Guidelines for Authors and Reviewers of Documents Containing YANG Data Models

General Documentation Guidelines YANG modules under review are likely to be contained in Internet- Drafts (I-Ds). All guidelines for I-D authors MUST be followed. The guidelines for RFCs should be followed and are defined in the following: (and any future RFCs that obsolete it), , and . The following sections MUST be present in an I-D containing a YANG module:

Narrative sections
Definition sections
Security Considerations section
IANA Considerations section
References section

There are three usage scenarios for YANG that can appear in an I-D or RFC:

normative module or submodule
example module or submodule
example YANG fragment not part of any module or submodule

The guidelines in this document refer mainly to a normative module or submodule but may be applicable to example modules and YANG fragments as well.

Module Copyright The module "description" statement MUST contain a reference to the latest approved IETF Trust Copyright statement, which is available online at: ]]>

Code Components Each normative YANG module or submodule contained within an I-D or RFC is considered to be a code component. The strings "<CODE BEGINS>" and "<CODE ENDS>" MUST be used to identify each code component. The "<CODE BEGINS>" tag SHOULD be followed by a string identifying the file name specified in . The name string form that includes the revision date SHOULD be used. The revision date MUST match the date used in the most recent revision of the module. The following example is for the "2016-03-20" revision of the "ietf-foo" module: file "ietf-foo@2016-03-20.yang" module ietf-foo { namespace "urn:ietf:params:xml:ns:yang:ietf-foo"; prefix "foo"; organization "..."; contact "..."; description "..."; revision 2016-03-20 { description "Latest revision"; reference "RFC FFFF: Foo Protocol"; } // ... more statements }

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          Example Modules
          Example modules are not code components.  The <CODE BEGINS>
convention MUST NOT be used for example modules.
          An example module SHOULD be named using the term "example", followed
by a hyphen, followed by a descriptive name, e.g., "example-toaster".
          See  regarding the namespace guidelines for example
modules.


      
        Terminology Section
        A terminology section MUST be present if any terms are defined in the
document or if any terms are imported from other documents.
      
      
        Tree Diagrams
        YANG tree diagrams provide a concise representation of a YANG module
and SHOULD be included to help readers understand YANG module
structure.  Guidelines on tree diagrams can be found in .
        If YANG tree diagrams are used, then an informative reference to the
YANG tree diagrams specification MUST be included in the document.
Refer to  for an example of such a reference.
      
      
        Narrative Sections
        The narrative part MUST include an overview section that describes
the scope and field of application of the module(s) defined by the
specification and that specifies the relationship (if any) of these
modules to other standards, particularly to standards containing
other YANG modules.  The narrative part SHOULD include one or more
sections to briefly describe the structure of the modules defined in
the specification.
        If the module or modules defined by the specification imports
definitions from other modules (except for those defined in 
or ) or are always implemented in conjunction with other
modules, then those facts MUST be noted in the overview section; any
special interpretations of definitions in other modules MUST be noted
as well.  Refer to  for an example of this
overview section.
        If the document contains major Network Management Datastore Architecture (NMDA) exceptions or include a temporary non-NMDA module , then the Introduction
section should mention this fact with the reasoning that motivated that design.
Refer to  for more NMDA-related guidance. Specifically,  includes a recommendation for designers to describe and justify any NMDA exceptions in detail as part of the module itself.
        Consistent indentation SHOULD be used for all examples, including
YANG fragments and protocol message instance data.  If line wrapping
is done for formatting purposes, then this SHOULD be noted following , as shown
in the following example:
        this is a long \
value so the line needs to wrap to stay within 72 characters
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        Native YANG features (e.g., breaking line, "+") SHOULD be used to fit a module into the line limits.
Exceptionally, RFC8792-folding of YANG modules MAY be used if and only if native YANG features are not sufficient.
A similar approach (e.g., use "--yang-line-length 69" or split a tree into subtrees) SHOULD be followed for tree diagrams.
        
          YANG Module Classification
          The narrative section SHOULD include a mention about the classification
of a given model. Such a mention is meant to ease positioning the
module in the overall operational ecosystem. Specifically, the following types
from  and  can be used:
          
            Service Model:
            
              Describes a service and the parameters of the service in a
portable way that can be used uniformly and independent of the
equipment and operating environment.
            
            
            

              Examples of service models are the L3VPN Service Model (L3SM)  and the L2VPN Service Model (L2SM) .
            
            Network Model:
            
              Describes a network-level abstraction (or a subset of aspects of a network infrastructure), including devices and their subsystems, and relevant protocols operating at the link and network layers across multiple devices. This model corresponds to the network configuration model discussed in .
            
            
            

              It can be used by a network operator to allocate resources (e.g., tunnel resource, topology resource) for the service or schedule resources to meet the service requirements defined in a service model.
            
            
            

              Examples of network models are the L3VPN Network Model (L3NM)  or the L2VPN Network Model (L2NM) .
            
            Device Model:
            
              Refers to the Network Element YANG data model described in  or the device configuration model discussed in .
            
            
            

              Device models are also used to refer to model a function embedded in a device (e.g., Access Control Lists (ACLs) ).
            
            
            

              A comprehensive list of device models is provided in Appendix 4.2 of .
            
          
        
      
      
        Definitions Section
        This section contains the module(s) defined by the specification.
These modules SHOULD be written using the YANG 1.1  syntax.
YANG 1.0  syntax MAY be used if no YANG 1.1 constructs or
semantics are needed in the module.  If any of the imported YANG
modules are written using YANG 1.1, then the module MUST be written
using YANG 1.1.
        A YIN syntax version of the module MAY also be present in the
document.  There MAY also be other types of modules present in the
document, such as Structure of Management Information Version 2
(SMIv2), which are not affected by these guidelines.
        Note that if the module itself is considered normative and not an
example module or example YANG fragment, then all YANG statements
within a YANG module are considered normative.  The use of keywords
defined in  and  apply to YANG "description"
statements in normative modules exactly as they would in any other
normative section.
        Example YANG modules and example YANG fragments MUST NOT contain any
normative text, including any all-uppercase reserved words from
 and .
        Consistent indentation and formatting SHOULD be used in all YANG
statements within a module.
        See  for guidelines on YANG usage.
      
      
        Security Considerations Section
        Each specification that defines one or more modules MUST contain a
   section that discusses security considerations relevant to those
   modules.
        Unless the modules comply with  or define YANG extensions (e.g., ), the security section MUST
   be patterned after the latest approved template
   (available at <https://trac.ietf.org/trac/ops/wiki/yang-security-guidelines>).   contains the security considerations
   template.  Authors
   MUST check the web page at the URL listed above in case there is a
   more recent version available.
        In particular:
        
          
            Writable data nodes that could be especially disruptive if abused
 MUST be explicitly listed by name, and the associated security
 risks MUST be explained.
          
          
            Readable data nodes 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.
          
          
            Operations (i.e., YANG "rpc" statements) that are potentially
 harmful to system behavior or that raise significant privacy
 concerns MUST be explicitly listed by name, and the reasons for
 the sensitivity/privacy concerns MUST be explained.
          
        
        Documents that define exclusively modules following the extension in  are not required to include the security template in . Likewise, following the template is not required for modules that define YANG extensions such as .
        
