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Internet Engineering Task Force (IETF) K. Davis
Request for Comments: 9562 Cisco Systems
Obsoletes: 4122 B. Peabody
Category: Standards Track Uncloud
ISSN: 2070-1721 P. Leach
University of Washington
May 2024
Universally Unique IDentifiers (UUIDs)
Abstract
This specification defines UUIDs (Universally Unique IDentifiers) --
also known as GUIDs (Globally Unique IDentifiers) -- and a Uniform
Resource Name namespace for UUIDs. 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 (NCS), later
in the Open Software Foundation's (OSF's) Distributed Computing
Environment (DCE), and then in Microsoft Windows platforms.
This specification is derived from the OSF DCE specification with the
kind permission of the OSF (now known as "The Open Group").
Information from earlier versions of the OSF DCE specification have
been incorporated into this document. This document obsoletes RFC
4122.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9562.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Motivation
2.1. Update Motivation
3. Terminology
3.1. Requirements Language
3.2. Abbreviations
4. UUID Format
4.1. Variant Field
4.2. Version Field
5. UUID Layouts
5.1. UUID Version 1
5.2. UUID Version 2
5.3. UUID Version 3
5.4. UUID Version 4
5.5. UUID Version 5
5.6. UUID Version 6
5.7. UUID Version 7
5.8. UUID Version 8
5.9. Nil UUID
5.10. Max UUID
6. UUID Best Practices
6.1. Timestamp Considerations
6.2. Monotonicity and Counters
6.3. UUID Generator States
6.4. Distributed UUID Generation
6.5. Name-Based UUID Generation
6.6. Namespace ID Usage and Allocation
6.7. Collision Resistance
6.8. Global and Local Uniqueness
6.9. Unguessability
6.10. UUIDs That Do Not Identify the Host
6.11. Sorting
6.12. Opacity
6.13. DBMS and Database Considerations
7. IANA Considerations
7.1. IANA UUID Subtype Registry and Registration
7.2. IANA UUID Namespace ID Registry and Registration
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Test Vectors
A.1. Example of a UUIDv1 Value
A.2. Example of a UUIDv3 Value
A.3. Example of a UUIDv4 Value
A.4. Example of a UUIDv5 Value
A.5. Example of a UUIDv6 Value
A.6. Example of a UUIDv7 Value
Appendix B. Illustrative Examples
B.1. Example of a UUIDv8 Value (Time-Based)
B.2. Example of a UUIDv8 Value (Name-Based)
Acknowledgements
Authors' Addresses
1. Introduction
This specification defines a Uniform Resource Name namespace for
Universally Unique IDentifiers (UUIDs), also known as Globally Unique
IDentifiers (GUIDs). 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;
in many cases, they can 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 either in combination with 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. 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 two formats may
leverage optional IEEE 802 Node IDs, 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 URNs. 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 database keys.
This stems from the increasingly distributed nature of modern
applications. In such cases, "auto-increment" schemes that are often
used by databases do not work well: 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, such as:
1. UUID versions that are not time ordered, 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; thus, they require inserts to be
performed at random locations. The resulting negative
performance effects on the common structures used for this
(B-tree and its variants) can be dramatic.
2. The 100-nanosecond Gregorian Epoch used in UUIDv1 timestamps
(described in Section 5.1) is uncommon and difficult to represent
accurately using a standard number format such as that described
in [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 Media
Access Control (MAC) address in the node field of UUIDv1.
Exposed MAC addresses can be used as an attack surface to locate
network interfaces and reveal various other information about
such machines (minimally, the manufacturer and, potentially,
other details). Additionally, with the advent of virtual
machines and containers, uniqueness of the MAC address 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 and encoding, timestamp type, timestamp
format, timestamp accuracy, node format and components, collision
handling, and multi-timestamp tick generation sequencing:
1. [ULID]
2. [LexicalUUID]
3. [Snowflake]
4. [Flake]
5. [ShardingID]
6. [KSUID]
7. [Elasticflake]
8. [FlakeID]
9. [Sonyflake]
10. [orderedUuid]
11. [COMBGUID]
12. [SID]
13. [pushID]
14. [XID]
15. [ObjectID]
16. [CUID]
An inspection of these implementations and the issues described above
has led to this document, in which new UUIDs are adapted to address
these issues.
