Network Working Group C. Elliott
Request for Comments: 3216 Cisco Systems
Category: Informational D. Harrington
Enterasys Networks
J. Jason
Intel Corporation
J. Schoenwaelder
F. Strauss
TU Braunschweig
W. Weiss
Ellacoya Networks
December 2001
SMIng Objectives
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 (2001). All Rights Reserved.
Abstract
This document describes the objectives for a new data definition
language, suitable for the modeling of network management constructs,
that can be directly mapped into SNMP and COPS-PR protocol
operations.
The purpose of this document is to serve as a set of objectives that
a subsequent language specification should try to address. It
captures the results of the working group discussions towards
consensus on the SMIng objectives.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Background . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Specific Objectives for SMIng . . . . . . . . . . . . . . 4
4.1 Accepted Objectives . . . . . . . . . . . . . . . . . . . 5
4.1.1 The Set of Specification Documents . . . . . . . . . . . . 6
4.1.2 Textual Representation . . . . . . . . . . . . . . . . . . 6
4.1.3 Human Readability . . . . . . . . . . . . . . . . . . . . 6
Elliott, et al. Informational [Page 1]RFC 3216 SMIng Objectives December 2001
4.1.4 Rigorously Defined Syntax . . . . . . . . . . . . . . . . 6
4.1.5 Accessibility . . . . . . . . . . . . . . . . . . . . . . 7
4.1.6 Language Extensibility . . . . . . . . . . . . . . . . . . 7
4.1.7 Special Characters in Text . . . . . . . . . . . . . . . . 7
4.1.8 Naming . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.9 Namespace Control . . . . . . . . . . . . . . . . . . . . 8
4.1.10 Modules . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.1.11 Module Conformance . . . . . . . . . . . . . . . . . . . . 9
4.1.12 Arbitrary Unambiguous Identities . . . . . . . . . . . . . 9
4.1.13 Protocol Independence . . . . . . . . . . . . . . . . . . 9
4.1.14 Protocol Mapping . . . . . . . . . . . . . . . . . . . . . 10
4.1.15 Translation to Other Data Definition Languages . . . . . . 10
4.1.16 Base Data Types . . . . . . . . . . . . . . . . . . . . . 10
4.1.17 Enumerations . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.18 Discriminated Unions . . . . . . . . . . . . . . . . . . . 11
4.1.19 Instance Pointers . . . . . . . . . . . . . . . . . . . . 11
4.1.20 Row Pointers . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.21 Constraints on Pointers . . . . . . . . . . . . . . . . . 12
4.1.22 Base Type Set . . . . . . . . . . . . . . . . . . . . . . 12
4.1.23 Extended Data Types . . . . . . . . . . . . . . . . . . . 12
4.1.24 Units, Formats, and Default Values of Defined Types and
Attributes . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.25 Table Existence Relationships . . . . . . . . . . . . . . 13
4.1.26 Table Existence Relationships (2) . . . . . . . . . . . . 14
4.1.27 Attribute Groups . . . . . . . . . . . . . . . . . . . . . 14
4.1.28 Containment . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.29 Single Inheritance . . . . . . . . . . . . . . . . . . . . 15
4.1.30 Reusable vs. Final Attribute Groups . . . . . . . . . . . 15
4.1.31 Events . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1.32 Creation/Deletion . . . . . . . . . . . . . . . . . . . . 16
4.1.33 Range and Size Constraints . . . . . . . . . . . . . . . . 16
4.1.34 Uniqueness . . . . . . . . . . . . . . . . . . . . . . . . 16
4.1.35 Extension Rules . . . . . . . . . . . . . . . . . . . . . 17
4.1.36 Deprecate Use of IMPLIED Keyword . . . . . . . . . . . . . 17
4.1.37 No Redundancy . . . . . . . . . . . . . . . . . . . . . . 17
4.1.38 Compliance and Conformance . . . . . . . . . . . . . . . . 17
4.1.39 Allow Refinement of All Definitions in Conformance
Statements . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.40 Categories . . . . . . . . . . . . . . . . . . . . . . . . 18
4.1.41 Core Language Keywords vs. Defined Identifiers . . . . . . 19
4.1.42 Instance Naming . . . . . . . . . . . . . . . . . . . . . 19
4.1.43 Length of Identifiers . . . . . . . . . . . . . . . . . . 19
4.1.44 Assign OIDs in the Protocol Mappings . . . . . . . . . . . 20
4.2 Nice-to-Have Objectives . . . . . . . . . . . . . . . . . 20
4.2.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2.2 Unions . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2.3 Float Data Types . . . . . . . . . . . . . . . . . . . . . 21
4.2.4 Comments . . . . . . . . . . . . . . . . . . . . . . . . . 22
Elliott, et al. Informational [Page 2]RFC 3216 SMIng Objectives December 2001
4.2.5 Referencing Tagged Rows . . . . . . . . . . . . . . . . . 22
4.2.6 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.7 Internationalization . . . . . . . . . . . . . . . . . . . 23
4.2.8 Separate Data Modelling from Management Protocol Mapping . 23
4.3 Rejected Objectives . . . . . . . . . . . . . . . . . . . 24
4.3.1 Incomplete Translations . . . . . . . . . . . . . . . . . 24
4.3.2 Attribute Value Constraints . . . . . . . . . . . . . . . 24
4.3.3 Attribute Transaction Constraints . . . . . . . . . . . . 25
4.3.4 Method Constraints . . . . . . . . . . . . . . . . . . . . 25
4.3.5 Agent Capabilities . . . . . . . . . . . . . . . . . . . . 25
4.3.6 Relationships . . . . . . . . . . . . . . . . . . . . . . 26
4.3.7 Procedures . . . . . . . . . . . . . . . . . . . . . . . . 26
4.3.8 Associations . . . . . . . . . . . . . . . . . . . . . . . 26
4.3.9 Association Cardinalities . . . . . . . . . . . . . . . . 27
4.3.10 Categories of Modules . . . . . . . . . . . . . . . . . . 27
4.3.11 Mapping Modules to Files . . . . . . . . . . . . . . . . . 27
4.3.12 Simple Grammar . . . . . . . . . . . . . . . . . . . . . . 28
4.3.13 Place of Module Information . . . . . . . . . . . . . . . 28
4.3.14 Module Namespace . . . . . . . . . . . . . . . . . . . . . 29
4.3.15 Hyphens in Identifiers . . . . . . . . . . . . . . . . . . 29
5. Security Considerations . . . . . . . . . . . . . . . . . 30
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 30
7. References . . . . . . . . . . . . . . . . . . . . . . . . 30
8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 31
9. Full Copyright Statement . . . . . . . . . . . . . . . . . 33
1. Introduction
This document describes the objectives for a new data definition
language that can be mapped into SNMP [1], [2] and COPS-PR [3]
protocol operations. It may also be translated into SMIv2 [4], [5],
[6] MIBs and SPPI [7] PIBs. Concepts such as attributes, attribute
groups, methods, conventions for organization into reusable data
structures, and mechanisms for representing relationships are
discussed.
