Internet Engineering Task Force A. Bierman
Internet-Draft Netconf Central
Intended status: BCP January 23, 2009
Expires: July 27, 2009
Guidelines for Authors and Reviewers of YANG Data Model Documents
draft-bierman-netmod-yang-usage-00
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Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract
This memo provides guidelines for authors and reviewers of standards
track specifications containing YANG data model modules. Applicable
portions may be used as a basis for reviews of other YANG data model
documents. Recommendations and procedures are defined, which are
intended to increase interoperability and usability of NETCONF
implementations which utilize YANG data model modules.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 5
2.2. NETCONF Terms . . . . . . . . . . . . . . . . . . . . . . 5
2.3. YANG Terms . . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. General Documentation Guidelines . . . . . . . . . . . . . . . 7
3.1. YANG Data Model Boilerplate Section . . . . . . . . . . . 7
3.2. Narrative Sections . . . . . . . . . . . . . . . . . . . . 7
3.3. Definitions Section . . . . . . . . . . . . . . . . . . . 8
3.4. Security Considerations Section . . . . . . . . . . . . . 8
3.5. IANA Considerations Section . . . . . . . . . . . . . . . 8
3.5.1. Documents that Create a New Name Space . . . . . . . . 8
3.5.2. Documents that Extend an Existing Name Space . . . . . 9
3.6. Reference Sections . . . . . . . . . . . . . . . . . . . . 9
3.7. Copyright Notices . . . . . . . . . . . . . . . . . . . . 9
3.8. Intellectual Property Section . . . . . . . . . . . . . . 9
4. YANG Usage Guidelines . . . . . . . . . . . . . . . . . . . . 10
4.1. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Defaults . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.3. Conditional Statements . . . . . . . . . . . . . . . . . . 10
4.4. Header Contents . . . . . . . . . . . . . . . . . . . . . 11
4.5. Data Types . . . . . . . . . . . . . . . . . . . . . . . . 12
4.6. Object Definitions . . . . . . . . . . . . . . . . . . . . 13
4.7. RPC Definitions . . . . . . . . . . . . . . . . . . . . . 13
4.8. Notification Definitions . . . . . . . . . . . . . . . . . 14
5. YANG Module Registry . . . . . . . . . . . . . . . . . . . . . 15
5.1. YANG Registry Data Model . . . . . . . . . . . . . . . . . 16
5.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 22
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 23
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
9.1. Normative References . . . . . . . . . . . . . . . . . . . 24
9.2. Informative References . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
The standardization of network configuration interfaces for use with
the NETCONF [RFC4741] protocol requires a modular set of data models,
which can be reused and extended over time.
This document defines a set of usage guidelines for standards track
documents containing YANG [I-D.ietf-netmod-yang] data models. It is
similar to the MIB usage guidelines specification [RFC4181] in intent
and structure.
Many YANG constructs are defined as optional to use, such as the
description clause. However, in order to maximize interoperability
of NETCONF implementations utilizing YANG data models, it is
desirable to define a set of usage guidelines which may require a
higher level of compliance than the minimum level defined in the YANG
specification.
A new IANA registry is needed to support YANG. This registry will
allow YANG module namespace and other definitions to be centrally
located, minimizing name collisions, and providing an authoritative
status of each YANG module.
The YANG Module Registry will support YANG modules, as well as YANG
submodules which utilize a 'virtual' module definition. A virtual
module contains only the module header, submodule include statements,
and meta statements. The Submodule Registration procedure [ed: IANA
procedure TBD] is used to publish specifications containing YANG
submodules which extend a virtual module. This procedure allows the
main module revision statement and include statement to be updated,
without requiring publication or a separate RFC to contain the main
module. Refer to Section 5 for more details.
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The NETCONF stack can be conceptually partitioned into four layers.
Layer Example
+-------------+ +--------------------+ +-------------------+
(4) | Content | | Configuration data | | Notification data |
+-------------+ +--------------------+ +-------------------+
| | |
+-------------+ +-----------------+ +---------------+
(3) | Operations | | <edit-config> | | <eventType> |
+-------------+ +-----------------+ +---------------+
| | |
+-------------+ +--------------------+ +----------------+
(2) | RPC | | <rpc>, <rpc-reply> | | <notification> |
+-------------+ +--------------------+ +----------------+
| | |
+-------------+ +-----------------------------+
(1) | Transport | | BEEP, SSH, SSL, console |
| Protocol | | |
+-------------+ +-----------------------------+
Figure 1
This document defines usage guidelines related to the NETCONF
operations layer (3), and NETCONF content layer (4).
