Defining and Using Metadata with YANG
draft-lhotka-netmod-yang-metadata-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Author | Ladislav Lhotka | ||
| Last updated | 2014-09-11 | ||
| Replaced by | draft-ietf-netmod-yang-metadata, RFC 7952 | ||
| RFC stream | (None) | ||
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| Consensus boilerplate | Unknown | ||
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draft-lhotka-netmod-yang-metadata-00
NETMOD Working Group L. Lhotka
Internet-Draft CZ.NIC
Intended status: Standards Track September 11, 2014
Expires: March 15, 2015
Defining and Using Metadata with YANG
draft-lhotka-netmod-yang-metadata-00
Abstract
This document defines a YANG extension statement that allows for
defining metadata annotions in YANG modules. The document also
specifies the encoding of annotations and rules for annotating
instances of YANG data nodes.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 15, 2015.
Copyright Notice
Copyright (c) 2014 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Defining Annotations in YANG . . . . . . . . . . . . . . . . 5
4. The Encoding of Annotations . . . . . . . . . . . . . . . . . 6
4.1. XML Encoding . . . . . . . . . . . . . . . . . . . . . . 6
4.2. JSON Encoding . . . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Metadata Object and Annotations . . . . . . . . . . . 7
4.2.2. Adding Annotations to Container, Anyxml and List
Instances . . . . . . . . . . . . . . . . . . . . . . 8
4.2.3. Adding Annotations to Leaf Instances . . . . . . . . 8
4.2.4. Adding Annotations to Leaf-list Instances . . . . . . 9
5. Representing Annotations in DSDL Schemas . . . . . . . . . . 9
6. Metadata YANG Module . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
9.1. Normative References . . . . . . . . . . . . . . . . . . 13
9.2. Informative References . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
There is a need to be able to annotate instances of YANG [3] data
nodes with various metadata. Typical use cases are:
o Deactivating a subtree in a configuration datastore while keeping
the data in place.
o Qualifying the data model information with instance-specific data.
For example, an annotation may be attached to an instance of a
leaf with the "union" type to indicate the member type to which
the instance belongs.
o RPC operations may use metadata annotations for different purposes
in both requests and responses. For example, the <edit-config>
operation in the NETCONF protocol (see section 7.2 of [5]) uses
annotations in the form of XML attributes for identifying the
point in the configuration and type of the operation.
However, metadata annotations could potentially lead to
interoperability problems if they are used in an ad hoc way by
different organizations and/or without proper documentation. A sound
metadata framework for YANG should therefore satisfy these
requirements:
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1. The set of annotations must be extensible in a distributed manner
so as to allow for defining new annotations without running into
the risk of collisions with annotations defined and used by
others.
2. Syntax and semantics of annotations must be documented and the
documentation must be easily accessible.
3. Clients of network management protocols such as NETCONF [5] or
RESTCONF [10] must be able to learn all annotations supported by
a given server and identify each of them correctly.
This document proposes a systematic way for defining and using
metadata annotations that satisfies the above requirements. For this
purpose, YANG extension statement "annotation" is defined in the
module "ietf-yang-metadata" (Section 6). Other YANG modules
importing this module can use the "annotation" statement for defining
one or more annotations.
The benefits of defining metadata annotations in a YANG module are as
follows:
o Each annotation is bound to a YANG module name, namespace URI and
prefix. This makes its encoding in instance documents (both XML
and JSON) straightforward and consistent with the encoding of YANG
data node instances.
o Annotations are indirectly registered through IANA YANG module
registration.
o Annotations are included in the data model. Specifically, servers
indicate support for certain annotations using standard module
advertisement methods, such as the <hello> message in NETCONF.
o Values of annotations need not be strings; any YANG built-in or
derived type may be used for them.
2. Terminology
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 [1].
The following terms are defined in [5]:
o client,
o datastore,
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o message,
o operation,
o server.
The following terms are defined in [3]:
o anyxml,
o built-in type,
o derived type,
o container,
o data model,
o data node,
o derived type,
o extension,
o leaf-list,
o list,
o module,
o RPC operation,
o submodule,
o type.
