Network Working Group P. Leach
Internet-Draft Microsoft
Expires: July 1, 2004 M. Mealling
VeriSign, Inc.
R. Salz
DataPower Technology, Inc.
January 2004
A UUID URN Namespace
draft-mealling-uuid-urn-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
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 provide a
guarantee of uniqueness across space and time. UUIDs were originally
used in the Network Computing System (NCS) [1] and later in the Open
Software Foundation's (OSF) Distributed Computing Environment [2].
This specification is derived from the latter specification with the
kind permission of the OSF (now known as The Open Group). Earlier
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versions of this document never left draft stage; this document
incorporates that information here.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Namespace Registration Template . . . . . . . . . . . . . . 3
4. Specification . . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.1 Variant . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1.2 Layout and byte order . . . . . . . . . . . . . . . . . . . 6
4.1.3 Version . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1.4 Timestamp . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.5 Clock sequence . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.6 Node . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1.7 Nil UUID . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Algorithms for creating a time-based UUID . . . . . . . . . 10
4.2.1 Basic algorithm . . . . . . . . . . . . . . . . . . . . . . 10
4.2.2 Generation details . . . . . . . . . . . . . . . . . . . . . 12
4.3 Algorithm for creating a name-based UUID . . . . . . . . . . 12
4.4 Algorithms for creating a UUID from truly random or
pseudo-random numbers . . . . . . . . . . . . . . . . . . . 13
4.5 Node IDs that do not identify the host . . . . . . . . . . . 14
5. Community Considerations . . . . . . . . . . . . . . . . . . 15
6. Security Considerations . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
Normative References . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
A. Appendix A - Sample Implementation . . . . . . . . . . . . . 16
B. Appendix B - Sample output of utest . . . . . . . . . . . . 27
C. Appendix C - Some name space IDs . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . 29
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1. 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 mean that it overrides the DCE standards that
defined UUIDs to begin with.
2. Motivation
One of the main reasons for using UUIDs is that no centralized
authority is required to administer them (although one format uses
IEEE 802.1 node identifiers, others do not). As a result, generation
on demand can be completely automated, and they can be used for a
wide variety of purposes. The UUID generation algorithm described
here supports very high allocation rates: 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
relative 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.
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.
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The internal representation of a UUID is a specific sequence of
bits in memory, as described in Section 4. In order 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 [6]:
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:
[2]
Identifier uniqueness considerations:
This document specifies three algorithms to generate UUIDs: the
first leverages the unique values of 802.1 MAC addresses to
guarantee uniqueness, the second another uses pseudo-random number
generators, and the third uses cryptographic hashing and
application-provided text strings. As a result, it is possible to
guarantee that 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
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persistent as possible.
Process of identifier assignment:
Generating a UUID does not require that it be 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, on many systems equality comparison can
be performed 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 an 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 if the timestamp portion of the UUID is in
the future and therefore not yet assignable, there is no mechanism
for determining if a UUID is 'valid' in any real sense.
Scope:
UUIDs are global in scope.
4. Specification
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4.1 Format
In its most general form, all that can be said of the UUID format is
that a UUID is 16 octets, and that some bits of the eight octet --
the variant field specified below -- determine finer structure.
4.1.1 Variant
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 the eighth octet of the UUID.
The following table lists the contents of the variant field, where
the letter "x" indicates a "don't-care" value.
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. This is unlikely to be an issue in
practice.
4.1.2 Layout and byte order
To minimize confusion about bit assignments within octets, 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.
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Field Data Type Octet Note
#
time_low unsigned 32 0-3 The low field of the
bit integer timestamp
time_mid unsigned 16 4-5 The middle field of the
bit integer timestamp
time_hi_and_version unsigned 16 6-7 The high field of the
bit integer timestamp multiplexed
with the version number
clock_seq_hi_and_rese unsigned 8 8 The high field of the
rved bit integer clock sequence
multiplexed with the
variant
clock_seq_low unsigned 8 9 The low field of the
bit integer clock sequence
node unsigned 48 10-15 The spatially unique
bit integer node identifier
In the absence of explicit application or presentation protocol
specification to the contrary, a UUID is encoded as a 128-bit object,
as follows: the fields are encoded as 16 octets, with the sizes and
order of the fields defined above, and with each field encoded with
the Most Significant Byte first (this is known as network byte
order). Note that the field names, particularly for multiplexed
fields, follow historical practice.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.3 Version
The version number is in the most significant four bits of the time
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stamp (bits four through seven of the time_hi_and_version field).
The following table lists the currently-defined versions for this
UUID variant.
Msb0 Msb1 Msb2 Msb3 Version Description
0 0 0 1 1 The time-based version
specified in this document.
0 0 1 0 2 DCE Security version, with
embedded POSIX UIDs.
0 0 1 1 3 The name-based version
specified in this document.
0 1 0 0 4 The randomly or pseudo-
randomly generated version
specified in this document.
The version is more accurately a sub-type; again, we retain the term
for compatibility.
4.1.4 Timestamp
The timestamp is a 60-bit value. For UUID version 1, this is
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).
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. This is not recommended however,
particularly since all that is needed to generate UTC from local time
is a time zone offset.
For UUID version 3, the timestamp is a 60-bit value constructed from
a name as described in Section 4.3.
For UUID version 4, it is a randomly or pseudo-randomly generated
60-bit value, as described in Section 4.4.
4.1.5 Clock sequence
For UUID version 1, the clock sequence is used to help avoid
duplicates that could arise when the clock is set backwards in time
or if the node ID changes.