Further, [RFC4122] itself was in need of an overhaul to address a
number of topics such as, but not limited to, the following:
1. Implementation of miscellaneous errata reports. Mostly around
bit-layout clarifications, which lead to inconsistent
implementations [Err1957], [Err3546], [Err4975], [Err4976],
[Err5560], etc.
2. Decoupling other UUID versions from the UUIDv1 bit layout so that
fields like "time_hi_and_version" do not need to be referenced
within a UUID that is not time based while also providing
definition sections similar to that for UUIDv1 for UUIDv3,
UUIDv4, and UUIDv5.
3. Providing implementation best practices around many real-world
scenarios and corner cases observed by existing and prototype
implementations.
4. Addressing security best practices and considerations for the
modern age as it pertains to MAC addresses, hashing algorithms,
secure randomness, and other topics.
5. Providing implementations a standard-based option for
implementation-specific and/or experimental UUID designs.
6. Providing more test vectors that illustrate real UUIDs created as
per the specification.
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:
ABNF Augmented Backus-Naur Form
CSPRNG Cryptographically Secure Pseudorandom Number Generator
DBMS Database Management System
IEEE Institute of Electrical and Electronics Engineers
ITU International Telecommunication Union
MAC Media Access Control
MD5 Message Digest 5
MSB Most Significant Bit
OID Object Identifier
SHA Secure Hash Algorithm
SHA-1 Secure Hash Algorithm 1 (with message digest of 160
bits)
SHA-3 Secure Hash Algorithm 3 (arbitrary size)
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
SHAKE Secure Hash Algorithm 3 based on the KECCAK algorithm
URN Uniform Resource Names
UTC Coordinated Universal Time
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
4. UUID Format
The UUID format is 16 octets (128 bits) in size; the variant bits in
conjunction with the version bits described in the next sections
determine finer structure. In terms of these UUID formats and
layout, bit definitions start at 0 and end at 127, while octet
definitions start at 0 and end at 15.
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").
Saving UUIDs to binary format is done by sequencing all fields in
big-endian format. However, there is a known caveat that Microsoft's
Component Object Model (COM) GUIDs leverage little-endian when saving
GUIDs. The discussion of this (see [MS_COM_GUID]) is outside the
scope of this specification.
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 uppercase or lowercase alphanumeric hexadecimal
characters separated by single dashes/hyphens. When used with
databases, please refer to Section 6.13.
The formal definition of the UUID string representation is provided
by the following ABNF [RFC5234]:
UUID = 4hexOctet "-"
2hexOctet "-"
2hexOctet "-"
2hexOctet "-"
6hexOctet
hexOctet = HEXDIG HEXDIG
DIGIT = %x30-39
HEXDIG = DIGIT / "A" / "B" / "C" / "D" / "E" / "F"
Note that the alphabetic characters may be all uppercase, all
lowercase, or mixed case, as per Section 2.3 of [RFC5234]. An
example UUID using this textual representation from the above ABNF is
shown in Figure 1.
f81d4fae-7dec-11d0-a765-00a0c91e6bf6
Figure 1: Example String UUID Format
The same UUID from Figure 1 is represented in binary (Figure 2), as
an unsigned integer (Figure 3), and as a URN (Figure 4) defined by
[RFC8141].
111110000001110101001111101011100111110111101100000100011101000\
01010011101100101000000001010000011001001000111100110101111110110
Figure 2: Example Binary UUID
329800735698586629295641978511506172918
Figure 3: Example Unsigned Integer UUID (Shown as a Decimal Number)
urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
Figure 4: Example URN Namespace for UUID
There are many other ways to define a UUID format; some examples are
detailed below. Please note that this is not an exhaustive list and
is only provided for informational purposes.
* Some UUID implementations, such as those found in [Python] and
[Microsoft], will output UUID with the string format, including
dashes, enclosed in curly braces.
* [X667] provides UUID format definitions for use of UUID with an
OID.
* [IBM_NCS] is a legacy implementation that produces a unique UUID
format compatible with Variant 0xx of Table 1.
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.