2. Motivation
As networking technology has evolved, a diverse set of technologies
has been deployed to manage the resulting products. These vary from
Web based products, to standard management protocols and text
scripts. The underlying systems to be manipulated are represented in
varying ways including implicitly in the system programming, via
proprietary data descriptions, or with standardized descriptions
using a range of technologies including MIBs, PIBs, or LDAP schemas.
The result is that management interfaces for network protocols,
services, and applications such as Differentiated Services may be
represented in many different, inconsistent fashions.
Elliott, et al. Informational [Page 3]
RFC 3216 SMIng Objectives December 2001
The SMIng working group has been chartered to define a new data
definition language that will eliminate the need for a separate SMIv2
and SPPI language. That is, the new language should address the
needs for the current SMIv2 and SPPI languages so that over time we
can all use the new language instead.
Another motivation is to permit a more expressive and complete
representation of the modeled information. Examples of additional
expressiveness and completeness that are considered are the ability
to formally define table existence relationships, the expression of
instance creation/deletion capabilities, and the ability to define
attribute groups using inheritance. These additional features are
discussed in subsequent sections.
It has been recognized that the two main goals of (a) merging
SMIv2/SPPI and (b) enhancing the state of art in network management
data modeling can lead to conflicts. In such cases, the SMIng
working group's consensus is to focus on enhancing the state of art
in network management data modeling.
3. Background
The Network Management Research Group (NMRG) of the Internet Research
Task Force (IRTF) has researched the issues of creating a protocol-
independent data definition language that could be used by multiple
protocols. Because SMIv2 and SPPI are very similar, the NMRG focused
on merging these two languages, but also researched ways to abstract
the objectives to produce a language that could be used for other
protocols, such as LDAP and Diameter. The NMRG has published the
results of their work in a meanwhile expired Internet Draft, but has
submitted their specification as one proposal to consider in the
development of the SMIng language.
The SMIng Working Group has accepted their submission for
consideration, and to use their proposal to better understand the
objectives and possible obstacles to be overcome. Where useful, the
NMRG proposal has been referenced in the details below.
4. Specific Objectives for SMIng
The following sections define the objectives for the definition of a
new data definition language. The objectives have been organized as
follows: accepted objectives (Section 4.1), nice-to-have objectives
(Section 4.2), and rejected objectives (Section 4.3). Each objective
has the following information:
o Type: a field that identifies the type of objective, using one of
the following values:
Elliott, et al. Informational [Page 4]RFC 3216 SMIng Objectives December 2001
* basic: considered a basic objective for SMIng and is contained
in SMIv2 and/or SPPI.
* align: supported in different ways in SMIv2 and SPPI and they
must be aligned.
* fix: considered a fix for a known problem in SMIv2 and/or SPPI.
* new: considered a new feature.
o From: a field that defines the origin of the objective and that
contains one or more of the following values:
* SMI: exists in SMIv2.
* SPPI: exists in SPPI.
* NMRG: exists in the NMRG proposal, but not in SMIv2 or SPPI.
* Charter: exists in working group charter.
* WG: proposed during working group discussions.
o Description: a quick description of the objective.
o Motivation: rationale for the objective.
o Notes: optional notes about an objective. For example, for nice-
to-have or rejected this may contain reasoning why this objective
is not required by the SMIng working group, but justification why
it should be considered anyway. Notes may be the opinions of the
participants in the discussion on objectives and as such should
not be taken as consensus of the working group or the
recommendation of the objectives editing team.
4.1 Accepted Objectives
This section represents the list of objectives that have been
accepted by the SMIng working group as worthwhile and therefore
deserving of further consideration. Each of these objectives must be
evaluated by the working group to determine if the benefit incurs an
acceptable level of cost. An accepted objective may subsequently be
rejected if the cost/benefit analysis determines that the benefit
does not justify the cost or that the objective is in direct conflict
with one or more other accepted objectives that are deemed more
important.
Elliott, et al. Informational [Page 5]RFC 3216 SMIng Objectives December 2001
4.1.1 The Set of Specification Documents
Type: new
From: NMRG
Description: SMIv2 is defined in three documents, based on an
obsolete ITU ASN.1 specification. SPPI is defined in one
document, based on SMIv2. The core of SMIng must be defined in
one document and must be independent of external specifications.
Motivation: Self-containment.