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If the clock is set backwards, or even 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 be just 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 UUID version 3, it is a 14-bit value constructed from a name as
described in Section 4.3.
For UUID version 4, it is a randomly or pseudo-randomly generated
14-bit value as described in Section 4.4.
4.1.6 Node
For UUID version 1, 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.
For systems with no IEEE address, a randomly or pseudo-randomly
generated value may be used; see Section 4.5. The multicast bit must
be set in such addresses, in order that they will never conflict with
addresses obtained from network cards.
For UUID version 3, the node field is a 48-bit value constructed from
a name as described in Section 4.3.
For UUID version 4, the node field is a randomly or pseudo-randomly
generated 48-bit value as described in Section 4.4.
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4.1.7 Nil UUID
The nil UUID is special form of UUID that is specified to have all
128 bits set to zero.
4.2 Algorithms for creating a time-based UUID
Various aspects of the algorithm for creating a version 1 UUID are
discussed in the following sections.
4.2.1 Basic algorithm
The following algorithm is simple, correct, and inefficient:
o Obtain a system-wide global lock
o From a system-wide shared stable store (e.g., a file), read the
UUID generator state: the values of the time stamp, clock
sequence, and node ID used to generate the last UUID.
o Get the current time as a 60-bit count of 100-nanosecond intervals
since 00:00:00.00, 15 October 1582
o Get the current node ID
o If the state was unavailable (e.g., non-existent or corrupted), or
the saved node ID is different than the current node ID, generate
a random clock sequence value
o If the state was available, but the saved time stamp is later than
the current time stamp, increment the clock sequence value
o Save the state (current time stamp, clock sequence, and node ID)
back to the stable store
o Release the global lock
o Format a UUID from the current time stamp, clock sequence, and
node ID values according to the steps in Section 4.2.2.
If UUIDs do not need to be frequently generated, the above algorithm
may be perfectly adequate. For higher performance requirements,
however, issues with the basic algorithm include:
o Reading the state from stable storage each time is inefficient
o The resolution of the system clock may not be 100-nanoseconds
o Writing the state to stable storage each time is inefficient
o Sharing the state across process boundaries may be inefficient
Each of these issues can be addressed in a modular fashion by local
improvements in the functions that read and write the state and read
the clock. We address each of them in turn in the following sections.
4.2.1.1 Reading stable storage
The 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).
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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 new clock sequence numbers, which increases the
probability of duplicates.
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.
4.2.1.2 System clock resolution
The time stamp is generated from the system time, whose resolution
may be less than the resolution of the UUID time stamp.
If UUIDs do not need to be frequently generated, the time stamp can
simply be the system time multiplied by the number of 100-nanosecond
intervals per system time interval.
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.
A high resolution time stamp can be simulated by keeping a count of
how many UUIDs have been generated with the same value of the system
time, and using it to construction the low-order bits of the time
stamp. The count will range between zero and the number of
100-nanosecond intervals per system time interval.
Note: 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.
4.2.1.3 Writing stable storage
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 time stamps 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
time stamp 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.
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4.2.1.4 Sharing state across processes
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, and a
per-process generator can allocate from that block until it is
exhausted.
4.2.2 Generation details
Version 1 UUIDs are generated according to the following algorithm:
o Determine the values for the UTC-based timestamp and clock
sequence to be used in the UUID, as described in Section 4.2.1.
o For the purposes of this algorithm, consider the timestamp to be a
60-bit unsigned integer and the clock sequence to be a 14-bit
unsigned integer. Sequentially number the bits in a field,
starting with zero for the least significant bit.
o Set the time_low field equal to the least significant 32 bits
(bits zero through 31) of the time stamp in the same order of
significance.
o Set the time_mid field equal to bits 32 through 47 from the time
stamp in the same order of significance.
o Set the 12 least significant bits (bits zero through 11) of the
time_hi_and_version field equal to bits 48 through 59 from the
time stamp in the same order of significance.
o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the four-bit version number
corresponding to the UUID version being created, as shown in the
table above.
o Set the clock_seq_low field to the eight least significant bits
(bits zero through seven) of the clock sequence in the same order
of significance.
o Set the six least significant bits (bits zero through five) of the
clock_seq_hi_and_reserved field to the six most significant bits
(bits eight through 13) of the clock sequence in the same order of
significance.
o Set the two most significant bits (bits six and seven) of the
clock_seq_hi_and_reserved to zero and one, respectively.
o Set the node field to the 48-bit IEEE address in the same order of
significance as the address.
4.3 Algorithm for creating a name-based UUID
The version 3 UUID is meant for generating UUIDs from "names" that
are drawn from, and unique within, some "name space." 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, ISO Object IDs (OIDs), X.500 Distinguished Names (DNs),
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and reserved words in a programming language. The mechanisms or
conventions for allocating names from, and ensuring their uniqueness
within, their name spaces are beyond the scope of this specification.
The requirements for version 3 UUIDs are as follows:
o The UUIDs generated at different times from the same name in the
same namespace MUST be equal
o The UUIDs generated from two different names in the same namespace
should be different (with very high probability)
o The UUIDs generated from the same name in two different namespaces
should be different with (very high probability)
o 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).
