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[tools/e2fsprogs.git] / doc / rfc4122.txt

Network Working Group                                           P. Leach
Request for Comments: 4122                                     Microsoft
Category: Standards Track                                    M. Mealling
                                                Refactored Networks, LLC
                                                                 R. Salz
                                              DataPower Technology, Inc.
                                                               July 2005


          A Universally Unique IDentifier (UUID) URN Namespace

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

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 guarantee
   uniqueness across space and time.  UUIDs were originally used in the
   Apollo Network Computing System and later in the Open Software
   Foundation's (OSF) Distributed Computing Environment (DCE), and then
   in Microsoft Windows platforms.

   This specification is derived from the DCE specification with the
   kind permission of the OSF (now known as The Open Group).
   Information from earlier versions of the DCE specification have been
   incorporated into this document.














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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3. Namespace Registration Template  . . . . . . . . . . . . . . .  3
   4. Specification  . . . . . . . . . . . . . . . . . . . . . . . .  5
      4.1. Format. . . . . . . . . . . . . . . . . . . . . . . . . .  5
           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 . . . . . . . . . . . . . . . . . . . . .  9
      4.2. Algorithms for Creating a Time-Based UUID . . . . . . . .  9
           4.2.1. Basic Algorithm. . . . . . . . . . . . . . . . . . 10
           4.2.2. Generation Details . . . . . . . . . . . . . . . . 12
      4.3. Algorithm for Creating a Name-Based UUID. . . . . . . . . 13
      4.4. Algorithms for Creating a UUID from Truly Random or
           Pseudo-Random Numbers . . . . . . . . . . . . . . . . . . 14
      4.5. Node IDs that Do Not Identify the Host. . . . . . . . . . 15
   5. Community Considerations . . . . . . . . . . . . . . . . . . . 15
   6. Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   8. Normative References . . . . . . . . . . . . . . . . . . . . . 16
   A. Appendix A - Sample Implementation . . . . . . . . . . . . . . 18
   B. Appendix B - Sample Output of utest  . . . . . . . . . . . . . 29
   C. Appendix C - Some Name Space IDs . . . . . . . . . . . . . . . 30

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 override the DCE standards that defined UUIDs.

   There is an ITU-T Recommendation and ISO/IEC Standard [3] that are
   derived from earlier versions of this document.  Both sets of
   specifications have been aligned, and are fully technically
   compatible.  In addition, a global registration function is being
   provided by the Telecommunications Standardisation Bureau of ITU-T;
   for details see <http://www.itu.int/ITU-T/asn1/uuid.html>.





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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 node identifiers, others do not).  As a result, generation
   on demand can be completely automated, and used for a variety of
   purposes.  The UUID generation algorithm described here supports very
   high allocation rates of up to 10 million per second per machine if
   necessary, so that they could even be used as transaction IDs.

   UUIDs are of a fixed size (128 bits) which is reasonably small
   compared to other alternatives.  This lends itself well to sorting,
   ordering, and hashing of all sorts, storing in databases, simple
   allocation, and ease of programming in general.

   Since UUIDs are unique and persistent, they make excellent Uniform
   Resource Names.  The unique ability to generate a new UUID without a
   registration process allows for UUIDs to be one of the URNs with the
   lowest minting cost.

3.  Namespace Registration Template

   Namespace ID:  UUID
   Registration Information:
      Registration date: 2003-10-01

   Declared registrant of the namespace:
      JTC 1/SC6 (ASN.1 Rapporteur Group)

   Declaration of syntactic structure:
      A UUID is an identifier that is unique across both space and time,
      with respect to the space of all UUIDs.  Since a UUID is a fixed
      size and contains a time field, it is possible for values to
      rollover (around A.D. 3400, depending on the specific algorithm
      used).  A UUID can be used for multiple purposes, from tagging
      objects with an extremely short lifetime, to reliably identifying
      very persistent objects across a network.

      The internal representation of a UUID is a specific sequence of
      bits in memory, as described in Section 4.  To accurately
      represent a UUID as a URN, it is necessary to convert the bit
      sequence to a string representation.