4.1.2 Textual Representation
Type: basic
From: SMI, SPPI, WG
Description: SMIng definitions must be represented in a textual
format.
Motivation: General IETF consensus.
4.1.3 Human Readability
Type: basic
From: WG
Description: The syntax must make it easy for humans to directly read
and write SMIng modules. It must be possible for SMIng module
authors to produce SMIng modules with text editing tools.
Motivation: The syntax must make it easy for humans to read and write
SMIng modules.
4.1.4 Rigorously Defined Syntax
Type: new
From: NMRG
Description: There must be a rigorously defined syntax for the SMIng
language.
Elliott, et al. Informational [Page 6]RFC 3216 SMIng Objectives December 2001
Motivation: An unambiguous language promotes consistency across
vendors so that different parsers produce the same results. It
also provides authoritative rules to SMIng modules designers.
4.1.5 Accessibility
Type: align
From: SMI, SPPI
Description: Attribute definitions must indicate whether attributes
can be read, written, created, deleted, and whether they are
accessible for notifications, or are not accessible. Align PIB-
ACCESS and MAX-ACCESS, and PIB-MIN-ACCESS and MIN-ACCESS.
Motivation: Alignment of SMIv2 and SPPI.
4.1.6 Language Extensibility
Type: new
From: NMRG
Description: The language must have characteristics, so that future
modules can contain information of future syntax without breaking
original SMIng parsers.
E.g., when SMIv2 introduced REFERENCEs it would have been nice if
it would not have broken SMIv1 parsers.
Motivation: Achieve language extensibility without breaking core
compatibility.
4.1.7 Special Characters in Text
Type: new
From: WG
Description: Allow an escaping mechanism to encode special
characters, e.g. double quotes and new-line characters, in text
such as DESCRIPTIONs or REFERENCEs.
Motivation: ABNF can contain literal characters enclosed in double
quotes; to provide the ABNF grammar, there must be the ability to
escape special characters.
Elliott, et al. Informational [Page 7]RFC 3216 SMIng Objectives December 2001
4.1.8 Naming
Type: basic
From: SMI, SPPI
Description: SMIng must provide mechanisms to uniquely identify
attributes, groups of attributes, and events. It is necessary to
specify how name collisions are handled.
Motivation: Already in SMIv2 and SPPI.
4.1.9 Namespace Control
Type: basic
From: SMI, SPPI
Description: There must be a hierarchical, centrally-controlled
namespace for standard named items, and a distributed namespace
must be supported to allow vendor-specific naming and to assure
unique module names across vendors and organizations.
Motivation: Need to unambiguously identify definitions of various
kinds. Some SMI implementations have problems with different
objects from multiple modules but with the same name.
Furthermore, the probability of module name clashes rises over
time (for example, different vendors defining their own SYSTEM-
MIB).
Notes: An example naming scheme is the one employed by the Java
programming language with a central naming authority assigning the
top-level names.
4.1.10 Modules
Type: basic
From: SMI, SPPI
Description: SMIng must provide a mechanism for uniquely identifying
a module, and specifying the status, contact person, revision
information, and the purpose of a module.
SMIng must provide mechanisms to group definitions into modules
and it must provide rules for referencing definitions from other
modules.
Elliott, et al. Informational [Page 8]RFC 3216 SMIng Objectives December 2001
Motivation: Modularity and independent advancement of documents.
Notes: Text about module conformance has been moved to Section
4.1.11.
4.1.11 Module Conformance
Type: basic
From: SMI, SPPI
Description: SMIng must provide mechanisms to detail the minimum
requirements implementers must meet to claim conformance to a
standard based on the module.
Motivation: Ability to convey conformance requirements.
4.1.12 Arbitrary Unambiguous Identities
Type: basic
From: SMI
Description: SMI allows the use of OBJECT-IDENTITIES to define
unambiguous identities without the need of a central registry.
SMI uses OIDs to represent values that represent references to
such identities. SMIng needs a similar mechanism (a statement to
register identities, and a base type to represent values).
Motivation: SMI Compatibility.
Notes: This is an obvious objective. Additionally, everything not on
the wire, such as modules, will still be assigned OIDs.
It is yet to be determined whether the assignment of the OID
occurs within the core or within a protocol-specific mapping.
4.1.13 Protocol Independence
Type: basic
From: Charter
Description: SMIng must define data definitions in support of the
SNMP and COPS-PR protocols. SMIng may define data definitions in
support of other protocols.
Elliott, et al. Informational [Page 9]RFC 3216 SMIng Objectives December 2001
Motivation: So data definitions may be used with multiple protocols
and multiple versions of those protocols.
4.1.14 Protocol Mapping
Type: basic
From: Charter
Description: The SMIng working group, in accordance with the working
group charter, will define mappings of protocol independent data
definitions to protocols based upon installed implementations.
The SMIng working group can define mappings to other protocols as
long as this does not impede the progress on other objectives.
Motivation: SMIng working group charter.
4.1.15 Translation to Other Data Definition Languages
Type: basic
From: Charter
Description: SMIng language constructs must, wherever possible, be
translatable to SMIv2 and SPPI. At the time of standardization of
a SMIng language, existing SMIv2 MIBs and SPPI PIBs on the
standards track will not be required to be translated to the SMIng
language. New MIBs/PIBs will be defined using the SMIng language.
Motivation: Provide best-effort backwards compatibility for existing
tools while not placing an unnecessary burden on MIBs/PIBs that
are already on the standards track.
4.1.16 Base Data Types
Type: basic
From: SMI, SPPI
Description: SMIng must support the base data types Integer32,
Unsigned32, Integer64, Unsigned64, Enumeration, Bits, OctetString,
and OID.
