]> Microsoft

paulle@microsoft.com

VeriSign, Inc.

michael@refactored-networks.com

brad@peabody.io

Cisco Systems

kydavis@cisco.com

ART uuidrev uuid This specification defines a Uniform Resource Name namespace for UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits long, and can guarantee uniqueness across space and time. UUIDs were originally used in the Apollo Network Computing System and later in the Open Software Foundation's (OSF) Distributed Computing Environment (DCE), and then in Microsoft Windows platforms. This specification is derived from the DCE specification with the kind permission of the OSF (now known as The Open Group). Information from earlier versions of the DCE specification have been incorporated into this document.

Introduction This specification defines a Uniform Resource Name namespace for UUIDs (Universally Unique IDentifier), also known as GUIDs (Globally Unique IDentifier). A UUID is 128 bits long, and requires no central registration process. The information here is meant to be a concise guide for those wishing to implement services using UUIDs as URNs . Nothing in this document should be construed to override the DCE standards that defined UUIDs. There is an ITU-T Recommendation and ISO/IEC Standard that are derived from earlier versions of this document. Both sets of specifications have been aligned, and are fully technically compatible. In addition, a global registration function is being provided by the Telecommunications Standardization Bureau of ITU-T; for details see .

Motivation One of the main reasons for using UUIDs is that no centralized authority is required to administer them (although one format uses IEEE 802 node identifiers, others do not). As a result, generation on demand can be completely automated, and used for a variety of purposes. The UUID generation algorithm described here supports very high allocation rates of up to 10 million per second per machine if necessary, so that they could even be used as transaction IDs. UUIDs are of a fixed size (128 bits) which is reasonably small compared to other alternatives. This lends itself well to sorting, ordering, and hashing of all sorts, storing in databases, simple allocation, and ease of programming in general. Since UUIDs are unique and persistent, they make excellent Uniform Resource Names. The unique ability to generate a new UUID without a registration process allows for UUIDs to be one of the URNs with the lowest minting cost. Furthermore, 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 UUIDs have gained popularity is as database keys. This stems from the increasingly distributed nature of modern applications. In such cases, "auto increment" schemes often used by databases do not work well, as the effort required to coordinate unique 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 synchronization makes them a good alternative, but UUID versions 1-5 lack certain other desirable characteristics:

1. Non-time-ordered UUID versions such as UUIDv4 have poor database index locality. Meaning new values created in succession are not close to each other in the index and thus require inserts to be performed at random locations. The negative performance effects of which on common structures used for this (B-tree and its variants) can be dramatic.
2. The 100-nanosecond, Gregorian epoch used in UUIDv1 timestamps is uncommon and difficult to represent accurately using a standard number format such as .
3. Introspection/parsing is required to order by time sequence; as opposed to being able to perform a simple byte-by-byte comparison.
4. Privacy and network security issues arise from using a MAC address in the node field of Version 1 UUIDs. Exposed MAC addresses can be used as an attack surface to locate machines and reveal various other information about such machines (minimally manufacturer, potentially other details). Additionally, with the advent of virtual machines and containers, MAC address uniqueness is no longer guaranteed.
5. Many of the implementation details specified in RFC4122 involved trade offs that are neither possible to specify for all applications nor necessary to produce interoperable implementations.
6. RFC4122 did not distinguish between the requirements for generation of a UUID versus an application which simply stores one, which are often different.

Due to the aforementioned issue, many widely distributed database applications and large application vendors have sought to solve the problem of creating a better time-based, sortable unique identifier for use as a database key. This has lead to numerous implementations over the past 10+ years solving the same problem in slightly different ways. While preparing this specification the following 16 different implementations were analyzed for trends in total ID length, bit Layout, lexical formatting/encoding, timestamp type, timestamp format, timestamp accuracy, node format/components, collision handling and multi-timestamp tick generation sequencing.

1. by A. Feerasta
2. by Twitter
3. by Twitter
4. by Boundary
5. by Instagram
6. by Segment
7. by P. Pearcy
8. by T. Pawlak
9. by Sony
10. by IT. Cabrera
11. by R. Tallent
12. by A. Chilton
13. by Google
14. by O. Poitrey
15. by MongoDB
16. by E. Elliott

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

Terminology

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 when, and only when, they appear in all capitals, as shown here.

Abbreviations The following abbreviations are used in this document:

UUID

Universally Unique Identifier

URN

Uniform Resource Names

ABNF

Augmented Backus-Naur Form

CSPRNG

Cryptographically Secure Pseudo-Random Number Generator

MAC

Media Access Control

MSB

Most Significant Bit

DBMS

Database Management System

IEEE

Institute of Electrical and Electronics Engineers, Inc.

ITU

International Telecommunication Union

MD5

Message Digest 5

SHA1

Secure Hash Algorithm 1

UTC

Coordinated Universal Time

changelog draft-00

• Merge RFC4122 with draft-peabody-dispatch-new-uuid-format-04.md
• Change: Reference RFC1321 to RFC6151
• Change: Reference RFC2141 to RFC8141
• Change: Reference RFC2234 to RFC4234
• Change: Reference FIPS 180-1 to FIPS 180-4 for SHA1
• Change: Converted UUIDv1 to match UUIDv6 section from Draft 04
• Change: Trimmed down the ABNF representation
• Change: http websites to https equivalent
• Errata: Bad Reference to RFC1750 | 3641 #4
• Errata: Change MD5 website to example.com | 3476 #6 (Also Fixes Errata: Fix uuid_create_md5_from_name() | 1352 #2)
• Errata: Typo in code comment | 6665 #11
• Errata: Fix BAD OID acronym | 6225 #9
• Errata: Incorrect Parenthesis usage Section 4.3 | 184 #5
• Errata: Lexicographically Sorting Paragraph Fix | 1428 #3
• Errata: Fix 4.1.3 reference to the correct bits | 1957 #13
• Errata: Fix reference to variant in octet 8 | 4975 #7
• Errata: Further clarify 3rd/last bit of Variant for spec | 5560 #8
• Errata: Fix clock_seq_hi_and_reserved most-significant bit verbiage | 4976 #10
• Errata: Better Clarify network byte order when referencing most significant bits | 3546 #12
• Draft 05: B.2. Example of a UUIDv7 Value two "var" in table #120
• Draft 05: MUST veribage in Reliability of 6.1 #121
• Draft 05: Further discourage centralized registry for distributed UUID Generation.
• New: Further Clarity of exact octet and bit of var/ver in this spec
• New: Block diagram, bit layout, test vectors for UUIDv4
• New: Block diagram, bit layout, test vectors for UUIDv3
• New: Block diagram, bit layout, test vectors for UUIDv5
• New: Add MD5 Security Considerations reference, RFC6151
• New: Add SHA1 Security Considerations reference, RFC6194

UUID Format The UUID format is 16 octets (128 bits); the variant bits in conjunction with the version bits described in the next sections in determine finer structure. UUIDs MAY be represented as binary data or integers. When in use with URNs or applications, any given 128 bit UUID SHOULD be represented by the "hex-and-dash" string format consisting of multiple groups of upper or lowercase alphanumeric hex characters separated by a single dash/hyphen. When used with databases please refer to . The formal definition of the UUID string representation is provided by the following (ABNF) . An example UUID using this textual representation from the previous table observed in . Note that in this example the alphabetic characters may be all uppercase, all lowercase or mixe case as per

Example Hex UUID

The same UUID from is represented in Binary , Integer and as a URN defined by .

