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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]