RFC 8791

YANG Data Structure Extensions

📅 published June 2020, from ietf

RFC 8791: YANG Data Structure Extensions 🎀

Okay bestie, this is RFC 8791, called "YANG Data Structure Extensions" 💅 It's an IETF Standards Track doc from June 2020 (ISSN 2070-1721). The authors are A. Bierman (YumaWorks), M. Bjorklund (Cisco), and K. Watsen (Watsen Networks). It updates RFC 8340 and doesn't obsolete anything 🦋

Abstract ✨

This doc is all about YANG mechanisms for defining abstract data structures with YANG 🧚

Status of This Memo 💖

This is an Internet Standards Track document. It's a product of the Internet Engineering Task Force (IETF) and it represents the consensus of the IETF community 🫶 It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). For more on Internet Standards, check Section 2 of RFC 7841.

For the current status, any errata, and how to send feedback, go to https://www.rfc-editor.org/info/rfc8791 🌸

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, because 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 they 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 ✨

Okay so we need standard ways to define abstract data that is NOT meant to be implemented as configuration or operational state 🥺 The "yang-data" extension statement from RFC 8040 [RFC8040] was made for this, but ngl it's kinda limited 💀

The "yang-data" extension was meant to model all or part of a protocol message, like the "errors" definition in the YANG module "ietf-restconf" [RFC8040], or the contents of a file. The thing is, protocols are often layered so that the header or payload can be extended by external documents. The YANG statements that model a protocol need to support that extensibility, which the protocol already has 🧚

This doc introduces "structure", a new YANG extension statement. It's similar to "yang-data" but more flexible, 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 🍓

It also introduces "augment-structure", a new YANG extension statement that lets abstract data structures be augmented from external modules. It's 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. These are the only ones with that special meaning, so lowercase versions don't count 💖

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 they are not redefined here:

  • configuration
  • operational state

1.1.2. YANG

The following terms are defined in [RFC7950], and they 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 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. Keep those names unique, bestie 🙈

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. It's giving freedom 🦋

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 ✨

okay bestie, this is the actual YANG module 💖 it defines two extensions, structure and augment-structure, and I copied the whole thing exactly so nothing gets lost in translation 🫶

   <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>

the description in that module has the usual BCP 14 key words, all sitting there in the code exactly as written 🎀 the actual rules for those key words come from the BCP 14 documents, so no need to re-explain them here 💅

5. IANA Considerations ✨

5.1. YANG Module Registry

okay so IANA did two registrations 🦋 the first one goes in the "ns" subregistry of the "IETF XML Registry" [RFC3688], and it looks like this:

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.

The second one goes in the "YANG Module Names" subregistry [RFC6020] inside 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 💀

okay this one is short but it's iconic 😭 the document just defines YANG extensions for conceptual YANG data structures. It doesn't add any new vulnerabilities beyond what's already in YANG 1.1 [RFC7950]. no cap, that's the whole thing 🫶

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 📇✨

Okay bestie, this one is a simple address book that could be stored as an artifact 💅 Here's the module:

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";
      }
    }
  }
}

And here's the tree diagram for this module 🌳 it's giving very organized:

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 one adds "county" and "zipcode" leafs to the address book, so the structure gets a little upgrade 🎀

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";
    }
  }
}

Here's the tree diagram of that module, and yes, the new leafs just slide right in 🫶

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 🍓

Same address book, but now it's JSON, and it's so pretty 😭✨

"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> 🔥 It's not the top level, but it's still iconic:

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;
    }
  }

}

And here's how that structure shows up inside an <rpc-error> 💀 the way it just nests in there, main character energy:

<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

That's the whole RFC, bestie 🌈💖 the squad that wrote it is literally iconic.

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