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Submission of <draft-ietf-ops-mib-review-guidelines-00.txt>
Greetings,
I wish to submit the attached file as an Internet Draft.
The filename is <draft-ietf-ops-mib-review-guidelines-00.txt>.
It is the initial submission of an OPS area draft that has
been authorized by OPS Area AD Bert Wijnen.
If you require verification that this posting is authorized,
you may contact Bert at <bwijnen@lucent.com>.
Regards and thanks,
Mike
--
C. M. Heard
heard@pobox.com
INTERNET DRAFT C. M. Heard, Editor
Consultant
February 2003
Guidelines for MIB Authors and Reviewers
<draft-ietf-ops-mib-review-guidelines-00.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This memo provides guidelines for authors and reviewers of IETF
specifications containing MIB modules.
OPS Area Expires August 2003 [Page 1]
Internet Draft Guidelines for MIB Authors and Reviewers February 2003
Table of Contents
1 Introduction ................................................. 3
2 Terminology .................................................. 3
3 General Documentation Guidelines ............................. 4
3.1 MIB Boilerplate Section .................................... 4
3.2 Narrative Sections ......................................... 4
3.3 Definitions Section ........................................ 5
3.4 Intellectual Property Section .............................. 5
3.5 References Sections ........................................ 5
3.6 Security Considerations Section ............................ 5
3.7 IANA Considerations Section ................................ 6
3.8 Copyright Notices .......................................... 7
4 SMIv2 Usage Guidelines ....................................... 8
4.1 Module Names ............................................... 8
4.2 Descriptors and Labels ..................................... 8
4.3 Naming Hierarchy ........................................... 9
4.4 IMPORTS Statement .......................................... 9
4.5 MODULE-IDENTITY Invocation ................................. 10
4.6 Textual Conventions and Object Definitions ................. 11
4.6.1 Usage of Data Types ...................................... 11
4.6.1.1 INTEGER, Integer32, Gauge32, and Unsigned32 ............ 11
4.6.1.2 Counter32 and Counter64 ................................ 13
4.6.1.3 CounterBasedGauge64 .................................... 14
4.6.1.4 OCTET STRING ........................................... 14
4.6.1.5 OBJECT IDENTIFIER ...................................... 15
4.6.1.6 The BITS Construct ..................................... 15
4.6.1.7 IpAddress .............................................. 16
4.6.1.8 TimeTicks .............................................. 16
4.6.1.9 TruthValue ............................................. 16
4.6.1.10 Other Data Types ...................................... 16
4.6.2 DESCRIPTION and REFERENCE Clauses ........................ 17
4.6.3 Conceptual Table Definitions ............................. 17
4.6.4 OID Values Assigned to Objects ........................... 19
4.6.5 OID Length Limitations and Table Indexing ................ 19
4.7 Notification Definitions ................................... 20
4.8 Compliance Statements ...................................... 21
4.9 Revisions to MIB Modules ................................... 23
Appendix A: MIB Review Checklist ............................. 26
Appendix B: Using smilint to compile MIB modules ............. 28
Appendix C: Using SMICng to compile MIB modules .............. 29
Appendix D: Commonly Used Textual Conventions ................ 31
Intellectual Property ......................................... 33
Normative References .......................................... 34
Informative References ........................................ 36
Security Considerations ....................................... 37
Acknowledgments ............................................... 37
Editor's Address .............................................. 37
Full Copyright Statement ...................................... 38
OPS Area Expires August 2003 [Page 2]
Internet Draft Guidelines for MIB Authors and Reviewers February 2003
1. Introduction
Some time ago the IESG instituted a policy of requiring OPS area
review of all IETF specifications containing MIB modules. These
reviews were established to ensure that such specifications follow
established IETF documentation practices and that the MIB modules
they contain meet certain generally accepted standards of quality,
including (but not limited to) compliance with all syntactic and
semantic requirements of SMIv2 (STD 58) [RFC2578] [RFC2579] [RFC2580]
that are applicable to "standard" MIB modules. The purpose of this
memo is to document the guidelines that are followed in such reviews.
Please note that the guidelines in this memo are not intended to
alter requirements or prohibitions (in the sense of "MUST", "MUST
NOT", "SHALL", or "SHALL NOT" as defined in RFC 2119 [RFC2119]) of
existing BCPs or Internet Standards except where those requirements
or prohibitions are ambiguous or contradictory. In the exceptional
cases where ambiguities or contradictions exist this memo documents
the current generally accepted interpretation. In certain instances
the guidelines in this memo do alter recommendations (in the sense of
"SHOULD", "SHOULD NOT", "RECOMMENDED", or "NOT RECOMMENDED" as
defined in RFC 2119) of existing BCPs or Internet Standards. This
has been done where practical experience has shown that the published
recommendations are suboptimal. In addition, this memo provides
guidelines for the selection of certain SMIv2 options (in the sense
of "MAY" or "OPTIONAL" as defined in RFC 2119) in cases where there
is a consensus on a preferred approach.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL", when used in the guidelines in this memo, are to be
interpreted as described in RFC 2119 [RFC2119].
The terms "MIB module" and "information module" are used
interchangeably in this memo. As used here, both terms refer to any
of the three types of information modules defined in Section 3 of RFC
2578 [RFC2578].
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
3. General Documentation Guidelines
In general, IETF specifications containing MIB modules MUST conform
to the requirements for IETF RFCs documented in [RFC2223bis].
Because the version under review will be an Internet Draft, the
notices on the front page will comply with the requirements of
http://www.ietf.org/ietf/1id-guidelines.txt and not with those of
[RFC2223bis]. The rest of the requirements in [RFC2223bis] will,
however, apply (see http://www.ietf.org/ID-nits.html for details).
Section 4 of [RFC2223bis] lists the sections that may exist in an
RFC. The "body of memo" part of an RFC in general contains multiple
sections, and in a MIB document MUST contain at least the following:
o MIB boilerplate section
o Narrative sections
o Definitions section
o Intellectual Property section
Section-by-section guidelines follow.
3.1. MIB Boilerplate Section
This section MUST contain a verbatim copy of the latest approved
Internet-Standard Management Framework boilerplate, which is
available on-line at http://www.ops.ietf.org/mib-boilerplate.html
3.2. Narrative Sections
The narrative part MUST include an overview section that describes
the scope and field of application of the MIB modules defined by the
specification and that specifies the relationship (if any) of these
MIB modules to other standards, particularly to standards containing
other MIB modules. The narrative part SHOULD include one or more
sections that briefly describes the structure of the MIB modules
defined in the specification.
If the MIB modules defined by the specification are always
implemented in conjunction with other MIB modules, then that fact
MUST be noted in the overview section, as MUST any special
interpretations of objects in other MIB modules. For instance, so-
called media-specific MIB modules are always implemented in
conjunction with the IF-MIB [RFC2863] and are required to document
how certain objects in the IF-MIB are used. In addition, media-
specific MIB modules that rely on the ifStackTable [RFC2863] and the
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
ifInvStackTable [RFC2864] to maintain information regarding
configuration and multiplexing of interface sublayers must contain a
description of the layering model.
3.3. Definitions Section
This section contains the MIB modules defined by the specification.
These MIB modules MUST be written in SMIv2 [RFC2578] [RFC2579]
[RFC2580]; SMIv1 [RFC1155] [RFC1212] [RFC1215] has "Not Recommended"
status [RFC3410] and is no longer acceptable in IETF MIB modules.
See Section 4 for guidelines on SMIv2 usage.
3.4. Intellectual Property Section
Bullets (A) and (B) in Section 10.4 of RFC 2026 [RFC2026] specify
that certain notices regarding intellectual or other property rights
appear in all standards-track RFCs. Verbatim copies of these so-
called IPR notices MUST appear in each MIB document that is destined
for the standards track. If proprietary rights are known to be
claimed with respect to the technology described in the document,
then the notice in bullet (D) MUST also appear in the document.
3.5. References Sections
Section 4.8 of [RFC2223bis] specifies the requirements for the
references sections. In particular, there MUST be separate lists of
normative and informative references, each in a separate section.
The style SHOULD follow that of recently published RFCs.
The standard MIB boilerplate available at
http://www.ops.ietf.org/mib-boilerplate.html includes lists of
normative and informative references that MUST appear in all
specifications that contain MIB modules. If items from other MIB
modules appear in an IMPORTS statement in the Definitions section,
then the specifications containing those other MIB modules MUST be
included in the list of normative references.
