Textual Conventions for the Representation of Floating-Point Numbers
draft-ietf-opsawg-mib-floats-02.txt

Abstract

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

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

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."

This Internet-Draft will expire on December 17, 2011.

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.

Table of Contents

• 1. Introduction
• 2. The Internet-Standard Management Framework
• 3. Applicability
• 4. Structure of the MIB Module
• 4.1. MIB modules required for IMPORTS
• 4.2. Documents required for REFERENCE clauses
• 5. Definitions
• 6. Security Considerations
• 7. IANA Considerations
• 8. Contributors
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Author's Address

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 [RFC3216] elaborates the need for these three floating-point data types in network management protocols.

• 32-bit,
• 64-bit,
• 128-bit,

has been recognized in network management. For example, Section 4.2.3 of the SMIng Objectives

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], and for a 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.

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:

• 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.
• 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.
• 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.
• Embedded systems sometimes lack floating point support, which can complicate the implementation of MIB objects using floating point numbers.
• 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.
• 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.
• 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 "201106010000Z"          -- June 1, 2011
    ORGANIZATION "None"
    CONTACT-INFO "Randy Presuhn
                  Email: randy_presuhn@mindspring.com"

    DESCRIPTION  "Textual conventions for the representation
		  of floating-point numbers.

                  Copyright (c) 2010 IETF Trust and the persons
                  identified as the document authors.  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 XXXX;
                  see the RFC itself for full legal notices."

    REVISION     "201106010000Z"          -- June 1, 2011
    DESCRIPTION  "Initial version, published as RFC XXXX."
     ::= { mib-2 XXX }
-- RFC Ed.: replace XXX with IANA-assigned number & remove this note
-- RFC Ed.: replace XXXX with the RFC number & remove this note

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
                

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 XXX }

      

Editor's Note (to be removed prior to publication): the IANA is requested to assign a value for "XXX" under the 'mib-2' subtree and to record the assignment in the SMI Numbers registry. When the assignment has been made, the RFC Editor is asked to replace "XXX" (here and in the MIB module) with the assigned value and to remove this note.

8. Contributors

The following people provided helpful comments during the development of this document:

• Andy Bierman
• Martin Dürst
• Alfred Hönes
• Jürgen Quittek
• Jürgen Schönwäder
• Dave Shield
• Robert Story

9. References

9.1. Normative References

9.2. Informative References

Author's Address