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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 Network Configuration Protocol (NETCONF) and RESTCONF protocol implementations that utilize YANG data model modules.

The standardization of network configuration interfaces for use with the Network Configuration Protocol and RESTCONF 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 data models. YANG is used to define the data structures, protocol operations, and notification content used within a NETCONF and/or RESTCONF server. A server that supports a particular YANG module will support client NETCONF and/or RESTCONF operation requests, as indicated by the specific content defined in the YANG module. This document is similar to the Structure of Management Information version 2 (SMIv2) usage guidelines specification in intent and structure. However, since that document was written a decade after SMIv2 modules had been in use, it was published as a 'Best Current Practice' (BCP). This document is not a BCP, but rather an informational reference, intended to promote consistency in documents containing YANG modules. Many YANG constructs are defined as optional to use, such as the description statement. However, in order to maximize interoperability of NETCONF and RESTCONF implementations utilizing YANG data models, it is desirable to define a set of usage guidelines that may require a higher level of compliance than the minimum level defined in the YANG specification. In addition, YANG allows constructs such as infinite length identifiers and string values, or top-level mandatory nodes, that a compliant server is not required to support. Only constructs that all servers are required to support can be used in IETF YANG modules. This document defines usage guidelines related to the NETCONF operations layer and NETCONF content layer, as defined in , and the RESTCONF methods and RESTCONF resources, as defined in , These guidelines are intended to be used by authors and reviewers to improve the readability and interoperability of published YANG data models. Note that this document is not a YANG tutorial and the reader is expected to know the YANG data modeling language before using this document.

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 . RFC 2119 language is used here to express the views of the NETMOD working group regarding content for YANG modules. YANG modules complying with this document will treat the RFC 2119 terminology as if it were describing best current practices.

The following terms are defined in and are not redefined here: capabilities client operation server

The following terms are defined in and are not redefined here: data node module namespace submodule version YANG YIN Note that the term 'module' may be used as a generic term for a YANG module or submodule. When describing properties that are specific to submodules, the term 'submodule' is used instead.

The following terms are used throughout this document: published: A stable release of a module or submodule. For example the "Request for Comments" described in section 2.1 of is considered a stable publication. unpublished: An unstable release of a module or submodule. For example the "Internet‑Draft" described in section 2.2 of is considered an unstable publication that is a work-in-progress, subject to change at any time. YANG fragment: A set of YANG statements that are not intended to represent a complete YANG module or submodule. These statements are not intended for actual use, except to provide an example of YANG statement usage. The invalid syntax "..." is sometimes used to indicate that additional YANG statements would be present in a real YANG module.

YANG tree diagrams provide a concise representation of a YANG module to help readers understand the module structure. The meaning of the symbols in YANG tree diagrams is as follows. Each node is printed as:

is one of: + for current x for deprecated o for obsolete is one of: rw for configuration data ro for non-configuration data -x for rpcs and actions -n for notifications is the name of the node () means that the node is a choice node :() means that the node is a case node ]]>

:. ]]>

is one of: ? for an optional leaf, choice, anydata or anyxml ! for a presence container * for a leaf-list or list [] for a list's keys is the name of the type for leafs and leaf-lists ]]>

TARGET", where TARGET is either the leafref path, with prefixed removed if possible. ]]>

is the list of features this node depends on, printed within curly brackets and a question mark "{...}?" ]]>

YANG data model modules under review are likely to be contained in Internet-Drafts. All guidelines for Internet-Draft authors MUST be followed. The RFC Editor provides guidelines for authors of RFCs, which are first published as Internet-Drafts. These guidelines should be followed and are defined in and updated in and "RFC Document Style" . The following sections MUST be present in an Internet-Draft containing a module: Narrative sections Definitions section Security Considerations section IANA Considerations section References section There are three usage scenarios for YANG that can appear in an Internet-Draft 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 complete module or submodule, but may be applicable to example modules and YANG fragments as well.

