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Versions: (draft-lengyel-netmod-yang-instance-data) 00 01 02 03 04

Netmod                                                        B. Lengyel
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               B. Claise
Expires: February 13, 2020                           Cisco Systems, Inc.
                                                         August 12, 2019


                     YANG Instance Data File Format
             draft-ietf-netmod-yang-instance-file-format-04

Abstract

   There is a need to document data defined in YANG models when a live
   server is not available.  Data is often needed already at design or
   implementation time or needed by groups that do not have a live
   running server available.  This document specifies a standard file
   format for YANG instance data (which follows the syntax and semantic
   from existing YANG models, re-using the same format as the reply to a
   <get> operation/request) and decorates it with metadata.

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 https://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 February 13, 2020.

Copyright Notice

   Copyright (c) 2019 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



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   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Principles  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Instance Data File Format . . . . . . . . . . . . . . . . . .   4
     3.1.  Specifying the Content Schema . . . . . . . . . . . . . .   6
       3.1.1.  Inline Method . . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  Simplified-Inline Method  . . . . . . . . . . . . . .   7
       3.1.3.  URI Method  . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Data Life cycle . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Delivery of Instance Data . . . . . . . . . . . . . . . . . .  13
   6.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .  13
   7.  Yang Instance Data Model  . . . . . . . . . . . . . . . . . .  13
     7.1.  Tree Diagram  . . . . . . . . . . . . . . . . . . . . . .  13
     7.2.  YANG Model  . . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  URI Registration  . . . . . . . . . . . . . . . . . . . .  19
     9.2.  YANG Module Name Registration . . . . . . . . . . . . . .  19
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     11.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  21
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  21
   Appendix C.  Detailed Use Cases - Non-Normative . . . . . . . . .  22
     C.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  22
       C.1.1.  Use Case 1: Early Documentation of Server
               Capabilities  . . . . . . . . . . . . . . . . . . . .  22
       C.1.2.  Use Case 2: Preloading Data . . . . . . . . . . . . .  24
       C.1.3.  Use Case 3: Documenting Factory Default Settings  . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24

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 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they
   appear in all capitals, as shown here.





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   Instance Data Set: A named set of data items decorated with metadata
   that can be used as instance data in a YANG data tree.

   Instance Data File: A file containing an instance data set formatted
   according to the rules described in this document.

   Content-schema: A set of YANG modules with their revision,suupported
   features and deviations for which the instance data set contains
   instance data

   Content defining Yang module(s): YANG module(s) that make up the
   content-schema

   YANG Instance Data, or just instance data for short, is data that
   could be stored in a datastore and whose syntax and semantics is
   defined by YANG models.

   The term Server is used as defined in [RFC8342]

2.  Introduction

   There is a need to document data defined in YANG models when a live
   server is not available.  Data is often needed already at design or
   implementation time or needed by groups that do not have a live
   running server available.  To facilitate this off-line delivery of
   data this document specifies a standard format for YANG instance data
   sets and YANG instance data files.

   The following is a list of already implemented and potential use
   cases.

   UC1  Documentation of server capabilities

   UC2  Preloading default configuration data

   UC3  Documenting Factory Default Settings

   UC4  Instance data used as backup

   UC5  Storing the configuration of a device, e.g. for archive or audit
        purposes

   UC6  Storing diagnostics data

   UC7  Allowing YANG instance data to potentially be carried within
        other IPC message formats

   UC8  Default instance data used as part of a templating solution



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   UC9  Providing data examples in RFCs or internet drafts

   In Appendix C we describe the first three use cases in detail.

   There are many and varied use cases where YANG instance data could be
   used.  We do not want to limit future uses of instance data sets, so
   specifying how and when to use Yang instance data is out of scope for
   this document.  It is anticipated that other documents will define
   specific use cases.  Use cases are listed here only to indicate the
   need for this work.