Example Hex UUID

Example Hex UUID

Example Hex UUID

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

Msb0	Msb1	Msb2	Description
0	x	x	Reserved, NCS backward compatibility.
1	0	x	The variant specified in this document.
1	1	0	Reserved, Microsoft Corporation backward compatibility
1	1	1	Reserved for future definition.

Interoperability, in any form, with variants other than the one defined here is not guaranteed, and is not likely to be an issue in practice. Specifically for UUIDs in this document bits 64 and 65 of octet 8 MUST be set to 1 and 0 as per row 2 of . As such all bit and field layouts will detail a 2 bit variant entry as guidance.

Version Field The version number is in the most significant 4 bits of octet 6. More specifically bits 48 through 51. The remaining 4 bits of Octet 6 are dynamic. lists all of the versions for this UUID variant specified in this document. UUID variant 10xx (8/9/A/B) versions defined by this specification

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

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

UUIDv4 Variant Examples

UUID Layouts To minimize confusion about bit assignments within octets and among differing versions, the UUID record definition is defined only in terms of fields that are integral numbers of octets. The fields are presented with the most significant one first. In the absence of explicit application or presentation protocol specification to the contrary, each field is encoded with the Most Significant Byte first (known as network byte order). Note that in some instances the field names, particularly for multiplexed fields, follow historical practice. While discussing UUID field layouts, bit definitions start at 0 and end at 127 while octets definitions start at 0 and end at 15.

UUID Version 1 UUID Version 1 is a time-based UUID featuring a 60-bit timestamp represented by Coordinated Universal Time (UTC) as a count of 100- nanosecond intervals since 00:00:00.00, 15 October 1582 (the date of Gregorian reform to the Christian calendar). UUID Version 1 also features clock sequence field which is used to help avoid duplicates that could arise when the clock is set backwards in time or if the node ID changes. Finally the node field consists of an IEEE 802 MAC address, usually the host address. For systems with multiple IEEE 802 addresses, any available one can be used. The lowest addressed octet (octet number 10) contains the global/local bit and the unicast/multicast bit, and is the first octet of the address transmitted on an 802.3 LAN.

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)

time_hi_and_version:

The first four most significant bits MUST contain the UUIDv1 version (0001) while the remaining 12 bits will contain the most significant 12 bits from the 60 bit starting timestamp. Occupies bits 48 through 63 (octets 6-7)

clock_seq_hi_and_res:

The first two bits MUST be set to the UUID variant (10) The remaining 6 bits contain the high portion of the clock sequence. Occupies bits 64 through 71 (octet 8) for a full 8 bits.

clock_seq_low:

The 8 bit low portion of the clock sequence. Occupies bits 72 through 79 (octet 9)

node:

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

For systems that do not have UTC available, but do have the local time, they may use that instead of UTC, as long as they do so consistently throughout the system. However, this is not recommended since generating the UTC from local time only needs a time zone offset. If the clock is set backwards, or might have been set backwards (e.g., while the system was powered off), and the UUID generator can not be sure that no UUIDs were generated with timestamps larger than the value to which the clock was set, then the clock sequence has to be changed. If the previous value of the clock sequence is known, it can just be incremented; otherwise it should be set to a random or high-quality pseudo-random value. Similarly, if the node ID changes (e.g., because a network card has been moved between machines), setting the clock sequence to a random number minimizes the probability of a duplicate due to slight differences in the clock settings of the machines. If the value of clock sequence associated with the changed node ID were known, then the clock sequence could just be incremented, but that is unlikely. The clock sequence MUST be originally (i.e., once in the lifetime of a system) initialized to a random number to minimize the correlation across systems. This provides maximum protection against node identifiers that may move or switch from system to system rapidly. The initial value MUST NOT be correlated to the node identifier. For systems with no IEEE address, a randomly or pseudo-randomly generated value may be used; see and .

UUID Version 2 UUID Version 2 is known as DCE Security UUIDs and . As such the definition of these UUIDs are outside the scope of this specification.

UUID Version 3 UUID Version 3 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "name space" as per . UUIDv3 values are created by computing an MD5 hash over a given name space value concatenated with the desired name value after both have been converted to a canonical sequence of octets in network byte order. This MD5 value is then uses to populate all 128 bits of the UUID layout. The UUID version and variant then replace the respective bits as defined by and . Some common name space values have been defined via . Where possible UUIDv5 SHOULD be used in lieu of UUIDv3. For more information on MD5 security considerations see .

UUIDv3 Field and Bit Layout

md5_high:

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

ver:

The 4 bit version field as defined by

md5_mid:

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

var:

The 2 bit variant field as defined by .

md5_low:

The final 62 bits of the layout immediately following the var field to be filled with the least-significant, right-most bits of the final 64 bits from the computed MD5 value.

UUID Version 4 The version 4 UUID is meant for generating UUIDs from truly-random or pseudo-random numbers. An implementation may generate 128 bits of random random data which is used to fill out the UUID fields in . The UUID version and variant then replace the respective bits as defined by and , Alternatively, an implementation MAY choose to randomly generate the exact required number of bits for for random_a, random_b, and random_c then concatenate the version and variant in the required position. For guidelines on random data generation see .

UUIDv4 Field and Bit Layout

random_a:

The first 48 bits of the layout that can be filled with random data as per fit.

ver:

The 4 bit version field as defined by

random_b:

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

var:

The 2 bit variant field as defined by .

random_c:

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

UUID Version 5 UUID Version 5 is meant for generating UUIDs from "names" that are drawn from, and unique within, some "name space" as per . UUIDv5 values are created by computing an SHA1 hash over a given name space value concatenated with the desired name value after both have been converted to a canonical sequence of octets in network byte order. This SHA1 value is then uses to populate all 128 bits of the UUID layout. Excess bits beyond 128 are discarded. The UUID version and variant then replace the respective bits as defined by and Some common name space values have been defined via . For more information on MD5 security considerations see .

UUIDv5 Field and Bit Layout

sha_high:

The first 48 bits of the layout are filled with the most significant, left-most 48 bits from the computed SHA1 value.

ver:

The 4 bit version field as defined by

sha_mid:

12 more bits of the layout consisting of the least significant, right-most 12 bits of 16 bits immediately following md5_high from the computed SHA1 value.

var:

The 2 bit variant field as defined by .

sha_low:

The final 62 bits of the layout immediately following the var field to be filled by skipping the 2 most significant, left-most bits of the remaining SHA1 hash and then using the next 62 most significant, left-most bits. Any leftover SHA1 bits are discarded and unused.

