NETMOD Working Group L. Lhotka Internet-Draft CZ.NIC Intended status: Standards Track February 28, 2015 Expires: September 1, 2015 Defining and Using Metadata with YANG draft-lhotka-netmod-yang-metadata-01 Abstract This document defines a YANG extension statement that allows for defining the syntax of metadata annotions in YANG modules. The document also specifies the XML and JSON encoding of annotations and other 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 working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 1, 2015. Copyright Notice Copyright (c) 2015 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 (http://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. Lhotka Expires September 1, 2015 [Page 1]
Internet-Draft YANG Metadata February 2015 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Defining Annotations in YANG . . . . . . . . . . . . . . . . 5 4. The Encoding of Annotations . . . . . . . . . . . . . . . . . 7 4.1. XML Encoding . . . . . . . . . . . . . . . . . . . . . . 7 4.2. JSON Encoding . . . . . . . . . . . . . . . . . . . . . . 7 4.2.1. Metadata Object and Annotations . . . . . . . . . . . 8 4.2.2. Adding Annotations to Container and List Instances . 8 4.2.3. Adding Annotations to Leaf or Anyxml Instances . . . 9 4.2.4. Adding Annotations to Leaf-list Instances . . . . . . 9 5. Representing Annotations in DSDL Schemas . . . . . . . . . . 10 6. Metadata YANG Module . . . . . . . . . . . . . . . . . . . . 11 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 8. Security Considerations . . . . . . . . . . . . . . . . . . . 14 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Normative References . . . . . . . . . . . . . . . . . . 14 9.2. Informative References . . . . . . . . . . . . . . . . . 15 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 15 A.1. Changes Between Revisions -00 and -01 . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 15 1. Introduction There is a need to be able to annotate instances of YANG [3] data nodes with metadata. Typical use cases are: o Deactivating a subtree in a configuration datastore while keeping the data in place. o Complementing data model information with instance-specific data. o RPC operations may use metadata annotations for various 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: 1. The set of annotations must be extensible in a distributed manner so as to allow for defining new annotations without running into Lhotka Expires September 1, 2015 [Page 2]
Internet-Draft YANG Metadata February 2015 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 the syntax of metadata annotations. 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 the syntax one or more annotations. Semantics of metadata annotations MUST be defined separately. How it is done is outside the scope of this document. The benefits of defining the syntax of metadata annotations in a YANG module are the following: 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 syntactic support for certain annotations using standard module advertisement methods, such as the <hello> message in NETCONF. o Values of annotations are not limited to 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, Lhotka Expires September 1, 2015 [Page 3]
Internet-Draft YANG Metadata February 2015 o datastore, 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, Lhotka Expires September 1, 2015 [Page 4]
Internet-Draft YANG Metadata February 2015 o prefix. The following terms are defined in [6]: o array, o member, o object, o primitive type. In the following text, 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. Lhotka Expires September 1, 2015 [Page 5]
Internet-Draft YANG Metadata February 2015 +--------------+------------------+-------------+ | substatement | RFC 6020 section | cardinality | +--------------+------------------+-------------+ | description | 7.19.3 | 0..1 | | if-feature | 7.18.2 | 0..n | | 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". 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. The "if-feature" statement, if present, makes the annotation conditional: it is supported only by servers that advertise the corresponding feature. 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, then the data subtree rooted at this node is deactivated."; } } By advertising a YANG module in which metadata annotation X is defined using the "md:annotation" statement, a server specifies just the syntax of annotation X. This means that configuration or state data, RPC messages and notifications will be considered syntactically valid if annotation X is attached to any data node instance, provided that all rules stated in this document are observed. However, the semantics of an annotation including the expected behavior of clients and servers MUST be specified by other means that are outside the scope of this document. Lhotka Expires September 1, 2015 [Page 6]
