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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