In general, each normative reference SHOULD point to the most recent
version of the specification in question.
3.6. Security Considerations Section
In order to comply with Section 4.9 of [RFC2223bis], each
specification that defines one or more MIB modules MUST contain a
section that discusses security considerations relevant to those MIB
modules. This section MUST be patterned after the latest approved
template available at http://www.ops.ietf.org/mib-security.html. In
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
particular, writeable MIB objects that could be especially disruptive
if abused MUST be explicitly listed by name and the associated
security risks MUST be spelled out; similarly, readable MIB objects
that contain especially sensitive information MUST be explicitly
listed by name and the reasons for the special sensitivity MUST be
explained.
3.7. IANA Considerations Section
In order to comply with Section 4.10 of [RFC2223bis], each
specification that defines a name space of assigned numbers MUST
include an IANA Considerations section conforming to the guidelines
set forth in [RFC2434] specifying how that name space is to be
administered.
Name spaces defined by MIB documents generally consist of the range
of values for some type (usually an enumerated INTEGER) defined by a
TEXTUAL-CONVENTION (TC) or of a set of administratively-defined
OBJECT IDENTIFIER (OID) values. In each case the definitions are
housed in stand-alone MIB modules that are maintained by IANA. These
IANA-maintained MIB modules are kept separate from the MIB modules
defined in standards-track specifications so that new assignments can
be made without having to republish a standards-track RFC. Examples
of IANA-maintained MIB modules include the IANAifType-MIB
(http://www.iana.org/assignments/ianaiftype-mib), which defines a
name space used by the IF-MIB [RFC2863], and the IANA-RTPROTO-MIB
(http://www.iana.org/assignments/ianaiprouteprotocol-mib), which
defined a name space used by the IPMROUTE-STD-MIB [RFC2932].
The current practice for such cases is to include a detailed IANA
Considerations section complying with [RFC2434] in the DESCRIPTION
clause of the MODULE-IDENTITY invocation in each IANA-maintained MIB
module and for the IANA Considerations section of the MIB document
that defines the name space to refer to the URLs for the relevant
modules. See RFC 2932 [RFC2932] for an example. This creates a
chicken-and-egg problem for MIB documents that have not yet been
published as RFCs because the relevant IANA-maintained MIB modules
will not yet exist. The accepted way out of this dilemma is to
include the initial content of the proposed IANA-maintained MIB
modules in appendices of the Internet Draft with a note to the RFC
Editor that those appendices are to be removed upon publication, when
the IANA-maintained modules go on-line.
Reviewers of draft MIB documents to which these considerations apply
MUST check that the IANA Considerations section proposed for
publication in the RFC is present and contains pointers to the
appropriate IANA-maintained MIB modules. Reviewers of Internet
Drafts that contain the proposed initial content of IANA-maintained
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
MIB modules MUST also verify that the DESCRIPTION clauses of the
MODULE-IDENTITY invocations contain an IANA Considerations section
compliant with the guidelines in [RFC2434].
Note that an IANA Considerations section is NOT required if the only
IANA action needed is the assignment of the object identifier for the
MIB module's MODULE-IDENTITY value. A note in the form of an ASN.1
comment requesting such an assignment is sufficient for this; see
Section 4.5 for an example.
3.8. Copyright Notices
IETF MIB documents MUST contain the copyright notices specified in
Section 4.3 of RFC 2223bis [RFC2223bis] and Section 10.4 Bullet (C)
of RFC 2026 [RFC2026]. Authors and reviewers MUST check make sure
that the correct year is inserted into these notices. In addition,
the DESCRIPTION clause of the MODULE-IDENTITY invocation of each MIB
module that will appear in the published RFC must contain a pointer
to the copyright notices in the RFC. A template suitable for
inclusion in an Internet Draft is as follows:
DESCRIPTION
" [ ... ]
Copyright (C) The Internet Society (date). This version
of this MIB module is part of RFC yyyy; see the RFC
itself for full legal notices."
-- RFC Ed.: replace yyyy with actual RFC number & remove this note
where the current year is inserted in place of the word "date".
OPS Area Expires August 2003 [Page 7]
Internet Draft Guidelines for MIB Authors and Reviewers February 2003
4. SMIv2 Usage Guidelines
In general, MIB modules in IETF standards-track specifications MUST
comply with all syntactic and semantic requirements of SMIv2
[RFC2578] [RFC2579] [RFC2580] that apply to "standard" MIB modules
and except as noted below SHOULD comply with SMIv2 recommendations.
The guidelines in this section are intended to supplement the SMIv2
documents in the following ways:
o to document the current generally accepted interpretation when
those documents contain ambiguities or contradictions;
o to update recommendations in those documents that have been shown
by practical experience to be out-of-date or otherwise suboptimal;
o to provide guidance in selection of SMIv2 options in cases where
there is a consensus on a preferred approach.
4.1. Module Names
RFC 2578 Section 3 specifies the rules for module names. Note in
particular that names of "standard" modules MUST be unique, MUST
follow the syntax rules in RFC 2578 Section 3, and MUST NOT be
changed when a MIB module is revised (see also RFC 2578 Section 10).
It is strongly RECOMMENDED that module names be mnemonic.
4.2. Descriptors and Labels
RFC 2578 Sections 3.1, 7.1.1, and 7.1.4 and RFC 2579 Section 3
recommend that descriptors associated with macro invocations and
labels associated with enumerated INTEGER and BITS values be no
longer than 32 characters, but require that they be no longer than 64
characters.
Restricting descriptors and labels to 32 characters often conflicts
with the recommendation that they be mnemonic and (for descriptors)
with the requirement that they be unique (see RFC 2578 Section 3.1
and RFC 2579 Section 3). The consensus of the current pool of OPS
Area MIB reviewers is that the SMIv2 recommendation to limit
descriptors and labels to 32 characters SHOULD be set aside in favor
of promoting clarity and uniqueness and that automated tools such as
MIB compilers SHOULD NOT by default generate warnings for violating
that recommendation.
Note that violations of the 64 character limit MUST NOT be ignored;
they MUST be treated as errors.
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
4.3. Naming Hierarchy
RFC 2578 Section 4 describes the object identifier subtrees that are
maintained by IANA and specifies the usages for those subtrees. In
particular, the mgmt subtree { iso 3 6 1 2 } is used to identify IETF
"standard" objects, while the experimental subtree { iso 3 6 1 3 } is
used to identify objects that are under development in the IETF. It
is REQUIRED that objects be moved from the experimental subtree to
the mgmt subtree when a MIB module enters the IETF standards track.
Experience has shown that it is impractical to move objects from one
subtree to another once those objects have seen large-scale use in an
operational environment. Hence any object that is targeted for
deployment in an operational environment MUST NOT be registered under
the experimental subtree, irrespective of the standardization status
of that object. The experimental subtree should be used only for
objects that are intended for limited experimental deployment. Such
objects typically are defined in Experimental RFCs.
Note: the term "object", as used here and in RFC 2578 Section 4, is
to be broadly interpreted as any construct that results in an OBJECT
IDENTIFIER registration. The list of such constructs is specified in
RFC 2578 Section 3.6.
4.4. IMPORTS Statement
RFC 2578 Section 3.2 specifies which symbols must be imported and
also lists certain pre-defined symbols that must not be imported.
The general requirement is that if an external symbol other than a
predefined ASN.1 type or the BITS construct is used, then it MUST be
mentioned in the module's IMPORTS statement. The words "external
object" in the first paragraph of that section may give the
impression that such symbols are limited to those that refer to
object definitions, but that is not the case, as subsequent
paragraphs should make clear.
Note that exemptions to this general requirement are granted by RFC
2580 Sections 5.4.3 and 6.5.2 for descriptors of objects appearing in
the OBJECT clause of a MODULE-COMPLIANCE statement or in the
VARIATION clause of an AGENT-CAPABILITIES statement. Some MIB
compilers also grant exemptions to descriptors of notifications
appearing in a VARIATION clause and to descriptors of object groups
and notification groups referenced by a MANDATORY-GROUPS clause, a
GROUP clause, or an INCLUDES clause, although RFC 2580 (through
apparent oversight) does not mention those cases. The exemptions are
sometimes seen as unhelpful because they make IMPORTS rules more
complicated and inter-module dependencies less obvious than they
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
otherwise would be. External symbols referenced by compliance
statements and capabilities statements MAY therefore be listed in the
IMPORTS statement; if this is done, it SHOULD be done consistently.