UUID Version 6 UUID version 6 is a field-compatible version of UUIDv1, reordered for improved DB locality. It is expected that UUIDv6 will primarily be used in contexts where there are existing v1 UUIDs. Systems that do not involve legacy UUIDv1 SHOULD consider using UUIDv7 instead. Instead of splitting the timestamp into the low, mid and high sections from UUIDv1, UUIDv6 changes this sequence so timestamp bytes are stored from most to least significant. That is, given a 60 bit timestamp value as specified for UUIDv1 in , for UUIDv6, the first 48 most significant bits are stored first, followed by the 4 bit version (same position), followed by the remaining 12 bits of the original 60 bit timestamp. The clock sequence bits remain unchanged from their usage and position in . The 48 bit node SHOULD be set to a pseudo-random value however implementations MAY choose to retain the old MAC address behavior from and . For more information on MAC address usage within UUIDs see the The format for the 16-byte, 128 bit UUIDv6 is shown in

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)

time_low_and_version:

The first four most significant bits MUST contain the UUIDv6 version (0110) while the remaining 12 bits will contain the least significant 12 bits from the 60 bit starting timestamp. Occupies bits 48 through 63 (octets 6-7)

clk_seq_hi_res:

The first two bits MUST be set to the UUID variant (10) The remaining 6 bits contain the high portion of the clock sequence. Occupies bits 64 through 71 (octet 8) for a full 8 bits.

clock_seq_low:

The 8 bit low portion of the clock sequence. Occupies bits 72 through 79 (octet 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.

UUID Version 7 UUID version 7 features a time-ordered value field derived from the widely implemented and well known Unix Epoch timestamp source, the number of milliseconds seconds since midnight 1 Jan 1970 UTC, leap seconds excluded. As well as improved entropy characteristics over versions 1 or 6. Implementations SHOULD utilize UUID version 7 over UUID version 1 and 6 if possible.

UUIDv7 Field and Bit Layout

unix_ts_ms:

48 bit big-endian unsigned number of Unix epoch timestamp as per .

ver:

4 bit UUIDv7 version set as per

rand_a:

12 bits pseudo-random data to provide uniqueness as per and .

var:

The 2 bit variant defined by .

rand_b:

The final 62 bits of pseudo-random data to provide uniqueness as per and .

UUID Version 8 UUID version 8 provides an RFC-compatible format for experimental or vendor-specific use cases. The only requirement is that the variant and version bits MUST be set as defined in and . UUIDv8's uniqueness will be implementation-specific and SHOULD NOT be assumed. The only explicitly defined bits are the Version and Variant leaving 122 bits for implementation specific time-based UUIDs. To be clear: UUIDv8 is not a replacement for UUIDv4 where all 122 extra bits are filled with random data. Some example situations in which UUIDv8 usage could occur:

• An implementation would like to embed extra information within the UUID other than what is defined in this document.
• An implementation has other application/language restrictions which inhibit the use of one of the current UUIDs.

UUIDv8 Field and Bit Layout

custom_a:

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

ver:

The 4 bit version field as defined by

custom_b:

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

var:

The 2 bit variant field as defined by .

custom_c:

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

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

Nil UUID Format

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

Max UUID Format

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

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

Reliability:

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

Source:

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

Sub-second Precision and Accuracy:

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

Length:

The length of a given timestamp directly impacts how long a given UUID will be valid. That is, how many timestamp ticks can be contained in a UUID before the maximum value for the timestamp field is reached. Care should be given to ensure that the proper length is selected for a given timestamp. UUID version 1 and 6 utilize a 60 bit timestamp and UUIDv7 features a 48 bit timestamp.

Altering, Fuzzing, or Smearing:

Implementations MAY alter the actual timestamp. Some examples included security considerations around providing a real clock value within a UUID, to correct inaccurate clocks or to handle leap seconds. This specification makes no requirement or guarantee about how close the clock value needs to be to actual time. If UUIDs do not need to be frequently generated, the UUIDv1 or UUIDv6 timestamp can simply be the system time multiplied by the number of 100-nanosecond intervals per system time interval.

Padding:

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

Truncating:

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

Error Handling:

If a system overruns the generator by requesting too many UUIDs within a single system time interval, the UUID service MUST either return an error, or stall the UUID generator until the system clock catches up. Note that if the processors overrun the UUID generation frequently, additional node identifiers can be allocated to the system, which will permit higher speed allocation by making multiple UUIDs potentially available for each time stamp value. Similar to techniques

Monotonicity and Counters Monotonicity is the backbone of time-based sortable UUIDs. Naturally 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. Additionally, care SHOULD be taken to ensure UUIDs generated in batches are also monotonic. That is, if one-thousand UUIDs are generated for the same timestamp; there is sufficient logic for organizing the creation order of those one-thousand UUIDs. For batch UUID creation implementations MAY utilize a monotonic counter which SHOULD increment for each UUID created during a given timestamp. For single-node UUID implementations that do not need to create batches of UUIDs, the embedded timestamp within UUID version 1, 6, and 7 can provide sufficient monotonicity guarantees by simply ensuring that timestamp increments before creating a new UUID. For the topic of Distributed Nodes please refer to Implementations SHOULD choose one method for single-node UUID implementations that require batch UUID creation.

Fixed-Length Dedicated Counter Bits (Method 1):

This references the practice of allocating 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. Positioning of a fixed bit-length counter SHOULD be immediately after the embedded timestamp. This promotes sortability and allows random data generation for each counter increment. With this method rand_a section of UUIDv7 SHOULD be utilized as fixed-length dedicated counter bits that are incremented by one for every UUID generation. The trailing random bits generated for each new UUID in rand_b can help produce unguessable UUIDs. In the event more counter bits are required the most significant, left-most, bits of rand_b MAY be leveraged as additional counter bits.

Monotonic Random (Method 2):

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

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

Fixed-Length Dedicated Counter Seeding:

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

Fixed-Length Dedicated Counter Length:

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

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

Counter Rollover Guards:

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

Counter Rollover Handling:

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

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

1. Compare the current timestamp against the previously stored timestamp.
2. If the current timestamp is equal to the previous timestamp; increment the counter according to the desired method.
3. If the current timestamp is greater than the previous timestamp; re-initialize the desired counter method to the new timestamp and generate new random bytes (if the bytes were frozen or being used as the seed for a monotonic counter).

Implementations SHOULD check if the 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, but not limited to, clock rollbacks, leap second handling or counter rollovers. Applications SHOULD embed sufficient logic to catch these scenarios and correct the problem ensuring the next UUID generated is greater than the previous. To handle this scenario, the general guidance is that application MAY reuse the previous timestamp and increment the previous counter method.

