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rfc:rfc6340

Internet Engineering Task Force (IETF) R. Presuhn Request for Comments: 6340 Independent Category: Standards Track August 2011 ISSN: 2070-1721

Textual Conventions for the Representation of Floating-Point Numbers

Abstract

 This memo defines a Management Information Base (MIB) module
 containing textual conventions (TCs) to represent floating-point
 numbers.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6340.

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Presuhn Standards Track [Page 1] RFC 6340 Floating-Point Textual Conventions August 2011

Table of Contents

 1. Introduction ....................................................2
 2. The Internet-Standard Management Framework ......................3
 3. Applicability ...................................................3
 4. Structure of the MIB Module .....................................4
    4.1. MIB Modules Required for IMPORTS ...........................4
    4.2. Documents Required for REFERENCE Clauses ...................4
 5. Definitions .....................................................4
 6. Security Considerations .........................................6
 7. IANA Considerations .............................................6
 8. Contributors ....................................................6
 9. References ......................................................7
    9.1. Normative References .......................................7
    9.2. Informative References .....................................7

1. Introduction

 This memo defines textual conventions for the representation of
 floating-point numbers.  All of these definitions are in terms of the
 IEEE "Standard for Floating-Point Arithmetic", IEEE 754-2008
 [IEEE.754.2008].
 The IEEE "Standard for Floating-Point Arithmetic", IEEE 754-2008
 [IEEE.754.2008], provides for a variety of interchange formats for
 floating-point numbers.  The need for three of these, namely
 o  32-bit,
 o  64-bit,
 o  128-bit,
 has been recognized in network management.  For example, Section
 4.2.3 of the SMIng Objectives [RFC3216] elaborates the need for these
 three floating-point data types in network management protocols.
 The selection of a floating-point format involves many considerations
 and trade-offs.  For an introduction to the fundamentals of floating-
 point representations see Chapter 4 of [KNUTH]; for a discussion of
 these issues specifically with respect to the IEEE formats, see
 [GOLDBERG].
 All of these textual conventions employ the binary interchange format
 defined in [IEEE.754.2008].  Specifically, this means that for all of
 them, the highest-order bit of the first byte is the sign bit, with
 the remaining bits of the octet string corresponding to the exponent
 and fraction parts, in network byte order.

Presuhn Standards Track [Page 2] RFC 6340 Floating-Point Textual Conventions August 2011

2. The Internet-Standard Management Framework

 For a detailed overview of the documents that describe the current
 Internet-Standard Management Framework, please refer to section 7 of
 RFC 3410 [RFC3410].
 Managed objects are accessed via a virtual information store, termed
 the Management Information Base or MIB.  MIB objects are generally
 accessed through the Simple Network Management Protocol (SNMP).
 Objects in the MIB are defined using the mechanisms defined in the
 Structure of Management Information (SMI).  This memo specifies a MIB
 module that is compliant to the SMIv2, which is described in STD 58,
 RFC 2578 [RFC2578], STD 58, RFC 2579 [RFC2579] and STD 58, RFC 2580
 [RFC2580].

3. Applicability

 The following list highlights some of the issues MIB designers need
 to consider when deciding whether to employ these textual
 conventions:
 o  Floating-point numbers are useful if the number space needs to
    cover a large dynamic range.  For number spaces with a limited
    range, fixed-point numbers can be more efficient and more precise.
 o  Floating-point numbers are typically the wrong answer for data
    that is truly decimal or can be handled adequately by re-thinking
    the units and representing the scaled numbers as integers.
 o  The SNMP "lexicographical" ordering for INDEX objects using these
    floating-point textual conventions will simply be that of the
    octet strings corresponding to the floating-point representations,
    which will not always reflect the numerical ordering of the
    corresponding floating-point values.  Even if MIB designers take
    this into account, users might still find the results of a MIB
    "walk" surprising.  Consequently, it is suggested that whenever
    one of these textual conventions is used for an INDEX object, that
    the DESCRIPTION clause should provide some warning.
 o  Embedded systems sometimes lack floating-point support, which can
    complicate the implementation of MIB objects using floating-point
    numbers.
 o  In choosing from among the types defined in this memo, MIB
    designers need to consider both the range and the precision
    needed, as well as recognize that it could be inefficient to use,
    for example, Float128TC when Float64TC would do.

Presuhn Standards Track [Page 3] RFC 6340 Floating-Point Textual Conventions August 2011

 o  Since these textual conventions are defined in terms of the OCTET
    STRING type, the SMI's mechanisms for formally setting range
    constraints are not available.  MIB designers using these textual
    conventions will need to use DESCRIPTION clauses to spell out any
    applicable range constraints beyond those implied by the
    underlying IEEE types.
 o  Whenever these textual conventions are used in a MIB module, the
    associated DESCRIPTION clause will need to clearly specify whether
    denormalized numbers, NaNs ("not a number") or infinities are
    permitted, along with any special semantics associated with these
    cases.  This is especially important for writeable objects.

4. Structure of the MIB Module

 This MIB module defines three textual conventions.  It defines no MIB
 objects.

4.1. MIB Modules Required for IMPORTS

 This MIB module employs definitions from [RFC2578] and [RFC2579].

4.2. Documents Required for REFERENCE Clauses

 This MIB module contains REFERENCE clauses making reference to IEEE
 754-2008 [IEEE.754.2008].