      Each field is treated as an integer and has its value printed as a
      zero-filled hexadecimal digit string with the most significant
      digit first.  The hexadecimal values "a" through "f" are output as
      lower case characters and are case insensitive on input.




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      The formal definition of the UUID string representation is
      provided by the following ABNF [7]:

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

   The following is an example of the string representation of a UUID as
   a URN:

   urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6

   Relevant ancillary documentation:
      [1][2]
   Identifier uniqueness considerations:
      This document specifies three algorithms to generate UUIDs: the
      first leverages the unique values of 802 MAC addresses to
      guarantee uniqueness, the second uses pseudo-random number
      generators, and the third uses cryptographic hashing and
      application-provided text strings.  As a result, the UUIDs
      generated according to the mechanisms here will be unique from all
      other UUIDs that have been or will be assigned.

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

   Process of identifier assignment:
      Generating a UUID does not require that a registration authority
      be contacted.  One algorithm requires a unique value over space
      for each generator.  This value is typically an IEEE 802 MAC
      address, usually already available on network-connected hosts.
      The address can be assigned from an address block obtained from
      the IEEE registration authority.  If no such address is available,



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      or privacy concerns make its use undesirable, Section 4.5
      specifies two alternatives.  Another approach is to use version 3
      or version 4 UUIDs as defined below.

   Process for identifier resolution:
      Since UUIDs are not globally resolvable, this is not applicable.

   Rules for Lexical Equivalence:
      Consider each field of the UUID to be an unsigned integer as shown
      in the table in section Section 4.1.2.  Then, to compare a pair of
      UUIDs, arithmetically compare the corresponding fields from each
      UUID in order of significance and according to their data type.
      Two UUIDs are equal if and only if all the corresponding fields
      are equal.

      As an implementation note, equality comparison can be performed on
      many systems by doing the appropriate byte-order canonicalization,
      and then treating the two UUIDs as 128-bit unsigned integers.

      UUIDs, as defined in this document, can also be ordered
      lexicographically.  For a pair of UUIDs, the first one follows the
      second if the most significant field in which the UUIDs differ is
      greater for the first UUID.  The second precedes the first if the
      most significant field in which the UUIDs differ is greater for
      the second UUID.

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

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

   Scope:
      UUIDs are global in scope.

4.  Specification

4.1.  Format

   The UUID format is 16 octets; some bits of the eight octet variant
   field specified below determine finer structure.




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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 octet 8 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, and is not likely 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.

   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





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   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 (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 4 bits of the time
   stamp (bits 4 through 7 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.



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    0     0     1     1        3     The name-based version
                                     specified in this document
                                     that uses MD5 hashing.

    0     1     0     0        4     The randomly or pseudo-
                                     randomly generated version
                                     specified in this document.

    0     1     0     1        5     The name-based version
                                     specified in this document
                                     that uses SHA-1 hashing.

   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.  However, this is not recommended
   since generating the UTC from local time only needs a time zone
   offset.

   For UUID version 3 or 5, the timestamp is a 60-bit value constructed
   from a name as described in Section 4.3.

   For UUID version 4, the timestamp 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.

   If the clock is set backwards, or might have been set backwards
   (e.g., while the system was powered off), and the UUID generator can
   not be sure that no UUIDs were generated with timestamps larger than
   the value to which the clock was set, then the clock sequence has to
   be changed.  If the previous value of the clock sequence is known, it
   can just be incremented; otherwise it should be set to a random or
   high-quality pseudo-random value.




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   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 or 5, the clock sequence is a 14-bit value
   constructed from a name as described in Section 4.3.

   For UUID version 4, clock sequence 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 or 5, 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.

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.



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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 timestamp, 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 timestamp is later than
      the current timestamp, increment the clock sequence value.

   o  Save the state (current timestamp, clock sequence, and node ID)
      back to the stable store.

   o  Release the global lock.

   o  Format a UUID from the current timestamp, 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.





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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).