Finally, even though it is not forbidden by the SMI, it is considered
poor style to import symbols that are not used, and "standard" MIB
modules SHOULD NOT do so.
4.5. MODULE-IDENTITY Invocation
RFC 2578 Section 3 requires that all SMIv2 MIB modules start with
exactly one invocation of the MODULE-IDENTITY macro. This invocation
MUST appear immediately after the IMPORTS statement.
RFC 2578 Section 5 describes how the various clauses are used. The
following additional guidelines apply to all MIB modules over which
the IETF has change control:
- If the module was developed by an IETF working group, then the
ORGANIZATION clause MUST provide the full name of the working
group, and the CONTACT-INFO clause MUST include working group
mailing list information. The CONTACT-INFO clause SHOULD also
provide a pointer to the working groups's web page.
- A REVISION clause MUST be present for each revision of the MIB
module, and the UTC time of the most recent REVISION clause MUST
match that of the LAST-UPDATED clause. The DESCRIPTION clause
associated with each revision MUST state in which RFC that revision
appeared and SHOULD provide a list of all significant changes.
When a MIB module is revised UTC times in all REVISION clauses
SHOULD be updated to use four-digit year notation.
- The value assigned to the MODULE-IDENTITY descriptor MUST be unique
and SHOULD reside under the mgmt subtree [RFC2578]. Most often it
will be an IANA-assigned value directly under mib-2 [RFC2578],
although for media-specific MIB modules that extend the IF-MIB
[RFC2863] it is customary to use an IANA-assigned value under
transmission [RFC2578]. It is also acceptable for a working group
to make its own assignments from a subtree delegated to it by IANA,
provided that adequate controls are in place to ensure that such
assignments are unique.
While a MIB module is under development, the RFC number in which it
will eventually be published is usually unknown and must be filled in
by the RFC Editor prior to publication. An appropriate form for the
REVISION clause applying to a version under development would be
something along the following lines:
OPS Area Expires August 2003 [Page 10]
Internet Draft Guidelines for MIB Authors and Reviewers February 2003
REVISION "200212132358Z" -- December 13, 2002
DESCRIPTION "Initial version, published as RFC yyyy."
-- RFC Ed.: replace yyyy with actual RFC number & remove this note
Note that after RFC publication a REVISION clause is present only for
published versions of a MIB module and not for interim versions that
existed only as Internet Drafts. Thus, a draft version of a MIB
module MUST contain just one new REVISION clause that covers all
changes since the last published version (if any).
When the initial version of a MIB module is under development, the
value assigned to the MODULE-IDENTITY descriptor will be unknown if
an IANA-assigned value is used, because the assignment is made just
prior to publication as an RFC. The accepted form for the MODULE-
IDENTITY statement in draft versions of such a module is something
along the following lines:
<descriptor> MODULE-IDENTITY
[ ... ]
::= { <subtree> XXX }
-- RFC Ed.: replace XXX with IANA-assigned number & remove this note
where <descriptor> is whatever descriptor has been selected for the
module and <subtree> is the subtree under which the module is to be
registered (e.g., mib-2 or transmission). Note that XXX must be
temporarily replaced by a number in order for the module to compile.
4.6. Textual Conventions and Object Definitions
4.6.1. Usage of Data Types
4.6.1.1. INTEGER, Integer32, Gauge32, and Unsigned32
The 32-bit integer data types INTEGER, Integer32, Gauge32, and
Unsigned32 are described in RFC 2578 Section 2 and further elaborated
in RFC 2578 Sections 7.1.1, 7.1.7, and 7.1.11. The following
guidelines apply when selecting one of these data types for an object
definition or a textual convention:
- For integer-valued enumerations:
- INTEGER is REQUIRED;
- Integer32, Unsigned32, and Gauge32 MUST NOT be used.
Note that RFC 2578 recommends (but does not require) that integer-
valued enumerations start at 1 and be numbered contiguously. This
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recommendation SHOULD be followed unless there is a valid reason to
do otherwise, e.g., to match values of external data or to indicate
special cases, and any such special-case usage SHOULD be clearly
documented. For an example see the InetAddressType TC [RFC3291].
- If the value range is between -2147483648..2147483647 (inclusive)
and negative values are possible, then:
- Integer32 is RECOMMENDED;
- INTEGER is acceptable;
- Unsigned32 and Gauge32 MUST NOT be used.
- If the value range is between 0..4294967295 (inclusive) and the
value of the information being modelled may increase above the
maximum value or decrease below the minimum value, then:
- Gauge32 is RECOMMENDED;
- Unsigned32 is acceptable;
- INTEGER and Integer32 MUST NOT be used if
values greater than 2147483647 are possible.
- If the value range is between 0..4294967295 (inclusive), and values
greater than 2147483647 are possible, and the value of the
information being modelled does not increase above the maximum
value nor decrease below the minimum value, then:
- Unsigned32 is RECOMMENDED;
- Gauge32 is acceptable;
- INTEGER and Integer32 MUST NOT be used.
- If the value range is between 0..2147483647 (inclusive), and the
value of the information being modelled does not increase above the
maximum value nor decrease below the minimum value, then:
- Unsigned32 is RECOMMENDED;
- INTEGER, Integer32, and Gauge32 are acceptable.
- For integer-valued objects that appear in an INDEX clause or for
integer-valued TCs that are to be used in an index column:
- Unsigned32 with a range that excludes zero is RECOMMENDED for
most index objects. It is acceptable to include zero in the
range when it is semantically significant or when it is used as
the index value for a unique row with special properties. Such
usage SHOULD be clearly documented in the DESCRIPTION clause.
- Integer32 or INTEGER with a non-negative range is acceptable.
Again, zero SHOULD be excluded from the range except when it is
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semantically significant or when it is used as the index value
for a unique row with special properties, and in such cases the
usage SHOULD be clearly documented in the DESCRIPTION clause.
- Use of Gauge32 is appropriate for index objects that have gauge
semantics.
The guidelines above combine both the usage rules for integer data
types and the INDEX rules in RFC 2578 Section 7.7 up to and including
bullet (1) plus the next-to-last paragraph on page 28.
Sometimes it will be necessary for external variables to represent
values of an index object -- e.g., ifIndex [RFC2863]. In such cases
authors of the module containing that object SHOULD consider defining
TCs such as InterfaceIndex and/or InterfaceIndexOrZero [RFC2863].
Note that INTEGER is a pre-defined ASN.1 type and MUST NOT be present
in a module's IMPORTS statement, whereas Integer32, Gauge32, and
Unsigned32 are defined by SNMPv2-SMI and MUST be imported from that
module if used.
4.6.1.2. Counter32 and Counter64
Counter32 and Counter64 have special semantics as described in RFC
2578 Sections 7.1.6 and 7.1.10 respectively. Object definitions MUST
(and textual conventions SHOULD) respect these semantics. That
means:
- It is OK to use Counter32/64 for counters that can reset themselves
on unusual/irregular events (e.g., counters maintained on a line
card may be reset when the line card is reset), and it is not
illegal if their value after the reset happens to be zero (i.e., it
does not have to be non-zero). So, "counters that can reset to
zero" is not automatically wrong for Counter32/64. However, if it
is possible for such resets to occur, then a discontinuity
indicator object SHOULD be provided to indicate when the last such
reset occurred.
- It is NOT OK to put in the DESCRIPTION clause of a Counter32/64
that there is a requirement that on a discontinuity the counter
MUST reset to zero.
- It is NOT OK to put in the DESCRIPTION clause of a Counter32/64
that there is a requirement that it MUST reset at any specific
time/event (e.g., midnight).
- It is NOT OK for one manager to request the agent to reset the
value of counter(s) to zero, and Counter32/64 is the wrong syntax
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
for "counters" which regularly reset themselves to zero. For the
latter it is better to define or use textual conventions such as
those in RFC 2493 [RFC2493].
There also exist closely-related textual conventions
ZeroBasedCounter32 and ZeroBasedCounter64 defined in RMON2-MIB
[RFC2021] and HCNUM-TC [RFC2856], respectively.
The only difference between ZeroBasedCounter32/64 TCs and
Counter32/64 is their starting value; at time=X, where X is their
minimum-wrap-time after they were created, the behaviour of
ZeroBasedCounter32/64 becomes exactly the same as Counter32/64.
Thus, the preceeding paragraphs/rules apply not only to Counter32/64,
but also to ZeroBasedCounter32/64 TCs.