+======+======+======+======+=========+=========================+
| MSB0 | MSB1 | MSB2 | MSB3 | Variant | Description |
+======+======+======+======+=========+=========================+
| 0 | x | x | x | 1-7 | Reserved. Network |
| | | | | | Computing System (NCS) |
| | | | | | backward compatibility, |
| | | | | | and includes Nil UUID |
| | | | | | as per Section 5.9. |
+------+------+------+------+---------+-------------------------+
| 1 | 0 | x | x | 8-9,A-B | The variant specified |
| | | | | | in this document. |
+------+------+------+------+---------+-------------------------+
| 1 | 1 | 0 | x | C-D | Reserved. Microsoft |
| | | | | | Corporation backward |
| | | | | | compatibility. |
+------+------+------+------+---------+-------------------------+
| 1 | 1 | 1 | x | E-F | Reserved for future |
| | | | | | definition and includes |
| | | | | | Max UUID as per |
| | | | | | Section 5.10. |
+------+------+------+------+---------+-------------------------+
Table 1: UUID Variants
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.
+======+======+======+======+=========+============================+
| 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 |
| | | | | | pseudorandomly 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. |
+------+------+------+------+---------+----------------------------+
Table 2: UUID Variant 10xx Versions Defined by This Specification
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
Figure 5: UUIDv4 Variant Examples
It should be noted that the other remaining UUID variants found in
Table 1 leverage different sub-typing or versioning mechanisms. The
recording and definition of the remaining UUID variant and sub-typing
combinations are outside of the scope of this document.
5. UUID Layouts
To minimize confusion about bit assignments within octets and among
differing versions, the UUID record definition is provided as a
grouping of fields within a bit layout consisting of four octets per
row. The fields are presented with the most significant one first.
5.1. UUID Version 1
UUIDv1 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 that 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 Sections 6.9 and 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: UUIDv1 Field and Bit Layout
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:
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).
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 if it might have been set backwards
(e.g., while the system was powered off), and the UUID generator
cannot 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 pseudorandom 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 IDs
that may move or switch from system to system rapidly. The initial
value MUST NOT be correlated to the Node ID.
Notes about nodes derived from IEEE 802:
* On systems with multiple IEEE 802 addresses, any available one MAY
be used.
* On systems with no IEEE address, a randomly or pseudorandomly
generated value MUST be used; see Sections 6.9 and 6.10.
* On systems utilizing a 64-bit MAC address, the least significant,
rightmost 48 bits MAY be used.
* Systems utilizing an IEEE 802.15.4 16-bit address SHOULD instead
utilize their 64-bit MAC address where the least significant,
rightmost 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
UUIDv2 is for DCE Security UUIDs (see [C309] and [C311]). As such,
the definition of these UUIDs is outside the scope of this
specification.
5.3. UUID Version 3
UUIDv3 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 hash [RFC1321] 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 Sections 4.2 and 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 under the heading "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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: UUIDv3 Field and Bit Layout
md5_high:
The first 48 bits of the layout are filled with the most
significant, leftmost 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,
rightmost 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, rightmost bits of
the final 64 bits from the computed MD5 value. Occupies bits 66
through 127 (octets 8-15)
5.4. UUID Version 4
UUIDv4 is meant for generating UUIDs from truly random or
pseudorandom numbers.
An implementation may generate 128 bits of random data that is used
to fill out the UUID fields in Figure 8. The UUID version and
variant then replace the respective bits as defined by Sections 4.1
and 4.2.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: UUIDv4 Field and Bit Layout
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
UUIDv5 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 hash [FIPS180-4] 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, leftmost 128
bits of the SHA-1 value are then used to populate all 128 bits of the
UUID layout, and the remaining 32 least significant, rightmost bits
of SHA-1 output are discarded. The UUID version and variant then
replace the respective bits as defined by Sections 4.2 and 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 under the heading "A
note on names".
There may be scenarios, usually depending on organizational security
policies, where SHA-1 libraries may not be available or may be 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 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: UUIDv5 Field and Bit Layout
sha1_high:
The first 48 bits of the layout are filled with the most
significant, leftmost 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,
rightmost 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 two most significant, leftmost
bits of the remaining SHA-1 hash and then using the next 62 most
significant, leftmost bits. Any leftover SHA-1 bits are discarded
and unused. Occupies bits 66 through 127 (octets 8-15).
5.6. UUID Version 6
UUIDv6 is a field-compatible version of UUIDv1 (Section 5.1),
reordered for improved DB locality. It is expected that UUIDv6 will
primarily be implemented in contexts where UUIDv1 is used. Systems
that do not involve legacy UUIDv1 SHOULD use UUIDv7 (Section 5.7)
instead.