Motivation: Most are already common. Unsigned64 and Integer64 are in
SPPI, must fix in SMI. Note that Counter and Gauge types can be
regarded as derived types instead of base types.
Elliott, et al. Informational [Page 10]RFC 3216 SMIng Objectives December 2001
4.1.17 Enumerations
Type: basic
From: SMI, SPPI
Description: SMIng must provide support for enumerations. Enumerated
values must be a part of the enumeration definition.
Motivation: SMIv2 already has enumerated numbers.
Notes: Enumerations have the implicit constraint that the attribute
is constrained to the values for the enumeration.
4.1.18 Discriminated Unions
Type: new
From: WG
Description: SMIng must support discriminated unions.
Motivation: Allows to group related attributes together, such as
InetAddressType (discriminator) and InetAddress, InetAddressIPv4,
InetAddressIPv6 (union). The lack of discriminated unions has
also lead to relatively complex sparse table work-around in some
DISMAN mid-level manager MIBs.
Notes: Discriminated unions have the property that the union
attribute type is constrained by the value of the discriminator
attribute.
4.1.19 Instance Pointers
Type: basic
From: SMI, SPPI
Description: SMIng must allow specifying pointers to instances (i.e.,
a pointer to a particular attribute in a row).
Motivation: It is common practice in MIBs and PIBs to point to other
instances.
Elliott, et al. Informational [Page 11]RFC 3216 SMIng Objectives December 2001
4.1.20 Row Pointers
Type: align
From: SMI, SPPI
Description: SMIng must allow specifying pointers to rows.
Motivation: It is common practice in MIBs and PIBs to point to other
rows (see RowPointer, PIB-REFERENCES).
4.1.21 Constraints on Pointers
Type: align
From: SPPI
Description: SMIng must allow specifying the types of objects to
which a pointer may point.
Motivation: Allows code generators to detect and reject illegal
pointers automatically. Can also be used to automatically
generate more reasonable implementation-specific data structures.
Notes: Pointer constraints are a special case of attribute value
constraints (Section 4.3.2) in which the prefix of the OID (row or
instance pointer) value is limited to be only from a particular
table.
4.1.22 Base Type Set
Type: basic
From: SMI, SPPI
Description: SMIng must support a fixed set of base types of fixed
size and precision. The list of base types must not be extensible
unless the SMI itself changes.
Motivation: Interoperability.
4.1.23 Extended Data Types
Type: align
From: SMI, SPPI
Elliott, et al. Informational [Page 12]RFC 3216 SMIng Objectives December 2001
Description: SMIng must support a mechanism to derive new types,
which provide additional semantics (e.g., Counters, Gauges,
Strings, etc.), from base types. It may be desirable to also
allow the derivation of new types from derived types. New types
must be as restrictive or more restrictive than the types that
they are specializing.
Motivation: SMI uses application types and textual conventions. SPPI
uses derived types.
4.1.24 Units, Formats, and Default Values of Defined Types and
Attributes
Type: fix
From: NMRG
Description: In SMIv2 OBJECT-TYPE definitions may contain UNITS and
DEFVAL clauses and TEXTUAL-CONVENTIONs may contain DISPLAY-HINTs.
In a similar fashion units and default values must be applicable
to defined types and format information must be applicable to
attributes.
Motivation: Some MIBs introduce TCs such as KBytes and every usage of
the TC then specifies the UNITS "KBytes". It would simplify
things if the UNITS were attached to the type definition itself.
Notes: The SMIng WG must clarify the behavior if an attribute uses a
defined type and both, the attribute and the defined type, have
units/default/format information.
4.1.25 Table Existence Relationships
Type: align
From: SMI, SPPI
Description: SMIng must support INDEX, AUGMENTS, and EXTENDS in the
SNMP/COPS-PR protocol mappings.
Motivation: These three table existence relationships exist either in
the SMIv2 or the SPPI.
Elliott, et al. Informational [Page 13]RFC 3216 SMIng Objectives December 2001
4.1.26 Table Existence Relationships (2)
Type: new
From: NMRG
Description: SMIng must support EXPANDS and REORDERS relationships in
the SNMP/COPS-PR protocol mappings.
Motivation: A REORDERS statement allows indexing orders to be
swapped. An EXPANDS statement formally states that there is a 1:n
existence relationship between table rows.
4.1.27 Attribute Groups
Type: new
From: NMRG
Description: An attribute group is a named, reusable set of
attributes that are meaningful together. It can be reused as the
type of attributes in other attribute groups (see also Section
4.1.28). This is similar to `structs' in C.
Motivation: Required to map the same grouping of attributes into SNMP
and COPS-PR tables. Allows to do index reordering without having
to redefine the attribute group. Allows to group related
attributes together (e.g. InetAddressType, InetAddress).
The ability to group attributes provides an indication that the
attributes are meaningful together.
4.1.28 Containment
Type: new
From: NMRG
Description: SMIng must provide support for the creation of new
attribute groups from attributes of more basic types and
potentially other attribute groups.
Motivation: Simplifies the reuse of attribute groups such as
InetAddressType and InetAddress pairs.
Elliott, et al. Informational [Page 14]RFC 3216 SMIng Objectives December 2001
Notes: Containment has the implicit existence constraint that if an
instance of a contained attribute group exists, then the
corresponding instance of the containing attribute group must also
exist.
4.1.29 Single Inheritance
Type: new
From: NMRG
Description: SMIng must provide support for mechanisms to extend
attribute groups through single inheritance.
Motivation: Allows to extend attribute groups, like a generic
DiffServ scheduler, with attributes for a specific scheduler,
without cut&paste.
Notes: Single inheritance with multiple levels (e.g., C derives from
B, and B derives from A) must be allowed.