   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 timestamp is generated from the system time, whose resolution may
   be less than the resolution of the UUID timestamp.

   If UUIDs do not need to be frequently generated, the timestamp 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 timestamp can be simulated by keeping a count of
   the number of UUIDs that have been generated with the same value of
   the system time, and using it to construct the low order bits of the
   timestamp.  The count will range between zero and the number of
   100-nanosecond intervals per system time interval.

   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 timestamps less than that value, and



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   the clock sequence and node ID remain unchanged, only the shared
   volatile copy of the state needs to be updated.  Furthermore, if the
   timestamp value in stable store is in the future by less than the
   typical time it takes the system to reboot, a crash will not cause a
   reinitialization of the clock sequence.

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; 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 timestamp in the same order of
      significance.

   o  Set the time_mid field equal to bits 32 through 47 from the
      timestamp 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
      timestamp 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 4-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 7) of the clock sequence in the same order of
      significance.







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   o  Set the 6 least significant bits (bits zero through 5) of the
      clock_seq_hi_and_reserved field to the 6 most significant bits
      (bits 8 through 13) of the clock sequence in the same order of
      significance.

   o  Set the two most significant bits (bits 6 and 7) 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 or 5 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 used for allocating names and ensuring
   their uniqueness within their name spaces are beyond the scope of
   this specification.

   The requirements for these types of UUIDs are as follows:

   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 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  Choose either MD5 [4] or SHA-1 [8] as the hash algorithm; If
      backward compatibility is not an issue, SHA-1 is preferred.




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   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 hash of the name space ID concatenated with the name.

   o  Set octets zero through 3 of the time_low field to octets zero
      through 3 of the hash.

   o  Set octets zero and one of the time_mid field to octets 4 and 5 of
      the hash.

   o  Set octets zero and one of the time_hi_and_version field to octets
      6 and 7 of the hash.

   o  Set the four most significant bits (bits 12 through 15) of the
      time_hi_and_version field to the appropriate 4-bit version number
      from Section 4.1.3.

   o  Set the clock_seq_hi_and_reserved field to octet 8 of the hash.

   o  Set the two most significant bits (bits 6 and 7) of the
      clock_seq_hi_and_reserved to zero and one, respectively.

   o  Set the clock_seq_low field to octet 9 of the hash.

   o  Set octets zero through five of the node field to octets 10
      through 15 of the 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.

   The algorithm is as follows:

   o  Set the two most significant bits (bits 6 and 7) 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 4-bit version number from
      Section 4.1.3.

   o  Set all the other bits to randomly (or pseudo-randomly) chosen
      values.



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   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
   <http://standards.ieee.org/regauth/oui/pilot-ind.html>, 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.)

   For compatibility with earlier specifications, note that this
   document uses the unicast/multicast bit, instead of the arguably more
   correct local/global bit.

   Advice on generating cryptographic-quality random numbers can be
   found in RFC1750 [5].

   In addition, items such as the computer's name and the name of the
   operating system, while not strictly speaking random, will help
   differentiate the results from those obtained by other systems.

   The exact algorithm to generate a node ID using these data is system
   specific, because both the data available and the functions to obtain
   them are often very system specific.  A generic approach, however, is
   to accumulate as many sources as possible into a buffer, use a
   message digest such as MD5 [4] or SHA-1 [8], take an arbitrary 6
   bytes from the hash value, and set the multicast bit as described
   above.

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.




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   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.  A predictable random number source will
   exacerbate the situation.

   Do not assume that it is easy to determine if a UUID has been
   slightly transposed in order to redirect a reference to another
   object.  Humans do not have the ability to easily check the integrity
   of a UUID by simply glancing at it.

   Distributed applications generating UUIDs at a variety of hosts must
   be willing to rely on the random number source at all hosts.  If this
   is not feasible, the namespace variant should be used.

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 Ralf
   S. Engelschall, John Larmouth, and Paul Thorpe.  Professor Larmouth
   was also invaluable in achieving coordination with ISO/IEC.

