RFC3216
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.
4.1.24 Units, Formats, and Default Values of Defined Types and
4.1.39 Allow Refinement of All Definitions in Conformance
4.2.8 Separate Data Modelling from Management Protocol Mapping . 23
Contents
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.
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.
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.
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.
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:
* 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.
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.
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.
Textual Representation
Type: basic
From: SMI, SPPI, WG
Description: SMIng definitions must be represented in a textual
format.
Motivation: General IETF consensus.
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.
Rigorously Defined Syntax
Type: new
From: NMRG
Description: There must be a rigorously defined syntax for the SMIng
language.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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.
Authors' Addresses
Chris Elliott Cisco Systems 7025 Kit Creek Road Research Triangle Park, NC 27709 USA
EMail: [email protected]
David Harrington Enterasys Networks 35 Industrial Way P.O. Box 5005 Rochester, NH 03866-5005 USA
EMail: [email protected]
Jamie Jason Intel Corporation MS JF3-206 2111 NE 25th Ave. Hillsboro, OR 97124 USA
EMail: [email protected]
Juergen Schoenwaelder TU Braunschweig Muehlenpfordtstr. 23 38106 Braunschweig Germany
EMail: [email protected] URI: http://www.ibr.cs.tu-bs.de/
Frank Strauss TU Braunschweig Muehlenpfordtstr. 23 38106 Braunschweig Germany
EMail: [email protected] URI: http://www.ibr.cs.tu-bs.de/
Walter Weiss Ellacoya Networks 7 Henry Clay Dr. Merrimack, NH. 03054 USA
EMail: [email protected]
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