Instead of splitting the timestamp into the low, mid, and high
sections from UUIDv1, UUIDv6 changes this sequence so timestamp bytes
are stored from most to least significant. That is, given a 60-bit
timestamp value as specified for UUIDv1 in Section 5.1, for UUIDv6
the first 48 most significant bits are stored first, followed by the
4-bit version (same position), followed by the remaining 12 bits of
the original 60-bit timestamp.
The clock sequence and node bits remain unchanged from their position
in Section 5.1.
The clock sequence and node bits SHOULD be reset to a pseudorandom
value for each new UUIDv6 generated; however, implementations MAY
choose to retain the old clock sequence and MAC address behavior from
Section 5.1. For more information on MAC address usage within UUIDs,
see the Section 8.
The format for the 16-byte, 128-bit UUIDv6 is shown in Figure 10.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time_high |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time_mid | ver | time_low |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| clock_seq | node |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| node |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: UUIDv6 Field and Bit Layout
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
UUIDv7 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.
Generally, UUIDv7 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: UUIDv7 Field and Bit Layout
unix_ts_ms:
48-bit big-endian unsigned number of the 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 of pseudorandom 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 pseudorandom 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
UUIDv8 provides a format for experimental or vendor-specific use
cases. The only requirement is that the variant and version bits
MUST be set as defined in Sections 4.1 and 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 and/or language
restrictions that inhibit the use of one of the current UUIDs.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: UUIDv8 Field and Bit Layout
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
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.
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 the
Nil UUID defined in Section 5.9.
FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF
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.
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. UUID Best Practices
The minimum requirements for generating UUIDs of each version are
described in this document. Everything else is an implementation
detail, and it is 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 were topics of great
debate while creating UUIDv7 for this specification. Choosing the
right timestamp for your application is very important. This section
will detail some of the most common points on this issue.
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:
UUIDv1 and UUIDv6 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, which is frequently less than the precision offered
by the operating system. With UUIDv1 and UUIDv6, 100 nanoseconds
of precision are present, while UUIDv7 features a 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 that
count 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. UUIDv1
and UUIDv6 utilize a 60-bit timestamp valid until 5623 AD; 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 1) correct inaccurate clocks, 2) handle
leap seconds, or 3) obtain a millisecond value by dividing by 1024
(or some other value) for performance reasons (instead of dividing
a number of microseconds by 1000). 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 (leftmost) with data. An example for this
padding data is to fill the most significant, leftmost 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, leftmost bits with the number of 32-bit Unix
timestamp rollovers 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, rightmost 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 knowingly return duplicate values due to a
counter rollover. Note that if the processors overrun the UUID
generation frequently, additional Node IDs can be allocated to the
system, which will permit higher speed allocation by making
multiple UUIDs potentially available for each timestamp 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 UUIDv6 and UUIDv7 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 Bit-Length Dedicated Counter (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. If present, a
fixed bit-length counter 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 leftmost bits) of UUIDv7 is
used as a fixed bit-length dedicated counter that is 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 that more counter bits are required, the most
significant (leftmost) 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" that 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 that 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 Leftmost 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 the 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 1 ms 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 2^12), 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 (leftmost)
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; 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; thus, it 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. Then, if any bits are to be used as a clock
sequence, they would follow next.
The following sub-topics cover issues related solely to creating
reliable fixed bit-length dedicated counters:
Fixed Bit-Length Dedicated Counter Seeding:
Implementations utilizing the fixed bit-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 bit-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, rightmost
position randomly initialized. The remaining most significant,
leftmost counter bit is initialized as zero for the sole purpose
of guarding against counter rollovers.
Fixed Bit-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 Bit-Length Dedicated Counter Seeding"
above that describes allocating a segment of the fixed bit-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, rightmost position is
less than the total length of the random value being incremented.
As such, the most significant, leftmost 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 they should at least
report an appropriate error. To handle this scenario, the general
guidance is that the 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 were 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 periodically be 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 timestamps 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 the utilization of multi-node,
clustered, applications that 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
increases, 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
pseudorandom 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 pseudorandom 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
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 are 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 term "name-based" in order to maintain consistency
with previously published 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 form for
the namespace if it does not exist. For example, at the time of
writing, Domain Name System (DNS) [RFC9499] has three conveyance
formats: common (www.example.com), presentation
(www.example.com.), and wire format (3www7example3com0). Looking
at [X500] Distinguished Names (DNs), [RFC4122] allowed either
text-based or binary DER-based names as inputs. For Uniform
Resource Locators (URLs) [RFC1738], 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 Object Identifiers (OIDs) [X660], 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 that
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 details the namespace IDs for some potentially
interesting namespaces such as those for DNS [RFC9499], URLs
[RFC1738], OIDs [X660], and DNs [X500].