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

Each normative YANG module or submodule contained within an Internet-Draft 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 Section 5.2 of . The following example is for the '2010‑01‑18' revision of the 'ietf‑foo' module: <CODE BEGINS> file "ietf-foo@2016-03-20.yang"

<CODE ENDS>

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".

A terminology section MUST be present if any terms are defined in the document or if any terms are imported from other documents. If YANG tree diagrams are used, then a sub-section explaining the YANG tree diagram syntax MUST be present, containing the following text:

YANG tree diagrams provide a concise representation of a YANG module, and SHOULD be included to help readers understand YANG module structure. Tree diagrams MAY be split into sections to correspond to document structure. The following example shows a simple YANG tree diagram:

The 'pyang' compiler can be used to produce the tree diagram, using the '-f tree' command line parameter. If the YANG module is comprised of groupings only, then the tree diagram SHOULD contain the groupings. The 'pyang' compiler can be used to produce a tree diagram with groupings using the '-f tree --tree-print-groupings" command line parameters. If the YANG module contains notifications, then the tree diagram SHOULD contain the notifications. If the YANG module contains RPC statements, then the tree diagram SHOULD contain the RPC statements.

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(s) defined by the specification imports definitions from other modules (except for those defined in the or documents), or are always implemented in conjunction with other modules, then those facts MUST be noted in the overview section, as MUST be noted any special interpretations of definitions in other modules.

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. 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 SMIv2, which are not affected by these guidelines. Note that all YANG statements within a YANG module are considered normative, if the module itself is considered normative, and not an example module. The use of keywords defined in apply to YANG description statements in normative modules exactly as they would in any other normative section. Example YANG modules MUST NOT contain any normative text, including any reserved words from . See for guidelines on YANG usage.

Each specification that defines one or more modules MUST contain a section that discusses security considerations relevant to those modules. This section MUST be patterned after the latest approved template (available at http://trac.tools.ietf.org/area/ops/trac/wiki/yang-security-guidelines). contains the security considerations template dated 2013-05-08. 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.

In order to comply with IESG policy as set forth in http://www.ietf.org/id-info/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 on what actions are required of the IANA. If there are no IANA considerations applicable to the document, then the IANA Considerations section stating that there are no actions is removed by the RFC Editor before publication. Refer to the guidelines in for more details.

Each specification that defines one or more modules SHOULD contain usage examples, either throughout the document or in an appendix. This includes example XML instance document snippets to demonstrate the intended usage of the YANG module(s).

If an Internet-Draft 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 YANG Namespace registry entry MUST be requested from the IANA. The specification includes the procedure for this purpose in its IANA Considerations section.

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.

For every import or include statement that appears in a module contained in the specification, which 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, which 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, which identifies a separate document, a corresponding informative reference to that document MAY appear in the Informative References section.

All modules need to be validated before submission in an Internet Draft. The 'pyang' YANG compiler is freely available from github:

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.

A version of 'rfcstrip' is available which will extract YANG modules from an Internet Draft or RFC. The 'rfcstrip' tool which supports YANG module extraction is freely available:

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.

Modules in IETF Standards Track specifications MUST comply with all syntactic and semantic requirements of 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 that clarify or restrict the minimum conformance requirements are included here.

Normative modules contained in Standards Track documents MUST be named according to the guidelines in the IANA Considerations section of . A distinctive word or acronym (e.g., protocol name or working group acronym) 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 acronym 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 representing 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 a new module, not a name change.

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 3 modules:

YANG defines the following rules for prefix usage: Prefixes are never allowed 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 also likely to be unique. Prefix values SHOULD NOT conflict with known modules that have been previously published.

Identifiers for 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 Section 13 of .

Identifiers SHOULD follow a consistent naming pattern throughout the module. Only lower-case letters, numbers, and dashes SHOULD be used in identifier names. Upper-case characters and the underscore character MAY be used if the identifier represents a well-known value that uses these characters. 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 the set of sibling nodes defined in the same module namespace. 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 modelling properties. Only YANG statements and YANG extension statements are designed to convey machine readable data modelling 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.