5. Definitions

FLOAT-TC-MIB DEFINITIONS ::= BEGIN
IMPORTS
    MODULE-IDENTITY,
    mib-2                                 FROM SNMPv2-SMI  -- RFC 2578
    TEXTUAL-CONVENTION                    FROM SNMPv2-TC;  -- RFC 2579
floatTcMIB    MODULE-IDENTITY
    LAST-UPDATED "201107270000Z"          -- July 27, 2011
    ORGANIZATION "IETF OPSAWG Working Group"
    CONTACT-INFO "WG Email: opsawg@ietf.org
                  Editor: Randy Presuhn
                  randy_presuhn@mindspring.com"
    DESCRIPTION  "Textual conventions for the representation
                  of floating-point numbers.

Presuhn Standards Track [Page 4] RFC 6340 Floating-Point Textual Conventions August 2011

                  Copyright (c) 2011 IETF Trust and the persons
                  identified as authors of the code.  All rights
                  reserved.
                  Redistribution and use in source and binary forms,
                  with or without modification, is permitted pursuant
                  to, and subject to the license terms contained in,
                  the Simplified BSD License set forth in Section
                  4.c of the IETF Trust's Legal Provisions Relating
                  to IETF Documents
                  (http://trustee.ietf.org/license-info).
                  This version of this MIB module is part of RFC 6340;
                  see the RFC itself for full legal notices."
    REVISION     "201107270000Z"          -- July 27, 2011
    DESCRIPTION  "Initial version, published as RFC 6340."
     ::= { mib-2 201 }
Float32TC ::= TEXTUAL-CONVENTION
    STATUS       current
    DESCRIPTION  "This type represents a 32-bit (4-octet) IEEE
                  floating-point number in binary interchange format."
    REFERENCE    "IEEE Standard for Floating-Point Arithmetic,
                  Standard 754-2008"
    SYNTAX       OCTET STRING (SIZE(4))
Float64TC ::= TEXTUAL-CONVENTION
    STATUS       current
    DESCRIPTION  "This type represents a 64-bit (8-octet) IEEE
                  floating-point number in binary interchange format."
    REFERENCE    "IEEE Standard for Floating-Point Arithmetic,
                  Standard 754-2008"
    SYNTAX       OCTET STRING (SIZE(8))
Float128TC ::= TEXTUAL-CONVENTION
    STATUS       current
    DESCRIPTION  "This type represents a 128-bit (16-octet) IEEE
                  floating-point number in binary interchange format."
    REFERENCE    "IEEE Standard for Floating-Point Arithmetic,
                  Standard 754-2008"
    SYNTAX       OCTET STRING (SIZE(16))
END

Presuhn Standards Track [Page 5] RFC 6340 Floating-Point Textual Conventions August 2011

6. Security Considerations

 This module does not define any management objects.  Instead, it
 defines a set of textual conventions that can be used by other MIB
 modules to define management objects.
 Meaningful security considerations can only be written in the MIB
 modules that define management objects.  Therefore, this memo has no
 impact on the security of the Internet.

7. IANA Considerations

 The MIB module in this document uses the following IANA-assigned
 OBJECT IDENTIFIER value recorded in the SMI Numbers registry:
    Descriptor        OBJECT IDENTIFIER value
    ----------        -----------------------
    floatTcMIB        { mib-2 201 }

8. Contributors

 The following people provided helpful comments during the development
 of this document:
 o  Andy Bierman
 o  Martin Duerst
 o  Alfred Hoenes
 o  Juergen Quittek
 o  Juergen Schoenwaeder
 o  Dave Shield
 o  Robert Story

Presuhn Standards Track [Page 6] RFC 6340 Floating-Point Textual Conventions August 2011

9. References

9.1. Normative References

 [IEEE.754.2008]  Institute of Electrical and Electronics Engineers,
                  "Standard for Floating-Point Arithmetic",
                  IEEE Standard 754, August 2008.
 [RFC2578]        McCloghrie, K., Ed., Perkins, D., Ed., and J.
                  Schoenwaelder, Ed., "Structure of Management
                  Information Version 2 (SMIv2)", STD 58, RFC 2578,
                  April 1999.
 [RFC2579]        McCloghrie, K., Ed., Perkins, D., Ed., and J.
                  Schoenwaelder, Ed., "Textual Conventions for SMIv2",
                  STD 58, RFC 2579, April 1999.
 [RFC2580]        McCloghrie, K., Perkins, D., and J. Schoenwaelder,
                  "Conformance Statements for SMIv2", STD 58,
                  RFC 2580, April 1999.

9.2. Informative References

 [GOLDBERG]       Goldberg, D., "What Every Computer Scientist Should
                  Know About Floating-Point Arithmetic", ACM Computing
                  Surveys Volume 23, Issue 1, March 1991.
 [KNUTH]          Knuth, D., "Seminumerical Algorithms", The Art of
                  Computer Programming (Second Edition) Vol. 2, 1981.
 [RFC3216]        Elliott, C., Harrington, D., Jason, J.,
                  Schoenwaelder, J., Strauss, F., and W. Weiss, "SMIng
                  Objectives", RFC 3216, December 2001.
 [RFC3410]        Case, J., Mundy, R., Partain, D., and B. Stewart,
                  "Introduction and Applicability Statements for
                  Internet-Standard Management Framework", RFC 3410,
                  December 2002.

Author's Address

 Randy Presuhn
 San Jose, CA  95120
 USA
 EMail: randy_presuhn@mindspring.com

Presuhn Standards Track [Page 7]

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