It also contains a definition for a registry for YANG Modules, which
can be used to locate documents which contain standards-track modules
or submodules.
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2. Terminology
2.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2.2. NETCONF Terms
The following terms are defined in [RFC4741] and are not redefined
here:
o agent
o application
o capabilities
o manager
o operation
o RPC
2.3. YANG Terms
The following terms are defined in [I-D.ietf-netmod-yang] and are not
redefined here:
o data node
o module
o submodule
o namespace
o version
2.4. Terms
The following terms are used throughout this document:
o YAM: Shorthand term for a YANG data model module or submodule,
used for properties which apply to both modules and submodules.
When describing properties which are specific to modules, the term
'YANG module', or simply 'module', is used instead. When
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describing properties which are specific to submodules, the term
'YANG submodule', or simply 'module' is used instead.
o Published Document: A stable release of a YAM, usually contained
in an RFC.
o Unpublished Document: An unstable release of a YAM, usually
contained in an Internet Draft.
o Virtual Module: A YANG module which does not contain any body
statements, and is maintained in a registry. The body statements
are defined in submodules, in one or more documents, and
'included' in the main module via a registry entry for the main
module.
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3. General Documentation Guidelines
YANG data model modules (YAMs) under review are likely to be
contained in Internet Drafts. All guidelines for Internet Draft
authors MUST be followed. These guidelines are available online at:
http://www.isi.edu/in-notes/rfc-editor/instructions2authors.txt
The following sections MUST be present in an Internet Draft
containing a YAM:
o YANG data model boilerplate section
o Narrative sections
o Definitions section
o Security Considerations section
o IANA Considerations section
o References section
3.1. YANG Data Model Boilerplate Section
This section MUST contain a verbatim copy of the latest approved
Internet-Standard Management Framework boilerplate, which is
available on-line at [ed: URL TBD].
3.2. Narrative Sections
The narrative part MUST include an overview section that describes
the scope and field of application of the YAM(s) defined by the
specification and that specifies the relationship (if any) of these
YAMs to other standards, particularly to standards containing other
YAM modules. The narrative part SHOULD include one or more sections
to briefly describe the structure of the YAMs defined in the
specification.
If the YAM(s) defined by the specification import definitions from
other YAMs (except for those defined in the YANG
[I-D.ietf-netmod-yang] or YANG Types [I-D.ietf-netmod-yang-types]
documents) or are always implemented in conjunction with other YAMs,
then those facts MUST be noted in the overview section, as MUST any
special interpretations of objects in other YAMs.
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3.3. Definitions Section
This section contains the YAM(s) defined by the specification. These
modules MUST be written in YANG [I-D.ietf-netmod-yang].
See Section 4 for guidelines on YANG usage.
3.4. Security Considerations Section
Each specification that defines one or more YAMs MUST contain a
section that discusses security considerations relevant to those
modules. This section MUST be patterned after the latest approved
template (available at [ed: URL TBD]).
In particular, writable YAM 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
YAM objects that contain especially sensitive information or that
raise significant privacy concerns MUST be explicitly listed by name
and the reasons for the sensitivity/privacy concerns MUST be
explained.
3.5. IANA Considerations Section
In order to comply with IESG policy as set forth in
http://www.ietf.org/ID-Checklist.html, every Internet-Draft that is
submitted to the IESG for publication MUST contain an IANA
Considerations section. The requirements for this section vary
depending what actions are required of the IANA.
Refer to [TBD] for details on the structure of the YANG registries
maintained by the IANA.
3.5.1. Documents that Create a New Name Space
If an Internet-Draft defines a new name space that is to be
administered by the IANA, then the document MUST include an IANA
Considerations section, specifies how the name space is to be
administered.
Specifically, if any YANG module namespace statement value contained
in the document is not already registered with IANA, then a new YANG
Namespace registry entry must be requested from the IANA [ed:
procedure TBD].
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3.5.2. Documents that Extend an Existing Name Space
If an Internet-Draft defines any extensions to a YANG Namespace
already administered by the IANA, then the document MUST include an
IANA Considerations section, specifies how the name space extension
is to be administered.
Specifically, if any YANG submodule belongs-to value contained in the
document is associated with a module that contains a namespace
statement value equal to a YANG Namespace already administered by the
IANA, then a new YANG Module registry entry and YANG Namespace Update
Procedure must be requested from the IANA [ed: procedure TBD].