2.1.  Principles

   The following is a list of the basic principles of the instance data
   format:

   P1  Two standard formats are based on the XML and the JSON encoding

   P2  Re-use existing formats similar to the response to a <get>
       operation/request

   P3  Add metadata about the instance data set (Section 3, Paragraph 9)

   P4  A YANG instance data set may contain data for many YANG modules

   P5  Instance data may include configuration data, state data or a mix
       of the two

   P6  Partial data sets are allowed

   P7  YANG instance data format may be used for any data for which YANG
       module(s) are defined and available to the reader, independent of
       whether the module is actually implemented by a server

3.  Instance Data File Format

   A YANG instance data file MUST contain a single instance data set and
   no additional data.

   The format of the instance data set is defined by the ietf-yang-
   instance-data YANG module.  It is made up of a header part and
   content-data.  The header part carries metadata for the instance data
   set.  The content-data, defined as an anydata data node, carries the
   "real data" that we want to document/provide.  The syntax and
   semantics of content-data is defined by the content-schema.






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   Two formats are specified based on the XML and JSON YANG encodings.
   Later as other YANG encodings (e.g.  CBOR) are defined further
   instance data formats may be specified.

   The content-data part SHALL follow the encoding rules defined in
   [RFC7950] for XML and [RFC7951] for JSON and MUST use UTF-8 character
   encoding.  Content-data MAY include:

      metadata as defined by [RFC7952].

      a default attribute as defined in [RFC6243] section 6. and in
      [RFC8040] section 4.8.9.

      origin metadata as specified in [RFC8526] and [RFC8527]

      implementation specific metadata.  Unknown metadata MUST be
      ignored by users of YANG instance data, allowing it to be used
      later for other purposes.

      in the XML format implementation specific XML attributes.  Unknown
      attributes MUST be ignored by users of YANG instance data,
      allowing them to be used later for other purposes.

   The content-data part will be very similar to the result returned for
   a NETCONF <get-data> or for a RESTCONF get operation.

   The content-data part MUST conform to the content-schema.  An
   instance data set MAY contain data for any number of YANG modules; if
   needed it MAY carry the complete configuration and state data set for
   a server.  Default values SHOULD NOT be included.

   Config=true and config=false data MAY be mixed in the instance data
   file.

   Instance data files MAY contain partial data sets.  This means
   mandatory, min-elements, require-instance=true, must and when
   constrains MAY be violated.

   The name of the instance data file SHOULD take one of the following
   two forms:

      If revision information inside the data set is present

      *  instance-data-set-name ['@' revision-date] '.filetype'

      *  E.g. acme-router-modules@2018-01-25.xml





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      If the leaf name is present in the instance data header this MUST
      be used.  Revision-date MUST be set to the latest revision date
      inside the instance data set.

      If timestamp information inside the data set is present

      *  instance-data-set-name ['@' timestamp] '.filetype'

      *  E.g. acme-router-modules@2018-01-25T15_06_34_3+01_00.json

      If the leaf name is present in the instance data header this MUST
      be used.  If the leaf timestamp is present in the instance data
      header this MUST be used; the semicolons and the decimal point if
      present shall be replaced by underscores.

   The revision date or timestamp is optional. ".filetype" SHALL be
   ".json" or ".xml" according to the format used.

   Metadata, information about the data set itself SHOULD be included in
   the instance data set.  Some metadata items are defined in the YANG
   module ietf-yang-instance-data, but other items MAY also be used.
   Metadata SHOULD include:

   o  Name of the data set

   o  Content schema specification

   o  Description of the instance data set.  The description SHOULD
      contain information whether and how the data can change during the
      lifetime of the server.

3.1.  Specifying the Content Schema

   To properly understand and use an instance data set the user needs to
   know the content-schema.  One of the following methods SHOULD be
   used:

      Inline method: Include the needed information as part of the
      instance data set.

      Simplified-Inline method: Include the needed information as part
      of the instance data set; short specification.

