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Versions: 00                                                            
INTERNET-DRAFT                                        Doug Wood
<draft-wood-ldapext-float-00.txt>                     Tivoli Systems Inc
December, 1999
Expires: June 2000

        Directory string representation for floating point values

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   Distribution of this memo is unlimited.  It is filed as <draft-
   ietf-ldapext-float-00.txt >, and expires on June, 2000.
   Please send comments to the authors.

1. Abstract

    This draft defines a way that floating point values may be
    represented as directory (ASCII) strings such that standard ordering
    rules can be used to sort the strings into the correct collating
    sequence for their numeric value.  The representation is intended
    for use in X.500 like directories, and has been developed to support
    mapping of the DMTF Common Information Model.

2. Introduction

    X.500 directories provide for use definable syntaxes, matching and
    ordering rules.  This provides for the definition of schema
    supporting any type and structure of data.  The definition of the
    LDAP protocol [RFC 2251] has encouraged the creation new generation
    of directories that support the X.500 structure, but donÆt support
    some of X.500Æs more heavy weight feature.  Among the unsupported
    feature are user definable syntaxes. This restricts schema designers
    to syntaxes provided by the directory vendor.  These typical do not
    include a syntax for floating point values.

    This draft defines a an ASCII format for floating point values that

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    allow them to be stored in attributes with Directory String syntax.
    And allows the standard case insensitive ordering rule to sort them
    in the correct collating sequence for their numeric value.  In
    addition, attributes are defined which can be used for values stored
    in the format described below, or as superiors for user defined
    attributes.  The attributes are provided both as a convenience, and
    as a method to document the storage format used.

    The format is defined specifically to support mapping the DMTF
    Common Information Model to directory schema, but also has general

3. String format

    Because string comparison is positional, it is necessary to define a
    fix format for representing the mantissa, and the exponent.  Because
    the collating sequence for string comparison is left to right, the
    most significant portion of the representation must be on the left.
    There are four separate cases that must be handled.

       o  Negative mantissa and positive exponent
       o  Negative mantissa and negative exponent
       o  Positive mantissa and negative exponent
       o  Positive mantissa, and positive exponent

    The above list is ordered by the desired collating sequence from
    smallest value to largest value.  A single representation does not
    provide the correct collating sequence for all cases.  Therefor it
    is necessary to sort by case, and then to sort within each case. To
    accomplish this, the cases are number from 1 to 5 as follows:

       1. Negative mantissa and positive exponent
       2. Negative mantissa and negative exponent
       3. Zero
       4. Positive mantissa and negative exponent
       5. Positive mantissa, and positive exponent

    For symmetry, zero is treated as its own case instead of a special
    sub-case of case 4.

    A 64 bit float has a range of 1.7976931348623158e+308 to
    2.2250738585072014e-308[1].  To represent this as a string, three
    digits are required for the exponent, and 17 for the mantissa not
    including the decimal point.  The directory representation is fixed
    format, zero padded, blank separated, with the most significant
    fields on the left.  The first character in the string is the case
    number. For readability, it is followed by a blank.  Next is a 3
    digit exponent, again followed by a blank. Next are a single digit,
    a decimal point, and 16 digits of decimal.

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                  | | |Exp| | | |16 digits       |
                  |c| |nnn| |n|.|nnnnnnnnnnnnnnnn|

    The way each of the fields is interpreted varies with the case.
    The cases are examined in reverse order so the simplest may be
    examined first.

3.1 Positive mantissa and positive exponent (case 5)

    This is relatively straightforward.  The exponent field has the
    exponent value expressed as a 3-digit integer string.  It is zero
    padded to the left if necessary.  The mantissa field as a seventeen-
    digit decimal string with exactly 1 digit to the left of the decimal
    point for a total of 18 characters.  It is zero padded to the right
    if necessary.

    - The first digit is a 5 to indicate the case
    - There is exactly one digit to the left of the decimal place.  It
      is always non zero.
    - Positions 2 through 4 have the exponent.  It is right justified,
      and zero padded to the left if it is less than three digits
    - Spaces are added to aid human readability
    - No signs are required for the exponent or the mantissa because
      they are expressed in the case number

3.2 Positive mantissa and negative exponent (case 4)

    When the exponent is negative, larger whole number values for the
    exponent produce smaller actual values.  For this case, a string
    comparison of the numeric representation of the exponent yields the
    reverse of the desired collating sequence.  To flip the collating
    sequence, the value of the exponent is added to 999, and the result
    stored as the exponent.  No sign is stored.  The sign of both the
    exponent and mantissa are indicated by the case character.

3.3 Zero (case 3)

    The case number is sufficient to insure the correct collating
    sequence.  To insure equality comparisons work correctly, all
    remaining digits are zero.

3.4 Negative mantissa and negative exponent (case 2)

    When both the exponent and the mantissa are negative, the collating
    order for the exponent is correct.  A larger exponent yields a
    number that is closer to zero and therefor larger.  However, the
    collating sequence for the mantissa is reversed.  To flip the
    collating sequence for the mantissa it is added to 10, and the
    result stored.

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3.5 Negative mantissa and positive exponent (case 1)

    When both the exponent and the mantissa are negative, the collating
    sequence is flipped for both of them.  This is achieved by adding
    the exponent to 999, and the mantissa to 10.

4. Examples

         | Value    | Representation           |
         | 3.25e5   | 5 005 3.2500000000000000 |
         | 8.4e-5   | 4 994 8.4000000000000000 |
         | 8.4e-7   | 4 992 8.4000000000000000 |
         | 7.23e-7  | 4 992 7.2300000000000000 |
         | 0.0e0    | 3 000 0.0000000000000000 |
         | -4.25e-4 | 2 004 5.7500000000000000 |
         | -6.35e-4 | 2 004 3.6500000000000000 |
         | -6.35e-3 | 2 003 3.6500000000000000 |
         | -4.0e104 | 1 895 6.0000000000000000 |
         | -4.0e105 | 1 894 6.0000000000000000 |
         | -6.0e105 | 1 894 4.0000000000000000 |

5.  32 bit vs. 64 bit values

    Both 32 and 64 bit floating point values are in common usage.  To
    allow comparisons between the two, both are stored in the 64-bit
    format described above.  This implies a greater degree of precision
    than is actually available for 32-bit values. The directory mapping
    described below provides implicit documentation of the actual
    precision of a value.

6.  Directory mapping

    The intent of the mapping is to simulate a new syntax.  The
    advantage of this approach is it may be utilized without any changes
    to existing directory servers.  To foster that illusion, and to aid
    in documentation, two new attributes are defined.  cimFloat32, and
    cimFloat64.  All floating-point attributes are derived from one of
    these.  They are defined as:

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                                        ; Need OID assigned
             NAME               æcimFloat32Æ
             DESC       '32 bit float encoded as sortable float format'
             EQUALITY   caseIgnoreeMatch
             ORDERING   caseIgnoreOrderingMatch

                                        ; Need OID assigned
             NAME       æcimFloat64Æ
             DESC       'Æ64 bit float encoded as sortable float formatÆ
             EQUALITY   caseIgnoreeMatch
             ORDERING   caseIgnoreOrderingMatch

7.  References

    [1] From sys/limits.h on AIX 4.3

8.  Acknowledgement

    This work is a product of the DMTF LDAP Mapping Working Group and
    has benefited from many comments and discussions during this groups

9.  Authors' Addresses

    Doug Wood
    Tivoli Systems
    9025 North River Rd.
    Indianapolis, IN 46240-7622

   Expires June 2000

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