The algorithm for generating the a UUID from a name and a name space
are as follows:
o Allocate a UUID to use as a "name space ID" for all UUIDs
generated from names in that name space; see Appendix C for some
pre-defined values
o Convert the name to a canonical sequence of octets (as defined by
the standards or conventions of its name space); put the name
space ID in network byte order
o Compute the MD5 [3] hash of the name space ID concatenated with
the name
o Set octets zero through three of the time_low field to octets zero
through three of the MD5 hash
o Set octets zero and one of the time_mid field to octets four and
five of the MD5 hash
o Set octets zero and one of the time_hi_and_version field to octets
six and seven of the MD5 hash
o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the four-bit version number from
Section 4.1.3.
o Set the clock_seq_hi_and_reserved field to octet eight of the MD5
hash
o Set the two most significant bits (bits six and seven) of the
clock_seq_hi_and_reserved to zero and one, respectively.
o Set the clock_seq_low field to octet nine of the MD5 hash
o Set octets zero through five of the node field to octets then
through fifteen of the MD5 hash
o Convert the resulting UUID to local byte order.
4.4 Algorithms for creating a UUID from truly random or pseudo-random
numbers
The version 4 UUID is meant for generating UUIDs from truly-random or
pseudo-random numbers.
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The algorithm is as follows:
o Set the two most significant bits (bits six and seven) of the
clock_seq_hi_and_reserved to zero and one, respectively.
o Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the four-bit version number from
Section 4.1.3.
o Set all the other bits to randomly (or pseudo-randomly) chosen
values.
See Section 4.5 for a discussion on random numbers.
4.5 Node IDs that do not identify the host
This section describes how to generate a version 1 UUID 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
[8], and the cost was US$550.
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.)
If a system does not have the capability to generate cryptographic
quality random numbers, then implementation advice can be found in
RFC1750 [4].
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, and use a
message digest such as MD5 [3] or SHA-1 [7], take an arbitrary six
bytes from the hash value, and set the multicast bit as described
above.
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5. 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.
6. 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.
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.
7. Acknowledgments
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
Ralph S. Engelschall, John Larmouth, and Paul Thorpe.
Normative References
[1] Zahn, L., Dineen, T. and P. Leach, "Network Computing
Architecture", ISBN 0-13-611674-4, January 1990.
[2] "DCE: Remote Procedure Call", Open Group CAE Specification C309,
ISBN 1-85912-041-5, August 1994.
[3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[4] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[5] Moats, R., "URN Syntax", RFC 2141, May 1997.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
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Specifications: ABNF", RFC 2234, November 1997.
[7] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995, <http://www.itl.nist.gov/
fipspubs/fip180-1.htm>.
[8] <http://standards.ieee.org/regauth/oui/pilot-ind.html>
Authors' Addresses
Paul J. Leach
Microsoft
1 Microsoft Way
Redmond, WA 98052
US
Phone: +1 425-882-8080
EMail: paulle@microsoft.com
Michael Mealling
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 21345
US
Phone: +1 770-717-0732
EMail: michael@neonym.net
URI: http://www.verisignlabs.com
Rich Salz
DataPower Technology, Inc.
1 Alewife Center
Cambridge, MA 02142
US
Phone: +1 617-864-0455
EMail: rsalz@datapower.com
URI: http://www.datapower.com
Appendix A. Appendix A - Sample Implementation
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
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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 a lot clearer.
All the following source files should be considered to have the
following copyright notice included:
copyrt.h
/*
** Copyright (c) 1990- 1993, 1996 Open Software Foundation, Inc.
** Copyright (c) 1989 by Hewlett-Packard Company, Palo Alto, Ca. &
** Digital Equipment Corporation, Maynard, Mass.
** Copyright (c) 1998 Microsoft.
** To anyone who acknowledges that this file is provided "AS IS"
** without any express or implied warranty: permission to use, copy,
** modify, and distribute this file for any purpose is hereby
** granted without fee, provided that the above copyright notices and
** this notice appears in all source code copies, and that none of
** the names of Open Software Foundation, Inc., Hewlett-Packard
** Company, or Digital Equipment Corporation be used in advertising
** or publicity pertaining to distribution of the software without
** specific, written prior permission. Neither Open Software
** Foundation, Inc., Hewlett-Packard Company, Microsoft, nor Digital
** Equipment Corporation makes any representations about the suitability
** of this software for any purpose.
*/
uuid.h
#include "copyrt.h"
#undef uuid_t
typedef struct {
unsigned32 time_low;
unsigned16 time_mid;
unsigned16 time_hi_and_version;
unsigned8 clock_seq_hi_and_reserved;
unsigned8 clock_seq_low;
byte node[6];
} uuid_t;
/* uuid_create -- generate a UUID */
int uuid_create(uuid_t * uuid);
/* uuid_create_from_name -- create a UUID using a "name"
from a "name space" */
void uuid_create_from_name(
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uuid_t *uuid, /* resulting UUID */
uuid_t nsid, /* UUID of the namespace */
void *name, /* the name from which to generate a UUID */
int namelen /* the length of the name */
);
/* uuid_compare -- Compare two UUID's "lexically" and return
-1 u1 is lexically before u2
0 u1 is equal to u2
1 u1 is lexically after u2
Note that lexical ordering is not temporal ordering!
