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3. Namespace Registration Template

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 Section 4. 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 [7]:

UUID                   = time-low "-" time-mid "-"
                         time-high-and-version "-"
                         clock-seq-and-reserved
                         clock-seq-low "-" node
time-low               = 4hexOctet
time-mid               = 2hexOctet
time-high-and-version  = 2hexOctet
clock-seq-and-reserved = hexOctet
clock-seq-low          = hexOctet
node                   = 6hexOctet
hexOctet               = hexDigit hexDigit
hexDigit =
      "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9" /
      "a" / "b" / "c" / "d" / "e" / "f" /
      "A" / "B" / "C" / "D" / "E" / "F"

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:

  • [1][2]

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, Section 4.5 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 table in section Section 4.1.2. 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 precedes 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.

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