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Ben Ramsey
102 lines
3.8 KiB
ReStructuredText
102 lines
3.8 KiB
ReStructuredText
.. _rfc4122.version2:
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=======================
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Version 2: DCE Security
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=======================
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Like a :ref:`version 1 UUID <rfc4122.version1>`, a version 2 UUID uses the
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current time, along with the MAC address (or *node*) for a network interface on
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the local machine. Additionally, a version 2 UUID replaces the low part of the
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time field with a local identifier such as the user ID or group ID of the local
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account that created the UUID. This serves three purposes:
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1. You can know *when* the identifier was created (see :ref:`rfc4122.version2.caveats`).
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2. You can know *where* the identifier was created.
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3. You can know *who* created the identifier.
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In a distributed system, these three pieces of information can be valuable. Not
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only is there no need for a central authority to generate identifiers, but you
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can determine what nodes in your infrastructure created the UUIDs, at what time
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they were created, and the account on the machine that created them.
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.. code-block::
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:caption: Use a domain to generate a version 2, DCE Security UUID
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use Ramsey\Uuid\Uuid;
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$uuid = Uuid::uuid2(Uuid::DCE_DOMAIN_PERSON);
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printf(
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"UUID: %s\nVersion: %d\nDate: %s\nNode: %s\nDomain: %s\nID: %s\n",
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$uuid->toString(),
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$uuid->getFields()->getVersion(),
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$uuid->getDateTime()->format('r'),
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$uuid->getFields()->getNode()->toString(),
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$uuid->getLocalDomainName(),
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$uuid->getLocalIdentifier()->toString()
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);
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This will generate a version 2 UUID and print out its string representation, the
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time the UUID was created, and the node used to create it, as well as the name
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of the local domain specified and the local domain identifier (in this case, a
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POSIX UID, automatically obtained from the local machine).
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It will look something like this:
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.. code-block:: text
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UUID: 000001f5-5e9a-21ea-9e00-0242ac130003
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Version: 2
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Date: Thu, 05 Mar 2020 04:30:10 +0000
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Node: 0242ac130003
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Domain: person
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ID: 501
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.. todo::
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Needs discussion on domains (list the domains), ability to specify the node
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and clock sequence (though the lower 8 bits of the clock sequence, originally
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a 14-bit integer, are replaced with the domain). In theory, 2^8-1 domains
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could be defined, but only 3 are registered by the DCE specification.
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Discuss ability to pass the local identifier.
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.. epigraph::
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Note that the [domain] can potentially hold values outside the range
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[0, 2\ :sup:`8` -- 1]; however, the only values currently registered are in
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the range [0, 2]… [DCE11SEC]_
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.. _rfc4122.version2.caveats:
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Problems With Version 2 UUIDs
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#############################
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Version 2 UUIDs can be useful for the data they contain. However, there are
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trade-offs in choosing to use them.
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Lossy Timestamps
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----------------
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Version 2 UUIDs are first generated in the same way as version 1 UUIDs, but then
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the low part of the timestamp (the ``time_low`` field) is replaced by a 32-bit
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integer that represents a local identifier, which refers to
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It is important to note that a version 2 UUID suffers from some loss of
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fidelity of the timestamp, due to replacing the time_low field with the
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local identifier. When constructing the timestamp value for date
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purposes, we replace the local identifier bits with zeros. As a result,
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the timestamp can be off by a range of 0 to 429.4967295 seconds (or 7
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minutes, 9 seconds, and 496730 microseconds).
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Astute observers might note this value directly corresponds to
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2\ :sup:`32` -- 1, or ``0xffffffff``. The local identifier is 32-bits, and we
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have set each of these bits to 0, so the maximum range of timestamp drift is
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``0x00000000`` to ``0xffffffff`` (counted in 100-nanosecond intervals).
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Limited Unique UUIDs
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--------------------
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With the inclusion of the local identifier comes a serious limitation in the
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amount of unique UUIDs that may be created.
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