UUID, GUID notes

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This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


For UUIDs used in filesystems, see Fstab#Device)


What

A UUID is a standardized way of generating a probably-universally-unique 128-bits number.


There are a few variants, some more anonymous (e.g. adding hashing, or simply being random) than others (e.g. contain network MAC address and time).


There are a few different ways of ensuring a value is unlikely to be generated twice.

One idea is to use your network card's MAC address (itself unique), add the current time (preferably in high resolution), and perhaps a (pseudo)random number or incremental counter for good measure. This is pretty simple and cheap to generate, and won't collide with a value generated later.

...but also makes it possible to extract information about its place and time of generation, which isn't ideal in some situations, so there are versions of UUID that are more anonymous.

Why

Usually used for identifiers.


One of the use cases is when want to add identifiers to a lot of things, and distributed among many computers.

And to be able to generate them without checking with a central authority, which may well need to store everything it generated/saw before. It may be inconvenient to have such a service, intractable to host one, and/or be a distributed system's primary bottleneck (though there are ways of making it scale rather better than the simple-and-stupid implementation).


If instead you generate number independently, in a space large enough to have a very very high probability of being unique, you eliminate the need for central and checked, at negligible risk of ever having colliding IDs (consider that 2128 is 340282366920938463463374607431768211456).


(And yes, generating 128-bit numbers yourself is functionally the same - though not directly a valid UUID because of a few special bits)

How

Use a UUID library. Recent languages may have it in your standard libary. It's easier, and more likely to stick to the standard than 5 minutes of DIY.


For example:

In Python: Use the uuid module (in the standard library since 2.5), for example
myuuid = uuid.uuid4()
...which is still an object. You can use str(myuuid) for the hexadecimal string with dashes.


In Java, use java.util.UUID, for example
UUID random_uuid = UUID.randomUUID();
, quite possibly followed by toString().


In .NET, use System.Guid, for example:
random_uuid = System.Guid.NewGuid()
, quite possibly followed by a .ToString()


When you have or want to do it yourself, the simplest (and probably most common) to generate are random UUIDs, i.e. version 4. You can generate a large random number, then twiddle the bits to have it be an RFC-compliant version 4 UUID.

For example, JavaScript has no standard-library function. Perhaps the simplest implementation to generate a random UUID is something like this or this, but there is a potential problem in Math.random() not giving randomness guarantees, which is why there are better libraries (with their own PRNG) which you might as well use. They include:

Layout

A UUID is a 128-bit number, conventionally grouped and dashed as 32-16-16-16-48 bits (8-4-4-4-12 hex characters), which reflects the way it is parsed.

The bit layout (from RFC 4122 ):

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                |         time_hi_and_version   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|clk_seq_hi_res |  clk_seq_low  |         node (0-1)            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         node (2-5)                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Note that fields vary purpose between versions. For example, the 60-bit time value can also be used to store a pseudorandom number, while the node field can be a MAC or a hash of something. (verify)


Versions and variants, a few layout details

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


The version specifies the means of generation, and is stored in the most significant four bits of time_hi_and_version (octet 6).

...which also means that, in hex representation, you can see the version right after the second dash. For example:
6e8bc430-9c3a-11d9-9669-0800200c9a66 is a version 1 UUID,
550e8400-e29b-41d4-a716-446655440000 is a version 4.


It seems there are currently:

  • 0001: Version 1: Based on time + node (MAC address of any of the cards present)
  • 0010: Version 2: DCE Security version (with embedded POSIX UIDs)
  • 0011: Version 3: Name-based (MD5 hash)
  • 0100: version 4: random
  • 0101: Version 5: Name-based (SHA1 hash)

Versions 1, 3, 4, and 5 are defined in RFC 4122. If the UUID variant is not RFC4122 based, the version value need not necessarily comply to the above.


The variant is specified by the most significant three bits in clk_seq_hi_res (octet 8), and controls the meaning/layout of the rest of the number. Apparently:

  • 001 ?
  • 000 NCS (reserved for backward compatibility)
  • 010 Current variant
  • 011 ?
  • 100 ?
  • 101 ?
  • 110 Microsoft (reserved for backward compatibility)
  • 111 Reserved for future use





See also

  • RFC 4122 (A Universally Unique IDentifier (UUID) URN Namespace)


And perhaps:

  • ISO/IEC 9834-8:2005, Information technology — Procedures for the operation of object identifier registration authorities — Part 8: Generation of universally unique identifiers (UUIDs) and their use in object identifiers
  • ISO/IEC 11578:1996 (Information technology -- Open Systems Interconnection -- Remote Procedure Call (RPC))
  • ITU-T Rec. X.667 (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)