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Internet Engineering Task Force (IETF) A. Bierman Request for Comments: 8791 YumaWorks Updates: 8340 M. Bjorklund Category: Standards Track Cisco ISSN: 2070-1721 K. Watsen

                                                       Watsen Networks
                                                             June 2020

                   YANG Data Structure Extensions

Abstract

 This document describes YANG mechanisms for defining abstract data
 structures with YANG.

Status of This Memo

 This is an Internet Standards Track document.

 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.

 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8791.

Copyright Notice

 Copyright (c) 2020 IETF Trust and the persons identified as the
 document authors.  All rights reserved.

 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction
   1.1.  Terminology
     1.1.1.  NMDA
     1.1.2.  YANG
 2.  Definitions
 3.  YANG Data Structures in YANG Tree Diagrams
 4.  YANG Data Structure Extensions Module
 5.  IANA Considerations
   5.1.  YANG Module Registry
 6.  Security Considerations
 7.  References
   7.1.  Normative References
   7.2.  Informative References
 Appendix A.  Examples
   A.1.  "structure" Example
   A.2.  "augment-structure" Example
   A.3.  XML Encoding Example
   A.4.  JSON Encoding Example
   A.5.  "structure" Example That Defines a Non-top-level Structure
 Authors' Addresses

1. Introduction

 There is a need for standard mechanisms to allow the definition of
 abstract data that is not intended to be implemented as configuration
 or operational state.  The "yang-data" extension statement from RFC
 8040 [RFC8040] was defined for this purpose, but it is limited in its
 functionality.

 The intended use of the "yang-data" extension was to model all or
 part of a protocol message, such as the "errors" definition in the
 YANG module "ietf-restconf" [RFC8040], or the contents of a file.
 However, protocols are often layered such that the header or payload
 portions of the message can be extended by external documents.  The
 YANG statements that model a protocol need to support this
 extensibility that is already found in that protocol.

 This document defines a new YANG extension statement called
 "structure", which is similar to but more flexible than the "yang-
 data" extension from [RFC8040].  There is no assumption that a YANG
 data structure can only be used as a top-level abstraction, and it
 may also be nested within some other data structure.

 This document also defines a new YANG extension statement called
 "augment-structure", which allows abstract data structures to be
 augmented from external modules and is similar to the existing YANG
 "augment" statement.  Note that "augment" cannot be used to augment a
 YANG data structure since a YANG compiler or other tool is not
 required to understand the "structure" extension.

1.1. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in BCP
 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

 The following term is used within this document:

 YANG data structure:  A data structure defined with the "structure"
    statement.

1.1.1. NMDA

 The following terms are defined in the Network Management Datastore
 Architecture (NMDA) [RFC8342] and are not redefined here:

configuration

operational state

1.1.2. YANG

 The following terms are defined in [RFC7950] and are not redefined
 here:

absolute-schema-nodeid

container

data definition statement

data node

leaf

leaf-list

list

2. Definitions

 A YANG data structure is defined with the "structure" extension
 statement, which is defined in the YANG module "ietf-yang-structure-
 ext".  The argument to the "structure" extension statement is the
 name of the data structure.  The data structures are considered to be
 in the same identifier namespace as defined in Section 6.2.1 of
 [RFC7950].  In particular, the seventh bullet states:

 |  All leafs, leaf-lists, lists, containers, choices, rpcs, actions,
 |  notifications, anydatas, and anyxmls defined (directly or through
 |  a "uses" statement) within a parent node or at the top level of
 |  the module or its submodules share the same identifier namespace.

 This means that data structures defined with the "structure"
 statement cannot have the same name as sibling nodes from regular
 YANG data definition statements or other "structure" statements in
 the same YANG module.

 This does not mean a YANG data structure, once defined, has to be
 used as a top-level protocol message or other top-level data
 structure.

 A YANG data structure is encoded in the same way as an "anydata"
 node.  This means that the name of the structure is encoded as a
 "container", with the instantiated children encoded as child nodes to
 this node.  For example, this structure:

   module example-errors {
     ...

     sx:structure my-error {
       leaf error-number {
         type int;
       }
     }
   }

 can be encoded in JSON as:

   "example-errors:my-error": {
     "error-number": 131
   }

3. YANG Data Structures in YANG Tree Diagrams

 A YANG data structure can be printed in a YANG tree diagram
 [RFC8340].  This document updates RFC 8340 [RFC8340] by defining two
 new sections in the tree diagram for a module:

 1.  YANG data structures, which are offset by two spaces and
     identified by the keyword "structure" followed by the name of the
     YANG data structure and a colon (":") character.

