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
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 RFC3198 [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.
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 RFC3290 [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 RFCwhereas the document defining
the DM usually follows the Standards Track [4]. In general, most of
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 RFC2863, 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].
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
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, RFC2578,
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)", RFC3159, 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, RFC2026, October 1996.
[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", RFC3198,
November 2001.
[9] Bernet, Y., Blake, S., Grossman, D. and A. Smith, "An Informal
Management Model for Diffserv Routers", RFC3290, May 2002.
[10] McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
RFC2863, 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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