Inheritance has the implicit existence constraint that if an
instance of a derived attribute group exists, then the
corresponding instance of the base attribute group must also
exist.
Inheritance could help to add attributes to an attribute group
that are specific to a certain protocol mapping and do not appear
in the protocol-neutral attribute group.
4.1.30 Reusable vs. Final Attribute Groups
Type: new
From: NMRG, WG
Description: SMIng must differentiate between "final" and reusable
attribute groups, where the reuse of attribute groups covers
inheritance and containment.
Motivation: This information gives people more information how
attribute groups can and should be used. It hinders them from
misusing them.
Notes: This objective attempts to convey the idea that some attribute
groups are not meant to stand on their own and instead only make
sense if contained within another attribute group.
Elliott, et al. Informational [Page 15]RFC 3216 SMIng Objectives December 2001
4.1.31 Events
Type: basic
From: SMI, SPPI
Description: SMIng must provide mechanisms to define events which
identify significant state changes.
Motivation: These represent the protocol-independent events that lead
to SMI notifications or SPPI reports.
4.1.32 Creation/Deletion
Type: align
From: SMI, SPPI
Description: SMIng must support a mechanism to define
creation/deletion operations for instances. Specific
creation/deletion errors, such as INSTALL-ERRORS, must be
supported.
Motivation: Available for row creation in SMI, and available in SPPI.
4.1.33 Range and Size Constraints
Type: basic
From: SMI, SPPI
Description: SMIng must allow specifying range and size constraints
where applicable.
Motivation: The SMI and SPPI both support range and size constraints.
4.1.34 Uniqueness
Type: basic
From: SPPI
Description: SMIng must allow the specification of uniqueness
constraints on attributes. SMIng must allow the specification of
multiple independent uniqueness constraints.
Elliott, et al. Informational [Page 16]RFC 3216 SMIng Objectives December 2001
Motivation: Knowledge of the uniqueness constraints on attributes
allows to verify protocol specific mappings (e.g. INDEX clauses).
The knowledge can also be used by code generators to improve
generated implementation-specific data structures.
4.1.35 Extension Rules
Type: basic
From: SMI, SPPI
Description: SMIng must provide clear rules how one can extend SMIng
modules without causing interoperability problems "over the wire".
Motivation: SMIv2 and SPPI have extension rules.
4.1.36 Deprecate Use of IMPLIED Keyword
Type: fix
From: WG
Description: The SMIng SNMP mapping must deprecate the use of the
IMPLIED indexing schema.
Motivation: IMPLIED is confusing and most people don't understand it.
The solution (IMPLIED) is worse than the problem it is trying to
solve and therefore for the sake of simplicity, the use of IMPLIED
should be deprecated.
4.1.37 No Redundancy
Type: fix
From: NMRG
Description: The SMIng language must avoid redundancy.
Motivation: Remove any textual redundancy for things like table
entries and SEQUENCE definitions, which only increase
specifications without providing any value.
4.1.38 Compliance and Conformance
Type: basic
From: SMI, SPPI
Elliott, et al. Informational [Page 17]RFC 3216 SMIng Objectives December 2001
Description: SMIng must provide a mechanism for compliance and
conformance specifications for protocol-independent definitions as
well as for protocol mappings.
Motivation: This capability exists in SMIv2 and SPPI. The NMRG
proposal has the ability to express much of this information at
the protocol-dependent layer. Some compliance or conformance
information may be protocol-independent, therefore there is also a
need to be able to express this information protocol-independent
part.
4.1.39 Allow Refinement of All Definitions in Conformance Statements
Type: fix
From: WG
Description: SMIv2, RFC 2580, Section 3.1 says:
The OBJECTS clause, which must be present, is used to specify
each object contained in the conformance group. Each of the
specified objects must be defined in the same information
module as the OBJECT-GROUP macro appears, and must have a MAX-
ACCESS clause value of "accessible-for-notify", "read-only",
"read-write", or "read-create".
The last sentence forbids to put a not-accessible INDEX object
into an OBJECT-GROUP. Hence, you can not refine its syntax in a
compliance definition. For more details, see
http://www.ibr.cs.tu-bs.de/ietf/smi-errata/
Motivation: This error should not be repeated in SMIng.
4.1.40 Categories
Type: basic
From: SPPI
Description: SMIng must provide a mechanism to group definitions into
subject categories. Concrete instances may only exist in the
scope of a given subject category or context.
Motivation: To scope the categories to which a module applies. In
SPPI this is used to allow a division of labor between multiple
client types.
Elliott, et al. Informational [Page 18]RFC 3216 SMIng Objectives December 2001
4.1.41 Core Language Keywords vs. Defined Identifiers
Type: fix
From: NMRG
Description: In SMI and SPPI modules some language keywords (macros
and a number of basetypes) have to be imported from different SMI
language defining modules, e.g. OBJECT-TYPE, MODULE-IDENTITY,
Integer32 must to be imported from SNMPv2-SMI and TEXTUAL-
CONVENTION must be imported from SNMPv2-TC, if used. MIB authors
are continuously confused about these import rules. In SMIng only
defined identifiers must be imported. All SMIng language keywords
must be implicitly known and there must not be a need to import
them from any module.
Motivation: Reduce confusion. Clarify the set of language keywords.
4.1.42 Instance Naming
Type: align
From: SMI, SPPI
Description: Instance naming in SMIv2 and SPPI is different. SMIng
must align the instance naming (either in the protocol neutral
model or the protocol mappings).
Motivation: COPS-PR and SNMP have different instance identification
schemes that must be handled.
Notes: A solution requires to investigate how close the naming
schemes dictated by the protocols are. Perhaps it is feasible to
have a single instance naming scheme in both SNMP and COPS-PR,
even though the current SPPI and SMIv2 are different.