UUID Generator States The 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). If an implementation does not have any stable store available, then it can always say that the values were unavailable. 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. For UUIDv1 and UUIDv6, if the node ID can never change (e.g., the net card is inseparable from the system), or if any change also reinitializes the clock sequence to a random value, then instead of keeping it in stable store, the current node ID may be returned For UUIDv1 and UUIDv6, the state does not always need to be written to stable store every time a UUID is generated. The timestamp in the stable store can be periodically set to a value larger than any yet used in a UUID. As long as the generated UUIDs have timestamps less than that value, and the clock sequence and node ID remain unchanged, only the shared volatile copy of the state needs to be updated. Furthermore, if the timestamp value in stable store is in the future by less than the typical time it takes the system to reboot, a crash will not cause a reinitialization of the clock sequence. If it is too expensive to access shared state each time a UUID is generated, then the system-wide generator can be implemented to allocate a block of time stamps each time it is called; a per- process generator can allocate from that block until it is exhausted.

Distributed UUID Generation Some implementations MAY desire to utilize multi-node, clustered, applications which involve two or more nodes independently generating UUIDs that will be stored in a common location. While UUIDs already feature sufficient entropy to ensure that the chances of collision are low as the total number of nodes increase; so does the likelihood of a collision. This section will detail the approaches that have been observed by by multi-node UUID implementations in distributed environments.

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. Utilization of shared knowledge schemes with central/global registries is outside the scope of this specification and should generally be discouraged.

Node IDs:

With this method, a pseudo-random Node ID value is placed within the UUID layout. This identifier helps ensure the bit-space for a given node is unique, resulting in UUIDs that do not conflict with any other UUID created by another node with a different node id. Implementations that choose to leverage an embedded node id SHOULD utilize UUIDv8. The node id SHOULD NOT be an IEEE 802 MAC address as per . 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.

Utilization of either a Centralized Registry or Node ID are not required for implementing UUIDs in this specification. However implementations SHOULD utilize one of the two aforementioned methods if distributed UUID generation is a requirement.

Name-Based UUID Generation TODO, define how to compute a namespace ID if I don't want to use one from The concept of name and name space should be broadly construed, and not limited to textual names. For example, some name spaces are the domain name system, URLs, Object Identifiers (OIDs), X.500 Distinguished Names (DNs), and reserved words in a programming language. The mechanisms or conventions used for allocating names and ensuring their uniqueness within their name spaces are beyond the scope of this specification. The requirements for these types of UUIDs are as follows:

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

Collision Resistance Implementations SHOULD weigh the consequences of UUID collisions within their application and when deciding between UUID versions that use entropy (random) versus the other components such as and . This is especially true for distributed node collision resistance as defined by . There are two example scenarios below which help illustrate the varying seriousness of a collision within an application.

Low Impact

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

High Impact:

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

Global and Local Uniqueness UUIDs created by this specification MAY be used to provide local uniqueness guarantees. For example, ensuring UUIDs created within a local application context are unique within a database MAY be sufficient for some implementations where global uniqueness outside of the application context, in other applications, or around the world is not required. Although true global uniqueness is impossible to guarantee without a shared knowledge scheme; a shared knowledge scheme is not required by UUID to provide uniqueness guarantees. Implementations MAY implement a shared knowledge scheme introduced in as they see fit to extend the uniqueness guaranteed this specification.

Unguessability TODO: Here or in security considerations, discuss security considerations with with "running out of random" Implementations SHOULD utilize a cryptographically secure pseudo-random number generator (CSPRNG) to provide values that are both difficult to predict ("unguessable") and have a low likelihood of collision ("unique"). Care SHOULD be taken to ensure the CSPRNG state is properly reseeded upon state changes, such as process forks, to ensure proper CSPRNG operation. CSPRNG ensures the best of and are present in modern UUIDs. Further advice on generating cryptographic-quality random numbers can be found in

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. One approach is to contact the IEEE and get a separate block of addresses. At the time of writing, the application could be found at . A better solution is to obtain a 47-bit cryptographic quality random number and use it as the low 47 bits of the node ID, with the least significant bit of the first octet of the node ID set to one. This bit is the unicast/multicast bit, which will never be set in IEEE 802 addresses obtained from network cards. Hence, there can never be a conflict between UUIDs generated by machines with and without network cards. (Recall that the IEEE 802 spec talks about transmission order, which is the opposite of the in-memory representation that is discussed in this document.) For compatibility with earlier specifications, note that this document uses the unicast/multicast bit, instead of the arguably more correct local/global bit. In addition, items such as the computer's name and the name of the operating system, while not strictly speaking random, will help differentiate the results from those obtained by other systems. The exact algorithm to generate a node ID using these data is system specific, because both the data available and the functions to obtain them are often very system specific. A generic approach, however, is to accumulate as many sources as possible into a buffer, use a message digest such as MD5 or SHA-1 , take an arbitrary 6 bytes from the hash value, and set the multicast bit as described above.

Sorting UUIDv6 and UUIDv7 are designed so that implementations that require sorting (e.g. database indexes) SHOULD sort as opaque raw bytes, without need for parsing or introspection. Time ordered monotonic UUIDs benefit from greater database index locality because the new values are near each other in the index. As a result objects are more easily clustered together for better performance. The real-world differences in this approach of index locality vs random data inserts can be quite large. UUIDs formats created by this specification SHOULD be Lexicographically sortable while in the textual representation. UUIDs created by this specification are crafted with big-ending byte order (network byte order) in mind. If Little-endian style is required a custom UUID format SHOULD be created using UUIDv8.

Opacity UUIDs SHOULD be treated as opaque values and implementations SHOULD NOT examine the bits in a UUID to whatever extent is possible. However, where necessary, inspectors should refer to and for more information on determining UUID version and variant.

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 such:

• Storing as binary requires less space and may result in faster data access.
• Storing as text requires more space but may require less translation if the resulting text form is to be used after retrieval and thus maybe simpler to implement.

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

IANA Considerations TODO: Q: Should Namespace Registration Template be here or in ? TODO: Need to ensure IANA doc, https://www.iana.org/assignments/urn-namespaces/urn-namespaces.xhtml, has this new document listed.

Community Considerations The use of UUIDs is extremely pervasive in computing. They comprise the core identifier infrastructure for many operating systems (Microsoft Windows) and applications (the Mozilla browser) and in many cases, become exposed to the Web 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.

Security Considerations Do not assume that UUIDs are hard to guess; they should not be used as security capabilities (identifiers whose mere possession grants access), for example. A predictable random number source will exacerbate the situation. Do not assume that it is easy to determine if a UUID has been slightly transposed in order to redirect a reference to another object. Humans do not have the ability to easily check the integrity of a UUID by simply glancing at it. Distributed applications generating UUIDs at a variety of hosts must be willing to rely on the random number source at all hosts. If this is not feasible, the namespace variant should be used. MAC addresses pose inherent security risks and SHOULD not be used within a UUID. Instead CSPRNG data SHOULD be selected from a source with sufficient entropy to ensure guaranteed uniqueness among UUID generation. See 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 it's 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, SHOULD be utilized. See for MD5 Security Considerations and for SHA1 security considerations.