*/
int uuid_compare(uuid_t *u1, uuid_t *u2);
uuid.c
#include "copyrt.h"
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#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_v3(uuid_t *uuid, unsigned char hash[16]);
static void get_current_time(uuid_time_t *timestamp);
static unsigned16 true_random(void);
/* uuid_create -- generator 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;
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/* 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;
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/* 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);
}
}
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/* 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;
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}
return rand();
}
/* uuid_create_from_name -- create a UUID using a "name" from a "name
space" */
void uuid_create_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;
htonl(net_nsid.time_low);
htons(net_nsid.time_mid);
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_v3(uuid, hash);
}
/* format_uuid_v3 -- make a UUID from a (pseudo)random 128-bit number */
void format_uuid_v3(uuid_t *uuid, unsigned char hash[16])
{
/* convert UUID to local byte order */
memcpy(uuid, hash, sizeof *uuid);
ntohl(uuid->time_low);
ntohs(uuid->time_mid);
ntohs(uuid->time_hi_and_version);
/* put in the variant and version bits */
uuid->time_hi_and_version &= 0x0FFF;
uuid->time_hi_and_version |= (3 << 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;
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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
#include "copyrt.h"
/* remove the following define if you aren't running WIN32 */
#define WININC 0
#ifdef WININC
#include <windows.h>
#else
#include <sys/types.h>
#include <sys/time.h>
#include <sys/sysinfo.h>
#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;
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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 "copyrt.h"
#include <stdio.h>
#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);
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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 1750 */
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);
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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 1750 */
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
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#include "copyrt.h"
#include "sysdep.h"
#include <stdio.h>
#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_from_name(&u, NameSpace_DNS, "www.widgets.com", 15);
printf("uuid_create_from_name(): "); puid(u);
}
Appendix B. Appendix B - Sample output of utest
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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_from_name(): e902893a-9d22-3c7e-a7b8-d6e313b71d9f
Appendix C. Appendix C - 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.
/* Name string is a fully-qualified domain name */
uuid_t NameSpace_DNS = { /* 6ba7b810-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b810,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
/* Name string is a URL */
uuid_t NameSpace_URL = { /* 6ba7b811-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b811,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
/* Name string is an ISO OID */
uuid_t NameSpace_OID = { /* 6ba7b812-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b812,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
/* Name string is an X.500 DN (in DER or a text output format) */
uuid_t NameSpace_X500 = { /* 6ba7b814-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b814,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
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Intellectual Property Statement
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The IETF invites any interested party to bring to its attention any
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Full Copyright Statement
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This document and translations of it may be copied and furnished to
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Leach, et al. Expires July 1, 2004 [Page 29]
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
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Leach, et al. Expires July 1, 2004 [Page 30]
<?xml version="1.0"?>
<!-- vim: set et sw=2 wm=5 fdm=syntax nofen: -->
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<!-- change this to no to create a document with lots of whitespace -->
<?rfc compact="yes"?>
<rfc ipr="full2026" docName="draft-mealling-uuid-urn-03.txt">
<front>
<title abbrev="UUID URN">A UUID URN Namespace</title>
<author initials="P." surname="Leach" fullname="Paul J. Leach">
<organization>Microsoft</organization>
<address>
<postal>
<street>1 Microsoft Way</street>
<city>Redmond</city> <region>WA</region> <code>98052</code>
<country>US</country>
</postal>
<phone>+1 425-882-8080</phone>
<email>paulle@microsoft.com</email>
</address>
</author>
<author initials="M." surname="Mealling" fullname="Michael Mealling">
<organization>VeriSign, Inc.</organization>
<address>
<postal>
<street>21345 Ridgetop Circle</street>
<city>Dulles</city> <region>VA</region> <code>21345</code>
<country>US</country>
</postal>
<phone>+1 770-717-0732</phone>
<email>michael@neonym.net</email>
<uri>http://www.verisignlabs.com</uri>
</address>
</author>
<author initials="R." surname="Salz" fullname="Rich Salz">
<organization>DataPower Technology, Inc.</organization>
<address>
<postal>
<street>1 Alewife Center</street>
<city>Cambridge</city> <region>MA</region> <code>02142</code>
<country>US</country>
</postal>
<phone>+1 617-864-0455</phone>
<email>rsalz@datapower.com</email>
<uri>http://www.datapower.com</uri>
</address>
</author>
<date month="January" year="2004" />
<keyword>URN, UUID</keyword>
<abstract>
<t>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 provide a
guarantee of uniqueness across space and time. UUIDs were originally
used in the Network Computing System (NCS) [1] and later in the Open
Software Foundation's (OSF) Distributed Computing Environment
<xref target="DCE"/>.</t>
<t>This specification is derived from the latter specification with the
kind permission of the OSF (now known as The Open Group). Earlier
versions of this document never left draft stage; this document
incorporates that information here.</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>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.</t>
<t>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 mean that it overrides the DCE standards that
defined UUIDs to begin with.</t>
</section>
<section anchor="motivation" title="Motivation">
<t>One of the main reasons for using UUIDs is that no centralized
authority is required to administer them (although one format uses
IEEE 802.1 node identifiers, others do not). As a result, generation
on demand can be completely automated, and they can be used for a wide
variety of purposes. The UUID generation algorithm described here
supports very high allocation rates: 10 million per second per machine
if necessary, so that they could even be used as transaction IDs.</t>
<t>UUIDs are of a fixed size (128 bits) which is reasonably small
relative 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.</t>
<t>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.</t>
</section>
<section anchor="template" title="Namespace Registration Template">
<t>
<list style="hanging">
<t hangText="Namespace ID: ">UUID</t>
<t hangText="Registration Information:"><vspace blankLines="0"/>
Registration date: 2003-10-01</t>
<t hangText="Declared registrant of the namespace:"><vspace blankLines="0"/>
JTC 1/SC6 (ASN.1 Rapporteur Group)</t>
<t hangText="Declaration of syntactic structure:"><vspace blankLines="0"/>
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.