Further, this section also details allocation, IANA registration, and
other details pertinent to Namespace IDs.
+=========+====================================+=========+==========+
|Namespace|Namespace ID Value |Name |Namespace |
| | |Reference|ID |
| | | |Reference |
+=========+====================================+=========+==========+
|DNS |6ba7b810-9dad-11d1-80b4-00c04fd430c8|[RFC9499]|[RFC4122],|
| | | |RFC 9562 |
+---------+------------------------------------+---------+----------+
|URL |6ba7b811-9dad-11d1-80b4-00c04fd430c8|[RFC1738]|[RFC4122],|
| | | |RFC 9562 |
+---------+------------------------------------+---------+----------+
|OID |6ba7b812-9dad-11d1-80b4-00c04fd430c8|[X660] |[RFC4122],|
| | | |RFC 9562 |
+---------+------------------------------------+---------+----------+
|X500 |6ba7b814-9dad-11d1-80b4-00c04fd430c8|[X500] |[RFC4122],|
| | | |RFC 9562 |
+---------+------------------------------------+---------+----------+
Table 3: Namespace IDs
Items may be added to this registry using the 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,
rightmost 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 are not defined by this
document or by [RFC4122]; thus, 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; instead, generating a UUIDv4 or UUIDv7
Namespace ID value is RECOMMENDED. 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
Sections 6.1 and 6.2. This is especially true for distributed node
collision resistance as defined by Section 6.4.
There are two example scenarios below that 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 UUIDs defined in this document.
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: 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
a UUID to provide uniqueness for practical implementation purposes.
Implementations MAY use 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
pseudorandom 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
Sections 6.7 and 8 are present in modern UUIDs.
Further advice on generating cryptographic-quality random numbers can
be found in [RFC4086], [RFC8937], and [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
[IEEE802.11bh] as an alternative to the pseudorandom 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 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 to 1. This bit is the unicast or 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 Appendix A. For more information about IEEE 802 address
and the unicast or multicast or local/global bits, please review
[RFC9542].
For compatibility with earlier specifications, note that this
document uses the unicast or multicast bit instead of the arguably
more correct local/global bit because MAC addresses with the local/
global bit set or not set are both possible in a network. This is
not the case with the unicast or 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. However, a generic approach 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 the
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 versus 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, avoiding parsing UUID values unnecessarily is
recommended; instead, treat UUIDs as opaquely as possible. Although
application-specific concerns could, of course, require some degree
of introspection (e.g., to examine Sections 4.1 or 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, be 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 as follows:
* Storing in binary form 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 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-generated 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 (see Sections 5.3 and 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 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 later 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,
to utilize time-based UUID surrogate keys based on the aforementioned
problems detailed in this section.
7. IANA Considerations
All references to [RFC4122] in IANA registries (outside of those
created by this document) have been replaced with references to this
document, including the IANA URN namespace registration
[URNNamespaces] for UUID. References to Section 4.1.2 of [RFC4122]
have been updated to refer to Section 4 of this document.
Finally, IANA should track UUID Subtypes and Special Case "Namespace
IDs Values" as specified in Sections 7.1 and 7.2 at the following
location: <https://www.iana.org/assignments/uuid>.
When evaluating requests, the designated expert should consider
community feedback, how well-defined the reference specification is,
and this specification's requirements. Vendor-specific, application-
specific, and deployment-specific values are unable to be registered.
Specification documents should be published in a stable, freely
available manner (ideally, located with a URL) but need not be
standards. The designated expert will either approve or deny the
registration request and communicate this decision to IANA. Denials
should include an explanation and, if applicable, suggestions as to
how to make the request successful.
7.1. IANA UUID Subtype Registry and Registration
This specification defines the "UUID Subtypes" registry for common
widely used UUID standards.