4.6.1.3. CounterBasedGauge64
SMIv2 unfortunately does not provide 64-bit integer base types. In
order to make up for this omission, the CounterBasedGauge64 textual
convention is defined in HCNUM-TC [RFC2856]. This TC uses Counter64
as a base type, but discards the special counter semantics, which is
allowed under the generally accepted interpretation of RFC 2579
Section 3.3. It does inherit all the syntactic restrictions of that
type, which means that it MUST NOT be subtyped and that objects
defined with it MUST NOT appear in an INDEX clause, MUST NOT have a
DEFVAL clause, and MUST have a MAX-ACCESS of read-only or
accessible-for-notify.
This TC SHOULD be used for object definitions that require a 64-bit
unsigned data type with gauge semantics. If a 64-bit unsigned data
type with different semantics is needed, then a different TC based on
Counter64 MUST be used, since one TC cannot refine another (cf. RFC
2579 Section 3.5).
4.6.1.4. OCTET STRING
The OCTET STRING type is described in RFC 2578 Section 7.1.2. It
represents arbitrary binary or textual data whose length is between 0
and 65535 octets inclusive. Objects and TCs whose SYNTAX is of this
type SHOULD have a size constraint when the actual bounds are more
restrictive than the SMI-imposed limits. This is particularly true
for index objects. Note, however, that size constraints SHOULD NOT
be imposed arbitrarily, as the SMI does not permit them to be changed
afterward.
There exist a number of standard TCs that cater to some of the more
common requirements for specialized OCTET STRING types. In
particular, SNMPv2-TC [RFC2579] contains the DisplayString,
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PhysAddress, MacAddress, and DateAndTime TCs, and the SNMP-
FRAMEWORK-MIB [RFC3411] contains the SnmpAdminString TC. When a
standard TC provides the desired semantics, it SHOULD be used in an
object's SYNTAX clause instead of OCTET STRING or an equivalent
locally-defined TC.
Note that OCTET STRING is a pre-defined ASN.1 type and MUST NOT be
present in a module's IMPORTS statement.
4.6.1.5. OBJECT IDENTIFIER
The OBJECT IDENTIFIER type is described in RFC 2578 Section 7.1.3.
Its instances represent administratively assigned names. Note that
both the SMI and the SNMP protocol limit instances of this type to
128 sub-identifiers and require that each sub-identifier be within
the range 0 to 4294967295 inclusive. Sub-typing is not allowed.
The purpose of OBJECT IDENTIFIER values is to provide authoritative
identification either for some type of item or for a specific
instance of some type of item. Among the items that can be
identified in this way are definitions in MIB modules created via the
MODULE-IDENTITY, OBJECT-IDENTITY, OBJECT-TYPE, NOTIFICATION-TYPE,
OBJECT-GROUP, NOTIFICATION-GROUP, MODULE-COMPLIANCE, and AGENT-
CAPABILITIES constructs, instances of objects defined in MIB modules,
protocols, languages, specifications, interface types, hardware, and
software. For some of these uses other possibilities exist, e.g.,
OCTET STRING or enumerated INTEGER values. The OBJECT IDENTIFIER
type SHOULD be used instead of the alternatives when the set of
identification values needs to be independently extensible without
the need for a registry to provide centralized coordination.
There exist a number of standard TCs that cater to some of the more
common requirements for specialized OBJECT IDENTIFIER types. In
particular, SNMPv2-TC [RFC2579] contains the AutonomousType,
VariablePointer, and RowPointerTCs. When a standard TC provides the
desired semantics, it SHOULD be used in an object's SYNTAX clause
instead of OBJECT IDENTIFIER or an equivalent locally-defined TC.
Note that OBJECT IDENTIFIER is a pre-defined ASN.1 type and MUST NOT
be present in a module's IMPORTS statement.
4.6.1.6. The BITS Construct
The BITS construct is described in RFC 2578 Section 7.1.4. It
represents an enumeration of named bits. The bit positions in a TC
or object definition whose SYNTAX is of this type MUST start at 0 and
MUST be contiguous.
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Note that the BITS construct is defined by the macros that use it and
therefore MUST NOT be present in a module's IMPORTS statement.
4.6.1.7. IpAddress
The IpAddress type described in RFC 2578 Section 7.1.5 SHOULD NOT be
used in new MIB modules. The InetAddress/InetAddressType textual
conventions [RFC3291] SHOULD be used instead.
4.6.1.8. TimeTicks
The TimeTicks type is described in RFC 2578 Section 7.1.8. It
represents the time in hundredths of a second between two epochs,
reduced modulo 2^32. It MUST NOT be sub-typed, and the DESCRIPTION
clause of any object or TC whose SYNTAX is of this type MUST identify
the reference epochs.
The TimeTicks type SHOULD NOT be used directly in definitions of
objects that are snapshots of sysUpTime [RFC3418]. The TimeStamp TC
[RFC2579] already conveys the desired semantics and SHOULD be used
instead.
4.6.1.9. TruthValue
The TruthValue TC is defined in SNMPv2-TC [RFC2579]. It is an
enumerated INTEGER type that assumes the values true(1) and false(2).
This TC SHOULD be used in the SYNTAX clause of object definitions
that require a Boolean type. MIB modules SHOULD NOT use enumerated
INTEGER types or define TCs that duplicate its semantics.
4.6.1.10. Other Data Types
There exist a number of standard TCs that cater to some of the more
common requirements for specialized data types. Some have been
mentioned above, and Appendix D contains a partial list that includes
those plus some others that are a bit more specialized. Note that
there exist many standard TCs that are not mentioned there; that
list can be expected to to grow as additional TCs are developed.
Whenever a standard TC already conveys the desired semantics, it
SHOULD be used in an object definition instead of the corresponding
base type or a locally-defined TC. This is especially true of the
TCs defined in SNMPv2-TC [RFC2579] and SNMP-FRAMEWORK-MIB [RFC3411]
because they are Internet Standards, and so modules that refer to
them will not suffer delay in advancement on the standards track on
account of such references.
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4.6.2. DESCRIPTION and REFERENCE Clauses
It is hard to overemphasize the importance of an accurate and
unambiguous DESCRIPTION clause for all objects and TCs. The
DESCRIPTION clause contains the instructions that implementors will
use to implement an object, and if they are inadequate or ambiguous,
then implementation quality will suffer. Probably the single most
important job of a MIB reviewer is to ensure that DESCRIPTION clauses
are sufficiently clear and unambiguous to allow interoperable
implementations to be created.
A very common problem is to see an object definition for, say,
"Number of poofpoofs" with no indication what a 'poofpoof' is. In
such cases it is strongly RECOMMENDED that there either be at least a
minimal explanation or else a REFERENCE clause to point to the
definition of a 'poofpoof'.
4.6.3. Conceptual Table Definitions
RFC 2578 Sections 7.1.12 and 7.1.12.1 specify the rules for defining
conceptual tables, and RFC 2578 Sections 7.7, 7.8, and 7.8.1 specify
conceptual table indexing rules. The following guidelines apply to
such definitions:
- For conceptual rows:
- If the row is an extension of a row in some other table, then an
AUGMENTS clause MUST be used if the relationship is one-to-one,
and an INDEX clause MUST be used if the relationship is sparse.
In the latter case the INDEX clause SHOULD be identical to that
of the original table.
- If the row is an element of an expansion table -- that is, if
multiple row instances correspond to a single row instance in
some other table -- then an INDEX clause MUST be used, and the
first-mentioned elements SHOULD be the indices of that other
table, listed in the same order.
- If objects external to the row are present in the INDEX clause,
then the conceptual row's DESCRIPTION clause MUST specify how
those objects are used in identifying instances of its columnar
objects, and in particular MUST specify for which values of those
index objects the conceptual row may exist.
- If dynamic row creation and/or deletion by management applications
is supported, then:
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- There MUST be one columnar object with a SYNTAX value of
RowStatus [RFC2579] and a MAX-ACCESS value of read-create. This
object is called the status column for the conceptual row. All
other columnar objects MUST have a MAX-ACCESS value of read-
create, read-only, accessible-for-notify, or not-accessible; a
MAX-ACCESS value of read-write is not allowed.
- There either MUST be one columnar object with a SYNTAX value of
StorageType [RFC2579] and a MAX-ACCESS value of read-create, or
else the row object DESCRIPTION clause MUST specify what happens
to dynamically-created rows after an agent restart.
- If the agent itself may also create and/or delete rows, then the
conditions under which this can occur MUST be clearly documented
in the row object DESCRIPTION clause.