      URI method: Include a URI that references another YANG instance
      data file.  This instance data file will use the same content-
      schema as the referenced YANG instance data file.  (if you don't
      want to repeat the info again and again)




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      EXTERNAL Method: Do not include the content-schema as it is
      already known, or the information is available through external
      documents.

   Additional methods e.g. a YANG-package based solution may be added
   later.

   Note, the specified content-schema only indicates the set of modules
   that were used to define this YANG instance data set.  Sometimes
   instance data may be used for a server supporting a different YANG
   module set. (e.g. for "UC2 Preloading Data" the instance data set may
   not be updated every time the YANG modules on the server are updated)
   Whether the instance data set is usable for a possibly different
   real-life YANG module set depends on many factors including the
   compatibility between the specified and the real-life YANG module set
   (considering modules, revisions, features, deviations), the scope of
   the instance data, etc.

3.1.1.  Inline Method

   One or more inline-target-spec elements define YANG module(s) used to
   specify the content defining YANG modules.

      E.g. ietf-yang-library@2016-06-21.yang

   The anydata inline-content-schema carries instance data (conforming
   to the inline-target-spec modules) that actually specifies the
   content defining YANG modules including revision, supported features,
   deviations and any relevant additional data (e.g. version labels)

3.1.2.  Simplified-Inline Method

   The instance data set contains a list of content defining YANG
   modules including the revision date for each.  Usage of this method
   implies that the modules are used without any deviations and with all
   features supported.

3.1.3.  URI Method

   A schema-uri leaf SHALL contain a URI that references another YANG
   instance data file.  The current instance data file will use the same
   content schema as the referenced file.

   The referenced instance data file MAY have no content-data if it is
   used solely for specifying the content-schema.  The referenced YANG
   instance data file might use the INLINE method or might use the URI
   method to reference further instance data file(s).  However at the




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   end of this reference chain there MUST be an instance data file using
   the INLINE method.

   If a referenced instance data file is not available the revision
   data, supported features and deviations for the target YANG modules
   are unknown.

   The URI method is advantageous when the user wants to avoid the
   overhead of specifying the content-schema in each instance data file:
   E.g.  In Use Case 6, when the system creates a diagnostic file every
   minute to document the state of the server.

3.2.  Examples

   The following example is based on "UC1, Documenting Server
   Capabilities".  It provides (a shortened) list of supported YANG
   modules and Netconf capabilities for a server.  It uses the inline
   method to specify the content-schema.

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=
    "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>acme-router-modules</name>
  <inline-spec>
    ietf-yang-library@2016-06-21.yang
  </inline-spec>
  <inline-content-schema>
    <module-state xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
      <module>
        <name>ietf-yang-library</name>
        <revision>2016-06-21</revision>
      </module>
      <module>
        <name>ietf-netconf-monitoring</name>
        <revision>2010-10-04</revision>
      </module>
    </module-state>
  </inline-content-schema>
  <revision>
    <date>1956-10-23</date>
    <description>Initial version</description>
  </revision>
  <description>Defines the minimal set of modules that any acme-router
      will contain.</description>
  <contact>info@acme.com</contact>
  <content-data>
    <!-- The example lists only 4 modules, but it could list the
      full set of supported modules for a server, potentially many