<vspace blankLines="1"/>
The internal representation of a UUID is a specific sequence of
bits in memory, as described in <xref target="specification"/>.
In order to accurately represent a UUID as a URN, it is necessary
to convert the bit sequence to a string representation.
<vspace blankLines="1"/>
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.
<vspace blankLines="1"/>
The formal definition of the UUID string representation is
provided by the following ABNF <xref
target="RFC2234"/>:<figure><artwork><![CDATA[
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"
]]></artwork></figure>
<vspace blankLines="1"/>
The following is an example of the string representation of a UUID
as a URN:<figure><artwork><![CDATA[
urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6
]]></artwork></figure>
</t>
<t hangText="Relevant ancillary documentation:"><vspace blankLines="0"/>
<xref target="DCE" />
</t>
<t hangText="Identifier uniqueness considerations:"><vspace blankLines="0"/>
This document specifies three algorithms to generate UUIDs: the
first leverages the unique values of 802.1 MAC addresses to
guarantee uniqueness, the second another uses pseudo-random number
generators, and the third uses cryptographic hashing and
application-provided text strings. As a result, it is possible to
guarantee that UUIDs generated according to the mechanisms here
will be unique from all other UUIDs that have been or will be
assigned.</t>
<t hangText="Identifier persistence considerations:"><vspace blankLines="0"/>
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.</t>
<t hangText="Process of identifier assignment:"><vspace blankLines="0"/>
Generating a UUID does not require that it be 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,
<xref target="node-id-no-id"/> specifies two
alternatives; another approach is to use version 3 or version 4
UUIDs as defined below.</t>
<t hangText="Process for identifier resolution:"><vspace blankLines="0"/>
Since UUIDs are not globally resolvable, this is not
applicable.</t>
<t hangText="Rules for Lexical Equivalence:"><vspace blankLines="0"/>
Consider each field of the UUID to be an unsigned integer as
shown in the table in section <xref target="uuid-layout"/>. 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.
<vspace blankLines="1"/>
As an implementation note, on many systems equality comparison can
be performed by doing the appropriate byte-order
canonicalization, and then treating the two UUIDs as 128-bit
unsigned integers.
<vspace blankLines="1"/>
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.</t>
<t hangText="Conformance with URN Syntax:"><vspace blankLines="0"/>
The string representation of a UUID is fully compatible with the
URN syntax. When converting from an 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.</t>
<t hangText="Validation mechanism:"><vspace blankLines="0"/>
Apart from determining if the timestamp portion of the UUID is in
the future and therefore not yet assignable, there is no mechanism
for determining if a UUID is 'valid' in any real sense.</t>
<t hangText="Scope:"><vspace blankLines="0"/>
UUIDs are global in scope.</t>
</list>
</t>
</section>
<section anchor="specification" title="Specification">
<section anchor="format" title="Format">
<t>In its most general form, all that can be said of the UUID format is
that a UUID is 16 octets, and that some bits of the eight octet --
the variant field specified below -- determine finer
structure.</t>
<section anchor="variant" title="Variant">
<t>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 the eighth octet of the UUID.</t>
<t>The following table lists the contents of the variant field,
where the letter "x" indicates a "don't-care" value.
<figure><artwork><![CDATA[
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.
]]></artwork></figure>
</t>
<t>Interoperability (in any form) with variants other than the one
defined here is not guaranteed. This is unlikely to be an issue
in practice.</t>
</section>
<section anchor="uuid-layout" title="Layout and byte order">
<t>To minimize confusion about bit assignments within octets, 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.<figure><artwork><![CDATA[
Field Data Type Octet Note
#
time_low unsigned 32 0-3 The low field of the
bit integer timestamp
time_mid unsigned 16 4-5 The middle field of the
bit integer timestamp
time_hi_and_version unsigned 16 6-7 The high field of the
bit integer timestamp multiplexed
with the version number
clock_seq_hi_and_rese unsigned 8 8 The high field of the
rved bit integer clock sequence
multiplexed with the
variant
clock_seq_low unsigned 8 9 The low field of the
bit integer clock sequence
node unsigned 48 10-15 The spatially unique
bit integer node identifier
]]></artwork></figure>
</t>
<t>In the absence of explicit application or presentation protocol
specification to the contrary, a UUID is encoded as a 128-bit
object, as follows: the fields are encoded as 16 octets, with the
sizes and order of the fields defined above, and with each field
encoded with the Most Significant Byte first (this is known as
network byte order). Note that the field names, particularly
for multiplexed fields, follow historical
practice.<figure><artwork><![CDATA[
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork></figure>
</t>
</section>
<section anchor="version" title="Version">
<t>The version number is in the most significant four bits of the
time stamp (bits four through seven of the time_hi_and_version
field).</t>
<t>The following table lists the currently-defined versions for
this UUID variant.<figure><artwork><![CDATA[
Msb0 Msb1 Msb2 Msb3 Version Description
0 0 0 1 1 The time-based version
specified in this document.
0 0 1 0 2 DCE Security version, with
embedded POSIX UIDs.
0 0 1 1 3 The name-based version
specified in this document.
0 1 0 0 4 The randomly or pseudo-
randomly generated version
specified in this document.