In general, it is suggested that substatements containing very common default values SHOULD NOT be present. The following substatements are commonly used with the default value, which would make the module difficult to read if used everywhere they are allowed. Statement Default Value config true mandatory false max-elements unbounded min-elements 0 ordered-by system status current yin-element false

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 to indicate that the NETCONF or RESTCONF capability is supported within the data model. 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 leaf 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.

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

YANG defines 5 separate contexts for evaluation of XPath statements: 1) 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". 2) 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. 3) Notification: an event notification document. The document root is the notification element. 4) RPC Input: The document root is the conceptual "input" node, which is the parent of all RPC input parameter definitions. 5) 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.

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 defining the must or when expression containing the 'local‑name' function. Example of a local-name function that should not be used:

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, '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 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'.

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:

It is possible to construct XPath expressions that will evaluate differently when combined with several modules within a server implementation, then 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 will 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.

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.

The status statement MUST be present 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 '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 submodules are used, then the document containing the main module MUST be updated so that the main module revision date is equal 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:

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 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 working group web and mailing information MUST 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. The description statement MUST be present. For modules published within IETF documents, the appropriate IETF Trust Copyright text MUST be present, as described in . 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. It is not required to keep the full revision history of draft versions (e.g., modules contained within Internet-Drafts). That is, within a sequence of draft versions, only the most recent revision need be recorded in the module. However, whenever a new (i.e. changed) version is made available (e.g., via a new version of an Internet-Draft), the revision date of that new version MUST be updated to a date later than that of the previous version.

It is RECOMMENDED that only valid YANG modules be included in documents, whether or not they 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 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 non-Standards-Track modules. The string SHOULD be selected according to the guidelines in . The following URIs exemplify what might be used by non Standards Track modules. Note that the domain "example.com" SHOULD be used by example modules in IETF drafts. Example URIs using URLs per RFC 3986 :

Example URIs using tags per RFC 4151 :

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 data 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 data was created. 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, is 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.

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. 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.

If the set of values is fixed and the data type contents are controlled by a single naming authority, then an enumeration data type SHOULD be used.

If extensibility 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.

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. 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:

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.

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:

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 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:

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.

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 it is defined, but 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 5 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" sub-statement on a leaf or choice unless the value applies on all possible contexts. Do not include a "config" sub-statement 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 absolute path from a "path", "must", or "when" statement.

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 '&lt;b&gt;' and '&lt;/b&gt;', 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 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.

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., "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 sub-directories in a file system. 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).

Use of top-level objects needs to be considered carefully -top-level siblings are not ordered -top-level siblings not are not static, and depends on the modules that are loaded for sub-tree 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

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.

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.

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 set of YANG features available in a module should be considered carefully. The description-stmt within a feature-stmt MUST specify any interactions with other 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 identity. Module names with a common prefix can help readers identity 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:

The "min‑elements" and "max‑elements" statements can be use 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.

The "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 the "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).

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.

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".

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 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 target data node because the client does not have any control over this external data. In the following dummy example, it is OK 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, and therefore not be adding 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.

The YANG "deviation" statement cannot 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:

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 sub-statements.

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 new a 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" sub-statement 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" sub-statement 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" sub-statement to "false". It is sometimes useful to separate configuration and operational data, so that they do not not even share the exact same naming characteristics. The correlation between configuration the operational data that is affected by changes in configuration is a complex problem. There may not be a simple 1:1 relationship between a configuration data node and an operational data node. Further work is needed in YANG to clarify this relationship. Protocol work may also be needed to allow a client to retrieve this type of information from a server. At this time the best practice is to clearly document any relationship to other data structures in the "description" statement.

