





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.









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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.



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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.




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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.





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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





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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














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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





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