]]></artwork></figure>
The version is more accurately a sub-type; again, we retain the
term for compatibility.</t>
</section>
<section anchor="timestamp" title="Timestamp">
<t>The timestamp is a 60-bit value. For UUID version 1, this is
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).</t>
<t>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. This is not recommended
however, particularly since all that is needed to generate UTC
from local time is a time zone offset.</t>
<t>For UUID version 3, the timestamp is a 60-bit value constructed
from a name as described in <xref target="alg-name"/>.</t>
<t>For UUID version 4, it is a randomly or pseudo-randomly
generated 60-bit value, as described in
<xref target="alg-rand"/>.</t>
</section>
<section anchor="clock-seq" title="Clock sequence">
<t>For UUID version 1, the clock sequence is used to help avoid
duplicates that could arise when the clock is set backwards in
time or if the node ID changes.</t>
<t>If the clock is set backwards, or even 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 be just incremented;
otherwise it should be set to a random or high-quality pseudo
random value.</t>
<t>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.)</t>
<t>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.</t>
<t>For UUID version 3, it is a 14-bit value constructed from a
name as described in <xref target="alg-name"/>.</t>
<t>For UUID version 4, it is a randomly or pseudo-randomly
generated 14-bit value as described in
<xref target="alg-rand"/>.</t>
</section>
<section anchor="node" title="Node">
<t>For UUID version 1, 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.</t>
<t>For systems with no IEEE address, a randomly or pseudo-randomly
generated value may be used; see <xref target="node-id-no-id"/>.
The multicast bit must be set in such addresses, in order that
they will never conflict with addresses obtained from network
cards.</t>
<t>For UUID version 3, the node field is a 48-bit value constructed
from a name as described in <xref target="alg-name"/>.</t>
<t>For UUID version 4, the node field is a randomly or
pseudo-randomly generated 48-bit value as described in
<xref target="alg-rand"/>.</t>
</section>
<section anchor="nil-uuid" title="Nil UUID">
<t>The nil UUID is special form of UUID that is specified to have
all 128 bits set to zero.</t>
</section>
</section>
<section anchor="alg-time" title="Algorithms for creating a time-based UUID">
<t>Various aspects of the algorithm for creating a version 1 UUID are
discussed in the following sections.</t>
<section anchor="alg-time-basic" title="Basic algorithm">
<t>The following algorithm is simple, correct, and inefficient:
<list style="symbols">
<t>Obtain a system-wide global lock</t>
<t>From a system-wide shared stable store (e.g., a file), read
the UUID generator state: the values of the time stamp, clock
sequence, and node ID used to generate the last UUID.</t>
<t>Get the current time as a 60-bit count of 100-nanosecond
intervals since 00:00:00.00, 15 October 1582</t>
<t>Get the current node ID</t>
<t>If the state was unavailable (e.g., non-existent or
corrupted), or the saved node ID is different than the current
node ID, generate a random clock sequence value</t>
<t>If the state was available, but the saved time stamp is later
than the current time stamp, increment the clock sequence
value</t>
<t>Save the state (current time stamp, clock sequence, and node
ID) back to the stable store</t>
<t>Release the global lock</t>
<t>Format a UUID from the current time stamp, clock sequence,
and node ID values according to the steps in
<xref target="alg-time-details"/>.</t>
</list>
</t>
<t>If UUIDs do not need to be frequently generated, the above
algorithm may be perfectly adequate. For higher performance
requirements, however, issues with the basic algorithm include:
<list style="symbols">
<t>Reading the state from stable storage each time is
inefficient</t>
<t>The resolution of the system clock may not be
100-nanoseconds</t>
<t>Writing the state to stable storage each time is
inefficient</t>
<t>Sharing the state across process boundaries may be
inefficient</t>
</list>
</t>
<t>Each of these issues can be addressed in a modular fashion by
local improvements in the functions that read and write the state
and read the clock. We address each of them in turn in the
following sections.</t>
<section anchor="read-stable" title="Reading stable storage">
<t>The 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).</t>
<t>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 new clock sequence numbers, which
increases the probability of duplicates.</t>
<t>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.</t>
</section>
<section anchor="clock-resolution" title="System clock resolution">
<t>The time stamp is generated from the system time, whose resolution
may be less than the resolution of the UUID time stamp.</t>
<t>If UUIDs do not need to be frequently generated, the time stamp
can simply be the system time multiplied by the number of
100-nanosecond intervals per system time interval.</t>
<t>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.</t>
<t>A high resolution time stamp can be simulated by keeping a count
of how many UUIDs have been generated with the same value of the
system time, and using it to construction the low-order bits of the
time stamp. The count will range between zero and the number of
100-nanosecond intervals per system time interval.</t>
<t>Note: 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.</t>
</section>
<section anchor="write-stable" title="Writing stable storage">
<t>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 time stamps 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 time stamp 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.</t>
</section>