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      dozens of modules -->
    <module-state xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
      <module>
        <name>ietf-yang-library</name>
        <revision>2016-06-21</revision>
        <namespace>
          urn:ietf:params:xml:ns:yang:ietf-yang-library
        </namespace>
        <conformance-type>implement</conformance-type>
      </module>
      <module>
        <name>ietf-system</name>
        <revision>2014-08-06</revision>
        <namespace>urn:ietf:params:xml:ns:yang:ietf-system</namespace>
        <feature>sys:authentication</feature>
        <feature>sys:local-users</feature>
        <deviation>
          <name>acme-system-ext</name>
          <revision>2018-08-06</revision>
        </deviation>
        <conformance-type>implement</conformance-type>
      </module>
      <module>
        <name>ietf-yang-types</name>
        <revision>2013-07-15</revision>
        <namespace>urn:ietf:params:xml:ns:yang:ietf-yang-types
          </namespace>
        <conformance-type>import</conformance-type>
      </module>
      <module>
        <name>acme-system-ext</name>
        <revision>2018-08-06</revision>
        <namespace>urn:rdns:acme.com:oammodel:acme-system-ext
          </namespace>
        <conformance-type>implement</conformance-type>
      </module>
    </module-state>
    <netconf-state>
      <capabilities>
        <capability>
          urn:ietf:params:netconf:capability:validate:1.1
        </capability>
      </capabilities>
    </netconf-state>
  </content-data>
</instance-data-set>





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     Figure 1: XML Instance Data Set - Use case 1, Documenting server
                               capabilities

   The following example is based on "UC2, Preloading Default
   Configuration".  It provides a (shortened) default rule set for a
   read-only operator role.  It uses the inline method for specifying
   the content-schema.












































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<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=
    "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data">
  <name>read-only-acm-rules</name>
  <inline-spec>ietf-yang-library@2019-01-04.yang</inline-spec>
  <inline-content-schema>
    <yang-library xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
      <module-set>
        <name>all</name>
        <module>
          <name>ietf-netconf-acm</name>
          <revision>2012-02-22</revision>
        </module>
      </module-set>
    </yang-library>
  </inline-content-schema>
  <revision>
    <date>1776-07-04</date>
    <description>Initial version</description>
  </revision>
  <description>Access control rules for a read-only role.</description>
  <content-data>
    <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
      <enable-nacm>true</enable-nacm>
      <read-default>deny</read-default>
      <exec-default>deny</exec-default>
      <rule-list>
        <name>read-only-role</name>
        <group>read-only-group</group>
        <rule>
          <name>read-all</name>
          <module-name>*</module-name>
          <access-operation>read</access-operation>
          <action>permit</action>
        </rule>
      </rule-list>
    </nacm>
  </content-data>
</instance-data-set>

      Figure 2: XML Instance Data Set - Use case 2, Preloading access
                               control data

   The following example is based on UC6 Storing diagnostics data.  An
   instance data set is produced by the server every 15 minutes that
   contains statistics about NETCONF.  As a new set is produced
   periodically many times a day a revision-date would be useless;
   instead a timestamp is included.



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   {
     "ietf-yang-instance-data:instance-data-set": {
       "name": "acme-router-netconf-diagnostics",
       "schema-uri": "file:///acme-netconf-diagnostics-yanglib.json",
       "timestamp": "2018-01-25T17:00:38Z",
       "description":
         "Netconf statistics",
       "content-data": {
         "ietf-netconf-monitoring:netconf-state": {
           "statistics": {
             "netconf-start-time ": "2018-12-05T17:45:00Z",
             "in-bad-hellos ": "32",
             "in-sessions ": "397",
             "dropped-sessions ": "87",
             "in-rpcs ": "8711",
             "in-bad-rpcs ": "408",
             "out-rpc-errors ": "408",
             "out-notifications": "39007"
           }
         }
       }
     }
   }

    Figure 3: JSON Instance Data File example - UC6 Storing diagnostics
                                   data

4.  Data Life cycle

   In UC2 "Preloading default configuration data" the loaded data may be
   changed later e.g. by management operations.  In UC6 "Storing
   Diagnostics data" the diagnostics values may change on device every
   second.

   YANG instance data is a snap-shot of information at a specific point
   of time.  If the data changes afterwards this is not represented in
   the instance data set anymore.  The valid values can be retrieved in
   run-time via NETCONF/RESTCONF or received e.g. in Yang-Push
   notifications.

   Whether the instance data changes and if so, when and how, SHOULD be
   described either in the instance data set's description statement or
   in some other implementation specific manner.