3.6. Reference Sections
[ed: 2223bis text TBD]
For every import or include statement which appears in a YAM
contained in the specification, which identifies a YAM in a separate
document, a corresponding normative reference to that document MUST
appear in the Normative References section. The reference MUST
correspond to the specific YAM version actually used within the
specification.
If any YANG submodule contained in the specification contains a
'belongs-to' statement value which identifies a 'virtual' YANG module
maintained in the IANA YANG Module Registry, then a corresponding
normative reference to the registry identifier MUST appear in the
Normative References section. The registry entry MUST be properly
updated, using the appropriate procedures [ed: IANA procedures TBD].
3.7. Copyright Notices
The proper copyright notices MUST be present in the module
description statement. [ed.: See RFC 4181, 3.7. Exact text for
insertion is TBD.]
3.8. Intellectual Property Section
The proper IPR statements MUST be present in the document, according
to the most current Internet Draft boilerplate. [ed.: actual IETF IPR
text reference TBD]
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4. YANG Usage Guidelines
In general, YAMs in IETF standards-track specifications MUST comply
with all syntactic and semantic requirements of YANG.
[I-D.ietf-netmod-yang]. The guidelines in this section are intended
to supplement the YANG specification, which is intended to define a
minimum set of conformance requirements.
In order to promote interoperability and establish a set of practices
based on previous experience, the following sections establish usage
guidelines for specific YANG constructs.
Only guidelines which clarify or restrict the minimum conformance
requirements are included here.
4.1. Identifiers
Identifiers for modules, submodules, typedefs, groupings, data
objects, rpcs, and notifications MUST be between 1 and 63 characters
in length.
4.2. Defaults
In general, it is suggested that sub-statements containing default
values SHOULD NOT be present. For example, 'status current;',
'config true;', 'mandatory false;', and 'max-elements unbounded;' are
common defaults which would make the YAM difficult to read if used
everywhere they are allowed.
Instead, it is suggested that common statements SHOULD only be used
when being set to a value other than the default value.
4.3. Conditional Statements
A YAM may be conceptually partitioned in several ways, using the 'if-
feature' and/or 'when' statements. In addition, NETCONF capabilities
are designed to identify optional functionality.
Data model designers need to carefully consider all modularity
aspects, including the use of YANG conditional statements.
Objects SHOULD NOT directly reference NETCONF capabilities, in order
to specify optional behavior. Instead, a 'feature' statement SHOULD
be defined to represent the NETCONF capability, and the 'if-feature'
statement SHOULD be used within the object definition.
If the condition associated with the desired semantics is not
dependent on any particular instance value within the database, then
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an 'if-feature' statement SHOULD be used instead of a 'when'
statement.
All 'must' and 'when' statements MUST contain valid XPath. If any
name tests are present, they MUST contain valid module prefixes
and/or data node names.
The 'attribute', 'namespace', 'preceding', 'preceding-sibling',
'following', and 'following-sibling' axis SHOULD NOT be used.
The 'position' and 'last' functions SHOULD NOT be used.
Implicit 'position' function calls within predicates SHOULD NOT be
used. (e.g., //chapter[42]).
Data nodes which use the 'int64' and 'uint64' built-in type SHOULD
NOT be used within relational expressions.
Data modelers need to be careful not to confuse the YANG value space
and the XPath value space. The data types are not the same in both,
and conversion between YANG and XPath data types SHOULD be considered
carefully.
Explicit XPath data type conversions SHOULD be used (e.g., 'string',
'boolean', or 'number' functions), instead of implicit XPath data
type conversions.
4.4. Header Contents
o The namespace MUST be a globally unique URI, usually assigned by
the IANA.
o Until a URI is assigned by the IANA, a temporary namespace string
SHOULD be selected which is not likely to collide with other YANG
namespaces, such as the filename of the Internet Draft containing
the YAM.
o The organization statement MUST be present.
o The contact statement MUST be present.
o The description statement MUST be present.
o If the YAM represents a model defined in one or more external
documents, then a reference statement SHOULD be present.
o A revision statement MUST be present for each published version of
the YAM.
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o Each new revision MUST include a revision date which is higher
than any other revision date in the YAM.
o It is acceptable to reuse the same revision statement within
unpublished versions (i.e., Internet Drafts), but the revision
date MUST be updated to a higher value each time the Internet
Draft is re-published.