- For conceptual rows that include a status column:
- The DESCRIPTION clause of the status column MUST specify which
columnar objects (if any) have to be set to valid values before
the row can be activated. If any objects in cascading tables
have to be populated with related data before the row can be
activated, then this MUST also be specified.
- The DESCRIPTION clause of the status column MUST specify whether
or not it is possible to modify other columns in the same
conceptual row when the status value is active(1). Note that in
many cases it will be possible to modify some writeable columns
when the row is active but not others. In such cases the
DESCRIPTION clause for each writeable column SHOULD state whether
or not that column can be modified when the row is active, and
the DESCRIPTION clause for the status column SHOULD state that
modifiability of other columns when the status value is active(1)
is specified in the DESCRIPTION clauses for those columns (rather
than listing the modifiable columns individually).
- For conceptual rows that include a StorageType column:
- The DESCRIPTION clause of the StorageType column MUST specify
which read-write or read-create columnar objects in permanent(4)
rows an agent must, at a minimum, allow to be writable.
Complete requirements for the RowStatus and StorageType TCs are can
be found in RFC 2579, in the DESCRIPTION clauses for those TCs.
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4.6.4. OID Values Assigned to Objects
RFC 2578 Section 7.10 specifies the rules for assigning OBJECT
IDENFIFIER (OID) values to OBJECT-TYPE definitions. In particular:
- A conceptual table MUST have exactly one subordinate object, which
is a conceptual row. The OID assigned to the conceptual row MUST
be derived by appending a sub-identifier of "1" to the OID assigned
to the conceptual table.
- A conceptual row has as many subordinate objects as there are
columns in the row; there MUST be at least one. The OID assigned
to each columnar object MUST be derived by appending a non-zero
sub-identifier, unique within the row, to the OID assigned to the
conceptual row.
- A columnar or scalar object MUST NOT have any subordinate objects.
- The last sub-identifier of an OID assigned to any object (be it
table, row, column, or scalar) MUST NOT be equal to zero. Note
that sub-identifiers of intermediate nodes MAY be equal to zero.
- The OID assigned to an object definition MUST NOT also be assigned
to another definition that results in OID registration. RFC 2578
Section 3.6 lists the constructs that create OID registrations.
4.6.5. OID Length Limitations and Table Indexing
As specified in RFC 2578 Section 3.5, all OIDs are limited to 128
sub-identifiers. While this is not likely to cause problems with
administrative assignments, it does place some limitations on table
indexing. That is true because the length limitation also applies to
OIDs for object instances, and these consist of the concatenation of
the "base" OID assigned in the object definition plus the index
components. When a table has multiple indices of types such as OCTET
STRING or OBJECT IDENTIFIER that resolve to multiple sub-identifiers,
then the 128 OID limitation can be quickly reached.
Despite its inconvenience, the 128 sub-identifier limit is not
something that can be ignored. In addition to being imposed by the
SMI, it is also imposed by the SNMP (see the last paragraph in
Section 4.1 of RFC 3416 [RFC3416]). It follows that any table with
enough indexing components to violate this limit cannot be read or
written using the SNMP and so is unusable. Hence table design MUST
take the 128 sub-identifier limit into account. In some cases it is
practical to use size constraints on the index variables to enforce
the limit, and their use is strongly RECOMMENDED in such cases.
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4.7. Notification Definitions
RFC 2578 Section 8 specifies the rules for notification definitions.
In particular:
- Inaccessible objects MUST NOT appear in the OBJECTS clause.
- For each object type mentioned in the OBJECTS clause, the
DESCRIPTION clause MUST specify which object instance is to be
present in the transmitted notification and MUST specify the
information/meaning conveyed.
- The OBJECT IDENTIFIER (OID) value assigned to each notification
type MUST have a next-to-last sub-identifier of zero, so that it is
possible to convert an SMIv2 notification definition into an SMIv1
trap definition and back again without information loss (see
[RFC2576] Section 2.1.2) and possible for a multilingual proxy
chain to translate an SNMPv2 trap into an SNMPv1 trap and back
again without information loss (see [RFC2576] Section 3). In
addition, the OID assigned to a notification definition MUST NOT
also be assigned to another definition that results in OID
registration. RFC 2578 Section 3.6 lists the constructs that
create OID registrations.
Although it is not specifically required by the SMI, it is customary
(and strongly RECOMMENDED) that notification definitions not be
registered beneath group definitions, compliance statements,
capabilities statements, or object definitions (this last is
especially unwise, as it may result in an object instance and a
notification definition sharing the same OID). It is also customary
(and strongly RECOMMENDED) that the OIDs assigned to notification
types be leaf OIDs (i.e., that there be no OID registrations
subordinate to a notification definition).
In many cases notifications will be triggered by external events, and
sometimes it will be possible for those external events to occur at a
sufficiently rapid rate that sending a notification for each
occurrence would overwhelm the network. In such cases a mechanism
MUST be provided for limiting the rate at which the notification can
be generated. One mechanism that has been widely used is to require
the notification generator to use throttling -- that is, to ensure
that no more than one notification is generated for each event source
in any given time interval of duration T. The throttling period T
MAY be configurable, in which case it would be specified in a MIB
object, or it MAY be fixed, in which case it would be specified in
the notification definition. Examples of the second technique can be
found in the SNMP-REPEATER-MIB [RFC2108] and in the ENTITY-MIB
[RFC2737].
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4.8. Compliance Statements
RFC 2580 Sections 3, 4, and 5 specify the rules for conformance
groups and compliance statements. In particular:
- Every object with a MAX-ACCESS value other than "not-accessible"
MUST be contained in at least one object group.
- Every notification MUST be contained in at least one notification
group.
- There MUST be at least one compliance statement defined for each
"standard" MIB module. It may reside either within that MIB module
or within a companion MIB module.
In writing compliance statements there are several points that are
easily overlooked:
- An object group or notification group that is not mentioned either
in the MANDATORY-GROUPS clause or in any GROUP clause of a MODULE-
COMPLIANCE statement is unconditionally optional with respect to
that compliance statement. An alternate way to indicate that an
object group or notification group is optional is to mention it in
a GROUP clause whose DESCRIPTION clause states that the group is
optional. The latter method is RECOMMENDED (for optional groups
that are relevant to the compliance statement) in order to make it
clear that the optional status is intended rather than being the
result of an act of omission.
- If there are any objects with a MAX-ACCESS value of read-write or
read-create for which there is no OBJECT clause that specifies a
MIN-ACCESS of read-only, then implementations must support write
access to those objects in order to be compliant with that MODULE-
COMPLIANCE statement. This fact sometimes catches MIB module
authors by surprise. When confronted with such cases, reviewers
SHOULD verify that this is indeed what the authors intended, since
it often is not.
- On the other side of the coin, MIB module authors need to be aware
that while a read-only compliance statement is sufficient to
support interoperable monitoring applications, it is not sufficient
to support interoperable configuration applications. A technique
commonly used in MIB modules that are intended to support both
monitoring and configuration is to provide both a read-only
compliance statement and a full compliance statement. A good
example is provided by the DIFFSERV-MIB [RFC3289]. Authors SHOULD
consider using this technique in situations where it is
appropriate.
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Sometimes MIB module authors will want to specify that a compliant
implementation needs to support only a subset of the values allowed
by an object's SYNTAX clause. For accessible objects this may be
done either by specifying the required values in an object's
DESCRIPTION clause or by providing an OBJECT clause with a refined
SYNTAX in a compliance statement. The latter method is RECOMMENDED
for most cases, and is REQUIRED if there are multiple compliance
statements with different value subsets required. The DIFFSERV-MIB
[RFC3289] illustrates this point. The diffServMIBFullCompliance
statement contains the following contains the OBJECT clause (*)
OBJECT diffServDataPathStatus
SYNTAX RowStatus { active(1) }
WRITE-SYNTAX RowStatus { createAndGo(4), destroy(6) }
DESCRIPTION
"Support for createAndWait and notInService is not required."
whereas the diffServMIBReadOnlyCompliance statement contains this:
OBJECT diffServDataPathStatus
SYNTAX RowStatus { active(1) }
MIN-ACCESS read-only
DESCRIPTION
"Write access is not required, and active is the only status that
needs to be supported."
One cannot do this for inaccessible index objects because they cannot
be present in object groups and cannot be mentioned in OBJECT
clauses. There are situations, however, in which one might wish to
indicate that an implementation is required to support only a subset
of the possible values of some index in a read-create table. In such
cases the requirements MUST be specified either in the index object's
DESCRIPTION clause (RECOMMENDED if there is only one value subset) or
in the DESCRIPTION clause of a MODULE-COMPLIANCE statement (REQUIRED
if the value subset is unique to the compliance statement).