          Security Considerations Section Template
          

 -- for all YANG modules you must evaluate whether any readable data
 -- nodes (those are all the "config false" nodes, but also all other
 -- nodes, because they can also be read via operations like get or
 -- get-config) are sensitive or vulnerable (for instance, if they
 -- might reveal customer information or violate personal privacy
 -- laws such as those of the European Union if exposed to
 -- unauthorized parties)

Some of the readable data nodes in this YANG module may be considered
sensitive or vulnerable in some network environments.  It is thus
important to control read access (e.g., via get, get-config, or
notification) to these data nodes. Specifically, the following
subtrees and data nodes have particular sensitivities/
vulnerabilities:



 -- if your YANG module has defined any RPC operations
 -- describe their specific sensitivity or vulnerability.

Some of the RPC operations in this YANG module may be considered
sensitive or vulnerable in some network environments.  It is thus
important to control access to these operations.  Specifically,
the following operations have particular sensitivities/
vulnerabilities:



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            Note:
            
              , , , , and  (or future RFCs that replace any of them) MUST be listed as normative references.
            
          
        
      
      
        IANA Considerations Section
        In order to comply with IESG policy as set forth in <https://www.ietf.org/id-info/checklist.html>, every I-D that is
submitted to the IESG for publication MUST contain an IANA
Considerations section.  The requirements for this section vary
depending on what actions are required of the IANA. If there are no
IANA considerations applicable to the document, then the IANA
Considerations section will state that "This document has no IANA
actions".  Refer to the guidelines in  for more details.
        Each normative YANG module MUST be registered in both the "IETF XML
Registry"   and the "YANG Module Names" registry
 . The registration request in the "YANG Module Names" registry
should indicate whether the module is IANA-maintained or not. This applies to new modules and updated
modules. An example of an update registration for the
"ietf-template" module can be found in .
        Additional IANA considerations applicable to IANA-maintained modules are provided in .
        
          Documents That Create a New Namespace
          If an I-D defines a new namespace that is to be administered by the
IANA, then the document MUST include an IANA Considerations section
that specifies how the namespace 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 entry
in the "ns" subregistry within the "IETF XML Registry" MUST be
requested from the IANA.
        
        
          Documents That Extend an Existing Namespace
          It is possible to extend an existing namespace using a YANG submodule
that belongs to an existing module already administered by IANA.  In
this case, the document containing the main module MUST be updated to
use the latest revision of the submodule.
        
      
      
        References Sections
        For every import or include statement that appears in a module
contained in the specification that identifies a module 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 module version actually used within the
specification.
        For every normative reference statement that appears in a module
contained in the specification that identifies a separate document, a
corresponding normative reference to that document SHOULD appear in
the Normative References section.  The reference SHOULD correspond to
the specific document version actually used within the specification.
If the reference statement identifies an informative reference that
identifies a separate document, a corresponding informative reference
to that document MAY appear in the Informative References section.
      
      
        Validation Tools
        All modules need to be validated before submission in an I-D.  The
'pyang' YANG compiler is freely available from GitHub:
        
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        If the 'pyang' compiler is used to validate a normative module, then
the "--ietf" command-line option MUST be used to identify any IETF
guideline issues.
        If the 'pyang' compiler is used to validate an example module, then
the "--ietf" command-line option MAY be used to identify any IETF
guideline issues.
        To ensure that a module fits into the line limits of an I-D, the command
"pyang -f yang --keep-comments --yang-line-length 69" should be used.
        The "yanglint" program is also freely available from GitHub.
        
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        This tool can be used to validate XPath statements within YANG
modules.
        To check that JSON-encoded examples  comply with the target data models,
programs such as "yangson" should be used. The "yangson" program is freely available from GitHub.
        
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        An example of integrating "yangson" in a tooling chain can be found at .
      
      
        Module Extraction Tools
        A version of 'rfcstrip' that will extract YANG modules from an I-D or
RFC is available.  The 'rfcstrip' tool that supports YANG module
extraction is freely available at:
        
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        This tool can be used to verify that the "<CODE BEGINS>" and "<CODE
ENDS>" tags are used correctly and that the normative YANG modules
can be extracted correctly.
        The "xym" tool is freely available on GitHub and can be used to
extract YANG modules from a document.
        
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        Module Usage Examples
        Each specification that defines one or more modules SHOULD contain
usage examples, either throughout the document or in an appendix.
This includes example instance document snippets in an appropriate
encoding (e.g., XML and/or JSON) to demonstrate the intended usage of
the YANG module(s).  Example modules MUST be validated.  Refer to
 for tools that validate YANG modules and examples. If IP addresses/prefixes
are used, then a mix of either IPv4 and IPv6 addresses/prefixes or IPv6
addresses/prefixes exclusively SHOULD be used in the examples.
        For some types (IP addresses, domain names, etc.), the IETF has reserved values for
documentation use. Authors SHOULD use these reserved values in the usage examples if these types are used. Examples of reserved values are listed below:
        
          
            IPv4 and IPv6 addresses/prefixes reserved for documentation are defined in  and .
          
          
            The Enterprise Number 32473 reserved for documentation use is defined in .
          
          
            Autonomous System Numbers (ASNs) reserved for documentation use are defined in .
          
          
            Reserved domain names for documentation are defined in .
          
        
        URI examples SHOULD be prefixed with "example:".

YANG Usage Guidelines Modules in IETF Standards Track specifications MUST comply with all syntactic and semantic requirements of YANG 1.1 . See the exception for YANG 1.0 in Section 3.6. 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 that clarify or restrict the minimum conformance requirements are included here.

Module Naming Conventions Normative modules contained in Standards Track documents MUST be named according to the guidelines in the IANA Considerations section of . A distinctive word or abbreviation (e.g., protocol name or working group abbreviation) SHOULD be used in the module name. If new definitions are being defined to extend one or more existing modules, then the same word or abbreviation should be reused, instead of creating a new one. All published module names MUST be unique. For a YANG module published in an RFC, this uniqueness is guaranteed by IANA. For unpublished modules, the authors need to check that no other work in progress is using the same module name. Example modules are non-normative and SHOULD be named with the prefix "example-". It is suggested that a stable prefix be selected that represents the entire organization. All normative YANG modules published by the IETF MUST begin with the prefix "ietf-". Another standards organization, such as the IEEE, might use the prefix "ieee-" for all YANG modules. Once a module name is published, it MUST NOT be reused, even if the RFC containing the module is reclassified to "Historic" status. A module name cannot be changed in YANG, and this would be treated as a new module, not a name change.

Prefixes All YANG definitions are scoped by the module containing the definition being referenced. This allows definitions from multiple modules to be used, even if the names are not unique. In the example below, the identifier "foo" is used in all three modules: YANG defines the following rules for prefix usage:

Prefixes are never used for built-in data types and YANG keywords.
A prefix MUST be used for any external statement (i.e., a statement defined with the YANG "extension" statement).
The proper module prefix MUST be used for all identifiers imported from other modules.
The proper module prefix MUST be used for all identifiers included from a submodule.

The following guidelines apply to prefix usage of the current (local) module:

The local module prefix SHOULD be used instead of no prefix in all path expressions.
The local module prefix MUST be used instead of no prefix in all "default" statements for an "identityref" or "instance-identifier" data type.
The local module prefix MAY be used for references to typedefs, groupings, extensions, features, and identities defined in the module.

Prefix values SHOULD be short but are also likely to be unique. Prefix values SHOULD NOT conflict with known modules that have been previously published. For convenience, prefix values of example modules MAY be prefixed with "ex" or similar patterns. In doing so, readers of example modules or tree diagrams that mix both example and standard modules can easily identify example parts.

