[nmrg] Draft version of Information - Data Model RFC

[Aiko Pras <[e-mail]>]

Hi everyone

Below you'll find the draft text Juergen and I have written for an 
informational RFC that should explain the differences between Information 
and Data models. This RFC is the outcome of our IRTF-NMRG meeting in 
december 2000, which was held in Austin (yes, you're right, I'm a little bit 
late). Before distributing this document outside the NMRG, I would like to 
give all of you the oppertunity to comment on it. Note that I will be away 
next week, so don't expect immediate answers.

Have a nice weekend

Aiko

--------------------------------------------------------------------
Network Working Group                                            A. Pras
Internet-Draft                                      University of Twente
Expires: December 27, 2002                              J. Schoenwaelder
                                                 University of Osnabrueck
                                                            June 28, 2002


       On the Difference between Information Models and Data Models
                       draft-irtf-nmrg-im-dm-00.txt

Status of this Memo

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Copyright Notice

    Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

    There has been ongoing confusion about the differences between
    Information Models and Data Models.  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 ITU or the 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).



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Table of Contents

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









































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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,
    there are still some feelings that none of these languages really
    suites all needs.  To discuss these feelings, the IETF organized for
    example at its 48th meeting (summer 2000) a BoF meeting on "Network
    Information Modeling" (NIM).

    To understand the advantages and disadvantages, as well as the main
    differences between the various languages, there have been many
    discussions, also outside the IETF.  Unfortunately these discussions
    were not always fruitful, primarily because it appeared that people
    had different understanding of main terms.  In particularly the terms
    "Information Model" (IM) and "Data Model" (DM) turned out to be
    controversial.

    In an attempt to stop this controversy and harmonize terminology, the
    IRTF Network Management Research Group (NMRG) [9] organized in
    December 2000 a special workshop.  For this workshop the IRTF-NMRG
    invited leading experts from the IETF, DMTF, ITU as well as the
    academic world (see the acknowledgements section for a list of
    participants).  The workshop was quite successful and its outcome,
    which is a better understanding of the terms "Information Model" and
    "Data Model", as presented in this document.

2. Overview

    One of the interesting observations at the IRTF-NMRG workshop was
    that IMs and DMs are different since they serve different purposes.
    The purpose of an IM is to model managed objects at a high conceptual
    level, which is easy to understand for the human designer or human
    manager.  In order to present the overall design as clear as
    possible, IMs try to abstract from protocol and implementation
    specific details.  One important aspect of an IM is that it also
    focuses on the relationships between managed objects.

    Compared to IMs, DMs are defined at a lower level of abstraction and
    with much more detail.  DMs are more intended for implementors, and
    include lower level and protocol specific constructs.








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                    IM                --> conceptual / abstract model
                     |                    targeted to the designer and
          +----------+---------+          human manager
          |          |         |
          DM        DM         DM     --> concrete / detailed model
                                          targeted to the implementor

    The relationship between an IM and DM is shown in the Figure above.
    Since conceptual models can be implemented in several different ways,
    multiple DMs can be derived from the same IM.

    Although IMs and DMs serve different purposes, it is not possible to
    precisely define what details should be expressed in the IM and what
    in the DM.  Therefore no principle difference exists between both
    models; in fact there is a grey area between both which makes it in
    certain cases impossible to determine if something is an IM or a DM.

3. Information Models

    An IM is primarily useful for designers and managers.  The terms
    "conceptual models" or "abstract models", which are often used in
    literature, relate to IMs.  An IM can be implemented in different
    ways and mapped upon different protocols; IMs are therefore protocol
    neutral.  An important characteristic of an IM is that it specifies
    the relationship between objects.

    IMs can be defined in an informal way, using natural languages like
    English.  A good example of an IM is provided by RFC 3290: "An
    Informal Management Model for Diffserv Routers" [4].  This RFC
    describes a conceptual model of a Diffserv Router, including the
    relationship between the components of such a router that need to be
    managed.  Within the IETF it is quite exceptional that an IM is
    described within a separate RFC, however; in such cases the status of
    such documents is usually "Informational" and not "Standards Track"
    [5].  In general most RFCs that define a MIB module also include some
    kind of informal description explaining the model behind that MIB
    module.  Such a model can be considered as an IM.  A good example of
    this is RFC 2863, which defines "The Interfaces Group MIB" [6].  Note
    that most RFCs include just a rudimentary, incomplete description of
    the underlying IM.

    Optionally IMs can also be defined "formally", using some kind of
    (semi) formal language.  Such formal definitions are not developed
    within the IETF.  The DMTF, however, uses UML class diagrams to
    define IMs in a semi-formal way.  An important advantage of UML class
    diagrams is that they represent objects and the relationship between
    them in a graphical way.  Because of this graphical representation,
    designers and operators may find it easier to understand the



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    underlying management model.  Although there are other techniques to
    graphically represent objects and the relationship between them
    (like, for example, entity-relationship diagrams), UML has the
    advantage that it is widely accepted by the industry and
    universities.  Because of this, there are also many tools that
    support the manipulation of UML diagrams.  UML itself is standardized
    by the Object Management Group (OMG) [10].