Acknowledgements The authors gratefully acknowledge the contributions of Rich Salz, Ben Campbell, Ben Ramsey, Fabio Lima, Gonzalo Salgueiro, Martin Thomson, Murray S. Kucherawy, Rick van Rein, Rob Wilton, Sean Leonard, Theodore Y. Ts'o., Robert Kieffer, sergeyprokhorenko, LiosK As well as all of those in the IETF community and on GitHub to who contributed to the discussions which resulted in this document. This document draws heavily on the OSF DCE specification for UUIDs. Ted Ts'o provided helpful comments, especially on the byte ordering section which we mostly plagiarized from a proposed wording he supplied (all errors in that section are our responsibility, however). We are also grateful to the careful reading and bit-twiddling of Ralf S. Engelschall, John Larmouth, and Paul Thorpe. Professor Larmouth was also invaluable in achieving coordination with ISO/IEC.

References Normative References Open Group CAE Specification C309 This document describes the MD5 message-digest algorithm. The algorithm takes as input a message of arbitrary length and produces as output a 128-bit "fingerprint" or "message digest" of the input. This memo provides information for the Internet community. It does not specify an Internet standard. This document updates the security considerations for the MD5 message digest algorithm. It also updates the security considerations for HMAC-MD5. This document is not an Internet Standards Track specification; it is published for informational purposes. Security systems are built on strong cryptographic algorithms that foil pattern analysis attempts. However, the security of these systems is dependent on generating secret quantities for passwords, cryptographic keys, and similar quantities. The use of pseudo-random processes to generate secret quantities can result in pseudo-security. A sophisticated attacker may find it easier to reproduce the environment that produced the secret quantities and to search the resulting small set of possibilities than to locate the quantities in the whole of the potential number space. Choosing random quantities to foil a resourceful and motivated adversary is surprisingly difficult. This document points out many pitfalls in using poor entropy sources or traditional pseudo-random number generation techniques for generating such quantities. It recommends the use of truly random hardware techniques and shows that the existing hardware on many systems can be used for this purpose. It provides suggestions to ameliorate the problem when a hardware solution is not available, and it gives examples of how large such quantities need to be for some applications. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. A Uniform Resource Name (URN) is a Uniform Resource Identifier (URI) that is assigned under the "urn" URI scheme and a particular URN namespace, with the intent that the URN will be a persistent, location-independent resource identifier. With regard to URN syntax, this document defines the canonical syntax for URNs (in a way that is consistent with URI syntax), specifies methods for determining URN-equivalence, and discusses URI conformance. With regard to URN namespaces, this document specifies a method for defining a URN namespace and associating it with a namespace identifier, and it describes procedures for registering namespace identifiers with the Internet Assigned Numbers Authority (IANA). This document obsoletes both RFCs 2141 and 3406. Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity, with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] This document includes security considerations for the SHA-0 and SHA-1 message digest algorithm. This document is not an Internet Standards Track specification; it is published for informational purposes. National Institute of Standards and Technology Open Group CAE Specification C311 In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References Twitter Twitter Boundary Instagram Engineering Segment Sony Google MongoDB IEEE

Namespace Registration Template TODO: Revise as per https://www.rfc-editor.org/rfc/rfc8141#appendix-A and https://www.rfc-editor.org/rfc/rfc8141#section-6.2

Namespace ID:

UUID

Registration Information:

Registration date: 2003-10-01

Declared registrant of the namespace:

JTC 1/SC6 (ASN.1 Rapporteur Group)

Declaration of syntactic structure:

A UUID is an identifier that is unique across both space and time, with respect to the space of all UUIDs. Since a UUID is a fixed size and contains a time field, it is possible for values to rollover (around A.D. 3400, depending on the specific algorithm used). A UUID can be used for multiple purposes, from tagging objects with an extremely short lifetime, to reliably identifying very persistent objects across a network. The internal representation of a UUID is a specific sequence of bits in memory, as described in . To accurately represent a UUID as a URN, it is necessary to convert the bit sequence to a string representation. Each field is treated as an integer and has its value printed as a zero-filled hexadecimal digit string with the most significant digit first. The hexadecimal values "a" through "f" are output as lower case characters and are case insensitive on input. The formal definition of the UUID string representation is provided by the following ABNF :

The following is an example of the string representation of a UUID as a URN: urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6

Relevant ancillary documentation:

Identifier uniqueness considerations:

This document specifies three algorithms to generate UUIDs: the first leverages the unique values of 802 MAC addresses to guarantee uniqueness, the second uses pseudo-random number generators, and the third uses cryptographic hashing and application-provided text strings. As a result, the UUIDs generated according to the mechanisms here will be unique from all other UUIDs that have been or will be assigned.

Identifier persistence considerations:

UUIDs are inherently very difficult to resolve in a global sense. This, coupled with the fact that UUIDs are temporally unique within their spatial context, ensures that UUIDs will remain as persistent as possible.

Process of identifier assignment:

Generating a UUID does not require that a registration authority be contacted. One algorithm requires a unique value over space for each generator. This value is typically an IEEE 802 MAC address, usually already available on network-connected hosts. The address can be assigned from an address block obtained from the IEEE registration authority. If no such address is available, or privacy concerns make its use undesirable, specifies two alternatives. Another approach is to use version 3 or version 4 UUIDs as defined below.

Process for identifier resolution:

Since UUIDs are not globally resolvable, this is not applicable.

Rules for Lexical Equivalence:

Consider each field of the UUID to be an unsigned integer as shown in the tables in section . Then, to compare a pair of UUIDs, arithmetically compare the corresponding fields from each UUID in order of significance and according to their data type. Two UUIDs are equal if and only if all the corresponding fields are equal. As an implementation note, equality comparison can be performed on many systems by doing the appropriate byte-order canonicalization, and then treating the two UUIDs as 128-bit unsigned integers. UUIDs, as defined in this document, can also be ordered lexicographically. For a pair of UUIDs, the first one follows the second if the most significant field in which the UUIDs differ is greater for the first UUID. The second follows the first if the most significant field in which the UUIDs differ is greater for the second UUID.

Conformance with URN Syntax:

The string representation of a UUID is fully compatible with the URN syntax. When converting from a bit-oriented, in-memory representation of a UUID into a URN, care must be taken to strictly adhere to the byte order issues mentioned in the string representation section.

Validation mechanism:

Apart from determining whether the timestamp portion of the UUID is in the future and therefore not yet assignable, there is no mechanism for determining whether a UUID is 'valid'.

Scope:

UUIDs are global in scope.