 2.  YANG data structure augmentations, which are offset by 2 spaces
     and identified by the keyword "augment-structure" followed by the
     augment target structure name and a colon (":") character.

 The new sections, including spaces conventions, appear as follows:

   structure <structure-name>:
     +--<node>
        +--<node>
        |  +--<node>
        +--<node>
   structure <structure-name>:
     +--<node>

   augment-structure <structure-name>:
     +--<node>
        +--<node>
        |  +--<node>
        +--<node>
   augment-structure <structure-name>:
     +--<node>

 Nodes in YANG data structures are printed according to the rules
 defined in Section 2.6 of [RFC8340].  The nodes in YANG data
 structures do not have any <flags>.

4. YANG Data Structure Extensions Module

 <CODE BEGINS> file "ietf-yang-structure-ext@2020-06-17.yang"
 module ietf-yang-structure-ext {
   yang-version 1.1;
   namespace "urn:ietf:params:xml:ns:yang:ietf-yang-structure-ext";
   prefix sx;

   organization
     "IETF NETMOD (NETCONF Data Modeling Language) Working Group";
   contact
     "WG Web:   <https://datatracker.ietf.org/wg/netmod/>
      WG List:  <mailto:netmod@ietf.org>

      Author:   Andy Bierman
                <mailto:andy@yumaworks.com>

      Author:   Martin Bjorklund
                <mailto:mbj+ietf@4668.se>

      Author:   Kent Watsen
                <mailto:kent+ietf@watsen.net>";
   description
     "This module contains conceptual YANG specifications for defining
      abstract data structures.

      The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL
      NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED',
      'MAY', and 'OPTIONAL' in this document are to be interpreted as
      described in BCP 14 (RFC 2119) (RFC 8174) when, and only when,
      they appear in all capitals, as shown here.

      Copyright (c) 2020 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 8791
      (https://www.rfc-editor.org/info/rfc8791); see the RFC itself
      for full legal notices.";

   revision 2020-06-17 {
     description
       "Initial revision.";
     reference
       "RFC 8791: YANG Data Structure Extensions.";
   }

   extension structure {
     argument name {
       yin-element true;
     }
     description
       "This extension is used to specify a YANG data structure that
        represents conceptual data defined in YANG.  It is intended to
        describe hierarchical data independent of protocol context or
        specific message encoding format.  Data definition statements
        within a 'structure' extension statement specify the generic
        syntax for the specific YANG data structure, whose name is the
        argument of the 'structure' extension statement.

        Note that this extension does not define a media type.  A
        specification using this extension MUST specify the message
        encoding rules, including the content media type, if
        applicable.

        The mandatory 'name' parameter value identifies the YANG data
        structure that is being defined.

        This extension is only valid as a top-level statement, i.e.,
        given as a substatement to 'module' or 'submodule'.

        The substatements of this extension MUST follow the ABNF
        rules below, where the rules are defined in RFC 7950:

must-stmt

[status-stmt]

          [description-stmt]
          [reference-stmt]
          *(typedef-stmt / grouping-stmt)
          *data-def-stmt

        A YANG data structure defined with this extension statement is
        encoded in the same way as an 'anydata' node.  This means
        that the name of the structure is encoded as a 'container',
        with the instantiated child statements encoded as child nodes
        to this node.

        The module name and namespace value for the YANG module using
        the extension statement are assigned to each of the data
        definition statements resulting from the YANG data structure.

        The XPath document element is the extension statement itself,
        such that the child nodes of the document element are
        represented by the data-def-stmt substatements within this
        extension.  This conceptual document is the context for the
        following YANG statements:

must-stmt
when-stmt
path-stmt
min-elements-stmt
max-elements-stmt
mandatory-stmt
unique-stmt
ordered-by
instance-identifier data type

        The following data-def-stmt substatements are constrained
        when used within a 'structure' extension statement.