In YANG, any data that has a "config" statement value of "false" could be considered operational data. The relationship between configuration (i.e., "config" statement has a value of "true") and operational data can be complex. One challenge for client developers is determining if the configured value is being used, which requires the developer to know which operational data parameters are associated with the particular configuration object(s). If possible, operational data 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 data to be retrieved together with minimal message overhead. Not preferred:

Preferred:

If it is not possible to combine configuration and operational data, then the keys used to represent list entries SHOULD be the same type. The "leafref" data type SHOULD be used in operational data 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 following example shows the use of the "leafref" data type: Not preferred:

Preferred:

In the simplest use-cases, there is no interaction between configuration and operational data. For example, the arbitrary administrative name or sequence number assigned to an access control rule. The configured value is always the value that is being used by the system. However, some configuration parameters interact with routing and other signalling protocols, such that the operational value in use by the system may not be the same as the configured value. Other parameters specify the desired state, but environmental and other factors can cause the actual state to be different. For example a "temperature" configuration setting only represents the desired temperature. An operational data parameter is needed that reports the actual temperature in order to determine if the cooling system is operating correctly. YANG has no mechanism other than the "description" statement to associate the desired temperature and the actual temperature. Careful consideration needs to be given to the location of operational data. It can either be located within the configuration subtree for which it applies, or it can be located outside the particular configuration subtree. Placing operational data within the configuration subtree is appropriate if the operational values can only exist if the configuration exists. Placing operational data outside the configuration subtree is appropriate if the operational values can exist without corresponding configuration (e.g., system generated interfaces). The "interfaces" and "interfaces‑state" subtrees defined in are an example of a complex relationship between configuration and operational data. The operational values can include interface entries that have been discovered or initialized by the system. An interface may be in use that has not been configured at all. Therefore, the operational data for an interface cannot be located within the configuration for that same interface. Sometimes the configured value represents some sort of procedure to be followed, in which the system will select an actual value, based on protocol negotiation. In this case it is RECOMMENDED to have a separate config false value to represented the resulting state. For instance:

For example a "duplex" mode configuration may be "auto" to auto-negotiate the actual value to be used. The operational parameter will never contain the value "auto". It will always contain the result of the auto-negotiation, such as "half" or "full". This is just one way in which the configuration data model is not exactly the same as the operational data model. Another is if the detailed properties of the data are different for configured vs. learned entries. If all the data model properties are aligned between configuration and operational data, then it can be useful to define the configuration parameters within a grouping, and then replicate that grouping within the operational data portion of the data model.

Note that this mechanism can also be used if the configuration and operational data are in separate sub-trees:

The need to replicate objects or define different operational data 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 the relationship in detail between configuration objects and any associated operational data objects. The "description" statements for both the configuration and the operational data SHOULD be used for this purpose.

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‑stmt" evaluation is generally more expensive than "if‑feature" or "choice" statements "must" statement is 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

A YANG module MUST NOT be designed such that the set of modules found on a server implementation can be predetermined in advance. 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.

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

Standard modules SHOULD NOT import multiple revisions of the same module into a module. This MAY be done if the authors can demonstrate that the "avoided" definitions from the most recent of the multiple revisions are somehow broken or harmful to interoperability.

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 if-feature 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 module is loaded and not changed at runtime). However every client edit might cause a "when" statement result to change.

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.

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 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 access control based on the value of the "interface" parameter, only the "reset" operation itself:

However, NACM can enforce access 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:

YANG modules can change over time. Typically, new data model definitions are needed to support new features. YANG update rules defined in section 11 of 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 which is considered to be stable, and unpublished work which is considered to be unstable. For example, in the IETF, the RFC document is used for published work, and the Internet-Draft is used for unpublished work.

This document registers one URI in the IETF XML registry . The following registration has been made in [RFC6087] and updated by this document.

The following assignment has been made in [RFC6087] and updated by this document in the YANG Module Names Registry, or the YANG module template in . Field Value Name ietf-template Namespace urn:ietf:params:xml:ns:yang:ietf-template Prefix temp Reference RFC XXXX

This document defines documentation guidelines for NETCONF or RESTCONF content defined with the YANG data modeling language. The guidelines for how to write a Security Considerations section for a YANG module are defined in the online document http://trac.tools.ietf.org/area/ops/trac/wiki/yang-security-guidelines This document does not introduce any new or increased security risks into the management system. The following section contains the security considerations template dated 2010-06-16. Be sure to check the webpage at the URL listed above in case there is a more recent version available. Each specification that defines one or more YANG modules MUST contain a section that discusses security considerations relevant to those modules. This section MUST be patterned after the latest approved template (available at

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 spelled out. Similarly, 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. Further, if new RPC operations have been defined, then the security considerations of each new RPC operation MUST be explained.