<section anchor="share-state" title="Sharing state across processes">
<t>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, and a
per-process generator can allocate from that block until it is
exhausted.</t>
</section>
</section>
<section anchor="alg-time-details" title="Generation details">
<t>Version 1 UUIDs are generated according to the following
algorithm:
<list style="symbols">
<t>Determine the values for the UTC-based timestamp and clock
sequence to be used in the UUID, as described in
<xref target="alg-time-basic"/>.</t>
<t>For the purposes of this algorithm, consider the timestamp
to be a 60-bit unsigned integer and the clock sequence to be
a 14-bit unsigned integer. Sequentially number the bits in a
field, starting with zero for the least significant bit.</t>
<t>Set the time_low field equal to the least significant 32
bits (bits zero through 31) of the time stamp in the same
order of significance.</t>
<t>Set the time_mid field equal to bits 32 through 47 from the
time stamp in the same order of significance.</t>
<t>Set the 12 least significant bits (bits zero through 11) of
the time_hi_and_version field equal to bits 48 through 59
from the time stamp in the same order of significance.</t>
<t>Set the four most significant bits (bits 12 through 15) of
the time_hi_and_version field to the four-bit version number
corresponding to the UUID version being created, as shown in
the table above.</t>
<t>Set the clock_seq_low field to the eight least significant
bits (bits zero through seven) of the clock sequence in the
same order of significance.</t>
<t>Set the six least significant bits (bits zero through five)
of the clock_seq_hi_and_reserved field to the six most
significant bits (bits eight through 13) of the clock
sequence in the same order of significance.</t>
<t>Set the two most significant bits (bits six and seven) of
the clock_seq_hi_and_reserved to zero and one,
respectively.</t>
<t>Set the node field to the 48-bit IEEE address in the same
order of significance as the address.</t>
</list>
</t>
</section>
</section>
<section anchor="alg-name" title="Algorithm for creating a name-based UUID">
<t>The version 3 UUID is meant for generating UUIDs from "names" that
are drawn from, and unique within, some "name space." 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, ISO Object IDs (OIDs), X.500 Distinguished Names
(DNs), and reserved words in a programming language. The mechanisms
or conventions for allocating names from, and ensuring their
uniqueness within, their name spaces are beyond the scope of this
specification.</t>
<t>The requirements for version 3 UUIDs are as follows:
<list style="symbols">
<t>The UUIDs generated at different times from the same name in
the same namespace MUST be equal</t>
<t>The UUIDs generated from two different names in the same
namespace should be different (with very high probability)</t>
<t>The UUIDs generated from the same name in two different
namespaces should be different with (very high probability)</t>
<t>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).</t>
</list>
</t>
<t>The algorithm for generating the a UUID from a name and a name
space are as follows:
<list style="symbols">
<t>Allocate a UUID to use as a "name space ID" for all UUIDs
generated from names in that name space; see
<xref target="appendixC"/> for some pre-defined values</t>
<t>Convert the name to a canonical sequence of octets (as defined
by the standards or conventions of its name space); put the name
space ID in network byte order</t>
<t>Compute the <xref target="RFC1321">MD5</xref> hash of the name
space ID concatenated with the name</t>
<t>Set octets zero through three of the time_low field to octets
zero through three of the MD5 hash</t>
<t>Set octets zero and one of the time_mid field to octets four
and five of the MD5 hash</t>
<t>Set octets zero and one of the time_hi_and_version field to
octets six and seven of the MD5 hash</t>
<t>Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the four-bit version number from
<xref target="version"/>.</t>
<t>Set the clock_seq_hi_and_reserved field to octet eight of the MD5
hash</t>
<t>Set the two most significant bits (bits six and seven) of the
clock_seq_hi_and_reserved to zero and one, respectively.</t>
<t>Set the clock_seq_low field to octet nine of the MD5 hash</t>
<t>Set octets zero through five of the node field to octets then
through fifteen of the MD5 hash</t>
<t>Convert the resulting UUID to local byte order.</t>
</list>
</t>
</section>
<section anchor="alg-rand" title="Algorithms for creating a UUID from truly random or pseudo-random numbers">
<t>The version 4 UUID is meant for generating UUIDs from truly-random or
pseudo-random numbers.</t>
<t>The algorithm is as follows:
<list style="symbols">
<t>Set the two most significant bits (bits six and seven) of the
clock_seq_hi_and_reserved to zero and one, respectively.</t>
<t>Set the four most significant bits (bits 12 through 15) of the
time_hi_and_version field to the four-bit version number from
<xref target="version"/>.</t>
<t>Set all the other bits to randomly (or pseudo-randomly) chosen
values.</t>
</list>
</t>
<t>See <xref target="node-id-no-id"/> for a discussion
on random numbers.</t>
</section>
<section anchor="node-id-no-id" title="Node IDs that do not identify the host">
<t>This section describes how to generate a version 1 UUID if an IEEE
802 address is not available, or its use is not desired.</t>
<t>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
<eref target="http://standards.ieee.org/regauth/oui/pilot-ind.html"/>,
and the cost was US$550.</t>
<t>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.)</t>
<t>If a system does not have the capability to generate cryptographic
quality random numbers, then implementation advice can be found in
<xref target="RFC1750">RFC1750</xref>.