The list-stmt is not required to have a key-stmt defined.
The config-stmt is ignored if present.

";

   extension augment-structure {
     argument path {
       yin-element true;
     }
     description
       "This extension is used to specify an augmentation to a YANG
        data structure defined with the 'structure' statement.  It is
        intended to describe hierarchical data independent of protocol
        context or specific message encoding format.

        This statement has almost the same structure as the
        'augment-stmt'.  Data definition statements within this
        statement specify the semantics and generic syntax for the
        additional data to be added to the specific YANG data
        structure, identified by the 'path' argument.

        The mandatory 'path' parameter value identifies the YANG
        conceptual data node that is being augmented and is
        represented as an absolute-schema-nodeid string, where the
        first node in the absolute-schema-nodeid string identifies the
        YANG data structure to augment, and the rest of the nodes in
        the string identifies the node within the YANG structure to
        augment.

        This extension is only valid as a top-level statement, i.e.,
        given as a substatement to 'module' or 'submodule'.

        The substatements of this extension MUST follow the ABNF
        rules below, where the rules are defined in RFC 7950:

          [status-stmt]
          [description-stmt]
          [reference-stmt]
          1*(data-def-stmt / case-stmt)

        The module name and namespace value for the YANG module using
        the extension statement are assigned to instance document data
        conforming to the data definition statements within this
        extension.

        The XPath document element is the augmented extension
        statement itself, such that the child nodes of the document
        element are represented by the data-def-stmt substatements
        within the augmented 'structure' statement.

        The context node of the 'augment-structure' statement is
        derived in the same way as the 'augment' statement, as defined
        in Section 6.4.1 of [RFC7950]. This conceptual node is
        considered the context node for the following YANG statements:

must-stmt
when-stmt
path-stmt
min-elements-stmt
max-elements-stmt
mandatory-stmt
unique-stmt
ordered-by
instance-identifier data type

        The following data-def-stmt substatements are constrained
        when used within an 'augment-structure' extension statement.

The list-stmt is not required to have a key-stmt defined.
The config-stmt is ignored if present.

        Example:

           module foo {
              import ietf-yang-structure-ext { prefix sx; }

              sx:structure foo-data {
                container foo-con { }
              }
           }

           module bar {
              import ietf-yang-structure-ext { prefix sx; }
              import foo { prefix foo; }

              sx:augment-structure /foo:foo-data/foo:foo-con {
                leaf add-leaf1 { type int32; }
                leaf add-leaf2 { type string; }
              }
           }
       ";
   }
 }
 <CODE ENDS>

5. IANA Considerations

5.1. YANG Module Registry

 IANA has registered the following URI in the "ns" subregistry within
 the "IETF XML Registry" [RFC3688]:

 URI:  urn:ietf:params:xml:ns:yang:ietf-yang-structure-ext
 Registrant Contact:  The IESG.
 XML:  N/A; the requested URI is an XML namespace.

 IANA has registered the following YANG module in the "YANG Module
 Names" subregistry [RFC6020] within the "YANG Parameters" registry:

 Name:  ietf-yang-structure-ext
 Namespace:  urn:ietf:params:xml:ns:yang:ietf-yang-structure-ext
 Prefix:  sx
 Reference:  RFC 8791

6. Security Considerations

 This document defines YANG extensions that are used to define
 conceptual YANG data structures.  It does not introduce any new
 vulnerabilities beyond those specified in YANG 1.1 [RFC7950].

7. References

7.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.

 [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
            RFC 7950, DOI 10.17487/RFC7950, August 2016,
            <https://www.rfc-editor.org/info/rfc7950>.

 [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
            Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
            <https://www.rfc-editor.org/info/rfc8040>.

 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.

 [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
            BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
            <https://www.rfc-editor.org/info/rfc8340>.

 [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
            and R. Wilton, "Network Management Datastore Architecture
            (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
            <https://www.rfc-editor.org/info/rfc8342>.

 [W3C.REC-xml-20081126]
            Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
            F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
            Edition)", World Wide Web Consortium Recommendation REC-
            xml-20081126, November 2008,
            <http://www.w3.org/TR/2008/REC-xml-20081126>.