Identifiers All YANG identifiers in published modules MUST be between 1 and 64 characters in length. These include any construct specified as an "identifier-arg-str" token in the ABNF in .

Identifier Naming Conventions Identifiers SHOULD follow a consistent naming pattern throughout the module. Only lowercase letters, numbers, and dashes SHOULD be used in identifier names. Uppercase characters, the period character, and the underscore character MAY be used if the identifier represents a well-known value that uses these characters. YANG does not permit any other characters in YANG identifiers. Identifiers SHOULD include complete words and/or well-known acronyms or abbreviations. Child nodes within a container or list SHOULD NOT replicate the parent identifier. YANG identifiers are hierarchical and are only meant to be unique within the set of sibling nodes defined in the same module namespace. List identifiers SHOULD be singular with the surrounding container name plural. Similarly, "leaf-list" identifiers SHOULD be singular. It is permissible to use common identifiers such as "name" or "id" in data definition statements, especially if these data nodes share a common data type. Identifiers SHOULD NOT carry any special semantics that identify data modeling properties. Only YANG statements and YANG extension statements are designed to convey machine-readable data modeling properties. For example, naming an object "config" or "state" does not change whether it is configuration data or state data. Only defined YANG statements or YANG extension statements can be used to assign semantics in a machine-readable format in YANG.

Defaults In general, it is suggested that substatements containing very common default values SHOULD NOT be present. The substatements listed in are commonly used with the default value, which would make the module difficult to read if used everywhere they are allowed. Statement Defaults

Statement	Default Value
config	true
mandatory	false
max-elements	unbounded
min-elements	0
ordered-by	system
status	current
yin-element	false

Conditional Statements A module may be conceptually partitioned in several ways, using the "if-feature" and/or "when" statements. Data model designers need to carefully consider all modularity aspects, including the use of YANG conditional statements. If a data definition is optional, depending on server support for a NETCONF or RESTCONF protocol capability, then a YANG "feature" statement SHOULD be defined. The defined "feature" statement SHOULD then be used in the conditional "if-feature" statement referencing the optional data definition. If any notification data, or any data definition, for a non- configuration data node is not mandatory, then the server may or may not be required to return an instance of this data node. If any conditional requirements exist for returning the data node in a notification payload or retrieval request, they MUST be documented somewhere. For example, a "when" or "if-feature" statement could apply to the data node, or the conditional requirements could be explained in a "description" statement within the data node or one of its ancestors (if any). If any "if-feature" statements apply to a list node, then the same "if-feature" statements MUST apply to any key leaf nodes for the list. There MUST NOT be any "if-feature" statements applied to any key leafs that do not also apply to the parent list node. There SHOULD NOT be any "when" statements applied to a key leaf node. It is possible that a "when" statement for an ancestor node of a key leaf will have the exact node-set result as the key leaf. In such a case, the "when" statement for the key leaf is redundant and SHOULD be avoided. Some modules use "case + when" construct but provide duplicated information (e.g., the "when" statements are constraining a single case in the choice as shown in the example below). Such constructs with duplicated information SHOULD NOT be used. Note that the use of "case + when" is still useful in cases where complementary modelling constraints should be expressed. See the example provided below. includes a provision for defining a constraint on state data and specifies that the constraint must be true in a valid state data. However, softens that behavior by allowing semantic constraints to be violated under some circumstances to help detecting anomalies. Relaxing validation constraints on state data is meant to reveal deviations of the observed behavior vs. intended behavior of a managed entity and hopefully trigger corrective actions by a management system. From that perspective, it is RECOMMENDED to avoid defining constraints on state data that would hinder the detection by a management system of abnormal behaviors of a managed entity.

XPath Usage This section describes guidelines for using the XML Path Language (XPath) within YANG modules.

XPath Evaluation Contexts YANG defines five separate contexts for evaluation of XPath statements:

The "running" datastore: collection of all YANG configuration data nodes. The document root is the conceptual container (e.g., "config" in the "edit-config" operation), which is the parent of all top-level data definition statements with a "config" statement value of "true".
State data + the "running" datastore: collection of all YANG data nodes. The document root is the conceptual container, parent of all top-level data definition statements.
Notification: an event notification document. The document root is the notification element.
RPC Input: The document root is the conceptual "input" node, which is the parent of all RPC input parameter definitions.
RPC Output: The document root is the conceptual "output" node, which is the parent of all RPC output parameter definitions.

Note that these XPath contexts cannot be mixed. For example, a "when" statement in a notification context cannot reference configuration data. It is especially important to consider the XPath evaluation context for XPath expressions defined in groupings. An XPath expression defined in a grouping may not be portable, meaning it cannot be used in multiple contexts and produce proper results. If the XPath expressions defined in a grouping are intended for a particular context, then this context SHOULD be identified in the "description" statement for the grouping.

Function Library The "position" and "last" functions SHOULD NOT be used. This applies to implicit use of the "position" function as well (e.g., '//chapter[42]'). A server is only required to maintain the relative XML document order of all instances of a particular user-ordered list or leaf-list. The "position" and "last" functions MAY be used if they are evaluated in a context where the context node is a user- ordered "list" or "leaf-list". The "id" function SHOULD NOT be used. The "ID" attribute is not present in YANG documents, so this function has no meaning. The YANG compiler SHOULD return an empty string for this function. The "namespace-uri" and "name" functions SHOULD NOT be used. Expanded names in XPath are different than YANG. A specific canonical representation of a YANG-expanded name does not exist. The "lang" function SHOULD NOT be used. This function does not apply to YANG because there is no "lang" attribute set with the document. The YANG compiler SHOULD return "false" for this function. The "local-name", "namespace-uri", "name", "string", and "number" functions SHOULD NOT be used if the argument is a node-set. If so, the function result will be determined by the document order of the node-set. Since this order can be different on each server, the function results can also be different. Any function call that implicitly converts a node-set to a string will also have this issue. The "local-name" function SHOULD NOT be used to reference local names outside of the YANG module that defines the must or when expression containing the "local-name" function. Example of a "local-name" function that should not be used: The "derived-from-or-self" function SHOULD be used instead of an equality expression for identityref values. This allows the identities to be conceptually augmented. Example:

Axes The "attribute" and "namespace" axes are not supported in YANG and MAY be empty in a NETCONF or RESTCONF server implementation. The "preceding" and "following" axes SHOULD NOT be used. These constructs rely on XML document order within a NETCONF or RESTCONF server configuration database, which may not be supported consistently or produce reliable results across implementations. Predicate expressions based on static node properties (e.g., element name or value, and "ancestor" or "descendant" axes) SHOULD be used instead. The "preceding" and "following" axes MAY be used if document order is not relevant to the outcome of the expression (e.g., check for global uniqueness of a parameter value). The "preceding-sibling" and "following-sibling" axes SHOULD NOT be used; however, they MAY be used if document order is not relevant to the outcome of the expression. A server is only required to maintain the relative XML document order of all instances of a particular user-ordered list or leaf-list. The "preceding-sibling" and "following-sibling" axes MAY be used if they are evaluated in a context where the context node is a user-ordered "list" or "leaf-list".