The following terms are defined in [8]:
o attribute,
o document,
o element,
o namespace,
o prefix.
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The following terms are defined in [6]:
o array,
o member,
o object,
o primitive type.
XML element names and YANG extension statements are always written
with explicit namespace prefixes that are assumed to be bound to URI
references as shown in Table 1.
+--------+------------------------------------------------+
| Prefix | URI Reference |
+--------+------------------------------------------------+
| rng | http://relaxng.org/ns/structure/1.0 |
| md | urn:ietf:params:xml:ns:yang:ietf-yang-metadata |
| ein | http://example.org/example-inactive |
+--------+------------------------------------------------+
Table 1: Used namespace prefixes and corresponding URI references
3. Defining Annotations in YANG
Metadata annotations are defined with YANG extension statement
"md:annotation". This YANG language extension is defined in the
module "ietf-yang-metadata" (Section 6).
Substatements of "md:annotation" are shown in Table 2. They are all
core YANG statements, and the numbers in the second column refer to
the corresponding sections in RFC 6020 [3] where each statement is
described.
+--------------+------------------+-------------+
| substatement | RFC 6020 section | cardinality |
+--------------+------------------+-------------+
| description | 7.19.3 | 0..1 |
| reference | 7.19.4 | 0..1 |
| status | 7.19.2 | 0..1 |
| type | 7.6.3 | 0..1 |
| units | 7.3.3 | 0..1 |
+--------------+------------------+-------------+
Table 2: Substatements of "md:annotation".
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Using the "type" statement, a type may be specified for the
annotation value according to the same rules as for YANG leaf or
leaf-list types. However, the "type" statement is optional as a
substatement of "md:annotation" statement. If it is not present, the
built-in "string" type is the default.
For example, the following module defines the "inactive" annotation:
module example-inactive {
namespace "http://example.org/example-inactive";
prefix "ein";
import ietf-yang-metadata {
prefix "md";
}
md:annotation inactive {
type boolean;
description
"If this annotation is attached to a configuration data node,
and its value is 'true', then the server MUST behave
as if the data subtree rooted at this node was not
present.";
}
}
Metadata annotations defined with the "md:annotation" statement may
be attached to any valid instance of a data node, i.e., container,
leaf, list, leaf-list or anyxml, throughout the data model. Metadata
annotations are always optional.
4. The Encoding of Annotations
XML attributes are a natural choice for encoding metadata in XML
instance documents. For JSON [6], there is no generally established
method for encoding metadata. This document thus introduces a
special encoding method that is consistent with the JSON encoding of
YANG data node instances as defined in [7].
4.1. XML Encoding
Metadata annotations are added to XML-encoded instances of YANG data
nodes as XML attributes according to these rules:
o The local name of the attribute SHALL be the same as the name of
the annotation specified in the argument of the corresponding
"md:annotation" statement.
o The namespace of the attribute SHALL be identified by the URI that
appears as the argument of the "namespace" statement in the YANG
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module where the annotation is defined. It is RECOMMENDED that
the prefix specified by the "prefix" statement in the same module
is used in the qualified name of the attribute.
o The attribute value SHALL be encoded in the same way as the value
of a YANG leaf instance having the same type.
For example, the "inactive" annotation as defined in Section 3 may be
encoded as follows:
<foo xmlns:ein="http://example.org/example-inactive"
ein:inactive="true">
...
</foo>
4.2. JSON Encoding
The metadata encoding defined in this section has the following
properties:
1. The encoding of YANG data node instances as defined in [7] does
not change.
2. Namespaces of metadata annotations are encoded in the same way as
namespaces of YANG data node instances, see [7].
4.2.1. Metadata Object and Annotations
All metadata annotations assigned to a YANG data node instance are
encoded as members (name/value pairs) of a single JSON object,
henceforth denoted as the metadata object. The placement and name of
this object depends on the type of the data node as specified in the
following subsections.
The name of a metadata annotation (member of the metadata object)
SHALL be of the following form:
MODULE_NAME:LOCAL_NAME
where MODULE_NAME is the name of the YANG module in which the
annotation is defined, and LOCAL_NAME is the name of the annotation
specified in the argument of the corresponding "md:annotation"
statement.