____________________
(*) There has been some dispute as to whether syntax refinements that
restrict enumerations (RFC 2578, Section 9) are permitted with TCs,
as shown in these examples, or are allowed only with the base types
INTEGER and BITS, as suggested by a strict reading of RFC 2578. The
rough consensus of the editors of the SMIv2 documents and the current
pool of OPS Area MIB reviewers is that they should be allowed with
TCs. MIB module authors should be aware that some MIB compilers
follow the strict reading of RFC 2578 and require that the TC be
replaced by its base type (INTEGER or BITS) when enumerations are
refined. That usage is legal, and it can be found in some older MIB
modules such as the IF-MIB [RFC2863].
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4.9. Revisions to MIB Modules
RFC 2578 Section 10 specifies general rules that apply any time a MIB
module is revised. Specifically:
- The MODULE-IDENTITY invocation must be updated to include
information about the revision. In particular, the LAST-UPDATED
clause value MUST be set to the revision time, a REVISION clause
with the same UTC time and an associated DESCRIPTION clause
describing the changes MUST be added, and any obsolete information
in the existing DESCRIPTION, ORGANIZATION, and CONTACT-INFO clauses
MUST be replaced with up-to-date information. See Section 4.5
above for additional requirements that apply to MIB modules that
are under IETF change control.
- On the other hand, the module name MUST NOT be changed (except to
correct typographical errors), existing definitions (even obsolete
ones) MUST NOT be removed from the MIB module, and descriptors and
OBJECT IDENTIFIER values associated with existing definitions MUST
NOT be changed or re-assigned.
It is important to note that the purpose in forbidding certain kinds
of changes is to ensure that a revised MIB module is compatible with
fielded implemantations based on previous versions of the module.
There are two distinct aspects of this backward compatibility
requirement. One is "over the wire" compatibility of agent and
manager implementations that are based on different revisions of the
MIB module. The other is "compilation" compatibility with MIB
modules that import definitions from the revised MIB module. The
rules forbidding changing or re-assigning OBJECT IDENTIFIER values
are necessary to ensure "over the wire" compatibility; the rules
against changing module names or descriptions or removing obsolete
definitions are necessary to ensure compilation compatibility.
RFC 2578 Section 10.2 specifies rules that apply to revisions of
object definitions. The following guidelines correct some errors in
these rules and provided some clarifications:
- Bullet (1) allows the labels of named numbers and named bits in
SYNTAX clauses of type enumerated INTEGER or BITS to be changed.
This can break compilation compatibility, since those labels may be
used by DEFVAL clauses in modules that import the definitions of
the affected objects. Therefore, labels of named numbers and named
bits MUST NOT be changed when revising IETF MIB modules, and they
SHOULD NOT be changed when revising enterprise MIB modules.
- Although not specifically permitted in bullets (1) through (8), it
is generally considered acceptable to add range constraints to the
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SYNTAX clause of an integer-valued object, provided that the
constraints simply make explicit some value restrictions that were
implicit in the definition of the object. The most common example
is an auxiliary object with a SYNTAX of INTEGER or Integer32 with
no range constraint. Since an auxiliary object is not permitted to
assume negative values, adding the range constraint (0..2147483647)
cannot possibly result in any "over the wire" change, nor will it
cause any compilation compatibility problems with a correctly
written MIB module. Such a change SHOULD be treated by a reviewer
as an editorial change, not as a semantic change.
RFC 2578 Section 10.3 specifies rules that apply to revisions of
notification definitions. No clarifications or corrections are
required.
RFC 2579 Section 5 specifies rules that apply to revisions of textual
convention definitions. The following guideline corrects an error in
these rules:
- Bullet (1) allows the labels of named numbers and named bits in
SYNTAX clauses of type enumerated INTEGER or BITS to be changed.
This can break compilation compatibility, since those labels may be
used by DEFVAL clauses in modules that import the definitions of
the affected TCs. Therefore, labels of named numbers and named
bits MUST NOT be changed when revising IETF MIB modules, and they
SHOULD NOT be changed when revising enterprise MIB modules.
RFC 2580 Section 7.1 specifies rules that apply to revisions of
conformance groups. Two point are worth re-iterating:
- Objects and notifications MUST NOT be added to or removed from an
existing object group or notification group. Doing so could cause
a compilation failure or (worse) a silent change in the meaning of
a compliance statement or capabilities statement that refers to
that group.
- The status of a conformance group is independent of the status of
its members. Thus, a current group MAY refer to deprecated objects
or notifications. This may be desirable in certain cases, e.g., a
set of widely-deployed objects or notifications may be deprecated
when they are replaced by a more up-to-date set of definitions, but
the conformance groups that contain them may remain current in
order to encourage continued implementation of the deprecated
objects and notifications.
RFC 2580 Section 7.2 specifies rules that apply to revisions of
compliance statements. The following guidelines correct an omission
from these rules and emphasize one important point:
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- RFC 2580 should (but does not) recommend that OBJECT clauses
specifying support for the original set of values be added to a
compliance statement when enumerated INTEGER objects or BITS
objects referenced by the compliance statement have enumerations
added, assuming that no such clauses are already present. This is
necessary in order to avoid a silent change to the meaning of the
compliance statement. MIB module authors and reviewers SHOULD
watch for this to ensure that such OBJECT clauses are added when
needed. Note that this may not always be possible to do, since
affected compliance statements may reside in modules other than the
one that contains the revised definition(s).
- The status of a compliance statement is independent of the status
of its members. Thus, a current compliance statement MAY refer to
deprecated object groups or notification groups. This may be
desirable in certain cases, e.g., a set of widely-deployed object
or notification groups may be deprecated when they are replaced by
a more up-to-date set of definitions, but compliance statements
that refer to them may remain current in order to encourage
continued implementation of the deprecated groups.
RFC 2580 Section 7.3 specifies rules that apply to revisions of
capabilities statements. The following guideline corrects an
omission from these rules:
- RFC 2580 should (but does not) recommend that VARIATION clauses
specifying support for the original set of values be added to a
capabilities statement when enumerated INTEGER objects or BITS
objects referenced by the capabilities statement have enumerations
added, assuming that no such clauses are already present. This is
necessary in order to avoid a silent change to the meaning of the
capabilities statement.
In certain exceptional situations the cost of strictly following the
SMIv2 rules governing MIB modules revisions may exceed the benefit.
In such cases the rules can be waived, but when that is done both the
change and the justification for it MUST be thoroughly documented.
One example is provided by Section 3.1.5 of RFC 2863, which documents
the semantic change that was made to ifIndex in the transition from
MIB-II [RFC1213] to the IF-MIB [RFC2863] and provides a detailed
justification for that change. Another example is provided by the
REVISION clause of the SONET-MIB [RFC2558] that documents raising the
MAX-ACCESS of several objects to read-write while adding MIN-ACCESS
of read-only for compatibility with the previous version [RFC1595].
Authors and reviewers may find it helpful to use tools that can list
the differences between two revisions of a MIB module. One such tool
is smidiff; see Appendix B for more information.
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Appendix A: MIB Review Checklist
The purpose of a MIB review is to review the MIB module both for
technical correctness and for adherence to IETF documentation
requirements. The following checklist may be helpful when reviewing
a draft document:
1.) I-D Boilerplate -- verify that the draft contains the required
Internet Draft boilerplate (see http://www.ietf.org/ietf/1id-
guidelines.txt), including the appropriate statement to permit
publication as an RFC, and that I-D boilerplate does not contain
references or section numbers.
2.) Abstract -- verify that the abstract does not contain references,
that it does not have a section number, and that its content follows
the guidelines in [RFC2223bis].
3.) MIB Boilerplate -- verify that the draft contains the latest
approved SNMP Network Management Framework boilerplate from the OPS
area web site (http://www.ops.ietf.org/mib-boilerplate.html).
4.) IPR Notices -- verify that the draft contains verbatim copies of
the IPR notices specified in bullets (A) and (B) and if needed bullet
(D) of Section 10.4 of RFC 2026.
5.) References -- verify that the references are properly divided
between normative and informative references, that RFC 2119 is
included as a normative reference if the terminology defined therein
is used in the document, that all references required by the
boilerplace are present, that all MIB modules containing imported
items are cited as normative references, and that all citations point
to the most current RFCs unless there is a valid reason to do
otherwise (for example, it is OK to include an informative reference
to a previous version of a specification to help explain a feature
included for backward compatibility).