Types Data nodes that use the "int64" and "uint64" built-in type SHOULD NOT be used within numeric or boolean expressions. There are boundary conditions in which the translation from the YANG 64-bit type to an XPath number can cause incorrect results. Specifically, an XPath "double" precision floating-point number cannot represent very large positive or negative 64-bit numbers because it only provides a total precision of 53 bits. The "int64" and "uint64" data types MAY be used in numeric expressions if the value can be represented with no more than 53 bits of precision. 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 MAY be used (e.g., "string", "boolean", or "number" functions), instead of implicit XPath data type conversions. XPath expressions that contain a literal value representing a YANG identity SHOULD always include the declared prefix of the module where the identity is defined. XPath expressions for "when" statements SHOULD NOT reference the context node or any descendant nodes of the context node. They MAY reference descendant nodes if the "when" statement is contained within an "augment" statement, and the referenced nodes are not defined within the "augment" statement. Example:

Wildcards It is possible to construct XPath expressions that will evaluate differently when combined with several modules within a server implementation rather than when evaluated within the single module. This is due to augmenting nodes from other modules. Wildcard expansion is done within a server against all the nodes from all namespaces, so it is possible for a "must" or "when" expression that uses the '*' operator to always evaluate to false if processed within a single YANG module. In such cases, the "description" statement SHOULD clarify that augmenting objects are expected to match the wildcard expansion.

Boolean Expressions The YANG "must" and "when" statements use an XPath boolean expression to define the test condition for the statement. It is important to specify these expressions in a way that will not cause inadvertent changes in the result if the objects referenced in the expression are updated in future revisions of the module. For example, the leaf "foo2" must exist if the leaf "foo1" is equal to "one" or "three": In the next revision of the module, leaf "foo1" is extended with a new enum named "four": Now the first XPath expression will allow the enum "four" to be accepted in addition to the "one" and "three" enum values.

YANG Definition Lifecycle Management The YANG status statement MUST be present within a definition if its value is "deprecated" or "obsolete". The status SHOULD NOT be changed from "current" directly to "obsolete". An object SHOULD be available for at least one year with a "deprecated" status before it is changed to "obsolete". The module or submodule name MUST NOT be changed, once the document containing the module or submodule is published. The module namespace URI value MUST NOT be changed, once the document containing the module is published. The revision date substatement within the import statement SHOULD be present if any groupings are used from the external module. The revision date substatement within the include statement SHOULD be present if any groupings are used from the external submodule. If an import statement is for a module from a stable source (e.g., an RFC for an IETF module), then a reference-stmt SHOULD be present within an import statement. If submodules are used, then the document containing the main module MUST be updated so that the main module revision date is equal to or more recent than the revision date of any submodule that is (directly or indirectly) included by the main module. Definitions for future use SHOULD NOT be specified in a module. Do not specify placeholder objects like the "reserved" example below:

Module Header, Meta, and Revision Statements For published modules, the namespace MUST be a globally unique URI, as defined in . This value is usually assigned by the IANA. The "organization" statement MUST be present. If the module is contained in a document intended for IETF Standards Track status, then the organization SHOULD be the IETF working group (WG) chartered to write the document. For other standards organizations, a similar approach is also suggested. The "contact" statement MUST be present. If the module is contained in a document intended for Standards Track status, then the WG web and mailing information SHOULD be present, and the main document author or editor contact information SHOULD be present. If additional authors or editors exist, their contact information MAY be present. There is no need to include the contact information for WG Chairs. The "description" statement MUST be present. For modules published within IETF documents, the appropriate IETF Trust Copyright text MUST be present, as described in Section 3.1. If the module relies on information contained in other documents, which are not the same documents implied by the import statements present in the module, then these documents MUST be identified in the reference statement. A "revision" statement MUST be present for each published version of the module. The "revision" statement MUST have a "reference" substatement. It MUST identify the published document that contains the module. Modules are often extracted from their original documents, and it is useful for developers and operators to know how to find the original source document in a consistent manner. The "revision" statement MAY have a "description" substatement. For convenience, the description text of a new published revision may summarize any changes made to a module compared to the previous published revision. Typically, that list is a YANG-specific subset of the summary of changes listing any changes made from the RFC being updated or obsoleted as per . The following example shows the revision statement for a published YANG module: The following example shows the revision statements for a published YANG module that updates a published module. The new revision statement summarizes the changes compared to the previous published revision. For an unpublished module, a complete history of each unpublished module revision is not required. That is, within a sequence of draft versions, only the most recent revision need be recorded in the module. Do not remove or reuse a revision statement for a published module. A new revision date is not required unless the module contents have changed. If the module contents have changed, then the revision date of that new module version MUST be updated to a date later than that of the previous version. The following example shows the revision statements for an unpublished update to a published YANG module. The latest revision statement of the unpublished module summarizes the changes compared to the previous revision.

Namespace Assignments It is RECOMMENDED that only valid YANG modules be included in documents, whether or not the modules are published yet. This allows:

the module to compile correctly instead of generating disruptive fatal errors.
early implementors to use the modules without picking a random value for the XML namespace.
early interoperability testing since independent implementations will use the same XML namespace value.

Until a URI is assigned by the IANA, a proposed namespace URI MUST be provided for the namespace statement in a YANG module. A value SHOULD be selected that is not likely to collide with other YANG namespaces. Standard module names, prefixes, and URI strings already listed in the "YANG Module Names" registry MUST NOT be used. A standard namespace statement value SHOULD have the following form: : ]]> The following URN prefix string SHOULD be used for published and unpublished YANG modules: The following example URNs would be valid namespace statement values for Standards Track modules: Note that a different URN prefix string SHOULD be used for modules that are not Standards Track. The string SHOULD be selected according to the guidelines in . The following URIs exemplify what might be used by modules that are not Standards Track. Note that the domain "example.com" SHOULD be used by example modules in IETF I-Ds. These URIs are not intended to be dereferenced. They are used for module namespace identification only. Example URIs using URLs per : Example URIs using tags per :

Top-Level Data Definitions The top-level data organization SHOULD be considered carefully, in advance. Data model designers need to consider how the functionality for a given protocol or protocol family will grow over time. The separation of configuration data and operational state SHOULD be considered carefully. It is sometimes useful to define separate top- level containers for configuration and non-configuration data. For some existing top-level data nodes, configuration data was not in scope, so only one container representing operational state was created. Refer to NMDA for details. The number of top-level data nodes within a module SHOULD be minimized. It is often useful to retrieve related information within a single subtree. If data is too distributed, it becomes difficult to retrieve all at once. The names and data organization SHOULD reflect persistent information, such as the name of a protocol. The name of the working group SHOULD NOT be used because this may change over time. A mandatory database data definition is defined as a node that a client must provide for the database to be valid. The server is not required to provide a value. Top-level database data definitions MUST NOT be mandatory. If a mandatory node appears at the top level, it will immediately cause the database to be invalid. This can occur when the server boots or when a module is loaded dynamically at runtime.

Data Types Selection of an appropriate data type (i.e., built-in type, existing derived type, or new derived type) is very subjective; therefore, few requirements can be specified on that subject. Data model designers SHOULD use the most appropriate built-in data type for the particular application. 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.

Fixed-Value Extensibility If the set of values is fixed and the data type contents are controlled by a single naming authority (e.g., IANA), then an enumeration data type SHOULD be used. If distributed extensibility or hierarchical organization of enumerated values is required, then the "identityref" data type SHOULD be used instead of an enumeration or other built-in type. Note that any module can declare an identity with base "foo-type" that is valid for the "foo" leaf. Identityref values are considered to be qualified names.

Patterns and Ranges 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. A single-quoted string SHOULD be used to specify the pattern, since a double-quoted string can modify the content. If the patterns used in a type definition have known limitations such as false negative or false positive matches, then these limitations SHOULD be documented within the typedef or data definition. The following typedef from demonstrates the proper use of the "pattern" statement: For string data types, if the length of the string is required to be bounded in all implementations, then a length statement MUST be present. The following typedef from demonstrates the proper use of the "length" statement: 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. The following typedef from demonstrates the proper use of the "range" statement:

Enumerations and Bits 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 present.