Example Code

Creating UUIDv1 through UUIDv5 Value This implementation consists of 5 files: uuid.h, uuid.c, sysdep.h, sysdep.c and utest.c. The uuid.* files are the system independent implementation of the UUID generation algorithms described above, with all the optimizations described above except efficient state sharing across processes included. The code has been tested on Linux (Red Hat 4.0) with GCC (2.7.2), and Windows NT 4.0 with VC++ 5.0. The code assumes 64-bit integer support, which makes it much clearer. All the following source files should have the following copyright notice included: copyrt.h uuid.h uuid.c #include #include #include #include "sysdep.h" #include "uuid.h" /* various forward declarations */ static int read_state(unsigned16 *clockseq, uuid_time_t *timestamp, uuid_node_t *node); static void write_state(unsigned16 clockseq, uuid_time_t timestamp, uuid_node_t node); static void format_uuid_v1(uuid_t *uuid, unsigned16 clockseq, uuid_time_t timestamp, uuid_node_t node); static void format_uuid_v3or5(uuid_t *uuid, unsigned char hash[16], int v); static void get_current_time(uuid_time_t *timestamp); static unsigned16 true_random(void); /* uuid_create -- generate a UUID */ int uuid_create(uuid_t *uuid) { uuid_time_t timestamp, last_time; unsigned16 clockseq; uuid_node_t node; uuid_node_t last_node; int f; /* acquire system-wide lock so we're alone */ LOCK; /* get time, node ID, saved state from non-volatile storage */ get_current_time(×tamp); get_ieee_node_identifier(&node); f = read_state(&clockseq, &last_time, &last_node); /* if no NV state, or if clock went backwards, or node ID changed (e.g., new network card) change clockseq */ if (!f || memcmp(&node, &last_node, sizeof node)) clockseq = true_random(); else if (timestamp < last_time) clockseq++; /* save the state for next time */ write_state(clockseq, timestamp, node); UNLOCK; /* stuff fields into the UUID */ format_uuid_v1(uuid, clockseq, timestamp, node); return 1; } /* format_uuid_v1 -- make a UUID from the timestamp, clockseq, and node ID */ void format_uuid_v1(uuid_t* uuid, unsigned16 clock_seq, uuid_time_t timestamp, uuid_node_t node) { /* Construct a version 1 uuid with the information we've gathered plus a few constants. */ uuid->time_low = (unsigned long)(timestamp & 0xFFFFFFFF); uuid->time_mid = (unsigned short)((timestamp >> 32) & 0xFFFF); uuid->time_hi_and_version = (unsigned short)((timestamp >> 48) & 0x0FFF); uuid->time_hi_and_version |= (1 << 12); uuid->clock_seq_low = clock_seq & 0xFF; uuid->clock_seq_hi_and_reserved = (clock_seq & 0x3F00) >> 8; uuid->clock_seq_hi_and_reserved |= 0x80; memcpy(&uuid->node, &node, sizeof uuid->node); } /* data type for UUID generator persistent state */ typedef struct { uuid_time_t ts; /* saved timestamp */ uuid_node_t node; /* saved node ID */ unsigned16 cs; /* saved clock sequence */ } uuid_state; static uuid_state st; /* read_state -- read UUID generator state from non-volatile store */ int read_state(unsigned16 *clockseq, uuid_time_t *timestamp, uuid_node_t *node) { static int inited = 0; FILE *fp; /* only need to read state once per boot */ if (!inited) { fp = fopen("state", "rb"); if (fp == NULL) return 0; fread(&st, sizeof st, 1, fp); fclose(fp); inited = 1; } *clockseq = st.cs; *timestamp = st.ts; *node = st.node; return 1; } /* write_state -- save UUID generator state back to non-volatile storage */ void write_state(unsigned16 clockseq, uuid_time_t timestamp, uuid_node_t node) { static int inited = 0; static uuid_time_t next_save; FILE* fp; if (!inited) { next_save = timestamp; inited = 1; } /* always save state to volatile shared state */ st.cs = clockseq; st.ts = timestamp; st.node = node; if (timestamp >= next_save) { fp = fopen("state", "wb"); fwrite(&st, sizeof st, 1, fp); fclose(fp); /* schedule next save for 10 seconds from now */ next_save = timestamp + (10 * 10 * 1000 * 1000); } } /* get-current_time -- get time as 60-bit 100ns ticks since UUID epoch. Compensate for the fact that real clock resolution is less than 100ns. */ void get_current_time(uuid_time_t *timestamp) { static int inited = 0; static uuid_time_t time_last; static unsigned16 uuids_this_tick; uuid_time_t time_now; if (!inited) { get_system_time(&time_now); uuids_this_tick = UUIDS_PER_TICK; inited = 1; } for ( ; ; ) { get_system_time(&time_now); /* if clock reading changed since last UUID generated, */ if (time_last != time_now) { /* reset count of uuids gen'd with this clock reading */ uuids_this_tick = 0; time_last = time_now; break; } if (uuids_this_tick < UUIDS_PER_TICK) { uuids_this_tick++; break; } /* going too fast for our clock; spin */ } /* add the count of uuids to low order bits of the clock reading */ *timestamp = time_now + uuids_this_tick; } /* true_random -- generate a crypto-quality random number. **This sample doesn't do that.** */ static unsigned16 true_random(void) { static int inited = 0; uuid_time_t time_now; if (!inited) { get_system_time(&time_now); time_now = time_now / UUIDS_PER_TICK; srand((unsigned int) (((time_now >> 32) ^ time_now) & 0xffffffff)); inited = 1; } return rand(); } /* uuid_create_md5_from_name -- create a version 3 (MD5) UUID using a "name" from a "name space" */ void uuid_create_md5_from_name(uuid_t *uuid, uuid_t nsid, void *name, int namelen) { MD5_CTX c; unsigned char hash[16]; uuid_t net_nsid; /* put name space ID in network byte order so it hashes the same no matter what endian machine we're on */ net_nsid = nsid; net_nsid.time_low = htonl(net_nsid.time_low); net_nsid.time_mid = htons(net_nsid.time_mid); net_nsid.time_hi_and_version = htons(net_nsid.time_hi_and_version); MD5Init(&c); MD5Update(&c, &net_nsid, sizeof net_nsid); MD5Update(&c, name, namelen); MD5Final(hash, &c); /* the hash is in network byte order at this point */ format_uuid_v3or5(uuid, hash, 3); } void uuid_create_sha1_from_name(uuid_t *uuid, uuid_t nsid, void *name, int namelen) { SHA_CTX c; unsigned char hash[20]; uuid_t net_nsid; /* put name space ID in network byte order so it hashes the same no matter what endian machine we're on */ net_nsid = nsid; net_nsid.time_low = htonl(net_nsid.time_low); net_nsid.time_mid = htons(net_nsid.time_mid); net_nsid.time_hi_and_version = htons(net_nsid.time_hi_and_version); SHA1_Init(&c); SHA1_Update(&c, &net_nsid, sizeof net_nsid); SHA1_Update(&c, name, namelen); SHA1_Final(hash, &c); /* the hash is in network byte order at this point */ format_uuid_v3or5(uuid, hash, 5); } /* format_uuid_v3or5 -- make a UUID from a (pseudo)random 128-bit number */ void format_uuid_v3or5(uuid_t *uuid, unsigned char hash[16], int v) { /* convert UUID to local byte order */ memcpy(uuid, hash, sizeof *uuid); uuid->time_low = ntohl(uuid->time_low); uuid->time_mid = ntohs(uuid->time_mid); uuid->time_hi_and_version = ntohs(uuid->time_hi_and_version); /* put in the variant and version bits */ uuid->time_hi_and_version &= 0x0FFF; uuid->time_hi_and_version |= (v << 12); uuid->clock_seq_hi_and_reserved &= 0x3F; uuid->clock_seq_hi_and_reserved |= 0x80; } /* uuid_compare -- Compare two UUID's "lexically" and return */ #define CHECK(f1, f2) if (f1 != f2) return f1 < f2 ? -1 : 1; int uuid_compare(uuid_t *u1, uuid_t *u2) { int i; CHECK(u1->time_low, u2->time_low); CHECK(u1->time_mid, u2->time_mid); CHECK(u1->time_hi_and_version, u2->time_hi_and_version); CHECK(u1->clock_seq_hi_and_reserved, u2->clock_seq_hi_and_reserved); CHECK(u1->clock_seq_low, u2->clock_seq_low) for (i = 0; i < 6; i++) { if (u1->node[i] < u2->node[i]) return -1; if (u1->node[i] > u2->node[i]) return 1; } return 0; } #undef CHECK ]]> sysdep.h #else #include #include #include #endif #include "global.h" /* change to point to where MD5 .h's live; RFC 1321 has sample implementation */ #include "md5.h" /* set the following to the number of 100ns ticks of the actual resolution of your system's clock */ #define UUIDS_PER_TICK 1024 /* Set the following to a calls to get and release a global lock */ #define LOCK #define UNLOCK typedef unsigned long unsigned32; typedef unsigned short unsigned16; typedef unsigned char unsigned8; typedef unsigned char byte; /* Set this to what your compiler uses for 64-bit data type */ #ifdef WININC #define unsigned64_t unsigned __int64 #define I64(C) C #else #define unsigned64_t unsigned long long #define I64(C) C##LL #endif typedef unsigned64_t uuid_time_t; typedef struct { char nodeID[6]; } uuid_node_t; void get_ieee_node_identifier(uuid_node_t *node); void get_system_time(uuid_time_t *uuid_time); void get_random_info(char seed[16]); ]]> sysdep.c #include "sysdep.h" /* system dependent call to get IEEE node ID. This sample implementation generates a random node ID. */ void get_ieee_node_identifier(uuid_node_t *node) { static inited = 0; static uuid_node_t saved_node; char seed[16]; FILE *fp; if (!inited) { fp = fopen("nodeid", "rb"); if (fp) { fread(&saved_node, sizeof saved_node, 1, fp); fclose(fp); } else { get_random_info(seed); seed[0] |= 0x01; memcpy(&saved_node, seed, sizeof saved_node); fp = fopen("nodeid", "wb"); if (fp) { fwrite(&saved_node, sizeof saved_node, 1, fp); fclose(fp); } } inited = 1; } *node = saved_node; } /* system dependent call to get the current system time. Returned as 100ns ticks since UUID epoch, but resolution may be less than 100ns. */ #ifdef _WINDOWS_ void get_system_time(uuid_time_t *uuid_time) { ULARGE_INTEGER time; /* NT keeps time in FILETIME format which is 100ns ticks since Jan 1, 1601. UUIDs use time in 100ns ticks since Oct 15, 1582. The difference is 17 Days in Oct + 30 (Nov) + 31 (Dec) + 18 years and 5 leap days. */ GetSystemTimeAsFileTime((FILETIME *)&time); time.QuadPart += (unsigned __int64) (1000*1000*10) // seconds * (unsigned __int64) (60 * 60 * 24) // days * (unsigned __int64) (17+30+31+365*18+5); // # of days *uuid_time = time.QuadPart; } /* Sample code, not for use in production; see RFC 4086 */ void get_random_info(char seed[16]) { MD5_CTX c; struct { MEMORYSTATUS m; SYSTEM_INFO s; FILETIME t; LARGE_INTEGER pc; DWORD tc; DWORD l; char hostname[MAX_COMPUTERNAME_LENGTH + 1]; } r; MD5Init(&c); GlobalMemoryStatus(&r.m); GetSystemInfo(&r.s); GetSystemTimeAsFileTime(&r.t); QueryPerformanceCounter(&r.pc); r.tc = GetTickCount(); r.l = MAX_COMPUTERNAME_LENGTH + 1; GetComputerName(r.hostname, &r.l); MD5Update(&c, &r, sizeof r); MD5Final(seed, &c); } #else void get_system_time(uuid_time_t *uuid_time) { struct timeval tp; gettimeofday(&tp, (struct timezone *)0); /* Offset between UUID formatted times and Unix formatted times. UUID UTC base time is October 15, 1582. Unix base time is January 1, 1970.*/ *uuid_time = ((unsigned64)tp.tv_sec * 10000000) + ((unsigned64)tp.tv_usec * 10) + I64(0x01B21DD213814000); } /* Sample code, not for use in production; see RFC 4086 */ void get_random_info(char seed[16]) { MD5_CTX c; struct { struct sysinfo s; struct timeval t; char hostname[257]; } r; MD5Init(&c); sysinfo(&r.s); gettimeofday(&r.t, (struct timezone *)0); gethostname(r.hostname, 256); MD5Update(&c, &r, sizeof r); MD5Final(seed, &c); } #endif ]]> utest.c #include "uuid.h" uuid_t NameSpace_DNS = { /* 6ba7b810-9dad-11d1-80b4-00c04fd430c8 */ 0x6ba7b810, 0x9dad, 0x11d1, 0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8 }; /* puid -- print a UUID */ void puid(uuid_t u) { int i; printf("%8.8x-%4.4x-%4.4x-%2.2x%2.2x-", u.time_low, u.time_mid, u.time_hi_and_version, u.clock_seq_hi_and_reserved, u.clock_seq_low); for (i = 0; i < 6; i++) printf("%2.2x", u.node[i]); printf("\n"); } /* Simple driver for UUID generator */ void main(int argc, char **argv) { uuid_t u; int f; uuid_create(&u); printf("uuid_create(): "); puid(u); f = uuid_compare(&u, &u); printf("uuid_compare(u,u): %d\n", f); /* should be 0 */ f = uuid_compare(&u, &NameSpace_DNS); printf("uuid_compare(u, NameSpace_DNS): %d\n", f); /* s.b. 1 */ f = uuid_compare(&NameSpace_DNS, &u); printf("uuid_compare(NameSpace_DNS, u): %d\n", f); /* s.b. -1 */ uuid_create_md5_from_name(&u, NameSpace_DNS, "www.example.com", 15); printf("uuid_create_md5_from_name(): "); puid(u); } ]]> Sample Output of utest ~~~ uuid_create(): 7d444840-9dc0-11d1-b245-5ffdce74fad2 uuid_compare(u,u): 0 uuid_compare(u, NameSpace_DNS): 1 uuid_compare(NameSpace_DNS, u): -1 uuid_create_md5_from_name(): 5df41881-3aed-3515-88a7-2f4a814cf09e ~~~

Some Name Space IDs This appendix lists the name space IDs for some potentially interesting name spaces, as initialized C structures and in the string representation defined above.