X. Security Considerations The YANG module defined in this memo is designed to be accessed via the NETCONF protocol . The lowest NETCONF layer is the secure transport layer and the mandatory-to-implement secure transport is SSH .

There are a number of data nodes defined in this YANG module which are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability:

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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. These are the subtrees and data nodes and their sensitivity/vulnerability:

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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. These are the operations and their sensitivity/vulnerability:

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The structure and contents of this document are adapted from , guidelines for MIB Documents, by C. M. Heard. The working group thanks Martin Bjorklund, Juergen Schoenwaelder, Ladislav Lhotka, and Jernej Tuljak for their extensive reviews and contributions to this document.

The following changes have been made to the guidelines published in : Updated NETCONF reference from RFC 4741 to RFC 6241 Updated NETCONF over SSH citation from RFC 4742 to RFC 6242 Updated YANG Types reference from RFC 6021 to RFC 6991 Updated obsolete URLs for IETF resources Changed top-level data node guideline Clarified XPath usage for a literal value representing a YANG identity Clarified XPath usage for a when-stmt Clarified XPath usage for 'proceeding‑sibling' and 'following‑sibling' axes Added terminology guidelines Added YANG tree diagram definition and guideline Updated XPath guidelines for type conversions and function library usage. Updated data types section Updated notifications section Clarified conditional key leaf nodes Clarify usage of 'uint64' and 'int64' data types Added text on YANG feature usage Added Identifier Naming Conventions Clarified use of mandatory nodes with conditional augmentations Clarified namespace and domain conventions for example modules Clarified conventions for identifying code components Added YANG 1.1 guidelines Added Data Model Constraints section Added mention of RESTCONF protocol

Harvard University In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document describes an IANA maintained registry for IETF standards which use Extensible Markup Language (XML) related items such as Namespaces, Document Type Declarations (DTDs), Schemas, and Resource Description Framework (RDF) Schemas. World Wide Web Consortium

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Applications uniform resource identifier URI URL URN WWW resource A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. The IETF policies about rights in Contributions to the IETF are designed to ensure that such Contributions can be made available to the IETF and Internet communities while permitting the authors to retain as many rights as possible. This memo details the IETF policies on rights in Contributions to the IETF. It also describes the objectives that the policies are designed to meet. This memo obsoletes RFCs 3978 and 4748 and, with BCP 79 and RFC 5377, replaces Section 10 of RFC 2026. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications. [STANDARDS TRACK] YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). This document introduces a collection of common data types to be used with the YANG data modeling language. This document obsoletes RFC 6021. This memo documents the process used by the Internet community for the standardization of protocols and procedures. It defines the stages in the standardization process, the requirements for moving a document between stages and the types of documents used during this process. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the "tag" Uniform Resource Identifier (URI) scheme. Tag URIs (also known as "tags") are designed to be unique across space and time while being tractable to humans. They are distinct from most other URIs in that they have no authoritative resolution mechanism. A tag may be used purely as an entity identifier. Furthermore, using tags has some advantages over the common practice of using "http" URIs as identifiers for non-HTTP-accessible resources. This memo provides information for the Internet community. This memo provides guidelines for authors and reviewers of IETF standards-track specifications containing MIB modules. Applicable portions may be used as a basis for reviews of other MIB documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Many protocols make use of identifiers consisting of constants and other well-known values. Even after a protocol has been defined and deployment has begun, new values may need to be assigned (e.g., for a new option type in DHCP, or a new encryption or authentication transform for IPsec). To ensure that such quantities have consistent values and interpretations across all implementations, their assignment must be administered by a central authority. For IETF protocols, that role is provided by the Internet Assigned Numbers Authority (IANA).</t><t> In order for IANA to manage a given namespace prudently, it needs guidelines describing the conditions under which new values can be assigned or when modifications to existing values can be made. If IANA is expected to play a role in the management of a namespace, IANA must be given clear and concise instructions describing that role. This document discusses issues that should be considered in formulating a policy for assigning values to a namespace and provides guidelines for authors on the specific text that must be included in documents that place demands on IANA.</t><t> This document obsoletes RFC 2434. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. 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 Network Configuration Protocol (NETCONF) implementations that utilize YANG data model modules. This document is not an Internet Standards Track specification; it is published for informational purposes. This document describes a method for invoking and running the Network Configuration Protocol (NETCONF) within a Secure Shell (SSH) session as an SSH subsystem. This document obsoletes RFC 4742. [STANDARDS-TRACK] This document defines a YANG data model for the management of network interfaces. It is expected that interface-type-specific data models augment the generic interfaces data model defined in this document. The data model includes configuration data and state data (status information and counters for the collection of statistics). The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF protocol access for particular users to a pre-configured subset of all available NETCONF protocol operations and content. This document defines such an access control model. [STANDARDS-TRACK] This document describes the fundamental and unique style conventions and editorial policies currently in use for the RFC Series. It captures the RFC Editor's basic requirements and offers guidance regarding the style and structure of an RFC. Additional guidance is captured on a website that reflects the experimental nature of that guidance and prepares it for future inclusion in the RFC Style Guide. This document obsoletes RFC 2223, "Instructions to RFC Authors". RFC documents contain a number of fixed elements such as the title page header, standard boilerplates, and copyright/IPR statements. This document describes them and introduces some updates to reflect current usage and requirements of RFC publication. In particular, this updated structure is intended to communicate clearly the source of RFC creation and review. This document obsoletes RFC 5741, moving detailed content to an IAB web page and preparing for more flexible output formats. This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in NETCONF.