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5.  Delivery of Instance Data

   Instance data sets that are produced as a result of some sort of
   specification or design effort SHOULD be available without the need
   for a live server e.g. via download from the vendor's website, or in
   any other way product documentation is distributed.

   Other instance data sets may be read from or produced by the YANG
   server itself e.g.  UC6 documenting diagnostic data.

6.  Backwards Compatibility

   The concept of backwards compatibility and what changes are backwards
   compatible are not defined for instance data sets as it is highly
   dependent on the specific use case and the content-schema.

   For instance data that is the result of a design or specification
   activity some changes that may be good to avoid are listed.  YANG
   uses the concept of managed entities identified by key values; if the
   connection between the represented entity and the key value is not
   preserved during an update this may lead to problems.

   o  If the key value of a list entry that represents the same managed
      entity as before is changed, the user may mistakenly identify the
      list entry as new.

   o  If the meaning of a list entry is changed, but the key values are
      not (e.g. redefining an alarm-type but not changing its alarm-
      type-id) the change may not be noticed.

   o  If the key value of a previously removed list entry is reused for
      a different entity, the change may be mis-interpreted as
      reintroducing the previous entity.

7.  Yang Instance Data Model

7.1.  Tree Diagram

   The following tree diagram [RFC8340] provides an overview of the data
   model.











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   module: ietf-yang-instance-data
     structure instance-data-set:
        +--rw name?                         string
        +--rw (content-schema-spec)?
        |  +--:(simplified-inline)
        |     +--rw module*                 string
        |  +--:(inline)
        |  |  +--rw inline-spec*            string
        |  |  +--rw inline-content-schema   <anydata>
        |  +--:(uri)
        |     +--rw schema-uri?           inet:uri
        +--rw description?          string
        +--rw contact?              string
        +--rw organization?         string
        +--rw datastore?            ds:datastore-ref
        +--rw revision* [date]
        |  +--rw date           string
        |  +--rw description?   string
        +--rw timestamp?            yang:date-and-time
        +--rw content-data?         <anydata>

7.2.  YANG Model

  <CODE BEGINS> file "ietf-yang-instance-data@2019-07-04.yang"
  module ietf-yang-instance-data {
    yang-version 1.1;
    namespace
      "urn:ietf:params:xml:ns:yang:ietf-yang-instance-data";
    prefix yid ;

    import ietf-yang-structure-ext { prefix sx; }
    import ietf-datastores { prefix ds; }
    import ietf-inet-types { prefix inet; }
    import ietf-yang-types { prefix yang; }
    import ietf-yang-metadata { prefix "md"; }

    organization  "IETF NETMOD Working Group";
    contact
     "WG Web:   <https://datatracker.ietf.org/wg/netmodf/>
      WG List:  <mailto:netmod@ietf.org>

      Author:  Balazs Lengyel
         <mailto:balazs.lengyel@ericsson.com>";

    description "The module defines the structure and content of YANG
       instance data sets.

       The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',



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       '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) 2019 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 2019-07-04 {
      description  "Initial revision.";
      reference "RFC XXXX: YANG Instance Data Format";
    }

    sx:structure instance-data-set {
      description "A data structure to define a format for a
        YANG instance data set.Consists of meta-data about
        the instance data set and the real content-data.";

      leaf name {
        type string;
        description "Name of the YANG instance data set.";
      }

      choice content-schema-spec {
        description "Specification of the content-schema";

        case simplified-inline {
          leaf-list module  {
            type string {
               pattern '.+@\d{4}-\d{2}-\d{2}\.yang';
             }
             description "The list of content defining YANG
               modules including the revision date for each.
               Usage of this leaf-list implies the modules are
               used without any deviations and with all features
               supported.";
          }



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        }
        case inline {
          leaf-list inline-spec {
            type string {
              pattern '.+@\d{4}-\d{2}-\d{2}\.yang';
            }
            min-elements 1;
            ordered-by user;
            description
              "Indicates that content defining Yang modules
              are specified inline.
              Each value MUST be a YANG Module name including the
              revision-date as defined for YANG file names in RFC7950.