4.1.43 Length of Identifiers
Type: fix
From: NMRG
Description: The allowed length of the various kinds of identifiers
must be extended from the current `should not exceed 32' (maybe
even from the `must not exceed 64') rule.
Motivation: Reflect current practice of definitions.
Elliott, et al. Informational [Page 19]RFC 3216 SMIng Objectives December 2001
Notes: The 32-rule was added back in the days where compilers could
not deal with long identifiers. This rule is continuously
violated these days and it does not make sense to keep it.
4.1.44 Assign OIDs in the Protocol Mappings
Type: new
From: NMRG
Description: SMIng must not assign OIDs to reusable definition of
attributes, attribute groups, events, etc. Instead, SNMP and
COPS-PR mappings must assign OIDs to the mapped items.
Motivation: Assignment of OIDs in protocol neutral definitions can
complicate reuse. OIDs of synonymous attributes are not the same
in SMI and SPPI definitions. MIBs and PIBs are already registered
in different parts of the OID namespace.
4.2 Nice-to-Have Objectives
This section represents the list of recommended objectives that would
be nice to have. However, these are not automatically thought of as
accepted objectives as, for example, they may entail a non-trivial
amount of work in underlying protocols to support or they may be
regarded as less important than other contradicting objectives that
are accepted.
4.2.1 Methods
Type: new
From: WG
Description: SMIng should support a mechanism to define method
signatures (parameters, return values, exception) that are
implemented on agents.
Motivation: Methods are needed to support the definition of
operational interfaces such as found in [RFC2925] (ping,
traceroute and lookup operations). Also, the ability to define
constructor/destructor interfaces could address issues such as
encountered with SNMP's RowStatus solution.
Notes: Is it possible to do methods without changing the underlying
protocol? There is agreement that methods are useful, but
disagreement upon the impact - one end of the spectrum sees this
as a documentation tool for existing SNMP capabilities, while the
Elliott, et al. Informational [Page 20]RFC 3216 SMIng Objectives December 2001
other end sees this as a protocol update, moving forward, to
natively support methods. The proposal is to wait and see if this
is practical to implement as a syntax that is useful and can map
to the protocol.
4.2.2 Unions
Type: new
From: WG
Description: SMIng should support a standard format for unions.
Motivation: Allows an attribute to contain one of many types of
values. The lack of unions has also lead to relatively complex
sparse table work-around in some DISMAN mid-level managers.
Despite from discriminated unions (see Section 4.1.18), this kind
of union has no accompanied explicit discriminator attribute that
selects the union's type of value.
Notes: The thought is that SNMP and COPS-PR can already support
unions because they do not care about what data type goes with a
particular OID.
4.2.3 Float Data Types
Type: new
From: WG, NMRG
Description: SMIng should support the base data types Float32,
Float64, Float128.
Motivation: Missing base types can hurt later on, because they cannot
be added without changing the language, even as an SMIng
extension. Lesson learned from the SMIv1/v2 debate about
Counter64/Integer64/...
Notes: There is no mention as to whether or not the underlying
protocols will have to natively support float data types. This is
left to the mapping. However, it seems imperative that the float
data type needs to be added to the set of intrinsic types in the
SMIng language at the creation of the language as it will be
impossible to add them later without changing the language.
Elliott, et al. Informational [Page 21]RFC 3216 SMIng Objectives December 2001
4.2.4 Comments
Type: fix
From: NMRG
Description: The syntax of comments should be well defined,
unambiguous and intuitive to most people, e.g., the C++/Java `//'
syntax.
Motivation: ASN.1 Comments (and thus SMI and SPPI comments) have been
a constant source of confusion. People use arbitrary lengthy
strings of dashes (`-----------') in the wrong assumption that
this is always treated as a comment. Some implementations try to
accept these syntactically wrong constructs which even raises
confusion. We should get rid of this problem.
Notes: If the SMIng working group adopts a C-like syntax, then the
C++/Java single-line comment should be adopted as well.
4.2.5 Referencing Tagged Rows
Type: align
From: SPPI
Description: PIB and MIB row attributes reference a group of entries
in another table. SPPI formalizes this by introducing PIB-TAG and
PIB-REFERENCES clauses. This functionality should be retained in
SMIng.
Motivation: SPPI formalizes tag references. Some MIBs also use tag
references (see SNMP-TARGET-MIB in RFC2573) even though SMIv2 does
not provide a formal notation.
4.2.6 Arrays
Type: new
From: WG
Description: SMIng should allow the definition of a SEQUENCE OF
attributes or attribute groups (Section 4.1.27).
Motivation: The desire for the ability to have variable-length,
multi-valued objects.
Elliott, et al. Informational [Page 22]RFC 3216 SMIng Objectives December 2001
Notes: Some issues with arrays are still unclear. As long as there
are no concepts to solve the problems with access semantics (how
to achieve atomic access to arbitrary-sized arrays) and their
mappings to SNMP and COPS-PR protocol operations, arrays cannot be
more than a nice to have objective.
4.2.7 Internationalization
Type: new
From: WG
Description: Informational text (DESCRIPTION, REFERENCE, ...) should
allow i18nized encoding, probably UTF-8.
Motivation: There has been some demand for i18n in the past. The BCP
RFC 2277 demands for internationalization.
Notes: Although English is the language of IETF documents, SMIng
should allow other languages for private use.
4.2.8 Separate Data Modelling from Management Protocol Mapping
Type: new
From: NMRG
Description: It should be possible to separate the domain specific
data modelling work from the network management protocol specific
work.