+======================+====+=========+================+============+
| Name | ID | Subtype | Variant | Reference |
+======================+====+=========+================+============+
| Gregorian Time-based | 1 | version | OSF DCE | [RFC4122], |
| | | | / IETF | RFC 9562 |
+----------------------+----+---------+----------------+------------+
| DCE Security | 2 | version | OSF DCE | [C309], |
| | | | / IETF | [C311] |
+----------------------+----+---------+----------------+------------+
| MD5 Name-based | 3 | version | OSF DCE | [RFC4122], |
| | | | / IETF | RFC 9562 |
+----------------------+----+---------+----------------+------------+
| Random | 4 | version | OSF DCE | [RFC4122], |
| | | | / IETF | RFC 9562 |
+----------------------+----+---------+----------------+------------+
| SHA-1 Name-based | 5 | version | OSF DCE | [RFC4122], |
| | | | / IETF | RFC 9562 |
+----------------------+----+---------+----------------+------------+
| Reordered Gregorian | 6 | version | OSF DCE | RFC 9562 |
| Time-based | | | / IETF | |
+----------------------+----+---------+----------------+------------+
| Unix Time-based | 7 | version | OSF DCE | RFC 9562 |
| | | | / IETF | |
+----------------------+----+---------+----------------+------------+
| Custom | 8 | version | OSF DCE | RFC 9562 |
| | | | / IETF | |
+----------------------+----+---------+----------------+------------+
Table 4: IANA UUID Subtypes
This table may be extended by Standards Action as 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
publication, "version" and "family" are the only 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
that one of four values will be included: the first three are "OSF
DCE / IETF", "Apollo NCS", and "Microsoft", and 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 IDs" 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.
8. Security Considerations
Implementations SHOULD NOT assume that UUIDs are hard to guess. For
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 modified 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 around privacy 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 Sections 6.9 and 6.10 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. References
9.1. Normative References
[C309] X/Open Company Limited, "X/Open DCE: Remote Procedure
Call", ISBN 1-85912-041-5, Open CAE Specification C309,
August 1994,
<https://pubs.opengroup.org/onlinepubs/9696999099/
toc.pdf>.
[C311] The Open Group, "DCE 1.1: Authentication and Security
Services", Open Group CAE Specification C311, August 1997,
<https://pubs.opengroup.org/onlinepubs/9696989899/
toc.pdf>.
[FIPS180-4]
National Institute of Standards and Technology (NIST),
"Secure Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS.180-4, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[FIPS202] National Institute of Standards and Technology (NIST),
"SHA-3 Standard: Permutation-Based Hash and Extendable-
Output Functions", FIPS PUB 202,
DOI 10.6028/NIST.FIPS.202, August 2015,
<https://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.202.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8141] Saint-Andre, P. and J. Klensin, "Uniform Resource Names
(URNs)", RFC 8141, DOI 10.17487/RFC8141, April 2017,
<https://www.rfc-editor.org/info/rfc8141>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[X667] ITU-T, "Information technology - Open Systems
Interconnection - 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 Recommendation X.667, September 2004.
9.2. Informative References
[COMBGUID] "Creating sequential GUIDs in C# for MSSQL or PostgreSql",
commit 2759820, December 2020,
<https://github.com/richardtallent/RT.Comb>.
[CUID] "Collision-resistant ids optimized for horizontal scaling
and performance.", commit 215b27b, October 2020,
<https://github.com/ericelliott/cuid>.
[Elasticflake]
Pearcy, P., "Sequential UUID / Flake ID generator pulled
out of elasticsearch common", commit dd71c21, January
2015, <https://github.com/ppearcy/elasticflake>.
[Err1957] RFC Errata, Erratum ID 1957, RFC 4122,
<https://www.rfc-editor.org/errata/eid1957>.
[Err3546] RFC Errata, Erratum ID 3546, RFC 4122,
<https://www.rfc-editor.org/errata/eid3546>.
[Err4975] RFC Errata, Erratum ID 4975, RFC 4122,
<https://www.rfc-editor.org/errata/eid4975>.
[Err4976] RFC Errata, Erratum ID 4976, RFC 4122,
<https://www.rfc-editor.org/errata/eid4976>.
[Err5560] RFC Errata, Erratum ID 5560, RFC 4122,
<https://www.rfc-editor.org/errata/eid5560>.
[Flake] Boundary, "Flake: A decentralized, k-ordered id generation
service in Erlang", commit 15c933a, February 2017,
<https://github.com/boundary/flake>.
