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rfc:rfc3444

Network Working Group A. Pras Request for Comments: 3444 University of Twente Category: Informational J. Schoenwaelder

                                              University of Osnabrueck
                                                          January 2003
                     On the Difference between
                 Information Models and Data Models

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 There has been ongoing confusion about the differences between
 Information Models and Data Models for defining managed objects in
 network management.  This document explains the differences between
 these terms by analyzing how existing network management model
 specifications (from the IETF and other bodies such as the
 International Telecommunication Union (ITU) or the Distributed
 Management Task Force (DMTF)) fit into the universe of Information
 Models and Data Models.
 This memo documents the main results of the 8th workshop of the
 Network Management Research Group (NMRG) of the Internet Research
 Task Force (IRTF) hosted by the University of Texas at Austin.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 3.  Information Models . . . . . . . . . . . . . . . . . . . . . . 3
 4.  Data Models  . . . . . . . . . . . . . . . . . . . . . . . . . 4
 5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 6
 6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 6
 7.  Normative References . . . . . . . . . . . . . . . . . . . . . 6
 8.  Informative References . . . . . . . . . . . . . . . . . . . . 7
 9.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 7
 10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 8

Pras & Schoenwaelder Informational [Page 1] RFC 3444 Information Models and Data Models January 2003

1. Introduction

 Currently multiple languages exist to define managed objects.
 Examples of such languages are the Structure of Management
 Information (SMI) [1], the Structure of Policy Provisioning
 Information (SPPI) [2] and, within the DMTF, the Managed Object
 Format (MOF) [3].  Despite the fact that multiple languages exist, a
 number of people still believe that none of these languages really
 suits all needs.
 There have been many discussions to understand the advantages and
 disadvantages, as well as the main differences, between various
 languages.  For instance, the IETF organized a BoF on "Network
 Information Modeling" (NIM) at its 48th meeting (Pittsburgh, August
 2000).  During these discussions, it turned out that people had a
 different understanding of the main terms, which caused confusion and
 long arguments.  In particular, the meaning of the terms "Information
 Model" (IM) and "Data Model" (DM) turned out to be controversial.
 In an attempt to address this issue, the IRTF Network Management
 Research Group (NMRG) dedicated its 8th workshop (Austin, December
 2000) to harmonizing the terminology used in information and data
 modeling.  Attendees included experts from the IETF, DMTF and ITU, as
 well as academics who do research in this field (see the
 Acknowledgments section for a list of participants).  The main
 outcome of this successful workshop -- a better understanding of the
 terms "Information Model" and "Data Model" -- is presented in this
 document.
 Short definitions of these terms can also be found elsewhere (e.g.,
 in RFC 3198 [8]).  Compared to most other documents, this one
 explains the rationale behind the proposed definitions and provides
 examples.

2. Overview

 One of the key observations made at the NMRG workshop was that IMs
 and DMs are different because they serve different purposes.
 The main purpose of an IM is to model managed objects at a conceptual
 level, independent of any specific implementations or protocols used
 to transport the data.  The degree of specificity (or detail) of the
 abstractions defined in the IM depends on the modeling needs of its
 designers.  In order to make the overall design as clear as possible,
 an IM should hide all protocol and implementation details.  Another
 important characteristic of an IM is that it defines relationships
 between managed objects.

Pras & Schoenwaelder Informational [Page 2] RFC 3444 Information Models and Data Models January 2003

 DMs, conversely, are defined at a lower level of abstraction and
 include many details.  They are intended for implementors and include
 protocol-specific constructs.
           IM                --> conceptual/abstract model
            |                    for designers and operators
 +----------+---------+
 |          |         |
 DM        DM         DM     --> concrete/detailed model
                                 for implementors
 The relationship between an IM and DM is shown in the Figure above.
 Since conceptual models can be implemented in different ways,
 multiple DMs can be derived from a single IM.
 Although IMs and DMs serve different purposes, it is not always
 possible to precisely define what kind of details should be expressed
 in an IM and which ones belong in a DM.  There is a gray area where
 IMs and DMs overlap -- just like there are gray areas between the
 models produced during the analysis, high-level design and low-level
 design phases in object-oriented software engineering.  In some
 cases, it is very difficult to determine whether an abstraction
 belongs to an IM or a DM.