Motivation: Today, working groups designing new protocols are forced
to care about the design of SNMP MIBs and maybe COPR-PR PIBs to
manage the new protocol. This means that experts in a specific
domain are faced with details of at least one foreign (network
management) technology. This leads to hard work and long revision
processes. It would be a win to separate the task of pure data
modelling which can be done by the domain experts easily from the
network management protocol specific mappings. The mapping to
SNMP and/or COPS-PR can be done (a) later separately and (b) by
network management experts. This required NM expertise no longer
hinders the progress of the domain specific working groups.
Elliott, et al. Informational [Page 23]RFC 3216 SMIng Objectives December 2001
4.3 Rejected Objectives
This section represents the list of objectives that were rejected
during the discussion on the objectives. Those objectives that have
been rejected need not be addressed by SMIng. This does not imply
that they must not be addressed.
4.3.1 Incomplete Translations
Type: basic
From: WG
Description: Reality sucks. All information expressed in SMIng may
not be directly translatable to a MIB or PIB construct, but all
information should be able to be conveyed in documentation or via
other mechanisms.
Motivation: SMIng working group requires this to ease transition.
Notes: The SMIng language itself cannot require what compilers do
that translate SMIng into something else. So this seems to fall
out of the scope of the current working group charter.
4.3.2 Attribute Value Constraints
Type: new
From: WG
Description: SMIng should provide mechanisms to formally specify
constraints between values of multiple attributes.
Motivation: Constraints on attribute values occur where one or more
attributes may affect the value or range of values for another
attribute. One such relationship exists in IPsec, where the type
of security algorithm determines the range of possible values for
other attributes such as the corresponding key size.
Notes: This objective as is has been rejected as too general, and
therefore virtually impossible to implement. However, constraints
that are implicit with discriminated unions (Section 4.1.18),
enumerated types (Section 4.1.17), pointer constraints (Section
4.1.21)), etc., are accepted and these implicit constraints are
mentioned in the respective objectives.
Elliott, et al. Informational [Page 24]RFC 3216 SMIng Objectives December 2001
4.3.3 Attribute Transaction Constraints
Type: new
From: WG
Description: SMIng should provide a mechanism to formally express
that certain sets of attributes can only be modified in
combination.
Motivation: COPS-PR always does operations on table rows in a single
transaction. There are SMIv2 attribute combinations that need to
be modified together (such as InetAddressType, InetAddress).
Notes: Alternative is to either use Methods (Section 4.2.1) or assume
that all attributes in an attribute group (Section 4.1.27) are to
be considered atomic.
4.3.4 Method Constraints
Type: new
From: WG
Description: Method definitions should provide constraints on
parameters.
Motivation: None.
Notes: Unless methods (Section 4.2.1) are done, there is no use for
this. Furthermore, this objective has not been motivated by any
proponent.
4.3.5 Agent Capabilities
Type: basic
From: SMI
Description: SMIng should provide mechanisms to describe agent
implementations.
Motivation: To permit manager to determine variations from the
standard for an implementation.
Notes: Agent capabilities should not be part of SMIng, but should
instead be a separate capabilities table.
Elliott, et al. Informational [Page 25]RFC 3216 SMIng Objectives December 2001
4.3.6 Relationships
Type: new
From: NMRG, WG
Description: Ability to formally depict existence dependency, value
dependency, aggregation, containment, and other relationships
between attributes or attribute groups.
Motivation: Helps humans to understand the conceptual model of a
module. Helps implementers of MIB compilers to generate more
`intelligent' code.
Notes: This objective was deemed too general to be useful and instead
the individual types of relationship objectives (e.g., pointers,
inheritance, containment, etc.) are evaluated on a case-by-case
basis with the specific relationships deemed useful being included
as accepted objectives.
4.3.7 Procedures
Type: new
From: WG
Description: SMIng should support a mechanism to formally define
procedures that are used by managers when interacting with an
agent.
Motivation: None.
Notes: This objective has not been motivated by any proponent.
4.3.8 Associations
Type: new
From: WG
Description: SMIng should provide mechanisms to explicitly specify
associations.
Motivation: None.
Notes: This objective has not been motivated by any proponent.
Elliott, et al. Informational [Page 26]RFC 3216 SMIng Objectives December 2001
4.3.9 Association Cardinalities
Type: new
From: WG
Description: Cardinalities between associations should be formally
defined.
Motivation: If you have an association between attribute groups A and
B, the cardinality of A indicates how many instances of A may be
associated with a single instance of B. Our discussions in
Minneapolis indicated that we want to convey "how many" instances
are associated in order to define the best mapping algorithm -
whether a new table, a single pointer, etc. For example, do we
use RowPointer or an integer index into another table? Do we map
to a table that holds instances of the association/relationship
itself?
Notes: Without associations (Section 4.3.8), this has no use.
4.3.10 Categories of Modules
Type: new
From: WG
Description: The SMIng documents should give clear guidance on which
kind of information (with respect to generality, type/attribute
group/extension/..) should be put in which kind of a module.
E.g., in SMIv2 we don't like to import Utf8String from SYSAPPL-
MIB, but we also do not like to introduce a redundant definition.
A module review process should probably be described that ensures
that generally useful definitions do not go into device or service
specific modules.
Motivation: Bad experience with SMIv2.
Notes: It is not clear how this can be done with the language to be
created by SMIng WG.
4.3.11 Mapping Modules to Files
Type: new
From: NMRG
Elliott, et al. Informational [Page 27]RFC 3216 SMIng Objectives December 2001
Description: There should be a clear statement how SMIng modules are
mapped to files (1:1, n:1?) and how files should be named (by
module name in case of 1:1 mapping?).
Motivation: SMI implementations show up a variety of filename
extensions (.txt, .smi, .my, none). Some expect all modules in a
single file, others don't. This makes it more difficult to
exchange modules.