Internet-Draft YANG Metadata February 2015 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 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 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 JSON 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]. Lhotka Expires September 1, 2015 [Page 7]
Internet-Draft YANG Metadata February 2015 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 (as a 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. 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 and List Instances For an instance that is encoded as a JSON object (i.e., a container or list entry), the metadata object is added as a new member of that object with the name "@". Examples: o "cask" is a container node: "cask": { "@": { "example-inactive:inactive": true }, ... } o "seq" is a list whose key is "name", annotation "inactive" is added only to the first entry: Lhotka Expires September 1, 2015 [Page 8]
Internet-Draft YANG Metadata February 2015 "seq": [ { "@": { "example-inactive:inactive": true }, "name": "one", ... }, { "name": "two", ... } ] 4.2.3. Adding Annotations to Leaf or Anyxml Instances For a leaf or anyxml instance, the metadata object is added as a sibling name/value pair whose the name is the symbol "@" concatenated with the name of the leaf or anyxml member that is being annotated. For example, if "flag" is a leaf node of the "boolean" type: "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 name of the leaf-list that is being annotated, 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: Lhotka Expires September 1, 2015 [Page 9]
Internet-Draft YANG Metadata February 2015 "bibliomod:folio": [6, 3, 7, 8], "@bibliomod: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 as 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. 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. Each "rng:attribute" pattern SHALL be wrapped in the Lhotka Expires September 1, 2015 [Page 10]
Internet-Draft YANG Metadata February 2015 <rng:optional> pattern, and this 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:optional> <rng:attribute name="ein:inactive"> <rng:choice> <rng:value>true</rng:value> <rng:value>false</rng:value> </rng:choice> </rng:attribute> </rng:optional> </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 <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 Editor: 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 "ietf-yang-metadata@2015-02-28.yang" module ietf-yang-metadata { Lhotka Expires September 1, 2015 [Page 11]
Internet-Draft YANG Metadata February 2015 namespace "urn:ietf:params:xml:ns:yang:ietf-yang-metadata"; prefix "md"; organization "IETF NETMOD (NETCONF Data Modeling Language) Working Group"; contact "WG Web: <http://tools.ietf.org/wg/netmod/> WG List: <mailto:netmod@ietf.org> WG Chair: Thomas Nadeau <mailto:tnadeau@lucidvision.com> WG Chair: Juergen Schoenwaelder <mailto:j.schoenwaelder@jacobs-university.de> Editor: Ladislav Lhotka <mailto:lhotka@nic.cz>"; description "This YANG module defines an extension statement that allows for defining metadata annotations. Copyright (c) 2014 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."; revision 2015-02-28 { description "Initial revision."; reference "RFC XXXX: Defining and Using Metadata with YANG"; } extension annotation { argument name; description "This extension allows for defining metadata annotations in Lhotka Expires September 1, 2015 [Page 12]
Internet-Draft YANG Metadata February 2015 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): 'description', 'if-feature', 'reference', 'status', 'type' and 'units'. If the 'type' statement is not present, the built-in 'string' type is used by default. A server announces syntactic 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). The 'description' and/or 'reference' statements should provide links to the specification of the annotation's semantics. 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]: Lhotka Expires September 1, 2015 [Page 13]
Internet-Draft YANG Metadata February 2015 ------------------------------------------------------------------- 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 the syntax of 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 in the specification of the annotation's semantics. 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-03 (work in progress), February 2015. Lhotka Expires September 1, 2015 [Page 14]
Internet-Draft YANG Metadata February 2015 [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>. 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., and K. Watsen, "RESTCONF Protocol", draft-ietf-netconf-restconf-04 (work in progress), January 2015. Appendix A. Change Log RFC Editor: Remove this section upon publication as an RFC. A.1. Changes Between Revisions -00 and -01 o Encoding of annotations for anyxml nodes was changed to be the same as for leafs. This was necessary because anyxml value now needn't be an object. o It is stated that "md:annotation" statement defines only the syntax of an annotation. o Allowed "if-feature" as a substatement of "md:annotation". Author's Address Ladislav Lhotka CZ.NIC Email: lhotka@nic.cz Lhotka Expires September 1, 2015 [Page 15]