3. Information Models

 IMs are primarily useful for designers to describe the managed
 environment, for operators to understand the modeled objects, and for
 implementors as a guide to the functionality that must be described
 and coded in the DMs.  The terms "conceptual models" and "abstract
 models", which are often used in the literature, relate to IMs.  IMs
 can be implemented in different ways and mapped on different
 protocols.  They are protocol neutral.
 An important characteristic of IMs is that they can (and generally
 should) specify relationships between objects.  Organizations may use
 the contents of an IM to delimit the functionality that can be
 included in a DM.
 IMs can be defined in an informal way, using natural languages such
 as English.  An example of such an IM is provided by RFC 3290 [9],
 which describes a conceptual model of a Diffserv Router and specifies
 the relationships between the components of such a router that need
 to be managed.  Within the IETF, however, it is exceptional that an
 IM be explicitly described, and even more that the IM and DM be
 specified in separate RFCs.  In such cases, the document specifying
 the IM is usually an Informational RFC whereas the document defining
 the DM usually follows the Standards Track [4].  In general, most of

Pras & Schoenwaelder Informational [Page 3] RFC 3444 Information Models and Data Models January 2003

 the RFCs that define an SNMP Management Information Base (MIB) module
 also include some kind of informal description explaining parts of
 the model behind that MIB module.  Such a model can be considered as
 a document of an IM.  An example of this is RFC 2863, which defines
 "The Interfaces Group MIB" [10].  But most MIB modules published to
 date include only a rudimentary and incomplete description of the
 underlying IM.
 Alternatively, IMs can be defined using a formal language or a semi-
 formal structured language.  One of the possibilities to formally
 specify IMs is to use class diagrams of the Unified Modeling Language
 (UML).  Although such diagrams are still rarely used within the IETF,
 several other organizations routinely use them for defining IMs,
 including the DMTF, the ITU-T SG 4, 3GPP SA5, the TeleManagement
 Forum, and the ATM Forum.  An important advantage of UML class
 diagrams is that they represent objects and the relationships between
 them in a standard graphical way.  Because of this graphical
 representation, designers and operators may find it easier to
 understand the underlying management model.  Although there are other
 techniques to graphically represent objects and relationships (e.g.,
 Entity-Relationship (ER) diagrams), UML presents the advantage of
 being widely adopted in the industry and taught in universities.
 Also, many tools for editing UML diagrams are now available.  UML is
 standardized by the Object Management Group (OMG) [5].
 In general, it seems advisable to use object-oriented techniques to
 describe an IM.  In particular, the notions of abstraction and
 encapsulation, as well as the possibility that object definitions
 include methods, are considered to be important.

4. Data Models

 Compared to IMs, DMs define managed objects at a lower level of
 abstraction.  They include implementation- and protocol-specific
 details, e.g. rules that explain how to map managed objects onto
 lower-level protocol constructs.
 Most of the management models standardized to date are DMs.  Examples
 include:
 o  Management Information Base (MIB) modules defined within the IETF.
    The language (syntax) used to define these DMs is called the
    "Structure of Management Information" (SMI) [1] and is derived
    from ASN.1 [6].

Pras & Schoenwaelder Informational [Page 4] RFC 3444 Information Models and Data Models January 2003

 o  Policy Information Base (PIB) modules, developed within the IETF.
    Their syntax is defined by the "Structure of Policy Provisioning
    Information" (SPPI) [2], which is close to SMI and is also derived
    from ASN.1 [6].
 o  Management Information Base (MIB) modules, originally defined by
    the ISO and currently maintained and enhanced by the ITU-T.  The
    syntax of these DMs is specified in the "Guidelines for the
    Definition of Managed Objects" (GDMO) [7].  GDMO MIB modules make
    use of object-oriented principles.
 o  CIM Schemas, developed within the DMTF.  The DMTF publishes them
    in two forms: graphical and textual.  The graphical forms use UML
    diagrams and are not normative (because not all details can be
    represented graphically).  They can be downloaded from the DMTF
    Web site in PDF and Visio formats.  (Visio is a tool to draw UML
    class diagrams.)  The textual forms are normative and written in a
    language called the "Managed Object Format" (MOF) [3].  CIM
    Schemas are object-oriented.
 Because CIM Schemas support a graphical notation whereas IETF MIB
 modules do not, designers and operators may find it easier to
 understand CIM Schemas than IETF MIB modules.  One could therefore
 argue that CIM Schemas are closer to IMs than IETF MIB modules.
 The Figure below summarizes these examples.  The languages that are
 used to define the DMs are shown between brackets.
                     IM                              --> IM
                      |
   +----------+-------+-------+--------------+
   |          |               |              |
  MIB        PIB          CIM schema      OSI-MIB    --> DM
 (SMI)      (SPPI)          (MOF)          (GDMO)
 To illustrate what details are included in a DM, let us consider the
 example of IETF MIB modules.  As opposed to IMs, IETF MIB modules
 include details such as OID assignments and indexing structures.  The
 relationships defined in the IM are implemented as OID pointers or
 realized through indexing relationships specified in INDEX clauses.
 Many other implementation-specific details are included, such as MAX-
 ACCESS and STATUS clauses and conformance statements.
 A special kind of DM language is the SMIng language defined by the
 NMRG.  This language was designed at a higher conceptual level than
 SMIv1/SMIv2 and SPPI.  In fact, one of the intentions behind SMIng
 was to stop the proliferation of different DM languages within the
 IETF and to harmonize the various models.  As a result, MIB and PIB