Notes: This is just an implementation detail and is best left to a
BCP and not made a part of the language definition.
4.3.12 Simple Grammar
Type: new
From: NMRG
Description: The grammar of the language should be as simple as
possible. It should be free of exception rules. A measurement of
simplicity is shortness of the ABNF grammar.
Motivation: Ease of implementation. Ease of learning/understanding.
Notes: This seems like an obvious objective, however shortness of the
ABNF grammar is not necessarily a reflection of the simplicity of
the grammar.
4.3.13 Place of Module Information
Type: fix
From: NMRG
Description: Module specific information (organization, contact,
description, revision information) should be bound to the module
itself and not to an artificial node (like SMIv2 MODULE-IDENTITY).
Motivation: Simplicity and design cleanup.
Notes: This does not seem to be a problem with the current SMI.
Although simplification is a good thing, this detail is not
considered an objective.
Elliott, et al. Informational [Page 28]RFC 3216 SMIng Objectives December 2001
4.3.14 Module Namespace
Type: new
From: WG
Description: Currently the namespace of modules is flat and there is
no structure in module naming causing the potential risk of name
clashes. Possible solutions:
* Assume module names are globally unique (just as SMIv1/v2),
just give some recommendations on module names.
* Force all organizations, WGs and vendors to apply a name prefix
(e.g. CISCO-GAGA-MIB, IETF-DISMAN-SCRIPT-MIB?).
* Force enterprises to apply a prefix based on the enterprise
number (e.g. ENT2021-SOME-MIB).
* Put module names in a hierarchical domain based namespace (e.g.
DISMAN-SCRIPT-MIB.ietf.org).
Motivation: Reduce risk of module name clashes.
Notes: Some aspects of this objective overlap with other objectives
(namespace control (Section 4.1.9)) and other aspects were thought
best left to a BCP.
4.3.15 Hyphens in Identifiers
Type: fix
From: NMRG
Description: There has been some confusion whether hyphens are
allowed in SMIv2 identifiers: Module names are allowed to contain
hyphens. Node identifiers usually are not. But for example
`mib-2' is a frequently used identifier that contains a hyphen due
to its SMIv1 origin, when hyphen were not disallowed. Similarly,
a number of named numbers of enumeration types contain hyphens
violating an SMIv2 rule.
SMIng should simply allow hyphens in all kinds of identifiers. No
exceptions.
Motivation: Reduce confusion and exceptions. Requires, however, that
implementation mappings properly quote hyphens where appropriate.
Elliott, et al. Informational [Page 29]RFC 3216 SMIng Objectives December 2001
Notes: This nit-picking is not worth to be subject to the discussion
on objectives. However, SMIng should care about the fact that
compilers have to map SMIng to programming languages where a
hyphen is a minus and thus not allowed in identifiers.
5. Security Considerations
This document defines objectives for a language with which to write
and read descriptions of management information. The language itself
has no security impact on the Internet.
6. Acknowledgements
Thanks to Dave Durham, whose work on the original NIM (Network
Information Model) draft was used in generating this document.
Thanks to Andrea Westerinen for her contributions on the original NIM
requirements and SMIng objectives drafts.
7. References
[1] Case, J., Fedor, M., Schoffstall, M. and J. Davin, "Simple
Network Management Protocol (SNMP)", STD 15, RFC 1157, May 1990.
[2] McCloghrie, K., Case, J., Rose, M. and S. Waldbusser, "Protocol
Operations for Version 2 of the Simple Network Management
Protocol (SNMPv2)", RFC 1905, January 1996.
[3] Chan, K., Seligson, J., Durham, D., Gai, S., McCloghrie, K.,
Herzog, S., Reichmeyer, F., Yavatkar, R. and A. Smith, "COPS
Usage for Policy Provisioning (COPS-PR)", RFC 3084, March 2001.
[4] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
M. and S. Waldbusser, "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[5] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J., Rose,
M. and S. Waldbusser, "Textual Conventions for SMIv2", STD 58,
RFC 2579, April 1999.
[6] McCloghrie, K., Perkins, D. and J. Schoenwaelder, "Conformance
Statements for SMIv2", STD 58, RFC 2580, April 1999.
[7] 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.
Elliott, et al. Informational [Page 30]RFC 3216 SMIng Objectives December 2001
8. Authors' Addresses
Chris Elliott
Cisco Systems
7025 Kit Creek Road
Research Triangle Park, NC 27709
USA
EMail: chelliot@cisco.com
David Harrington
Enterasys Networks
35 Industrial Way
P.O. Box 5005
Rochester, NH 03866-5005
USA
EMail: dbh@enterasys.com
Jamie Jason
Intel Corporation
MS JF3-206
2111 NE 25th Ave.
Hillsboro, OR 97124
USA
EMail: jamie.jason@intel.com
Juergen Schoenwaelder
TU Braunschweig
Muehlenpfordtstr. 23
38106 Braunschweig
Germany
EMail: schoenw@ibr.cs.tu-bs.de
URI: http://www.ibr.cs.tu-bs.de/
Elliott, et al. Informational [Page 31]
RFC 3216 SMIng Objectives December 2001
Frank Strauss
TU Braunschweig
Muehlenpfordtstr. 23
38106 Braunschweig
Germany
EMail: strauss@ibr.cs.tu-bs.de
URI: http://www.ibr.cs.tu-bs.de/
Walter Weiss
Ellacoya Networks
7 Henry Clay Dr.
Merrimack, NH. 03054
USA
EMail: wweiss@ellacoya.com
Elliott, et al. Informational [Page 32]
RFC 3216 SMIng Objectives December 2001
9. Full Copyright Statement
Copyright (C) The Internet Society (2001). 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.
Elliott, et al. Informational [Page 33]