updated YANG tree diagram syntax to align with pyang 1.7.1 added general guideline to include module usage examples

clarified <CODE BEGINS> is only for normative modules clarified example module namespace URI conventions clarified pyang usage for normative and example modules updated YANG tree diagrams section with text from RFC 8022

fixed references added mention of RESTCONF to abstract and intro created separate section for code components fixed document status

changed CODE BEGINS guideline for example modules updated tree diagram guidelines clarified published and unpublished terms added section on Empty and Boolean data types clarified how to update the revision statement updated operational state guidelines added 'YANG fragment' to terminology section

update contact statement guideline update example modules guidelines add guidelines on top-level data nodes add guideline on use of NP containers added guidelines on union types add guideline on deviations added section on open systems considerations added guideline about definitions reserved for future use

Changed example 'my‑module' to 'example‑module' Added section Updating YANG Modules (Published vs. Unpublished) Added Example Modules section Added "<EXAMPLE BEGINS>" convention for full example modules Added section on using action vs. rpc Changed term "operational state" to "operational data" Added section on YANG Data Node Constraints Added guidelines on using must vs. when statements Made ietf-foo module validate for I-D submission

Clarified that YANG 1.1 SHOULD be used but YANG 1.0 MAY be used if no YANG 1.1 features needed Changed SHOULD follow YANG naming conventions to MUST follow (for standards track documents only) Clarified module naming conventions for normative modules, example modules, and modules from other SDOs. Added prefix value selection guidelines Added new section on guidelines for reusable groupings Made header guidelines less IETF-specific Added new section on guidelines for extension statements Added guidelines for nested "choice" statement within a "case" statement

Added sections for deviation statements and performance considerations Added YANG 1.1 section Updated YANG reference from 1.0 to 1.1

Updated draft based on github data tracker issues added by Benoit Clause (Issues 12 - 18)

Updated draft based on mailing list comments.

All issues from the issue tracker have been addressed.