              E.g. ietf-yang-library@2016-06-21.yang

              The first item is either ietf-yang-library or some other
              YANG module that contains a list of YANG modules with
              their name, revision-date, supported-features and
              deviations.
              As some versions of ietf-yang-library MAY contain
              different module-sets for different datastores, if
              multiple module-sets are included, the instance data set's
              meta-data MUST contain the datastore information and
              instance data for the ietf-yang-library MUST also contain
              information specifying the module-set for the relevant
              datastore.

              Subsequent items MAY specify YANG modules augmenting the
              first module with useful data (e.g. a version label).";
          }
          anydata inline-content-schema {
            mandatory true;
            description "Instance data corresponding to the YANG modules
              specified in the inline-spec nodes defining the set
              of content defining Yang YANG modules for this
              instance-data-set.";
          }
        }

        case uri {
          leaf schema-uri {
            type inet:uri;
            description
              "A reference to another YANG instance data file.
              This instance data file will use the same set of target
              YANG modules, revisions, supported features and deviations
              as the referenced YANG instance data file.";



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          }
        }
      }

      leaf-list description {
        type string;
        description "Description of the instance data set.";
      }

      leaf contact {
        type string;
        description "Contact information for the person or
          organization to whom queries concerning this
          instance data set should be sent.";
      }

      leaf organization {
        type string;
        description "Organization responsible for the instance
          data set.";
      }

      leaf datastore {
        type ds:datastore-ref;
        description  "The identity of the datastore with which the
          instance data set is associated e.g. the datastore from
          where the data was read or the datastore where the data
          could be loaded or the datastore which is being documented.
          If a single specific datastore can not be specified, the
          leaf MUST be absent.

          If this leaf is absent, then the datastore to which the
          instance data belongs is undefined.";
      }

      list revision {
        key date;
        description "Instance data sets that are produced as
          a result of some sort of specification or design effort
          SHOULD have at least one revision entry.  For every
          published editorial change, a new one SHOULD be added
          in front of the revisions sequence so that all
          revisions are in reverse chronological order.

          For instance data sets that are read from
          or produced by a server or otherwise
          subject to frequent updates or changes, revision
          SHOULD NOT be present";



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        leaf date {
          type string {
            pattern '\d{4}-\d{2}-\d{2}';
          }
          description "Specifies the date the instance data set
            was last modified. Formatted as YYYY-MM-DD";
        }

        leaf description {
          type string;
          description
            "Description of this revision of the instance data set.";
        }
      }

      leaf timestamp {
        type yang:date-and-time;
        description "The date and time when the instance data set
          was last modified.

          For instance data sets that are read from or produced
          by a server or otherwise subject to frequent
          updates or changes, timestamp SHOULD be present";
      }

      anydata content-data {
        description "Contains the real instance data.
          The data MUST conform to the relevant YANG Modules specified
          either in the content-schema-spec or in some other
          implementation specific manner.";
      }
    }
  }
  <CODE ENDS>

8.  Security Considerations

   Depending on the nature of the instance data, instance data files MAY
   need to be handled in a secure way.  The same type of handling should
   be applied, that would be needed for the result of a <get> operation
   returning the same data.

9.  IANA Considerations

   This document registers one URI and one YANG module.






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9.1.  URI Registration

   This document registers one URI in the IETF XML registry [RFC3688].
   Following the format in RFC 3688, the following registration is
   requested to be made:

   URI: urn:ietf:params:xml:ns:yang:ietf-yang-instance-data

   Registrant Contact: The IESG.

   XML: N/A, the requested URI is an XML namespace.

