Movatterモバイル変換

Network Working Group                                            A. PrasRequest for Comments: 3444                          University of TwenteCategory: Informational                                 J. Schoenwaelder                                                University of Osnabrueck                                                            January 2003On the Difference betweenInformation Models and Data ModelsStatus 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 Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .23.  Information Models . . . . . . . . . . . . . . . . . . . . . .34.  Data Models  . . . . . . . . . . . . . . . . . . . . . . . . .45.  Security Considerations  . . . . . . . . . . . . . . . . . . .66.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .67.  Normative References . . . . . . . . . . . . . . . . . . . . .68.  Informative References . . . . . . . . . . . . . . . . . . . .79.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .710. Full Copyright Statement . . . . . . . . . . . . . . . . . . .8Pras & Schoenwaelder         Informational                      [Page 1]

RFC 3444           Information Models and Data Models       January 20031. 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.,   inRFC 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 byRFC 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 ofPras & 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 isRFC 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 PIBPras & 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",BCP9,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.dePras & Schoenwaelder         Informational                      [Page 7]

RFC 3444           Information Models and Data Models       January 200310.  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]