6.) Security Considerations Section -- verify that the draft uses the
latest approved template from the OPS area web site
(http://www.ops.ietf.org/mib-security.html) and that the guidelines
therein have been followed.
7.) IANA Considerations Section -- if the draft contains the initial
version of an IANA-maintained module, verify that the MODULE-IDENTITY
invocation contains maintenance instructions that comply with RFC
2434. Note that module itself must be in an appendix that will
disappear after publication and that the IANA Considerations section
that will appear in the RFC must contain a pointer to that module.
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8.) Copyrights -- verify that the draft contains a copyright notice
on the front page and in the DESCRIPTION clause of the MODULE-
IDENTITY invocation and there is a full copyright notice section with
text matching that in recently published RFCs. Make sure that the
year is up-to-date in all three places.
9.) MIB compilation -- examine all error or warning messages
generated by SMICng and smilint when set to maximum complaint levels
(exception: warnings for names longer than 32 characters should be
ignored). In general, error messages (E from SMICng, severity <= 4
from smilint) indicate conditions that MUST be corrected, and warning
messages (W from SMICng, severity >= 5 from smilint) indicate
conditions that SHOULD be corrected. Judgment is required, however,
because there are situations when a diagnostic message will be issued
for something that is in fact legitimate (the converse is also true).
10.) Technical content -- review the actual technical content for
compliance with the guidelines in this document. It is particularly
important to check that DESCRIPTION clauses are sufficiently clear
and unambiguous to allow interoperable implementations to be created.
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Appendix B: Using smilint to compile MIB modules
smilint is part of the open-source libsmi package maintained by Frank
Strauss of the Technical University of Braunschweig; see
http://www.ibr.cs.tu-bs.de/projects/libsmi/ for more information and
downloading instructions.
By default, the smilint program does only minimal checking, and in a
MIB review one generally wishes to run the program at maximum
complaint level, discarding only those warnings pertaining to names
longer than 32 characters:
smilint -m -s -l 9 -i namelength-32 <module>
where <module> is replaced with the file name of the module being
compiled. A full pathname is required unless it resides in one of
the standard MIB directories (see the program documentation for more
information).
There is a related utility called smidiff that is useful for checking
whether updates to a previously-published MIB module conform to the
SMIv2 rules on revisions. Command line options similar to those
above are recommended, except that smidiff does not honor the switch
"-i namelength-32". The same effect can be obtained by merging
stdout and stderr and piping the result through the command
grep -v 'name .* longer than 32 characters$'
For those who are unable or unwilling to compile and install smilint
and smidiff locally, there is an e-mail service that can be used
instead. In order to check a MIB module all that is necessary is to
send an e-mail message to smilint@ibr.cs.tu-bs.de with a blank
subject line and the MIB module in the message body. The service
will run smilint on that MIB module using the default switches shown
above and will send back an error report by return e-mail. There is
also an expert mode that allows multiple MIB module attachments and
supports arbitrary commands. Instructions on using it can be
obtained by sending an e-mail message to smilint@ibr.cs.tu-bs.de with
"help" in the subject line (less the quotes) and an empty message
body. The service will send back a help page via return e-mail.
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Appendix C: Using SMICng to compile MIB modules
SMICng is a commercial program available from SNMPinfo; see
http://www.snmpinfo.com/ for more information.
As a rule, MIB modules are not compiled directly with SMICng but
rather are referenced by a SMICng include file, which mentions the
modules from which symbols are imported and contains any module-
specific compilation options. For the purposes of doing MIB reviews,
selecting maximally picky compilation switches is usually the right
thing to do. A sample include file (for the SONET-MIB [RFC2558]) is
shown below. Note that this include file was developed for use with
SMICng 2.2.07, and some changes (particularly to the names of
included files) may be required for use with later versions.
-- File: rfc2558.inc -- SONET-MIB
-- IMPORTS
#condInclude "rfc1902.inc" -- SNMPv2-SMI
#condInclude "rfc1903.inc" -- SNMPv2-TC
#condInclude "rfc1904.inc" -- SNMPv2-CONF
#condInclude "rfc2863.inc" -- IF-MIB
#condInclude "rfc2493.inc" -- PerfHist-TC-MIB
-- MIB module
#pushOpt
-- Add extra strict checking beyond that mandated by SMI
-- C - check size/range present
-- W - don't allow size/range for items in a sequence
-- H - strict ASN.1 for comments
-- 7 - restrict INTEGER values below 2G-1
-- R - check (in V1) that INDEX objs are read-only
-- S - require (in V2) that items in compliances be imported
-- B - strong checking for size/range of items in index clause
-- 0B - act as BITS is builtin
#addOpt "C W H 7 R S B 0B"
-- Remove strict checking for benign practices permitted by SMI
-- 9 - no dup OID values with OVAs
-- 0M - check if index items used in VARIABLES or OBJECTS clause
#removeOpt "9 0M"
-- Remove relaxed checking beyond that forbidden by SMI
-- 4 - allow non-standard access for objects
-- 5 - allow 'optional' status
-- 6 - no check table, row, seq names
-- J - allow DEFVAL on Counters
-- K - allow (in v1) zero valued enums
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-- U - allow underbars in names
-- L - allow (in v1) neg enum values
-- 2 - allow (in v2) use of RFC 1442 types
-- 3 - allow (in v1) INDEX clause on objects
-- O - allow (in v2) hyphens in labels for enumerated values
-- P - allow (in v2) hyphens in descriptors(identifiers)
-- T - no check (in v2) of proper access for items in groups
-- M - no check (in v2) that NTs and accessible OTs are in a group
-- F - allow integer/integer32 index items without a range
-- A - allow (in v2) MIB modules missing a MODULE-IDENTITY
-- G - allow unused IMPORTS and textual conventions
-- N - no check (in v2) of access of objects in notifications
-- I - use (in v1) the v2 rules for checking ACCESS of index items
-- Z - allow bad table modelling practices
-- 0I - allow use of SMI items and macros without being imported
-- 0N - no check (in v1) of access of leaf items in traps
-- 0S - allow non-matching syntax of sequence items
#removeOpt "4 5 6 J K U L 2 3 O P T M F A G N I Z 0I 0N 0S"
-- Relax strict checking for practices permitted (or required) by SMI
-- V - allow (in v2) a V1 OID or OT as V2 group
-- E - allow seq item syntax to match TC
-- 0V - allow use dup items in VARIABLES or OBJECTS clause
#addOpt "V E 0V"
#condinclude "rfc2558.mi2"
#condExcludeModule SONET-MIB 0
#popOpt
In some cases the switches selected above will cause SMICng to
complain about something that is not an error. For instance,
#removeOpt "G" causes complaints about unused TCs in a module such as
PerfHist-TC-MIB [RFC2493] that contains nothing but TCs, while
#removeOpt "Z" will cause complaints about the ifInvStackTable in the
IF-INVERTED-STACK-MIB [RFC2864] because the indices appear in the
opposite order than in the IF-MIB's [RFC2863] ifStackTable (which is
by design). In such cases the sense of the switches in question
should be reversed.
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Appendix D: Commonly Used Textual Conventions
The following TCs are defined in SNMPv2-TC [RFC2579]:
DisplayString OCTET STRING (SIZE (0..255))
PhysAddress OCTET STRING
MacAddress OCTET STRING (SIZE (6))
TruthValue enumerated INTEGER
TestAndIncr INTEGER (0..2147483647)
AutonomousType OBJECT IDENTIFIER
VariablePointer OBJECT IDENTIFIER
RowPointer OBJECT IDENTIFIER
RowStatus enumerated INTEGER
TimeStamp TimeTicks
TimeInterval INTEGER (0..2147483647)
DateAndTime OCTET STRING (SIZE (8 | 11))
StorageType enumerated INTEGER
TDomain OBJECT IDENTIFIER
TAddress OCTET STRING (SIZE (1..255))
Note: InstancePointer is obsolete and MUST NOT be used.