4.5. Data Types
o Selection of an appropriate data type (i.e., built-in type,
existing derived type, or new derived type) is very subjective and
therefore few requirements can be specified on that subject.
o Data model designers SHOULD use the most appropriate built-in data
type for the particular application.
o If extensibility of enumerated values is required, then the
identityref data type SHOULD be used instead of an enumeration or
other built-in type.
o If an appropriate derived type exists in any standard YAM, such as
[I-D.ietf-netmod-yang-types], then it SHOULD be used instead of
defining a new derived type.
o The description statement MUST be present.
o If the type semantics are defined in an external document, then a
reference statement SHOULD be present.
o For string data types, if a machine-readable pattern can be
defined for the desired semantics, then one or more pattern
statements SHOULD be present.
o For string data types, if the length of the string is not
unbounded in all implementations, then a length statement SHOULD
be present.
o For numeric data types, if the values allowed by the intended
semantics are different than those allowed by the unbounded
intrinsic data type (e.g., int32), then a range statement SHOULD
be present.
o The signed numeric data types (i.e., 'int8', 'int16', 'int32', and
'int64') SHOULD NOT be used unless negative values are allowed for
the desired semantics.
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o The 'float32' and 'float64' data types SHOULD only be used if the
other numeric data types do not fully represent the desired
semantics.
o For enumeration or bits data types, the semantics for each enum or
bit SHOULD be documented. A separate description statement
(within each enum or bit statement) SHOULD be used, instead of the
description statement for the type itself.
o If an appropriate units identifier can be associated with the
desired semantics, then a units statement SHOULD be present.
o If an appropriate default value can be associated with the desired
semantics, then a default statement SHOULD be present.
o If a significant number of derived types are defined, and it is
anticipated that these data types will be reused by multiple YAMs,
then these derived types SHOULD be contained in a separate module
or submodule, to allow easier reuse without unnecessary coupling.
4.6. Object Definitions
o The description statement MUST be present.
o If the object semantics are defined in an external document, then
a reference statement SHOULD be present.
o The 'anyxml' construct MUST NOT be used within configuration data.
o If there are referential integrity constraints associated with the
desired semantics that can be represented with XPath, then one or
more must statements SHOULD be present.
o For list and leaf-list objects, if the number of possible
instances for all implementations is not unbounded, then the min-
elements and/or max-elements statements SHOULD be present.
4.7. RPC Definitions
o The description statement MUST be present.
o If the RPC method semantics are defined in an external document,
then a reference statement SHOULD be present.
o If the RPC method impacts system behavior in some way, it SHOULD
be mentioned in the description statement.
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o If the RPC method is potentially harmful to system behavior in
some way, it MUST be mentioned in the Security Considerations
section of the document.
4.8. Notification Definitions
o The description statement MUST be present.
o If the notification semantics are defined in an external document,
then a reference statement SHOULD be present.
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5. YANG Module Registry
This section contains a YANG module registry specification, which can
be used to document each release of a module. It can also be used to
maintain virtual modules, in which all the body statements are
contained in submodules specified in the registry, not in a YANG
module within a published RFC or Internet Draft.
In order for YANG submodules to be used effectively within standards
track documents, it is desirable to avoid re-publishing an RFC
containing the 'main' module, each time a submodule is added or
changed.
The use of submodules can effectively reduce the number of XML
namespaces used within NETCONF PDUs, but their primary use is to
allow flexible partitioning of a single XML namespace into multiple,
independent documents, which can be easily extended over time.
The YANG Module Registry is an XML instance document which contains
minimal information about the modules represented in the registry.
Each registry has a unique ID, called the 'registry-id'. There are
also zero or more 'module' entries.
Each 'module' entry contains the module name, XML namespace, and
optional 'url' field to identify its location. If this is a virtual
module, then the 'virtual' field will be present.
Within each module entry, there are one or more 'release' entries.
Each 'release' entry contains the publication date of the release.
It also contains zero or more 'submodule' entries.
For each submodule included by the main module represented by each
'module' entry, a 'submodule' entry SHOULD be present. Each entry
provides the name, release date and an optional 'url' if the
submodule is available online.
It is expected that the IANA will maintain the official YANG Module
Registry for YAMs contained in published standards-track documents.
It is also expected that procedures for adding a new YANG module, and
adding a new release of an existing module, will also be specified.
[ed: A YANG data model and example XML instance document are provided
below to demonstrate how such a registry might work. This work is
very preliminary and subject to change.]