9.2.  YANG Module Name Registration

   This document registers one YANG module in the YANG Module Names
   registry [RFC6020].

   name: ietf-yang-instance-data
   namespace: urn:ietf:params:xml:ns:yang:ietf-yang-instance-data
   prefix: yid
   reference: RFC XXXX

10.  Acknowledgments

   For their valuable comments, discussions, and feedback, we wish to
   acknowledge Andy Bierman, Juergen Schoenwaelder, Rob Wilton, Joe
   Clarke, Kent Watsen Martin Bjorklund, Ladislav Lhotka, Qin Wu and
   other members of the Netmod WG.

11.  References

11.1.  Normative References

   [I-D.ietf-netmod-yang-data-ext]
              Bierman, A., Bjorklund, M., and K. Watsen, "YANG Data
              Structure Extensions", draft-ietf-netmod-yang-data-ext-04
              (work in progress), July 2019.

   [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>.





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   [RFC6243]  Bierman, A. and B. Lengyel, "With-defaults Capability for
              NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,
              <https://www.rfc-editor.org/info/rfc6243>.

   [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>.

   [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              RFC 7951, DOI 10.17487/RFC7951, August 2016,
              <https://www.rfc-editor.org/info/rfc7951>.

   [RFC7952]  Lhotka, L., "Defining and Using Metadata with YANG",
              RFC 7952, DOI 10.17487/RFC7952, August 2016,
              <https://www.rfc-editor.org/info/rfc7952>.

   [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>.

   [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>.

   [RFC8526]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "NETCONF Extensions to Support the Network
              Management Datastore Architecture", RFC 8526,
              DOI 10.17487/RFC8526, March 2019,
              <https://www.rfc-editor.org/info/rfc8526>.

   [RFC8527]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "RESTCONF Extensions to Support the Network
              Management Datastore Architecture", RFC 8527,
              DOI 10.17487/RFC8527, March 2019,
              <https://www.rfc-editor.org/info/rfc8527>.

11.2.  Informative References

   [I-D.ietf-ccamp-alarm-module]
              Vallin, S. and M. Bjorklund, "YANG Alarm Module", draft-
              ietf-ccamp-alarm-module-09 (work in progress), April 2019.





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   [I-D.ietf-netconf-rfc7895bis]
              Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", draft-ietf-netconf-
              rfc7895bis-07 (work in progress), October 2018.

   [I-D.ietf-netconf-yang-push]
              Clemm, A. and E. Voit, "Subscription to YANG Datastores",
              draft-ietf-netconf-yang-push-25 (work in progress), May
              2019.

   [I-D.wu-netconf-restconf-factory-restore]
              Wu, Q., Lengyel, B., and Y. Niu, "Factory default
              Setting", draft-wu-netconf-restconf-factory-restore-03
              (work in progress), October 2018.

   [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>.

   [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>.

Appendix A.  Open Issues

   o  -

Appendix B.  Changes between revisions

   v03 - v04

   o  removed entity-tag and last-modified timestamp

   o  Added simplified-inline method of content-schema specification

   v02 - v03

   o  target renamed to "content-schema" and "content defining Yang
      module(s)"

   o  Made name of instance data set optional

   o  Updated according to draft-ietf-netmod-yang-data-ext-03

   o  Clarified that entity-tag and last-modified timestamp are encoded
      as metadata.  While they contain useful data, the HTTP-header
      based encoding from Restconf is not suitable.



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   v01 - v02

   o  Removed design time from terminology

   o  Defined the format of the content-data part by referencing various
      RFCs and drafts instead of the result of the get-data and get
      operations.

   o  Changed target-ptr to a choice

   o  Inline target-ptr may include augmenting modules and alternatives
      to ietf-yang-library

   o  Moved list of target modules into a separate <target-modules>
      element.

   o  Added backwards compatibility considerations

   v00 - v01

   o  Added the target-ptr metadata with 3 methods

   o  Added timestamp metadata

   o  Removed usage of dedicated .yid file extension

   o  Added list of use cases

   o  Added list of principles

   o  Updated examples

   o  Moved detailed use case descriptions to appendix

Appendix C.  Detailed Use Cases - Non-Normative

C.1.  Use Cases

   We present a number of use cases were YANG instance data is needed.