Note that unlike YANG data node instances, for annotations the
explicit namespace identifier (MODULE_NAME) must always be used.
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The value of a metadata annotation SHALL be encoded in exactly the
same way as the value of a YANG leaf node having the same type as the
annotation.
4.2.2. Adding Annotations to Container, Anyxml and List Instances
For an instance that is translated to a JSON object (i.e., a
container, anyxml or list entry), the metadata object is added as a
new member of that object with the name "@".
Examples:
o "cask" is a container or anyxml node:
"cask": {
"@": {
"example-inactive:inactive": true
},
...
}
o "seq" is a list whose key is "name", annotation "inactive" is
added only to the first entry:
"seq": [
{
"@": {
"example-inactive:inactive": true
},
"name": "one",
...
},
{
"name": "two",
...
}
]
4.2.3. Adding Annotations to Leaf Instances
For a leaf instance, the metadata object is added as a sibling name/
value pair whose the name is the symbol "@" concatenated with the
identifier of the leaf.
For example, if "flag" is a leaf node:
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"flag": true,
"@flag": {
"example-inactive:inactive": true
}
4.2.4. Adding Annotations to Leaf-list Instances
For a leaf-list instance, which is represented as a JSON array with
values of a primitive type, annotations may be assigned to one or
more entries by adding a name/array pair as a sibling the leaf-list
instance, where the name is the symbol "@" concatenated with the
identifier of the leaf-list, and the value is a JSON array whose i-th
element is the metadata object with annotations assigned to the i-th
entry of the leaf-list instance, or null if the i-th entry has no
annotations.
Trailing null values in the array, i.e., those following the last
non-null metadata object, MAY be omitted.
For example, in the following leaf-list instance with four entries,
the "inactive" annotation is added to the second and third entry in
the following way:
"folio": [6, 3, 7, 8],
"@folio": [
null,
{"example-inactive:inactive": true},
{"example-inactive:inactive": true}
]
5. Representing Annotations in DSDL Schemas
RFC 6110 [4] defines a standard mapping of YANG data models to
Document Schema Definition Languages (DSDL) [9]. This section
specifies the mapping for the extension statement "md:annotation"
(Section 6), which enables validation of XML instance documents
containing metadata annotations.
The first step of the DSDL mapping procedure, i.e., the
transformation of the YANG data model to the hybrid schema (see
sec. 6 in [4]), is modified as follows:
1. If the data model contains at least one "md:annotation"
statement, then a RELAX NG named pattern definition MUST be added
a child of the root <rng:grammar> element in the hybrid schema.
It is RECOMMENDED to use the name "__yang_metadata__" for this
named pattern.
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2. A reference to the named pattern described in item 1 MUST be
included as a child of every <rng:element> pattern that
corresponds to a container, leaf, list or leaf-list data node.
3. Every metadata annotation definition in the form
md:annotation ARGUMENT;
or
md:annotation ARGUMENT {
...
}
is mapped to the following RELAX NG pattern:
<rng:attribute name="PREFIX:ARGUMENT">
...
</rng:attribute>
where PREFIX is the namespace prefix bound to the namespace URI
of the YANG module that contains the "md:annotation" statement.
The "rng:attribute" pattern SHALL be inserted as a child of the
named pattern definition described in item 1.
4. Substatements of "md:annotation", if there are any, SHALL be
mapped to children of the "rng:attribute" pattern exactly as
described in sec. 10 of [4].
For example, the named pattern definition (item 1), when constructed
only for the "inactive" annotation, will have the following form:
<rng:define name="__yang_metadata__">
<rng:attribute name="ein:inactive">
<rng:choice>
<rng:value>true</rng:value>
<rng:value>false</rng:value>
</rng:choice>
</rng:attribute>
</rng:define>
Every "rng:element" pattern that corresponds to a container, leaf,
list or leaf-list data node will then contain a reference to the
above named pattern, for example
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<rng:element name="foo:bar">
<rng:ref name="__yang_metadata__"/>
...