Pras & Schoenwaelder Informational [Page 5] RFC 3444 Information Models and Data Models January 2003

 modules defined in SMIng can be mapped on different underlying
 protocols.  There is a mapping on SNMP and another mapping on COPS-
 PR.  SMIng is therefore more protocol neutral than other IETF
 approaches.  It also supports some object-oriented principles and
 provides extension mechanisms that allow the addition of new features
 (e.g., the support for methods).  New features can then be used when
 they are supported by the underlying protocols, without breaking
 SMIng implementations.  Still, SMIng should be considered a DM.  For
 instance, to express relationships between managed objects,
 techniques such as UML and ER diagrams still give better results
 because these diagrams are easier to understand.
 Note that the IETF SMING Working Group took a different approach and
 decided not to use the SMIng language defined by the NMRG.  Instead,
 the SMING Working Group is developing a third version of SMI (SMIv3)
 that is primarily targeted towards SNMP, and which incorporates only
 some of the ideas developed within the NMRG.

5. Security Considerations

 The meaning of the terms Information Model and Data Model has no
 direct security impact on the Internet.

6. Acknowledgments

 The authors would like to thank everyone who participated in the 8th
 NMRG workshop (in alphabetic order): Szabolcs Boros, Marcus Brunner,
 David Durham, Dave Harrington, Jean-Philippe Martin-Flatin, George
 Pavlou, Robert Parhonyi, David Perkins, David Sidor, Andrea
 Westerinen and Bert Wijnen.

7. Normative References

 [1]  McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Structure of
      Management Information Version 2 (SMIv2)", STD 58, RFC 2578,
      April 1999.
 [2]  McCloghrie, K., Fine, M., Seligson, J., Chan, K., Hahn, S.,
      Sahita, R., Smith, A. and F. Reichmeyer, "Structure of Policy
      Provisioning Information (SPPI)", RFC 3159, August 2001.
 [3]  Distributed Management Task Force, "Common Information Model
      (CIM) Specification Version 2.2", DSP 0004, June 1999.
 [4]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

Pras & Schoenwaelder Informational [Page 6] RFC 3444 Information Models and Data Models January 2003

 [5]  Object Management Group, "Unified Modeling Language (UML),
      Version 1.4", formal/2001-09-67, September 2001.
 [6]  International Organization for Standardization, "Information
      processing systems - Open Systems Interconnection -
      Specification of Abstract  Syntax Notation One (ASN.1)",
      International Standard 8824, December 1987.
 [7]  International Telecommunication Union, "Information technology -
      Open Systems Interconnection  - Structure of Management
      Information:  Guidelines for the Definition of Managed Objects",
      Recommendation X.722, 1992.

8. Informative References

 [8]  Westerinen, A., Schnizlein, J., Strassner, J., Scherling, M.,
      Quinn, B., Herzog, S., Huynh, A., Carlson, M., Perry, J. and S.
      Waldbusser, "Terminology for Policy-Based Management", RFC 3198,
      November 2001.
 [9]  Bernet, Y., Blake, S., Grossman, D. and A. Smith, "An Informal
      Management Model for Diffserv Routers", RFC 3290, May 2002.
 [10] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
      RFC 2863, June 2000.

9. Authors' Addresses

 Aiko Pras
 University of Twente
 PO Box 217
 7500 AE Enschede
 The Netherlands
 Phone: +31 53 4893778
 EMail: pras@ctit.utwente.nl
 Juergen Schoenwaelder
 University of Osnabrueck
 Albrechtstr. 28
 49069 Osnabrueck
 Germany
 Phone: +49 541 969-2483
 EMail: schoenw@informatik.uni-osnabrueck.de

Pras & Schoenwaelder Informational [Page 7] RFC 3444 Information Models and Data Models January 2003

10. Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Pras & Schoenwaelder Informational [Page 8]

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