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RFC 3444

On the Difference between Information Models and Data Models

Pages: 8
Informational

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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
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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.
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   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
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   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].
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   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
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   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.
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   [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
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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.