been left in the document for reference purposes.

# Gregorian to Unix Offset:
# The number of 100-ns intervals between the
# UUID epoch 1582-10-15 00:00:00
# and the Unix epoch 1970-01-01 00:00:00
# Greg_Unix_offset = 0x01b21dd213814000 or 122192928000000000

# Unix 64 bit Nanosecond Timestamp:
# Unix NS: Tuesday, February 22, 2022 2:22:22 PM GMT-05:00
# Unix_64_bit_ns = 0x16D6320C3D4DCC00 or 1645557742000000000

# Unix Nanosecond precision to Gregorian 100-nanosecond intervals
# Greg_100_ns = (Unix_64_bit_ns/100)+Greg_Unix_offset

# Work:
# Greg_100_ns = (1645557742000000000/100)+122192928000000000
# Unix_64_bit_ns = (138648505420000000-122192928000000000)*100

# Final:
# Greg_100_ns = 0x1EC9414C232AB00 or 138648505420000000
Figure 15: Test Vector Timestamp Pseudo-code

C.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   b10
clock_seq  14   b11, 0x3C8
node       48   0x9E6BDECED846
total      128
final: C232AB00-9414-11EC-B3C8-9E6BDECED846
Figure 16: UUIDv1 Example Test Vector

C.2. Example of a UUIDv3 Value

The MD5 computation from is detailed in Figure 17 using the DNS NameSpace and the Name "". 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.

Name Space (DNS): 6ba7b810-9dad-11d1-80b4-00c04fd430c8
MD5:              5df418813aed051548a72f4a814cf09e
Figure 17: UUIDv3 Example MD5
field     bits value
md5_high  48   0x5df418813aed
ver        4   0x3
md5_mid   12   0x515
var        2   b10
md5_low   62   b00, 0x8a72f4a814cf09e
total     128
final: 5df41881-3aed-3515-88a7-2f4a814cf09e
Figure 18: UUIDv3 Example Test Vector
MD5 hex and dash:      5df41881-3aed-0515-48a7-2f4a814cf09e
Ver and Var Overwrite: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
Final:                 5df41881-3aed-3515-88a7-2f4a814cf09e
Figure 19: UUIDv3 Example Ver Var bit swaps

C.3. Example of a UUIDv4 Value

This UUIDv4 example was created by generating 16 bytes of random data resulting in the hex 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   b10
random_c  62   b01, 0xbacf847db4148a8
total     128
final: 919108f7-52d1-4320-9bac-f847db4148a8
Figure 20: UUIDv4 Example Test Vector
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
Figure 21: UUIDv4 Example Ver/Var bit swaps

C.4. Example of a UUIDv5 Value

The SHA-1 computation from is detailed in Figure 22 using the DNS NameSpace and the Name "". 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.

Name Space (DNS): 6ba7b810-9dad-11d1-80b4-00c04fd430c8
SHA-1:            2ed6657de927468b55e12665a8aea6a22dee3e35
Figure 22: UUIDv5 Example SHA-1
field      bits value
sha1_high  48   0x2ed6657de927
ver         4   0x5
sha1_mid   12   0x68b
var         2   b10
sha1_low   62   b01, 0x5e12665a8aea6a2
total      128
final: 2ed6657d-e927-568b-95e1-2665a8aea6a2
Figure 23: UUIDv5 Example Test Vector
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
Figure 24: UUIDv5 Example Ver/Var bit swaps and discarded SHA-1 segment

C.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   b10
clock_seq   14   b11, 0x3C8
node        48   0x9E6BDECED846
total       128
final: 1EC9414C-232A-6B00-B3C8-9E6BDECED846
Figure 25: UUIDv6 Example Test Vector

C.6. Example of a UUIDv7 Value

This example UUIDv7 test vector utilizes a well-known 32 bit Unix epoch with additional 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 0x17F22E279B0 or 1645557742000

field       bits value
unix_ts_ms  48   0x17F22E279B0
ver          4   0x7
rand_a      12   0xCC3
var          2   b10
rand_b      62   b01, 0x8C4DC0C0C07398F
total       128
final: 017F22E2-79B0-7CC3-98C4-DC0C0C07398F
Figure 26: UUIDv7 Example Test Vector

C.7. Example of a UUIDv8 Value (time-based)

This example UUIDv8 test vector utilizes a well-known 64 bit Unix epoch with nanosecond precision, truncated to the least-significant, right-most, bits to fill the first 48 bits through version.

The next two segments of custom_b and custom_c are are filled with random data.

Timestamp is Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00 represented as 0x16D6320C3D4DCC00 or 1645557742000000000

It should be noted that this example is just to illustrate one scenario for UUIDv8. Test vectors will likely be implementation specific and vary greatly from this simple example.

field     bits value
custom_a  48   0x320C3D4DCC00
ver        4   0x8
custom_b  12   0x75B
var        2   b10
custom_c  62   b00, 0xEC932D5F69181C0
total     128
final: 320C3D4D-CC00-875B-8EC9-32D5F69181C0
Figure 27: UUIDv8 Example Time-based Test Vector

C.8. Example of a UUIDv8 Value (name-based)

A SHA-256 version of Appendix C.4 is detailed in Figure 28 to detail the usage of hash spaces alongside namespace and names. 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.

Hash Space (SHA2_256): 3fb32780-953c-4464-9cfd-e85dbbe9843d
Name Space (DNS):      6ba7b810-9dad-11d1-80b4-00c04fd430c8
Figure 28: UUIDv8 Example SHA256
field     bits value
custom_a  48   0x401835fda627
ver        4   0x8
custom_b  12   0x627
var        2   b10
custom_c  62   b0, 0x73fed73f2bc5b2c
total     128
final: 401835fd-a627-870a-873f-ed73f2bc5b2c
Figure 29: UUIDv8 Example Name-Based SHA-256 Test Vector
A: 401835fd-a627-a70a-073f-ed73f2bc5b2c-2a8936385a38a9c133de0ca4af0dfaed
B: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
C: 401835fd-a627-870a-873f-ed73f2bc5b2c
D:                                     -2a8936385a38a9c133de0ca4af0dfaed
Figure 30: UUIDv8 Example Ver/Var bit swaps and discarded SHA-256 segment

Examining Figure 30:

  • Line A details the full SHA-256 as a hex value with the dashes inserted.
  • Line B details the version and variant hex 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.

Authors' Addresses

P. Leach
M. Mealling
VeriSign, Inc.
Brad G. Peabody
Kyzer R. Davis
Cisco Systems