8.  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]  ISO/IEC 9834-8:2004 Information Technology, "Procedures for the
        operation of OSI Registration Authorities: Generation and
        registration of Universally Unique Identifiers (UUIDs) and their
        use as ASN.1 Object Identifier components" ITU-T Rec. X.667,
        2004.

   [4]  Rivest, R., "The MD5 Message-Digest Algorithm ", RFC 1321, April
        1992.




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   [5]  Eastlake, D., 3rd, Schiller, J., and S. Crocker, "Randomness
        Requirements for Security", BCP 106, RFC 4086, June 2005.

   [6]  Moats, R., "URN Syntax", RFC 2141, May 1997.

   [7]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [8]  National Institute of Standards and Technology, "Secure Hash
        Standard", FIPS PUB 180-1, April 1995,
        <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.








































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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
   sharing across processes included.  The code has been tested on Linux
   (Red Hat 4.0) with GCC (2.7.2), and Windows NT 4.0 with VC++ 5.0.
   The code assumes 64-bit integer support, which makes it much clearer.

   All the following source files should have the following copyright
   notice included:

copyrt.h

/*
** 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, Microsoft, 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;




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/* uuid_create -- generate a UUID */
int uuid_create(uuid_t * uuid);

/* uuid_create_md5_from_name -- create a version 3 (MD5) UUID using a
   "name" from a "name space" */
void uuid_create_md5_from_name(
    uuid_t *uuid,         /* 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_create_sha1_from_name -- create a version 5 (SHA-1) UUID
   using a "name" from a "name space" */
void uuid_create_sha1_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);



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static void format_uuid_v3or5(uuid_t *uuid, unsigned char hash[16],
    int v);
static void get_current_time(uuid_time_t *timestamp);
static unsigned16 true_random(void);

/* uuid_create -- 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(&timestamp);
     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 =



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        (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;




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    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;
        }



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        /* going too fast for our clock; spin */
    }
    /* add the count of uuids to low order bits of the clock reading */
    *timestamp = time_now + uuids_this_tick;
}

/* true_random -- generate a crypto-quality random number.
   **This sample doesn't do that.** */
static unsigned16 true_random(void)
{
    static int inited = 0;
    uuid_time_t time_now;

    if (!inited) {
        get_system_time(&time_now);
        time_now = time_now / UUIDS_PER_TICK;
        srand((unsigned int)
               (((time_now >> 32) ^ time_now) & 0xffffffff));
        inited = 1;
    }

    return rand();
}

/* uuid_create_md5_from_name -- create a version 3 (MD5) UUID using a
   "name" from a "name space" */
void uuid_create_md5_from_name(uuid_t *uuid, uuid_t nsid, void *name,
                               int namelen)
{
    MD5_CTX c;
    unsigned char hash[16];
    uuid_t net_nsid;

    /* put name space ID in network byte order so it hashes the same
       no matter what endian machine we're on */
    net_nsid = nsid;
    net_nsid.time_low = htonl(net_nsid.time_low);
    net_nsid.time_mid = htons(net_nsid.time_mid);
    net_nsid.time_hi_and_version = htons(net_nsid.time_hi_and_version);

    MD5Init(&c);
    MD5Update(&c, &net_nsid, sizeof net_nsid);
    MD5Update(&c, name, namelen);
    MD5Final(hash, &c);

    /* the hash is in network byte order at this point */
    format_uuid_v3or5(uuid, hash, 3);
}



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void uuid_create_sha1_from_name(uuid_t *uuid, uuid_t nsid, void *name,
                                int namelen)
{
    SHA_CTX c;
    unsigned char hash[20];
    uuid_t net_nsid;

    /* put name space ID in network byte order so it hashes the same
       no matter what endian machine we're on */
    net_nsid = nsid;
    net_nsid.time_low = htonl(net_nsid.time_low);
    net_nsid.time_mid = htons(net_nsid.time_mid);
    net_nsid.time_hi_and_version = htons(net_nsid.time_hi_and_version);

    SHA1_Init(&c);
    SHA1_Update(&c, &net_nsid, sizeof net_nsid);
    SHA1_Update(&c, name, namelen);
    SHA1_Final(hash, &c);