    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 like, for example, rules that explain how to map managed
    objects on lower level protocol constructs.

    The MIB modules defined within the IETF are in fact DMs.  The
    language (syntax) used to define these DMs is called the "Structure
    of Management Information" (SMI) [1], which in turn is based on ASN.1
    [7].

    Not only IETF MIBs, but also most other standardized management
    models are DMs.  Examples are:

    o  Policy Information Bases (PIBs), which are also developed within
       the IETF.  PIBs use as syntax the "Structure of Policy
       Provisioning Information" (SPPI) [2], which is similar to the SMI
       and is also based on ASN.1.

    o  Management Information Bases (MIBs), as defined by ISO.  These DMs
       use the syntax as defined by the "Guidelines for the Definition of
       Managed Objects" (GDMO) [8].  ISO MIBs make also use of object-
       oriented principles.

    o  CIM Schemas, as developed within the DMTF.  These DMs use the
       syntax as defined by the "Managed Object Formats (MOFs)" [3].  The
       DMTF publishes CIM Schemas in the form of graphical UML documents
       in addition to this MOF syntax.  Because of this graphical
       notation, designers and managers may find it easier to understand
       CIM Schemas than IETF MIBs.  One could therefore argue that CIM
       Schemas are closer to IMs then IETF MIBs, which lack such
       graphical notation.  The UML diagrams can be downloaded from the
       DMTF website in PDF as well as VISIO format.  (VISIO is one of the
       tools to draw UML class diagrams).  Note that, in contrast to IETF



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       MIBS, CIM Schemas make use of object-oriented principles.

    The Figure below shows these examples.  The languages that are used
    to define the DMs are shown between brackets.

                          IM                              --> IM
                           |
        +----------+-------+-------+--------------+
        |          |               |              |
       MIB        PIB          CIM schema      OSI-MIBs   --> 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 like OID assignments and indexing structures.  The
    "relationships" that existed at the IM level are now "implemented" in
    terms of OID pointers and indexing relationships manifested in INDEX
    clauses.  Also many other implementation specific details are
    included, like for example MAX-ACCESS and STATUS clauses and
    conformance statements.

    A special kind of DM language is the SMIng language designed by the
    NMRG.  This language was particularly designed at a higher conceptual
    level then SMIv1/SMIv2 and SPPI.  In fact one of the intentions
    behind SMIng was to stop the proliferation of different DM languages
    and harmonize the various models.  As a result MIBs/PIBs defined in
    SMIng can be mapped on different underlying protocols; there is a
    mapping on SNMP and there is a mapping on COPS-PR.  SMIng is
    therefore more protocol neutral than other IETF approaches.  SMIng
    also supports some object-oriented principles and provides an
    extension mechanism which allows to add more features such as support
    for methods when the protocols support them without breaking SMIng
    implementations.  Still SMIng should be considered as a DM; to
    express for example the relationship between managed objects,
    techniques like UML or ER diagrams give still better results since
    these diagrams are easier to understand.

    It should be noted that the SMIng working group within the IETF
    decided to not adapt the SMIng language defined by the NMRG.
    Instead, the SMIng working group currently focusses resources on
    developing a third version of the SMI (SMIv3) which is primarily
    targeted towards SNMP and which only incorporates some of the ideas
    developed within the NMRG.

5. Acknowledgments

    The authors would like to thank everyone who participated at the 8th
    IRTF-NMRG meeting (in alphabetic order): Szabolcs Boros, Mark



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    Brunner, David Durham, Dave Harrington, Jean-Philippe Martin-Flatin,
    George Pavlou, Robert Parhonyi, David Perkins, David Sidor, Andrea
    Westerinen and Bert Wijnen.

References

    [1]  McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
         M. and S. Waldbusser, "Structure of Management Information
         Version 2 (SMIv2)", RFC 2578, STD 59, 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]  Bernet, Y., Blake, S., Grossman, D. and A. Smith, "An Informal
         Management Model for Diffserv Routers", RFC 3290, May 2002.

    [5]  Bradner, S., "The Internet Standards Process -- Revision 3", RFC
         2026, October 1996.

    [6]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
         RFC 2863, June 2000.

    [7]  International Organization for Standardization, "Information
         processing systems - Open Systems Interconnection -
         Specification of Abstract  Syntax Notation One (ASN.1)",
         International Standard 8824, December 1987.

    [8]  International Organization for Standardization, "Information
         technology - Open Systems Interconnection  - Structure of
         Management Information - Part 4:  Guidelines for the Definition
         of Managed Objects", International Standard 10165-4, 1992.

    [9]   <http://www.irtf.org/>

    [10]  <http://www.omg.org/>












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Authors' Addresses

    Aiko Pras
    University of Twente
    PO Box 217
    7500 AE Enschede
    The Netherlands

    Phone: [phone]
    EMail: [e-mail]


    Juergen Schoenwaelder
    University of Osnabrueck
    Albrechtstr. 28
    49069 Osnabrueck
    Germany

    Phone: [phone]
    EMail: [e-mail]































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Acknowledgement

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



















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