Union Types The YANG "union" type is evaluated by testing a value against each member type in the union. The first type definition that accepts a value as valid is the member type used. In general, member types SHOULD be ordered from most restrictive to least restrictive types. In the following example, the "enumeration" type will never be matched because the preceding "string" type will match everything. Incorrect: Correct: It is possible for different member types to match, depending on the input encoding format. In XML, all values are passed as string nodes; but in JSON, there are different value types for numbers, booleans, and strings. In the following example, a JSON numeric value will always be matched by the "int32" type, but in XML the string value representing a number will be matched by the "string" type. The second version will match the "int32" member type no matter how the input is encoded. Incorrect: Correct:

Empty and Boolean YANG provides an "empty" data type, which has one value (i.e., present). The default is "not present", which is not actually a value. When used within a list key, only one value can (and must) exist for this key leaf. The type "empty" SHOULD NOT be used for a key leaf since it is pointless. There is really no difference between a leaf of type "empty" and a leaf-list of type "empty". Both are limited to one instance. The type "empty" SHOULD NOT be used for a leaf-list. The advantage of using type "empty" instead of type "boolean" is that the default (not present) does not take up any bytes in a representation. The disadvantage is that the client may not be sure if an empty leaf is missing because it was filtered somehow or not implemented. The client may not have a complete and accurate schema for the data returned by the server and may not be aware of the missing leaf. The YANG "boolean" data type provides two values ("true" and "false"). When used within a list key, two entries can exist for this key leaf. Default values are ignored for key leafs, but a default statement is often used for plain boolean leafs. The advantage of the "boolean" type is that the leaf or leaf-list has a clear representation for both values. The default value is usually not returned unless explicitly requested by the client, so no bytes are used in a typical representation. In general, the "boolean" data type SHOULD be used instead of the "empty" data type, as shown in the example below: Incorrect: Correct:

Reusable Type Definitions If an appropriate derived type exists in any standard module, such as , then it SHOULD be used instead of defining a new derived type. If an appropriate units identifier can be associated with the desired semantics, then a units statement SHOULD be present. If an appropriate default value can be associated with the desired semantics, then a default statement SHOULD be present. If a significant number of derived types are defined, and it is anticipated that these data types will be reused by multiple modules, then these derived types SHOULD be contained in a separate module or submodule, to allow easier reuse without unnecessary coupling. The "description" statement MUST be present. If the type definition semantics are defined in an external document (other than another YANG module indicated by an import statement), then the reference statement MUST be present.

Reusable Groupings A reusable grouping is a YANG grouping that can be imported by another module and is intended for use by other modules. This is not the same as a grouping that is used within the module in which it is defined, but it happens to be exportable to another module because it is defined at the top level of the YANG module. The following guidelines apply to reusable groupings, in order to make them as robust as possible:

Clearly identify the purpose of the grouping in the "description" statement.
There are five different XPath contexts in YANG (rpc/input, rpc/output, notification, "config true" data nodes, and all data nodes). Clearly identify which XPath contexts are applicable or excluded for the grouping.
Do not reference data outside the grouping in any "path", "must", or "when" statements.
Do not include a "default" substatement on a leaf or choice unless the value applies on all possible contexts.
Do not include a "config" substatement on a data node unless the value applies on all possible contexts.
Clearly identify any external dependencies in the grouping "description" statement, such as nodes referenced by an absolute path from a "path", "must", or "when" statement.

Data Definitions The "description" statement MUST be present in the following YANG statements:

anyxml
augment
choice
container
extension
feature
grouping
identity
leaf
leaf-list
list
notification
rpc
typedef

If the data definition semantics are defined in an external document, (other than another YANG module indicated by an import statement), then a reference statement MUST be present. The "anyxml" construct may be useful to represent an HTML banner containing markup elements, such as "<b>" and "</b>", and MAY be used in such cases. However, this construct SHOULD NOT be used if other YANG data node types can be used instead to represent the desired syntax and semantics. It has been found that the "anyxml" statement is not implemented consistently across all servers. It is possible that mixed-mode XML will not be supported or that configuration anyxml nodes will not supported. 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. For list and leaf-list data definitions, if the number of possible instances is required to be bounded for all implementations, then the max-elements statements SHOULD be present. If any "must" or "when" statements are used within the data definition, then the data definition "description" statement SHOULD describe the purpose of each one. The "choice" statement is allowed to be directly present within a "case" statement in YANG 1.1. This needs to be considered carefully. Consider simply including the nested "choice" as additional "case" statements within the parent "choice" statement. Note that the "mandatory" and "default" statements within a nested "choice" statement only apply if the "case" containing the nested "choice" statement is first selected. If a list defines any key leafs, then these leafs SHOULD be defined in order, as the first child nodes within the list. The key leafs MAY be in a different order in some cases, e.g., they are defined in a grouping, and not inline in the list statement.

Non-Presence Containers A non-presence container is used to organize data into specific subtrees. It is not intended to have semantics within the data model beyond this purpose, although YANG allows it (e.g., a "must" statement within the non-presence container). Example using container wrappers: Example without container wrappers: Use of non-presence containers to organize data is a subjective matter similar to use of subdirectories in a file system. Although these containers do not have any semantics, they can impact protocol operations for the descendant data nodes within a non-presence container, so use of these containers SHOULD be considered carefully. The NETCONF and RESTCONF protocols do not currently support the ability to delete all list (or leaf-list) entries at once. This deficiency is sometimes avoided by use of a parent container (i.e., deleting the container also removes all child entries).

Top-Level Data Nodes Use of top-level objects needs to be considered carefully:

top-level siblings are not ordered
top-level siblings are not static and depend on the modules that are loaded
for subtree filtering, retrieval of a top-level leaf-list will be treated as a content-match node for all top-level-siblings
a top-level list with many instances may impact performance

Operation Definitions If the operation semantics are defined in an external document (other than another YANG module indicated by an import statement), then a reference statement MUST be present. If the operation impacts system behavior in some way, it SHOULD be mentioned in the "description" statement. If the operation is potentially harmful to system behavior in some way, it MUST be mentioned in the Security Considerations section of the document.

Notification Definitions The "description" statement MUST be present. If the notification semantics are defined in an external document (other than another YANG module indicated by an import statement), then a reference statement MUST be present. If the notification refers to a specific resource instance, then this instance SHOULD be identified in the notification data. This is usually done by including "leafref" leaf nodes with the key leaf values for the resource instance. For example: Note that there are no formal YANG statements to identify any data node resources associated with a notification. The "description" statement for the notification SHOULD specify if and how the notification identifies any data node resources associated with the specific event.