[FlakeID] "Flake ID Generator", commit fcd6a2f, April 2020,
<https://github.com/T-PWK/flake-idgen>.
[IBM_NCS] IBM, "uuid_gen Command (NCS)", March 2023,
<https://www.ibm.com/docs/en/aix/7.1?topic=u-uuid-gen-
command-ncs>.
[IEEE754] IEEE, "IEEE Standard for Floating-Point Arithmetic.", IEEE
Std 754-2019, DOI 10.1109/IEEESTD.2019.8766229, July 2019,
<https://standards.ieee.org/ieee/754/6210/>.
[IEEE802.11bh]
IEEE, "IEEE Draft Standard for Information technology--
Telecommunications and information exchange between
systems Local and metropolitan area networks--Specific
requirements - Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) Specifications Amendment:
Enhancements for Extremely High Throughput (EHT)",
Electronic ISBN 978-1-5044-9520-2, March 2023,
<https://standards.ieee.org/ieee/802.11bh/10525/>.
[KSUID] Segment, "K-Sortable Globally Unique IDs", commit bf376a7,
July 2020, <https://github.com/segmentio/ksuid>.
[LexicalUUID]
Twitter, "Cassie", commit f6da4e0, November 2012,
<https://github.com/twitter-archive/cassie>.
[Microsoft]
Microsoft, "2.3.4.3 GUID - Curly Braced String
Representation", April 2023, <https://learn.microsoft.com/
en-us/openspecs/windows_protocols/ms-
dtyp/222af2d3-5c00-4899-bc87-ed4c6515e80d>.
[MS_COM_GUID]
Chen, R., "Why does COM express GUIDs in a mix of big-
endian and little-endian? Why can't it just pick a side
and stick with it?", September 2022,
<https://devblogs.microsoft.com/
oldnewthing/20220928-00/?p=107221>.
[ObjectID] MongoDB, "ObjectId",
<https://docs.mongodb.com/manual/reference/method/
ObjectId/>.
[orderedUuid]
Cabrera, I. B., "Laravel: The mysterious "Ordered UUID"",
January 2020, <https://itnext.io/laravel-the-mysterious-
ordered-uuid-29e7500b4f8>.
[pushID] Lehenbauer, M., "The 2^120 Ways to Ensure Unique
Identifiers", February 2015,
<https://firebase.googleblog.com/2015/02/the-2120-ways-to-
ensure-unique_68.html>.
[Python] Python, "uuid - UUID objects according to RFC 4122",
<https://docs.python.org/3/library/uuid.html>.
[RANDOM] Occil, P., "Random Number Generator Recommendations for
Applications", June 2023,
<https://peteroupc.github.io/random.html>.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, "Uniform
Resource Locators (URL)", RFC 1738, DOI 10.17487/RFC1738,
December 1994, <https://www.rfc-editor.org/info/rfc1738>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/info/rfc4122>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.
[RFC6194] 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, March 2011,
<https://www.rfc-editor.org/info/rfc6194>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8937] Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
and C. Wood, "Randomness Improvements for Security
Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
<https://www.rfc-editor.org/info/rfc8937>.
[RFC9499] Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
RFC 9499, DOI 10.17487/RFC9499, March 2024,
<https://www.rfc-editor.org/info/rfc9499>.
[RFC9542] Eastlake 3rd, D., Abley, J., and Y. Li, "IANA
Considerations and IETF Protocol and Documentation Usage
for IEEE 802 Parameters", BCP 141, RFC 9542,
DOI 10.17487/RFC9542, April 2024,
<https://www.rfc-editor.org/info/rfc9542>.
[ShardingID]
Instagram Engineering, "Sharding & IDs at Instagram",
December 2012, <https://instagram-engineering.com/
sharding-ids-at-instagram-1cf5a71e5a5c>.
[SID] "sid : generate sortable identifiers", Commit 660e947,
June 2019, <https://github.com/chilts/sid>.
[Snowflake]
Twitter, "Snowflake is a network service for generating
unique ID numbers at high scale with some simple
guarantees.", commit ec40836, May 2014,
<https://github.com/twitter-archive/snowflake>.
[Sonyflake]
Sony, "A distributed unique ID generator inspired by
Twitter's Snowflake", commit 848d664, August 2020,
<https://github.com/sony/sonyflake>.