Issue 1: Tree Diagrams: Added so RFCs with YANG modules can use an Informative reference to this RFC for tree diagrams. Updated guidelines to reference this RFC when tree diagrams are used Issue 2: XPath function restrictions: Added paragraphs in XPath usage section for 'id', 'namespace‑uri', 'name', and 'lang' functions Issue 3: XPath function document order issues: Added paragraph in XPath usage section about node-set ordering for 'local‑name', 'namespace‑uri', 'name', 'string' and 'number' functions. Also any function that implicitly converts a node-set to a string. Issue 4: XPath preceding-sibling and following-sibling: Checked and text in XPath usage section already has proposed text from Lada. Issue 5: XPath 'when‑stmt' reference to descendant nodes: Added exception and example in XPath Usage section for augmented nodes. Issue 6: XPath numeric conversions: Changed 'numeric expressions' to 'numeric and boolean expressions' Issue 7: XPath module containment: Added sub-section on XPath wildcards Issue 8: status-stmt usage: Added text to Lifecycle Management section about transitioning from active to deprecated and then to obsolete. Issue 9: resource identification in notifications: Add text to Notifications section about identifying resources and using the leafref data type. Issue 10: single quoted strings: Added text to Data Types section about using a single-quoted string for patterns.

This section is adapted from RFC 4181. The purpose of a YANG module review is to review the YANG module both for technical correctness and for adherence to IETF documentation requirements. The following checklist may be helpful when reviewing an Internet-Draft: I-D Boilerplate -- verify that the draft contains the required Internet-Draft boilerplate (see http://www.ietf.org/id-info/guidelines.html), including the appropriate statement to permit publication as an RFC, and that I-D boilerplate does not contain references or section numbers. Abstract -- verify that the abstract does not contain references, that it does not have a section number, and that its content follows the guidelines in http://www.ietf.org/id-info/guidelines.html. Copyright Notice -- verify that the draft has the appropriate text regarding the rights that document contributers provide to the IETF Trust . Verify that it contains the full IETF Trust copyright notice at the beginning of the document. The IETF Trust Legal Provisions (TLP) can be found at:

Security Considerations section -- verify that the draft uses the latest approved template from the OPS area website (http://trac.tools.ietf.org/area/ops/trac/wiki/yang-security-guidelines) and that the guidelines therein have been followed. IANA Considerations section -- this section must always be present. For each module within the document, ensure that the IANA Considerations section contains entries for the following IANA registries:

References -- verify that the references are properly divided between normative and informative references, that RFC 2119 is included as a normative reference if the terminology defined therein is used in the document, that all references required by the boilerplate are present, that all YANG modules containing imported items are cited as normative references, and that all citations point to the most current RFCs unless there is a valid reason to do otherwise (for example, it is OK to include an informative reference to a previous version of a specification to help explain a feature included for backward compatibility). Be sure citations for all imported modules are present somewhere in the document text (outside the YANG module). License -- verify that the draft contains the Simplified BSD License in each YANG module or submodule. Some guidelines related to this requirement are described in . Make sure that the correct year is used in all copyright dates. Use the approved text from the latest Trust Legal Provisions (TLP) document, which can be found at:

Other Issues -- check for any issues mentioned in http://www.ietf.org/id-info/checklist.html that are not covered elsewhere. Technical Content -- review the actual technical content for compliance with the guidelines in this document. The use of a YANG module compiler is recommended when checking for syntax errors. A list of freely available tools and other information can be found at:

<CODE BEGINS> file "ietf-template@2016-03-20.yang"

WG List: Editor: your-name "; // replace the first sentence in this description statement. // replace the copyright notice with the most recent // version, if it has been updated since the publication // of this document description "This module defines a template for other YANG modules. Copyright (c) IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX; see the RFC itself for full legal notices."; // RFC Ed.: replace XXXX with actual RFC number and remove // this note reference "RFC XXXX"; // RFC Ed.: remove this note // Note: extracted from RFC XXXX // replace '2016-03-20' with the module publication date // The format is (year-month-day) revision "2016-03-20" { description "what changed in this revision"; reference "document containing this module"; } // extension statements // feature statements // identity statements // typedef statements // grouping statements // data definition statements // augment statements // rpc statements // notification statements // DO NOT put deviation statements in a published module } ]]>

<CODE ENDS>