C.1.1.  Use Case 1: Early Documentation of Server Capabilities

   A server has a number of server-capabilities that are defined in YANG
   modules and can be retrieved from the server using protocols like
   NETCONF or RESTCONF. server capabilities include






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   o  data defined in ietf-yang-library: YANG modules, submodules,
      features, deviations, schema-mounts, datastores supported
      ([I-D.ietf-netconf-rfc7895bis])

   o  alarms supported ([I-D.ietf-ccamp-alarm-module])

   o  data nodes, subtrees that support or do not support on-change
      notifications ([I-D.ietf-netconf-yang-push])

   o  netconf-capabilities in ietf-netconf-monitoring

   While it is good practice to allow a client to query these
   capabilities from the live server, that is often not possible.

   Often when a network node is released an associated NMS (network
   management system) is also released with it.  The NMS depends on the
   capabilities of the server.  During NMS implementation information
   about server capabilities is needed.  If the information is not
   available early in some off-line document, but only as instance data
   from the live network node, the NMS implementation will be delayed,
   because it has to wait for the network node to be ready.  Also
   assuming that all NMS implementors will have a correctly configured
   network node available to retrieve data from, is a very expensive
   proposition.  (An NMS may handle dozens of node types.)

   Network operators often build their own home-grown NMS systems that
   needs to be integrated with a vendor's network node.  The operator
   needs to know the network node's server capabilities in order to do
   this.  Moreover the network operator's decision to buy a vendor's
   product may even be influenced by the network node's OAM feature set
   documented as the Server's capabilities.

   Beside NMS implementors, system integrators and many others also need
   the same information early.  Examples could be model driven testing,
   generating documentation, etc.

   Most server-capabilities are relatively stable and change only during
   upgrade or due to licensing or addition or removal of HW.  They are
   usually defined by a vendor at design time, before the product is
   released.  It feasible and advantageous to define/document them early
   e.g. in a YANG instance data File.

   It is anticipated that a separate IETF document will define in detail
   how and which set of server capabilities should be documented.







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C.1.2.  Use Case 2: Preloading Data

   There are parts of the configuration that must be fully configurable
   by the operator, however for which often a simple default
   configuration will be sufficient.

   One example is access control groups/roles and related rules.  While
   a sophisticated operator may define dozens of different groups often
   a basic (read-only operator, read-write system administrator,
   security-administrator) triplet will be enough.  Vendors will often
   provide such default configuration data to make device configuration
   easier for an operator.

   Defining Access control data is a complex task.  To help the device
   vendor pre-defines a set of default groups (/nacm:nacm/groups) and
   rules for these groups to access specific parts of common models
   (/nacm:nacm/rule-list/rule).

   YANG instance data files are used to document and/or preload the
   default configuration.

C.1.3.  Use Case 3: Documenting Factory Default Settings

   Nearly every server has a factory default configuration.  If the
   system is really badly misconfigured or if the current configuration
   is to be abandoned the system can be reset to this default.

   In Netconf the <delete-config> operation can already be used to reset
   the startup datastore.  There are ongoing efforts to introduce a new,
   more generic reset-datastore operation for the same purpose
   [I-D.wu-netconf-restconf-factory-restore]

   The operator currently has no way to know what the default
   configuration actually contains.  YANG instance data can be used to
   document the factory default configuration.

Authors' Addresses

   Balazs Lengyel
   Ericsson
   Magyar Tudosok korutja 11
   1117 Budapest
   Hungary

   Phone: +36-70-330-7909
   Email: balazs.lengyel@ericsson.com





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   Benoit Claise
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   1831 Diegem
   Belgium

   Phone: +32 2 704 5622
   Email: bclaise@cisco.com











































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