The following TC is defined in SNMP-FRAMEWORK-MIB [RFC3411]:
SnmpAdminString OCTET STRING (SIZE (0..255))
The following TCs are defined in INET-ADDRESS-MIB [RFC3291]:
InetAddressType enumerated INTEGER
InetAddress OCTET STRING (SIZE (0..255))
InetAddressPrefixLength Unsigned32
InetPortNumber Unsigned32 (0..65535))
InetAutonomousSystemNumber Unsigned32
The following TCs are defined in TRANSPORT-ADDRESS-MIB [RFC3419]:
TransportDomain OBJECT IDENTIFIER
TransportAddressType enumerated INTEGER
TransportAddress OCTET STRING (SIZE (0..255))
The following TC is defined in RMON2-MIB [RFC2021]:
ZeroBasedCounter32 Gauge32
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The following TCs are defined in HCNUM-TC [RFC2856]:
ZeroBasedCounter64 Counter64
CounterBasedGauge64 Counter64
The following TCs are defined in IF-MIB [RFC2863]:
InterfaceIndex Integer32 (1..2147483647)
InterfaceIndexOrZero Integer32 (0..2147483647)
The following TCs are defined in PerfHist-TC-MIB [RFC2493]:
PerfCurrentCount Gauge32
PerfIntervalCount Gauge32
PerfTotalCount Gauge32
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Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
OPS Area Expires August 2003 [Page 33]
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Normative References
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirements Levels", BCP 14, RFC 2119, March 1997.
[RFC2223bis]
Reynolds, J., and R. Braden, "Instructions to Request for
Comments (RFC) Authors", work in progress (currently
<draft-rfc-editor-rfc2223bis-03.txt>).
[RFC2434] Alvestrand, H., "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[RFC2578] 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.
[RFC2579] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J.,
Rose, M. and S. Waldbusser, "Textual Conventions for
SMIv2", STD 58, RFC 2579, April 1999.
[RFC2580] McCloghrie, K., Perkins, D., Schoenwaelder, J., Case, J.,
Rose, M. and S. Waldbusser, "Conformance Statements for
SMIv2", STD 58, RFC 2580, April 1999.
[RFC2863] McCloghrie, K. and F. Kastenholz, "The Interfaces Group
MIB", RFC 2863, June 2000.
[RFC2864] McCloghrie, K. and G. Hanson, "The Inverted Stack Table
Extension to the Interfaces Group MIB", RFC 2864, June 2000.
[RFC2493] Tesink, K., "Textual Conventions for MIB Modules Using
Performance History Based on 15 Minute Intervals", RFC 2493,
January 1999.
[RFC2021] Waldbusser, S., "Remote Network Monitoring Management
Information Base Version 2 using SMIv2", RFC 2021, January
1997.
[RFC2856] Bierman, A., McCloghrie, K. and R. Presuhn, "Textual
Conventions for Additional High Capacity Data Types", RFC
2856, June 2000.
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
[RFC3291] Daniele, M., Haberman, B., Routhier, S. and J.
Schoenwaelder, "Textual Conventions for Internet Network
Addresses", RFC 3291, May 2002.
[RFC3411] Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture
for Describing Simple Network Management Protocol (SNMP)
Management Frameworks", STD 62, RFC 3411, December 2002.
[RFC3416] Presuhn, R., Case, J., McCloghrie, K., Rose, M. and S.
Waldbusser, "Protocol Operations for the Simple Network
Management Protocol (SNMP)", STD 62, RFC 3416, December
2002.
[RFC3418] Presuhn, R., Case, J., McCloghrie, K., Rose, M. and S.
Waldbusser, "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62, RFC
3418, December 2002.
[RFC3419] M. Daniele, M. and J. Schoenwaelder, "Textual Conventions
for Transport Addresses", RFC 3419, December 2002.
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Informative References
[RFC3410] Case, J., Mundy, R., Partain, D. and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[RFC1155] Rose, M. and K. McCloghrie, "Structure and Identification of
Management Information for TCP/IP-based Internets", STD 16,
RFC 1155, May 1990.
[RFC1212] Rose, M. and K. McCloghrie, "Concise MIB Definitions", STD
16, RFC 1212, March 1991.
[RFC1215] Rose, M., "A Convention for Defining Traps for use with the
SNMP", RFC 1215, March 1991.
[RFC2932] McCloghrie, K., Farinacci, D., and D. Thaler, "IPv4
Multicast Routing MIB", RFC 2932, October 2000.
[RFC2576] Frye, R., Levi, D., Routhier, S. and B. Wijnen, "Coexistence
between Version 1, Version 2, and Version 3 of the
Internet-standard Network Management Framework", RFC 2576,
March 2000.
[RFC2108] de Graaf, K., Romascanu, D., McMaster, D. and K. McCloghrie,
"Definitions of Managed Objects for IEEE 802.3 Repeater
Devices using SMIv2", RFC 2108, February 1997.
[RFC2737] McCloghrie, K. and A. Bierman, "Entity MIB (Version 2)", RFC
2737, December 1999.
[RFC3289] Baker, F., Chan, K. and A. Smith, "Management Information
Base for the Differentiated Services Architecture", RFC
3289, May 2002.
[RFC1213] McCloghrie, K. and M. Rose, "Management Information Base for
Network Management of TCP/IP-based internets - MIB-II", STD
17, RFC 1213, March 1991.
[RFC1595] Brown, T. and K. Tesink, "Definitions of Managed Objects for
the SONET/SDH Interface Type", RFC 1595, March 1994.
[RFC2558] Tesink, K., "Definitions of Managed Objects for the
SONET/SDH Interface Type", RFC 2558, March 1999.
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Security Considerations
Implementation and deployment of a MIB module in a system may result
in security risks that would not otherwise exist. It is important
for authors and reviewers of documents that define MIB modules to
ensure that those documents fully comply with the guidelines in
http://www.ops.ietf.org/mib-security.html so that all such risks are
adequately disclosed.
Acknowledgments
Most of the material on usage of data types was based on input
provided by Bert Wijnen with assistance from Keith McCloghrie, David
T. Perkins, and Juergen Schoenwaelder. Much of the other material on
SMIv2 usage was taken from an unpublished guide for MIB authors and
reviewers by Juergen Schoenwaelder. Some of the recommendations in
these guidelines are based on material drawn from the on-line SMIv2
errata list at http://www.ibr.cs.tu-bs.de/ietf/smi-errata/. Thanks
to Frank Strauss and Juergen Schoenwaelder for maintaining that list
and to the contributors who supplied the material for that list.
Finally, thanks are due to the following individuals whose comments
on earlier versions of this memo contained many valuable suggestions
for additions, clarifications, and corrections: Andy Bierman, David
Harrington, Keith McCloghrie, David T. Perkins, Randy Presuhn, Dan
Romascanu, Juergen Schoenwaelder, Frank Strauss, Dave Thaler, and
Bert Wijnen.
Editor's Address
C. M. Heard
600 Rainbow Dr. #141
Mountain View, CA 94041-2542
USA
Phone: +1 650 964 8391
EMail: heard@pobox.com
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Internet Draft Guidelines for MIB Authors and Reviewers February 2003
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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************************************************************
* NOTES TO RFC Editor (to be removed prior to publication) *
* *
* 1.) The normative reference [RFC2223bis] currently *
* points to a work in progress that is intended to replace *
* RFC 2223. Please update that reference to point to the *
* forthcoming RFC that replaces RFC 2223, and replace all *
* occurrences of "2223bis" with the number of that RFC. *
* *
* 2.) The I-D <draft-ietf-atommib-rfc2493bis-01.txt> is *
* expected to eventually replace RFC 2493. If that draft *
* (or a successor) is published as an RFC prior to or *
* concurrently with this document, then the normative *
* occurrences of [RFC2493] (i.e., those in Section 4.6.1.2 *
* and in Appendix D) should be changed to point to that *
* RFC, a normative reference to that RFC should be added, *
* and the reference [RFC2493] should be moved to the *
* informative reference section. Note that [RFC2493] and *
* [RFC2558] must remain as informative references *
* supporting Appendix C. *
* *
* 3.) The I-D <draft-ietf-snmpv3-coex-v2-02.txt> (or a *
* successor) is expected to eventually replace RFC 2576. *
* If that draft (or a successor) is published as an RFC *
* prior to or concurrently with this document, then the *
* informative reference [RFC2576] should be updated to *
* point to the replacement RFC, and the reference tag *
* [RFC2576] should be updated to match. *
* *
* 4.) The I-D <draft-ietf-entmib-v3-00.txt> (or a *
* successor) is expected to eventually replace RFC 2737. *
* If that draft (or a successor) is published as an RFC *
* prior to or concurrently with this document, then the *
* informative reference [RFC2737] should be updated to *
* point to the replacement RFC, and the reference tag *
* [RFC2737] should be updated to match. *
* *
************************************************************
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