Feature Definitions The YANG "feature" statement is used to define a label for a set of optional functionality within a module. The "if-feature" statement is used in the YANG statements associated with a feature. The description-stmt within a feature-stmt MUST specify any interactions with other features. The set of YANG features defined in a module should be considered carefully. Very fine granular features increase interoperability complexity and should be avoided. A likely misuse of the feature mechanism is the tagging of individual leafs (e.g., counters) with separate features. If there is a large set of objects associated with a YANG feature, then consider moving those objects to a separate module, instead of using a YANG feature. Note that the set of features within a module is easily discovered by the reader, but the set of related modules within the entire YANG library is not as easy to identify. Module names with a common prefix can help readers identify the set of related modules, but this assumes the reader will have discovered and installed all the relevant modules. Another consideration for deciding whether to create a new module or add a YANG feature is the stability of the module in question. It may be desirable to have a stable base module that is not changed frequently. If new functionality is placed in a separate module, then the base module does not need to be republished. If it is designed as a YANG feature, then the module will need to be republished. If one feature requires implementation of another feature, then an "if-feature" statement SHOULD be used in the dependent "feature" statement. For example, feature2 requires implementation of feature1:

YANG Data Node Constraints

Controlling Quantity The "min-elements" and "max-elements" statements can be used to control how many list or leaf-list instances are required for a particular data node. YANG constraint statements SHOULD be used to identify conditions that apply to all implementations of the data model. If platform-specific limitations (e.g., the "max-elements" supported for a particular list) are relevant to operations, then a data model definition statement (e.g., "max-ports" leaf) SHOULD be used to identify the limit.

"must" versus "when" "must" and "when" YANG statements are used to provide cross-object referential tests. They have very different behavior. The "when" statement causes data node instances to be silently deleted as soon as the condition becomes false. A false "when" expression is not considered to be an error. The "when" statement SHOULD be used together with "augment" or "uses" statements to achieve conditional model composition. The condition SHOULD be based on static properties of the augmented entry (e.g., list key leafs). The "must" statement causes a datastore validation error if the condition is false. This statement SHOULD be used for enforcing parameter value restrictions that involve more than one data node (e.g., end-time parameter must be after the start-time parameter).

"augment" Statements The YANG "augment" statement is used to define a set of data definition statements that will be added as child nodes of a target data node. The module namespace for these data nodes will be the augmenting module, not the augmented module. A top-level "augment" statement SHOULD NOT be used if the target data node is in the same module or submodule as the evaluated "augment" statement. The data definition statements SHOULD be added inline instead.

Conditional Augment Statements The "augment" statement is often used together with the "when" statement and/or "if-feature" statement to make the augmentation conditional on some portion of the data model. The following example from shows how a conditional container called "ethernet" is added to the "interface" list only for entries of the type "ethernetCsmacd".

Conditionally Mandatory Data Definition Statements YANG has very specific rules about how configuration data can be updated in new releases of a module. These rules allow an "old client" to continue interoperating with a "new server". If data nodes are added to an existing entry, the old client MUST NOT be required to provide any mandatory parameters that were not in the original module definition. It is possible to add conditional "augment" statements such that the old client would not know about the new condition and would not specify the new condition. The conditional "augment" statement can contain mandatory objects only if the condition is false, unless explicitly requested by the client. Only a conditional "augment" statement that uses the "when" statement form of a condition can be used in this manner. The YANG features enabled on the server cannot be controlled by the client in any way, so it is not safe to add mandatory augmenting data nodes based on the "if-feature" statement. The XPath "when" statement condition MUST NOT reference data outside of the target data node because the client does not have any control over this external data. In the following dummy example, it is okay to augment the "interface" entry with "mandatory-leaf" because the augmentation depends on support for "some-new-iftype". The old client does not know about this type, so it would never select this type; therefore, it would not add a mandatory data node. Note that this practice is safe only for creating data resources. It is not safe for replacing or modifying resources if the client does not know about the new condition. The YANG data model MUST be packaged in a way that requires the client to be aware of the mandatory data nodes if it is aware of the condition for this data. In the example above, the "some-new-iftype" identity is defined in the same module as the "mandatory-leaf" data definition statement. This practice is not safe for identities defined in a common module such as "iana-if-type" because the client is not required to know about "my-module" just because it knows about the "iana-if-type" module.

Deviation Statements Per , the YANG "deviation" statement is not allowed to appear in IETF YANG modules, but it can be useful for documenting server capabilities. Deviation statements are not reusable and typically not shared across all platforms. There are several reasons that deviations might be needed in an implementation, e.g., an object cannot be supported on all platforms, or feature delivery is done in multiple development phases. Deviation statements can also be used to add annotations to a module, which does not affect the conformance requirements for the module. It is suggested that deviation statements be defined in separate modules from regular YANG definitions. This allows the deviations to be platform specific and/or temporary. The order that deviation statements are evaluated can affect the result. Therefore, multiple deviation statements in the same module, for the same target object, SHOULD NOT be used. The "max-elements" statement is intended to describe an architectural limit to the number of list entries. It is not intended to describe platform limitations. It is better to use a "deviation" statement for the platforms that have a hard resource limit. Example documenting platform resource limits:

Extension Statements The YANG "extension" statement is used to specify external definitions. This appears in the YANG syntax as an "unknown-statement". Usage of extension statements in a published module needs to be considered carefully. The following guidelines apply to the usage of YANG extensions:

The semantics of the extension MUST NOT contradict any YANG statements. Extensions can add semantics not covered by the normal YANG statements.
The module containing the extension statement MUST clearly identify the conformance requirements for the extension. It should be clear whether all implementations of the YANG module containing the extension need to also implement the extension. If not, identify what conditions apply that would require implementation of the extension.
The extension MUST clearly identify where it can be used within other YANG statements.
The extension MUST clearly identify if YANG statements or other extensions are allowed or required within the extension as substatements.

Data Correlation Data can be correlated in various ways, using common data types, common data naming, and common data organization. There are several ways to extend the functionality of a module, based on the degree of coupling between the old and new functionality:

inline:: update the module with new protocol-accessible objects. The naming and data organization of the original objects is used. The new objects are in the original module namespace.
augment:: create a new module with new protocol-accessible objects that augment the original data structure. The naming and data organization of the original objects is used. The new objects are in the new module namespace.
mirror:: create new objects in a new module or the original module, except use a new naming scheme and data location. The naming can be coupled in different ways. Tight coupling is achieved with a "leafref" data type, with the "require-instance" substatement set to "true". This method SHOULD be used.

If the new data instances are not limited to the values in use in the original data structure, then the "require-instance" substatement MUST be set to "false". Loose coupling is achieved by using key leafs with the same data type as the original data structure. This has the same semantics as setting the "require-instance" substatement to "false". The relationship between configuration and operational state has been clarified in NMDA .

Use of "leafref" for Key Correlation Sometimes it is not practical to augment a data structure. For example, the correlated data could have different keys or contain mandatory nodes. The following example shows the use of the "leafref" data type for data correlation purposes: Not preferred: Preferred:

Operational State The modeling of operational state with YANG has been refined over time. At first, only data that has a "config" statement value of "false" was considered to be operational state. This data was not considered to be part of any datastore, which made the YANG XPath definition much more complicated. Operational state is now modeled using YANG according to the NMDA and conceptually contained in the operational state datastore, which also includes the operational values of configuration data. There is no longer any need to duplicate data structures to provide separate configuration and operational state sections. This section describes some data modeling issues related to operational state and guidelines for transitioning YANG data model design to be NMDA compatible.

Combining Operational State and Configuration Data If possible, operational state SHOULD be combined with its associated configuration data. This prevents duplication of key leafs and ancestor nodes. It also prevents race conditions for retrieval of dynamic entries and allows configuration and operational state to be retrieved together with minimal message overhead.

