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Internet Engineering Task Force (IETF)                        R. PresuhnRequest for Comments: 6340                                   IndependentCategory: Standards Track                                    August 2011ISSN: 2070-1721Textual Conventions for the Representation of Floating-Point NumbersAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 2011Table of Contents1. Introduction ....................................................22. The Internet-Standard Management Framework ......................33. Applicability ...................................................34. Structure of the MIB Module .....................................44.1. MIB Modules Required for IMPORTS ...........................44.2. Documents Required for REFERENCE Clauses ...................45. Definitions .....................................................46. Security Considerations .........................................67. IANA Considerations .............................................68. Contributors ....................................................69. References ......................................................79.1. Normative References .......................................79.2. Informative References .....................................71.  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,Section4.2.3 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 20112.  The Internet-Standard Management Framework   For a detailed overview of the documents that describe the current   Internet-Standard Management Framework, please refer tosection 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 inSection4.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 ofRFC 6340;                    see the RFC itself for full legal notices."      REVISION     "201107270000Z"          -- July 27, 2011      DESCRIPTION  "Initial version, published asRFC 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))  ENDPresuhn                      Standards Track                    [Page 5]

RFC 6340           Floating-Point Textual Conventions        August 20116.  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 StoryPresuhn                      Standards Track                    [Page 6]

RFC 6340           Floating-Point Textual Conventions        August 20119.  References9.1.  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.comPresuhn                      Standards Track                    [Page 7]