Creating a UUIDv6 Value This section details a function in C which converts from a UUID version 1 to version 6:

UUIDv6 Function in C #include #include #include #include /* Converts UUID version 1 to version 6 in place. */ void uuidv1tov6(uuid_t u) { uint64_t ut; unsigned char *up = (unsigned char *)u; // load ut with the first 64 bits of the UUID ut = ((uint64_t)ntohl(*((uint32_t*)up))) << 32; ut |= ((uint64_t)ntohl(*((uint32_t*)&up[4]))); // dance the bit-shift... ut = ((ut >> 32) & 0x0FFF) | // 12 least significant bits (0x6000) | // version number ((ut >> 28) & 0x0000000FFFFF0000) | // next 20 bits ((ut << 20) & 0x000FFFF000000000) | // next 16 bits (ut << 52); // 12 most significant bits // store back in UUID *((uint32_t*)up) = htonl((uint32_t)(ut >> 32)); *((uint32_t*)&up[4]) = htonl((uint32_t)(ut)); } ]]>

Creating a UUIDv7 Value

UUIDv7 Function in C #include #include #include #include // ... // csprng data source FILE *rndf; rndf = fopen("/dev/urandom", "r"); if (rndf == 0) { printf("fopen /dev/urandom error\n"); return 1; } // ... // generate one UUIDv7E uint8_t u[16]; struct timespec ts; int ret; ret = clock_gettime(CLOCK_REALTIME, &ts); if (ret != 0) { printf("clock_gettime error: %d\n", ret); return 1; } uint64_t tms; tms = ((uint64_t)ts.tv_sec) * 1000; tms += ((uint64_t)ts.tv_nsec) / 1000000; memset(u, 0, 16); fread(&u[6], 10, 1, rndf); // fill everything after the timestamp with random bytes *((uint64_t*)(u)) |= htonll(tms << 16); // shift time into first 48 bits and OR into place u[8] = 0x80 | (u[8] & 0x3F); // set variant field, top two bits are 1, 0 u[6] = 0x70 | (u[6] & 0x0F); // set version field, top four bits are 0, 1, 1, 1 ]]>

Creating a UUIDv8 Value UUIDv8 will vary greatly from implementation to implementation. The following example utilizes:

• 32 bit custom-epoch timestamp (seconds elapsed since 2020-01-01 00:00:00 UTC)
• 16 bit exotic resolution (~15 microsecond) subsecond timestamp encoded using the fractional representation
• 58 bit random number
• 8 bit application-specific unique node ID
• 8 bit rolling sequence number

UUIDv8 Function in C #include int get_random_bytes(uint8_t *buffer, int count) { // ... } int generate_uuidv8(uint8_t *uuid, uint8_t node_id) { struct timespec tp; if (clock_gettime(CLOCK_REALTIME, &tp) != 0) return -1; // real-time clock error // 32 bit biased timestamp (seconds elapsed since 2020-01-01 00:00:00 UTC) uint32_t timestamp_sec = tp.tv_sec - 1577836800; uuid[0] = timestamp_sec >> 24; uuid[1] = timestamp_sec >> 16; uuid[2] = timestamp_sec >> 8; uuid[3] = timestamp_sec; // 16 bit subsecond fraction (~15 microsecond resolution) uint16_t timestamp_subsec = ((uint64_t)tp.tv_nsec << 16) / 1000000000; uuid[4] = timestamp_subsec >> 8; uuid[5] = timestamp_subsec; // 58 bit random number and required ver and var fields if (get_random_bytes(&uuid[6], 8) != 0) return -1; // random number generator error uuid[6] = 0x80 | (uuid[6] & 0x0f); uuid[8] = 0x80 | (uuid[8] & 0x3f); // 8 bit application-specific node ID to guarantee application-wide uniqueness uuid[14] = node_id; // 8 bit rolling sequence number to help ensure process-wide uniqueness static uint8_t sequence = 0; uuid[15] = sequence++; // NOTE: unprotected from race conditions return 0; } ]]>

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 clk_seq_hi_res, clock_seq_low, and node. All of which have been generated with random data.

Test Vector Timestamp Pseudo-code

Example of UUIDv1 Value

UUIDv1 Example Test Vector

Example of UUIDv3 Value The MD5 computation from is detailed in while the field mapping and all values are illustrated in . Finally to further illustrate the bit swaping for version and variant see .

UUIDv3 Example MD5

UUIDv3 Example Test Vector

UUIDv3 Example Ver Var bit swaps

Example of UUIDv4 Value This UUIDv4 example was created by generating 16 bytes of random data resulting in the hex value of 919108F752D133205BACF847DB4148A8. This is then used to fill out the feilds as shown in . Finally to further illustrate the bit swapping for version and variant see .

UUIDv4 Example Test Vector

UUIDv4 Example Ver/Var bit swaps

Example of UUIDv5 Value The SHA1 computation from is detailed in while the field mapping and all values are illustrated in . Finally to further illustrate the bit swapping for version and variant and the unused/discarded part of the SHA1 value see .

UUIDv5 Example SHA1

UUIDv5 Example Test Vector

UUIDv5 Example Ver/Var bit swaps and discarded SHA1 segment

Example of a UUIDv6 Value

UUIDv6 Example Test Vector

Example of a UUIDv7 Value This example UUIDv7 test vector utilizes a well-known 32 bit Unix epoch with additional millisecond precision to fill the first 48 bits rand_a and rand_b are filled with random data. The timestamp is Tuesday, February 22, 2022 2:22:22.00 PM GMT-05:00 represented as 0x17F22E279B0 or 1645557742000

UUIDv7 Example Test Vector

Example of a UUIDv8 Value This example UUIDv8 test vector utilizes a well-known 64 bit Unix epoch with nanosecond precision, truncated to the least-significant, right-most, bits to fill the first 48 bits through version. The next two segments of custom_b and custom_c are are filled with random data. Timestamp is Tuesday, February 22, 2022 2:22:22.000000 PM GMT-05:00 represented as 0x16D6320C3D4DCC00 or 1645557742000000000 It should be noted that this example is just to illustrate one scenario for UUIDv8. Test vectors will likely be implementation specific and vary greatly from this simple example.

UUIDv8 Example Test Vector