[ULID] "Universally Unique Lexicographically Sortable
Identifier", Commit d0c7170, May 2019,
<https://github.com/ulid/spec>.
[URNNamespaces]
IANA, "Uniform Resource Names (URN) Namespaces",
<https://www.iana.org/assignments/urn-namespaces/>.
[X500] ITU-T, "Information technology - Open Systems
Interconnection - The Directory: Overview of concepts,
models and services", ISO/IEC 9594-1, ITU-T
Recommendation X.500, October 2019.
[X660] ITU-T, "Information technology - Procedures for the
operation of object identifier registration authorities:
General procedures and top arcs of the international
object identifier tree", ISO/IEC 9834-1, ITU-T
Recommendation X.660, July 2011.
[X680] ITU-T, "Information Technology - Abstract Syntax Notation
One (ASN.1) & ASN.1 encoding rules", ISO/IEC 8824-1:2021,
ITU-T Recommendation X.680, February 2021.
[XID] "Globally Unique ID Generator", commit efa678f, October
2020, <https://github.com/rs/xid>.
Appendix A. 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. For the
randomized node, the least significant bit of the first octet is set
to a value of 1 as per Section 6.10. Thus, the starting value
0x9E6BDECED846 was changed to 0x9F6BDECED846.
The pseudocode used for converting from a 64-bit Unix timestamp to a
100 ns 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 Pseudocode
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
Figure 16: UUIDv1 Example Test Vector
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". 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
Figure 17: UUIDv3 Example MD5
-------------------------------------------
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
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
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
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
A.4. Example of a UUIDv5 Value
The SHA-1 computation form is detailed in Figure 22, using the DNS
Namespace ID value and the Name "www.example.com". 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
Figure 22: UUIDv5 Example SHA-1
-------------------------------------------
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
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
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
Figure 25: UUIDv6 Example Test Vector
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
Figure 26: UUIDv7 Example Test Vector
Appendix B. Illustrative Examples
The following sections contain illustrative examples that serve to
show how one may use UUIDv8 (Section 5.8) for custom and/or
experimental application-based logic. The examples below have not
been through the same rigorous testing, prototyping, and feedback
loop that other algorithms in this document have undergone. The
authors encourage implementers to create their own UUIDv8 algorithm
rather than use the items defined in this section.
B.1. Example of a UUIDv8 Value (Time-Based)
This example UUIDv8 test vector utilizes a well-known 64-bit Unix
Epoch timestamp with 10 ns precision, truncated to the least
significant, rightmost 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 (10 ns-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
Figure 27: UUIDv8 Example Time-Based Illustrative Example
B.2. Example of a UUIDv8 Value (Name-Based)
As per Section 5.5, name-based UUIDs that want 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 (as
defined by [FIPS180-4]), SHA-3 or SHAKE (as defined by [FIPS202]), or
even algorithms that have not been defined yet.
A SHA-256 version of the SHA-1 computation in 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,
is that the output hash MUST be 128 bits or larger.
Namespace (DNS): 6ba7b810-9dad-11d1-80b4-00c04fd430c8
Name: www.example.com
----------------------------------------------------------------
SHA-256:
5c146b143c524afd938a375d0df1fbf6fe12a66b645f72f6158759387e51f3c8
Figure 28: UUIDv8 Example SHA256
-------------------------------------------
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
Figure 29: UUIDv8 Example Name-Based SHA-256 Illustrative Example
A: 5c146b14-3c52-4afd-938a-375d0df1fbf6-fe12a66b645f72f6158759387e51f3c8
B: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx
C: 5c146b14-3c52-8afd-938a-375d0df1fbf6
D: -fe12a66b645f72f6158759387e51f3c8
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 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 and var have been
overwritten.
* Line D details the discarded leftover values from the original
SHA-256 computation.
Acknowledgements
The authors gratefully acknowledge the contributions of Rich Salz,
Michael Mealling, 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, and LiosK.
As well as all of those in the IETF community and on GitHub to who
contributed to the discussions that resulted in this document.
This document draws heavily on the OSF DCE specification (Appendix A
of [C309]) for UUIDs. Ted Ts'o provided helpful comments.
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.
Authors' Addresses
Kyzer R. Davis
Cisco Systems
Email: kydavis@cisco.com
Brad G. Peabody
Uncloud
Email: brad@peabody.io
Paul J. Leach
University of Washington
Email: pjl7@uw.edu