</rng:element>
Note that it is not necessary to use such a reference for
"rng:element" patterns corresponding to anyxml data nodes because
they already permit any XML attributes to be attached to their
instances.
The second step of the DSDL mapping procedure, i.e., the
transformation of the hybrid schema to RELAX NG, Schematron and DSRL
schemas, is unaffected by the inclusion of "md:annotation".
6. Metadata YANG Module
RFC Ed.: In this section, replace all occurrences of 'XXXX' with the
actual RFC number and all occurrences of the revision date below with
the date of RFC publication (and remove this note).
<CODE BEGINS> file "yang-metadata@2014-09-11.yang"
module ietf-yang-metadata {
namespace "urn:ietf:params:xml:ns:yang:ietf-yang-metadata";
prefix "md";
organization
"IETF NETMOD (NETCONF Data Modeling Language) Working Group";
contact
"Editor: Ladislav Lhotka
<mailto:lhotka@nic.cz>";
description
"This YANG module defines an extension statement that allows for
defining metadata annotations.";
revision 2014-09-11 {
description
"Initial revision.";
reference
"RFC XXXX: Defining and Using Metadata with YANG";
}
extension annotation {
argument name;
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description
"This extension allows for defining metadata annotations in
YANG modules. The 'md:annotation' statement can appear only at
the top level of a YANG module.
An annotation defined with this extension statement inherits
the namespace and other context from the YANG module in which
it is defined.
Other properties of the annotation and documentation may be
specified using the following standard YANG substatements (all
are optional and may appear only once): 'type', 'description',
'reference', 'status' and 'units'. If the 'type' statement is
not present, the built-in 'string' type is used by default.
A server announces support for a particular annotation by
including the module in which the annotation is defined among
the advertised YANG modules (e.g. in NETCONF hello message).
Depending on the prescribed usage patterns, the annotation
then may be attached by the server and/or client to any valid
instance of a data node defined by the server's data model.
XML and JSON encoding of annotations is defined in
RFC XXXX.";
}
}
<CODE ENDS>
7. IANA Considerations
RFC Ed.: In this section, replace all occurrences of 'XXXX' with the
actual RFC number (and remove this note).
This document registers the following namespace URI in the IETF XML
registry [2]:
----------------------------------------------------------
URI: urn:ietf:params:xml:ns:yang:ietf-yang-metadata
Registrant Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
----------------------------------------------------------
This document registers the following YANG module in the YANG Module
Names registry [3]:
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-------------------------------------------------------------------
name: ietf-yang-metadata
namespace: urn:ietf:params:xml:ns:yang:ietf-yang-metadata
prefix: md
reference: RFC XXXX
-------------------------------------------------------------------
8. Security Considerations
This document introduces a mechanism for defining metadata
annotations in YANG modules and using them with instances of YANG
data nodes. By itself, this mechanism represents no security threat.
Security implications of a particular annotation defined using this
mechanism have to be duly considered and documented.
9. References
9.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[2] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[3] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF)", RFC 6020,
October 2010.
[4] Lhotka, L., "Mapping YANG to Document Schema Definition
Languages and Validating NETCONF Content", RFC 6110,
February 2011.
[5] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)", RFC
6241, June 2011.
[6] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[7] Lhotka, L., "JSON Encoding of Data Modeled with YANG",
draft-ietf-netmod-yang-json-00 (work in progress), April
2014.
[8] Cowan, J. and R. Tobin, "XML Information Set (Second
Edition)", World Wide Web Consortium Recommendation REC-
xml-infoset-20040204, February 2004,
<http://www.w3.org/TR/2004/REC-xml-infoset-20040204>.
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9.2. Informative References
[9] International Organization for Standardization, "Document
Schema Definition Languages (DSDL) - Part 1: Overview",
ISO/IEC 19757-1, November 2004.
[10] Bierman, A., Bjorklund, M., Watsen, K., and R. Fernando,
"RESTCONF Protocol", draft-ietf-netconf-restconf-01 (work
in progress), July 2014.
Author's Address
Ladislav Lhotka
CZ.NIC
Email: lhotka@nic.cz
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