Representing Operational Values of Configuration Data If possible, the same data type SHOULD be used to represent the configured value and the operational value, for a given leaf or leaf- list object. Sometimes the configured value set is different than the operational value set for that object, for example, the "admin-status" and "oper-status" leafs in . In this case, a separate object MAY be used to represent the configured and operational values. Sometimes the list keys are not identical for configuration data and the corresponding operational state. In this case, separate lists MAY be used to represent the configured and operational values. If it is not possible to combine configuration and operational state, then the keys used to represent list entries SHOULD be the same type. The "leafref" data type SHOULD be used in operational state for key leafs that have corresponding configuration instances. The "require-instance" statement MAY be set to "false" (in YANG 1.1 modules only) to indicate instances are allowed in the operational state that do not exist in the associated configuration data. The need to replicate objects or define different operational state objects depends on the data model. It is not possible to define one approach that will be optimal for all data models. Designers SHOULD describe and justify any NMDA exceptions in detail, such as the use of separate subtrees and/or separate leafs. The "description" statements for both the configuration and the operational state SHOULD be used for this purpose.

NMDA Transition Guidelines YANG modules SHOULD be designed with the assumption that they will be used on servers supporting the operational state datastore. With this in mind, YANG modules SHOULD define "config false" nodes wherever they make sense to the data model. "Config false" nodes SHOULD NOT be defined to provide the operational value for configuration nodes, except when the value space of a configured and operational value may differ, in which case a distinct "config false" node SHOULD be defined to hold the operational value for the configured node. The following guidelines are meant to help modelers develop YANG modules that will maximize the utility of the model with both current and new implementations. New modules and modules that are not concerned with the operational state of configuration information SHOULD immediately be structured to be NMDA compatible, as described in Section 4.23.1. This transition MAY be deferred if the module does not contain any configuration datastore objects. The remaining are options that MAY be followed during the time that NMDA mechanisms are being defined.

Modules that require immediate support for the NMDA features SHOULD be structured for NMDA. A temporary non-NMDA version of this type of module MAY exist, as either an existing model or a model created by hand or with suitable tools that mirror the current modeling strategies. Both the NMDA and the non-NMDA modules SHOULD be published in the same document, with NMDA modules in the document main body and the non-NMDA modules in a non-normative appendix. The use of the non-NMDA module will allow temporary bridging of the time period until NMDA implementations are available.
For published models, the model should be republished with an NMDA-compatible structure, deprecating non-NMDA constructs. For example, the "ietf-interfaces" model in has been restructured as an NMDA-compatible model in . The "/interfaces-state" hierarchy has been marked "status deprecated". Models that mark their "/foo-state" hierarchy with "status deprecated" will allow NMDA-capable implementations to avoid the cost of duplicating the state nodes, while enabling non-NMDA-capable implementations to utilize them for access to the operational values.
For models that augment models that have not been structured with the NMDA, the modeler will have to consider the structure of the base model and the guidelines listed above. Where possible, such models should move to new revisions of the base model that are NMDA compatible. When that is not possible, augmenting "state" containers SHOULD be avoided, with the expectation that the base model will be re-released with the state containers marked as deprecated. It is RECOMMENDED to augment only the "/foo" hierarchy of the base model. Where this recommendation cannot be followed, then any new "state" elements SHOULD be included in their own module.

Temporary Non-NMDA Modules A temporary non-NMDA module allows a non-NMDA-aware client to access operational state from an NMDA-compliant server. It contains the top-level "config false" data nodes that would have been defined in a legacy YANG module (before NMDA). A server that needs to support both NMDA and non-NMDA clients can advertise both the new NMDA module and the temporary non-NMDA module. A non-NMDA client can use separate "foo" and "foo-state" subtrees, except the "foo-state" subtree is located in a different (temporary) module. The NMDA module can be used by a non-NMDA client to access the conventional configuration datastores and the deprecated <get> operation to access nested "config false" data nodes. To create the temporary non-NMDA model from an NMDA model, the following steps can be taken:

Change the module name by appending "-state" to the original module name
Change the namespace by appending "-state" to the original namespace value
Change the prefix by appending "-s" to the original prefix value
Add an import to the original module (e.g., for typedef definitions)
Retain or create only the top-level nodes that have a "config" statement value "false". These subtrees represent "config false" data nodes that were combined into the configuration subtree; therefore, they are not available to non-NMDA aware clients. Set the "status" statement to "deprecated" for each new node.
The module description SHOULD clearly identify the module as a temporary non-NMDA module

Example: Create a New NMDA Module Create an NMDA-compliant module, using combined configuration and state subtrees, whenever possible.

Example: Convert an Old Non-NMDA Module Do not remove non-compliant objects from existing modules. Instead, change the status to "deprecated". At some point, usually after 1 year, the status MAY be changed to "obsolete". Old Module:

Example: Create a Temporary NMDA Module Create a new module that contains the top-level operational state data nodes that would have been available before they were combined with configuration data nodes (to be NMDA compliant).

Performance Considerations It is generally likely that certain YANG statements require more runtime resources than other statements. Although there are no performance requirements for YANG validation, the following information MAY be considered when designing YANG data models:

Lists are generally more expensive than containers
"when" statement evaluation is generally more expensive than "if-feature" or "choice" statements
"must" statements are generally more expensive than "min-entries", "max-entries", "mandatory", or "unique" statements
"identityref" leafs are generally more expensive than "enumeration" leafs
"leafref" and "instance-identifier" types with "require-instance" set to "true" are generally more expensive than if "require-instance" is set to "false"

Open Systems Considerations Only the modules imported by a particular module can be assumed to be present in an implementation. An open system MAY include any combination of YANG modules.

Guidelines for Constructs Specific to YANG 1.1 The set of guidelines for YANG 1.1 will grow as operational experience is gained with the new language features. This section contains an initial set of guidelines for new YANG 1.1 language features.

Importing Multiple Revisions Standard modules SHOULD NOT import multiple revisions of the same module into a module. This MAY be done if independent definitions (e.g., enumeration typedefs) from specific revisions are needed in the importing module.

Using Feature Logic The YANG 1.1 feature logic is much more expressive than YANG 1.0. A "description" statement SHOULD describe the "if-feature" logic in text, to help readers understand the module. YANG features SHOULD be used instead of the "when" statement, if possible. Features are advertised by the server, and objects conditional by the "if-feature" statement are conceptually grouped together. There is no such commonality supported for "when" statements. Features generally require less server implementation complexity and runtime resources than objects that use "when" statements. Features are generally static (i.e., set when a module is loaded and not changed at runtime). However, every client edit might cause a "when" statement result to change.

"anyxml" versus "anydata" The "anyxml" statement MUST NOT be used to represent a conceptual subtree of YANG data nodes. The "anydata" statement MUST be used for this purpose.

"action" versus "rpc" The use of "action" statements or "rpc" statements is a subjective design decision. RPC operations are not associated with any particular data node. Actions are associated with a specific data node definition. An "action" statement SHOULD be used if the protocol operation is specific to a subset of all data nodes instead of all possible data nodes. The same action name MAY be used in different definitions within different data node. For example, a "reset" action defined with a data node definition for an interface might have different parameters than for a power supply or a VLAN. The same action name SHOULD be used to represent similar semantics. The NETCONF Access Control Model (NACM) does not support parameter-based access control for RPC operations. The user is given permission (or not) to invoke the RPC operation with any parameters. For example, if each client is only allowed to reset their own interface, then NACM cannot be used. For example, NACM cannot enforce access control based on the value of the "interface" parameter, only the "reset" operation itself: However, NACM can enforce access control for individual interface instances, using a "reset" action. If the user does not have read access to the specific "interface" instance, then it cannot invoke the "reset" action for that interface instance:

Updating YANG Modules (Published versus Unpublished) YANG modules can change over time. Typically, new data model definitions are needed to support new features. YANG update rules defined in MUST be followed for published modules. They MAY be followed for unpublished modules. The YANG update rules only apply to published module revisions. Each organization will have their own way to identify published work that is considered to be stable and unpublished work that is considered to be unstable. For example, in the IETF, the RFC document is used for published work, and the I-D is used for unpublished work.