    /* the hash is in network byte order at this point */
    format_uuid_v3or5(uuid, hash, 5);
}

/* format_uuid_v3or5 -- make a UUID from a (pseudo)random 128-bit
   number */
void format_uuid_v3or5(uuid_t *uuid, unsigned char hash[16], int v)
{
    /* convert UUID to local byte order */
    memcpy(uuid, hash, sizeof *uuid);
    uuid->time_low = ntohl(uuid->time_low);
    uuid->time_mid = ntohs(uuid->time_mid);
    uuid->time_hi_and_version = ntohs(uuid->time_hi_and_version);

    /* put in the variant and version bits */
    uuid->time_hi_and_version &= 0x0FFF;
    uuid->time_hi_and_version |= (v << 12);
    uuid->clock_seq_hi_and_reserved &= 0x3F;
    uuid->clock_seq_hi_and_reserved |= 0x80;
}

/* uuid_compare --  Compare two UUID's "lexically" and return */
#define CHECK(f1, f2) if (f1 != f2) return f1 < f2 ? -1 : 1;
int uuid_compare(uuid_t *u1, uuid_t *u2)
{
    int i;

    CHECK(u1->time_low, u2->time_low);
    CHECK(u1->time_mid, u2->time_mid);



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    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



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#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);
            }
        }



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        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();



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    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"



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uuid_t NameSpace_DNS = { /* 6ba7b810-9dad-11d1-80b4-00c04fd430c8 */
    0x6ba7b810,
    0x9dad,
    0x11d1,
    0x80, 0xb4, 0x00, 0xc0, 0x4f, 0xd4, 0x30, 0xc8
};

/* puid -- print a UUID */
void puid(uuid_t u)
{
    int i;

    printf("%8.8x-%4.4x-%4.4x-%2.2x%2.2x-", u.time_low, u.time_mid,
    u.time_hi_and_version, u.clock_seq_hi_and_reserved,
    u.clock_seq_low);
    for (i = 0; i < 6; i++)
        printf("%2.2x", u.node[i]);
    printf("\n");
}

/* Simple driver for UUID generator */
void main(int argc, char **argv)
{
    uuid_t u;
    int f;

    uuid_create(&u);
    printf("uuid_create(): "); puid(u);

    f = uuid_compare(&u, &u);
    printf("uuid_compare(u,u): %d\n", f);     /* should be 0 */
    f = uuid_compare(&u, &NameSpace_DNS);
    printf("uuid_compare(u, NameSpace_DNS): %d\n", f); /* s.b. 1 */
    f = uuid_compare(&NameSpace_DNS, &u);
    printf("uuid_compare(NameSpace_DNS, u): %d\n", f); /* s.b. -1 */
    uuid_create_md5_from_name(&u, NameSpace_DNS, "www.widgets.com", 15);
    printf("uuid_create_md5_from_name(): "); puid(u);
}

Appendix B.  Appendix B - Sample Output of utest

     uuid_create(): 7d444840-9dc0-11d1-b245-5ffdce74fad2
     uuid_compare(u,u): 0
     uuid_compare(u, NameSpace_DNS): 1
     uuid_compare(NameSpace_DNS, u): -1
     uuid_create_md5_from_name(): e902893a-9d22-3c7e-a7b8-d6e313b71d9f





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RFC 4122                  A UUID URN Namespace                 July 2005


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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Authors' Addresses

   Paul J. Leach
   Microsoft
   1 Microsoft Way
   Redmond, WA  98052
   US

   Phone: +1 425-882-8080
   EMail: paulle@microsoft.com


   Michael Mealling
   Refactored Networks, LLC
   1635 Old Hwy 41
   Suite 112, Box 138
   Kennesaw, GA 30152
   US

   Phone: +1-678-581-9656
   EMail: michael@refactored-networks.com
   URI: http://www.refactored-networks.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


















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RFC 4122                  A UUID URN Namespace                 July 2005


Full Copyright Statement

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