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5.1. YANG Registry Data Model
This section contains a YANG module definition which represents the
information stored in the IANA YANG Module Registry. It is provided
for informational purposes only. The actual definition is [TBD].
module yang-registry {
namespace "yang-registry-TBD";
prefix "yr";
// for the uri data type
import yang-types { prefix "yang"; }
organization "IETF";
contact
"Send comments to <andy@netconfcentral.com>.";
description
"YANG Module Registry Data Structure";
revision "2009-01-22" {
description
"Initial version.";
}
container registry {
leaf registry-id {
description
"Contains the identity of this registry.";
type yang:uri;
mandatory true;
}
list module {
key "name";
unique "namespace";
leaf name {
description "YANG module name.";
// TBD: imported name string type
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type string { length "1..63"; }
}
leaf namespace {
description "YANG module namespace.";
type yang:uri;
mandatory true;
}
leaf url {
description
"URL for this YANG module, if one
is available.";
type yang:uri;
}
leaf virtual {
description
"If present, then this registry entry
represents a virtual YANG module,
which is a YANG module which does not
contain any body statements. Instead,
submodules are used to contain all
body statements.
Each release entry within this entry
is expected to contain all
the submodule content information for
this virtual module.";
type empty;
}
list release {
description
"Describes the contents of a specific
release of a YANG module. At least
one entry MUST exist for the most
current version of the module.";
min-elements 1;
key version;
leaf version {
description "YANG module release date.";
// TBD: imported date string type
// YYYY-MM-DD
type string { length "10"; }
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}
list submodule {
key "name";
leaf name {
description "YANG submodule name.";
// TBD: imported name string type
type string { length "1..63"; }
}
leaf version {
description "YANG module revision date.";
// TBD: imported date string type
// YYYY-MM-DD
type string { length "10"; }
mandatory true;
}
leaf url {
description
"URL for this YANG submodule, if one
is available.";
type yang:uri;
}
} // list submodule
} // list release
} // list module
} // container registry
} // module yang-registry
Figure 2
5.2. Examples
This section contains some example registry entries, demonstrating
the basic use cases.
<?xml version="1.0" encoding="UTF-8"?>
<registry xmlns="yang-registry-TBD">
<registry-id>
http://example.com/yang-registry
</registry-id>
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<module>
<name>notification</name>
<namespace>
urn:ietf:params:xml:ns:netconf:notification:1.0
</namespace>
<url>
ftp://ftp.rfc-editor.org/in-notes/rfc5277.txt
</url>
<release>
<version>2008-07-01</version>
</release>
</module>
<module>
<name>notification-content</name>
<namespace>
urn:ietf:params:xml:ns:netmod:notification
</namespace>
<url>
ftp://ftp.rfc-editor.org/in-notes/rfc5277.txt
</url>
<release>
<version>2008-07-01</version>
</release>
</module>
<module>
<name>services</name>
<namespace>
http://example.com/yang/services
</namespace>
<url>
http://example.com/yang/monitor-tools.yang
</url>
<virtual/>
<release>
<version>2009-01-23</version>
<submodule>
<name>common-types</name>
<version>2008-11-14</version>
<url>
http://example.com/yang/common-types.yang
</url>
</submodule>
<submodule>
<name>ping</name>
<version>2008-11-14</version>
<url>
http://example.com/yang/ping.yang
</url>
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</submodule>
<submodule>
<name>traceroute</name>
<version>2009-01-23</version>
<url>
http://example.com/yang/traceroute.yang
</url>
</submodule>
</release>
<release>
<version>2008-11-14</version>
<submodule>
<name>common-types</name>
<version>2008-11-14</version>
<url>
http://example.com/yang/common-types.yang
</url>
</submodule>
<submodule>
<name>ping</name>
<version>2008-11-14</version>
<url>
http://example.com/yang/ping.yang
</url>
</submodule>
</release>
</module>
</registry>
Figure 3
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6. IANA Considerations
There are no actions requested of IANA at this time. [ed.: If the
YANG Registry approach is pursued, then details for those procedures
will need to be defined.]
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7. Security Considerations
This document defines documentation guidelines for NETCONF content
defined with the YANG data modeling language. It does not introduce
any new or increased security risks into the management system. [ed:
RFC 4181 style security section TBD]
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8. Acknowledgments
The structure and contents of this document are adapted from
Guidelines for MIB Documents [RFC4181], by C. M. Heard.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[I-D.ietf-netmod-yang]
Bjorklund, M., "YANG - A data modeling language for
NETCONF", draft-ietf-netmod-yang-03 (work in progress),
January 2009.
[I-D.ietf-netmod-yang-types]
Schoenwaelder, J., "Common YANG Data Types",
draft-ietf-netmod-yang-types-01 (work in progress),
November 2008.
9.2. Informative References
[RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB
Documents", BCP 111, RFC 4181, September 2005.
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Author's Address
Andy Bierman
Netconf Central
Simi Valley, CA
USA
Email: andy@netconfcentral.com
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