7.2. Informative References

 [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
            DOI 10.17487/RFC3688, January 2004,
            <https://www.rfc-editor.org/info/rfc3688>.

 [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
            the Network Configuration Protocol (NETCONF)", RFC 6020,
            DOI 10.17487/RFC6020, October 2010,
            <https://www.rfc-editor.org/info/rfc6020>.

Appendix A. Examples

A.1. "structure" Example

 This example shows a simple address book that could be stored as an
 artifact:

 module example-module {
   yang-version 1.1;
   namespace "urn:example:example-module";
   prefix exm;

   import ietf-yang-structure-ext {
     prefix sx;
   }

   sx:structure address-book {
     list address {
       key "last first";
       leaf last {
         type string;
         description "Last name";
       }
       leaf first {
         type string;
         description "First name";
       }
       leaf street {
         type string;
         description "Street name";
       }
       leaf city {
         type string;
         description "City name";
       }
       leaf state {
         type string;
         description "State name";
       }
     }
   }
 }

 Below is the tree diagram of this module:

 module: example-module

   structure address-book:
     +-- address* [last first]
        +-- last      string
        +-- first     string
        +-- street?   string
        +-- city?     string
        +-- state?    string

A.2. "augment-structure" Example

 This example adds "county" and "zipcode" leafs to the address book:

 module example-module-aug {
   yang-version 1.1;
   namespace "urn:example:example-module-aug";
   prefix exma;

   import ietf-yang-structure-ext {
     prefix sx;
   }
   import example-module {
     prefix exm;
   }

   sx:augment-structure "/exm:address-book/exm:address" {
     leaf county {
       type string;
       description "County name";
     }
     leaf zipcode {
       type string;
       description "Postal zipcode";
     }
   }
 }

 Below is the tree diagram of this module:

 module: example-module-aug

   augment-structure /exm:address-book/exm:address:
     +-- county?    string
     +-- zipcode?   string

A.3. XML Encoding Example

 This example shows how an address book can be encoded in XML
 [W3C.REC-xml-20081126]:

 <address-book xmlns="urn:example:example-module">
   <address>
     <last>Flintstone</last>
     <first>Fred</first>
     <street>301 Cobblestone Way</street>
     <city>Bedrock</city>
     <zipcode xmlns="urn:example:example-module-aug">70777</zipcode>
   </address>
   <address>
     <last>Root</last>
     <first>Charlie</first>
     <street>4711 Cobblestone Way</street>
     <city>Bedrock</city>
     <zipcode xmlns="urn:example:example-module-aug">70777</zipcode>
   </address>
 </address-book>

A.4. JSON Encoding Example

 This example shows how an address book can be encoded in JSON:

 "example-module:address-book": {
   "address": [
     {
       "city": "Bedrock",
       "example-module-aug:zipcode": "70777",
       "first": "Fred",
       "last": "Flintstone",
       "street": "301 Cobblestone Way"
     },
     {
       "city": "Bedrock",
       "example-module-aug:zipcode": "70777",
       "first": "Charlie",
       "last": "Root",
       "street": "4711 Cobblestone Way"
     }
   ]
 }

A.5. "structure" Example That Defines a Non-top-level Structure

 The following example defines a data structure with error information
 that can be included in an <error-info> element in an <rpc-error>:

 module example-error-info {
   yang-version 1.1;
   namespace "urn:example:example-error-info";
   prefix exei;

   import ietf-yang-structure-ext {
     prefix sx;
   }

   sx:structure my-example-error-info {
     leaf error-code {
       type uint32;
     }
   }

 The example below shows how this structure can be used in an
 <rpc-error>:

 <rpc-reply message-id="101"
      xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
   <rpc-error>
     <error-type>protocol</error-type>
     <error-tag>operation-failed</error-tag>
     <error-severity>error</error-severity>
     <error-info>
       <my-example-error-info
           xmlns="urn:example:example-error-info">
         <error-code>42</error-code>
       </my-example-error-info>
     </error-info>
   </rpc-error>
 </rpc-reply>

Authors' Addresses

 Andy Bierman
 YumaWorks

 Email: andy@yumaworks.com

 Martin Bjorklund
 Cisco

 Email: mbj+ietf@4668.se

 Kent Watsen
 Watsen Networks

 Email: kent+ietf@watsen.net