</t>
<t>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.</t>
<t>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, and use
a message digest such as <xref target="RFC1321">MD5</xref> or
<xref target="FIPS.180-1.1995">SHA-1</xref>, take an arbitrary six
bytes from the hash value, and set the multicast bit as described
above.</t>
</section>
</section>
<section anchor="community-considerations" title="Community Considerations">
<t>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.</t>
</section>
<section anchor="security-considerations" title="Security Considerations">
<t>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.</t>
<t>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.</t>
</section>
<section anchor="acknowledgments" title="Acknowledgments">
<t>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).</t>
<t>We are also grateful to the careful reading and bit-twiddling of
Ralph S. Engelschall, John Larmouth, and Paul Thorpe.</t>
</section>
</middle>
<back>
<references title="Normative References">
<reference anchor="NCA">
<front>
<title abbrev="NCA">Network Computing Architecture</title>
<author initials="L." surname="Zahn" fullname="Lisa Zahn">
<organization/>
</author>
<author initials="T." surname="Dineen" fullname="Terence Dineen">
<organization />
</author>
<author initials="P." surname="Leach" fullname="Paul Leach">
<organization/>
</author>
<date month="January" year="1990"/>
</front>
<seriesInfo name="ISBN" value="0-13-611674-4"/>
</reference>
<reference anchor="DCE">
<front>
<title>DCE: Remote Procedure Call</title>
<author>
<organization/>
</author>
<date month="August" year="1994"/>
</front>
<seriesInfo name="Open Group CAE Specification" value="C309" />
<seriesInfo name="ISBN" value="1-85912-041-5"/>
</reference>
<?rfc include="reference.RFC.1321.xml" ?>
<?rfc include="reference.RFC.1750.xml" ?>
<?rfc include="reference.RFC.2141.xml" ?>
<?rfc include="reference.RFC.2234.xml" ?>
<?rfc include="reference.FIPS.180-1.1995.xml" ?>
</references>
<section anchor="appendixA" title="Appendix A - Sample Implementation">
<t>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 a lot clearer.</t>
<t>All the following source files should be considered to have the
following copyright notice included:<figure><artwork><![CDATA[
copyrt.h
/*
** Copyright (c) 1990- 1993, 1996 Open Software Foundation, Inc.
** Copyright (c) 1989 by Hewlett-Packard Company, Palo Alto, Ca. &
** Digital Equipment Corporation, Maynard, Mass.
** Copyright (c) 1998 Microsoft.
** To anyone who acknowledges that this file is provided "AS IS"
** without any express or implied warranty: permission to use, copy,
** modify, and distribute this file for any purpose is hereby
** granted without fee, provided that the above copyright notices and
** this notice appears in all source code copies, and that none of
** the names of Open Software Foundation, Inc., Hewlett-Packard
** Company, or Digital Equipment Corporation be used in advertising
** or publicity pertaining to distribution of the software without
** specific, written prior permission. Neither Open Software
** Foundation, Inc., Hewlett-Packard Company, Microsoft, nor Digital
** Equipment Corporation makes any representations about the suitability
** of this software for any purpose.
*/
uuid.h
#include "copyrt.h"
#undef uuid_t
typedef struct {
unsigned32 time_low;
unsigned16 time_mid;
unsigned16 time_hi_and_version;
unsigned8 clock_seq_hi_and_reserved;
unsigned8 clock_seq_low;
byte node[6];
} uuid_t;
/* uuid_create -- generate a UUID */
int uuid_create(uuid_t * uuid);
/* uuid_create_from_name -- create a UUID using a "name"
from a "name space" */
void uuid_create_from_name(
uuid_t *uuid, /* resulting UUID */
uuid_t nsid, /* UUID of the namespace */
void *name, /* the name from which to generate a UUID */
int namelen /* the length of the name */
);
/* uuid_compare -- Compare two UUID's "lexically" and return
-1 u1 is lexically before u2
0 u1 is equal to u2
1 u1 is lexically after u2
Note that lexical ordering is not temporal ordering!
*/
int uuid_compare(uuid_t *u1, uuid_t *u2);
uuid.c
#include "copyrt.h"
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#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_v3(uuid_t *uuid, unsigned char hash[16]);
static void get_current_time(uuid_time_t *timestamp);
static unsigned16 true_random(void);
/* uuid_create -- generator 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_from_name -- create a UUID using a "name" from a "name
space" */
void uuid_create_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;
htonl(net_nsid.time_low);
htons(net_nsid.time_mid);
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_v3(uuid, hash);
}
/* format_uuid_v3 -- make a UUID from a (pseudo)random 128-bit number */
void format_uuid_v3(uuid_t *uuid, unsigned char hash[16])
{
/* convert UUID to local byte order */
memcpy(uuid, hash, sizeof *uuid);
ntohl(uuid->time_low);
ntohs(uuid->time_mid);
ntohs(uuid->time_hi_and_version);
/* put in the variant and version bits */
uuid->time_hi_and_version &= 0x0FFF;
uuid->time_hi_and_version |= (3 << 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
#include "copyrt.h"
/* remove the following define if you aren't running WIN32 */
#define WININC 0
#ifdef WININC
#include <windows.h>
#else
#include <sys/types.h>
#include <sys/time.h>
#include <sys/sysinfo.h>
#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 "copyrt.h"
#include <stdio.h>
#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 1750 */
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 1750 */
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 "copyrt.h"
#include "sysdep.h"
#include <stdio.h>
#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_from_name(&u, NameSpace_DNS, "www.widgets.com", 15);
printf("uuid_create_from_name(): "); puid(u);
}
]]></artwork></figure>
</t>
</section>
<section anchor="appendixB" title="Appendix B - Sample output of utest">
<t><figure><artwork><![CDATA[
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_from_name(): e902893a-9d22-3c7e-a7b8-d6e313b71d9f
]]></artwork></figure>
</t>
</section>
<section anchor="appendixC" title="Appendix C - Some name space IDs">
<t>This appendix lists the name space IDs for some potentially
interesting name spaces, as initialized C structures and in the string
representation defined above.
<figure><artwork><![CDATA[
/* Name string is a fully-qualified domain name */
uuid_t NameSpace_DNS = { /* 6ba7b810-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b810,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
/* Name string is a URL */
uuid_t NameSpace_URL = { /* 6ba7b811-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b811,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
/* Name string is an ISO OID */
uuid_t NameSpace_OID = { /* 6ba7b812-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b812,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
/* Name string is an X.500 DN (in DER or a text output format) */
uuid_t NameSpace_X500 = { /* 6ba7b814-9dad-11d1-80b4-00c04fd430c8 */
0x6ba7b814,
0x9dad,
0x11d1,
0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};
]]></artwork></figure>
</t>
</section>
</back>
</rfc>