Defining Standard Tags specifies a method for associating tags with YANG modules. Tags may be defined and associated at module design time, at implementation time, or via user administrative control. Design-time tags are indicated using the module-tag extension statement. A module MAY indicate, using module-tag extension statements, a set of tags that are to be automatically associated with it (i.e., not added through configuration). Authors can use existing standard tags or use new tags defined in the model definition, as appropriate. For IETF modules, new tags MUST be assigned in the IANA "IETF YANG Module Tags" registry within the "YANG Module Tags" registry .

Modeling Abstract Data Structures For contexts where YANG is used to model abstract data structures (e.g., protocol messages), the use of is RECOMMENDED compared to the "yang-data" extension statement .

Examples of modules that rely upon are or .

Abstract data structures can be augmented using the "augment-structure" statement .

Examples of modules that augment abstract data structures are and .

IANA-Maintained Modules

Context IANA maintains a set of registries that are key for interoperability. The content of these registries are usually available using various formats (e.g., plain text, XML). However, there were some confusion in the past about whether the content of some registries is dependent on a specific representation format. For example, Section 5 of was published to clarify that MIB and YANG modules are merely additional formats in which the "Interface Types (ifType)" and "Tunnel Types (tunnelType)" registries are available. The MIB and YANG modules are not separate registries, and the same values are always present in all formats of the same registry. Also, some YANG modules include parameters and values directly in a module that is not maintained by IANA while these are populated in an IANA registry. Such a design is suboptimal as it creates another source of information that may deviate from the IANA registry as new values are assigned or some values are deprecated. For the sake of consistency, better flexibility to support new values, and maintaining IANA registries as the unique authoritative source of information, when such an information is maintained in a registry, this document encourages the use of IANA-maintained modules. The following section provides a set of guidelines for YANG module authors related to the design of IANA-maintained modules. These guidelines are meant to leverage existing IANA registries and use YANG as another format to present the content of these registries when appropriate.

Guidelines for IANA-Maintained Modules When designing a YANG module for a functionality governed by a protocol for which IANA maintains a registry, it is RECOMMENDED to specify an IANA-maintained module that echoes the content of that registry. This is superior to including that content in an IETF-maintained module. When one or multiple sub-registries are available under the same registry, it is RECOMMENDED to define an IANA-maintained module for each sub-registry. However, module designers MAY consider defining one single IANA-maintained module that covers all sub-registries if maintaining that single module is manageable (e.g., very few values are present or expected to be present for each sub-registry). An example of such a module is documented in . An IANA-maintained module may use the "identityref" data type (e.g., ) or an enumeration data type (e.g., ). See for a guidance on which data type to use. The decision about which type to use should be made based upon specifics related to the intended use of the IANA-maintained module. For example, identities are useful if the registry entries are organized hierarchically, possibly including multiple inheritances. It is RECOMMENDED that the reasoning for the design choice is documented in the companion specification that registers an IANA-maintained module. For example, defines an IANA-maintained module that uses enumerations for the following reason: Designers of IANA-maintained modules MAY supply the full initial version of the module in a specification document that registers the module or only a script to be used (including by IANA) for generating the module (e.g., an XSLT stylesheet as in Appendix A of ). For both cases, the document that defines an IANA-maintained module MUST include a note indicating that the document is only documenting the initial version of the module and that the authoritative version is to be retrieved from the IANA registry. Also, the IANA-maintained module MUST include the following note indicating the RFC that registered the initial version of the IANA-maintained module:

The initial version of this YANG module is part of RFC IIII; see the RFC itself for full legal notices.

It is RECOMMENDED to include the URL from where to retrieve the recent version of the module. When a script is used, the Internet-Draft that defines an IANA-maintained module SHOULD include an appendix with the initial full version of the module. Including such an appendix in pre-RFC versions is meant to assess the correctness of the outcome of the supplied script. The authors MUST include a note to the RFC Editor requesting that the appendix be removed before publication as RFC and that RFC IIII is replaced with the RFC number that is assigned to the document. Initial versions of IANA-maintained modules that are published in RFCs may be misused despite the appropriate language to refer to the IANA registry to retrieve the up-to-date module. This is problematic for interoperability, e.g., when values are deprecated or are associated with a new meaning. If an IANA-maintained module is imported by another module, a normative reference with the IANA URL from where to retrieve the IANA-maintained module SHOULD be included. Although not encouraged, referencing the RFC that defines the initial version of the IANA module is acceptable in specific cases (e.g., the imported version is specifically the initial version, the RFC includes useful description about the usage of the module). Examples of IANA URLs from where to retrieve the latest version of an IANA-maintained module are: , , and . "IANA_FOO_URL" is used in the following to refer to such URLs. These URLs are expected to be sufficiently permanent and stable. Whenever referencing a specific version of an IANA-maintained module is needed, then URLs such as are used. "IANA_FOO_URL_With_REV" is used in the following to refer to such URLs. A template for IANA-maintained modules is provided in .

Guidance for Writing the IANA Considerations for RFCs Defining IANA-Maintained Modules In addition to the IANA considerations in , the IANA Considerations Section of an RFC that includes an IANA-maintained module MUST provide the required instructions for IANA to automatically perform the maintenance of that IANA module. These instructions describe how to proceed with updates to the IANA-maintained module that are triggered by a change to the authoritative registry. Concretely, the IANA Considerations Section SHALL at least provide the following information:

An IANA request to add a note to the page displaying the information about the IANA-maintained module that new values must not be directly added to the module, but to an authoritative IANA registry.
An IANA request to add a note to the authoritative IANA registry to indicate that any change to the registry must be reflected into the corresponding IANA-maintained module.
Details about the required actions (e.g., add a new "identity" or "enum" statement) to update the IANA-maintained module to reflect changes to an authoritative IANA registry. Typically, these details have to include the procedure to create a new "identity" statement name and sub-statements ("base", "status", "description", and "reference") or a new "enum" statement and sub- statements ("value", "status", "description", and "reference").
- When creating a new "identity" statement name or a new "enum" statement, it is RECOMMENDED to mirror the name (if present) as recorded in the IANA registry.
- If the name in the IANA registry does not comply with the naming conventions listed in , the procedure MUST detail how IANA can generate legal identifiers from such a name. For example, if the name begins with a number, it is RECOMMENDED to spell out the number when used as an identifier (e.g., "3des-cbc" will be "triple-des-cbc" ()).
A note that unassigned or reserved values must not be present in the IANA-maintained module.
An indication whether experimental values are included in the IANA-maintained module. Absent such an indication, experimental values MUST NOT be listed in the IANA-maintained module.
An instruction about how to generate the "revision" statement.

A template for the IANA Considerations is provided in for IANA-maintained modules with identities and for IANA-maintained modules with enumerations. Authors may modify the template to reflect specifics of their modules (e.g., Multiple registries can be listed for a single IANA-maintained module, no explicit description (or name) field is listed under the authoritative IANA registry, or the name does not comply with YANG naming conventions ()). The following templates are to be considered in addition to the required information that is provided in .

Template for IANA-Maintained Modules with Identities

Template for IANA-Maintained Modules with Enumerations