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

Netmod                                                        B. Lengyel
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               B. Claise
Expires: June 9, 2019                                Cisco Systems, Inc.
                                                        December 6, 2018


                     YANG Instance Data File Format
             draft-ietf-netmod-yang-instance-file-format-01

Abstract

   There is a need to document data defined in YANG models when a live
   YANG server is not available.  Data is often needed already in design
   time or needed by groups that do not have a live running YANG 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 existing formats from <get> operation/request
   and decorates them 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 June 9, 2019.

Copyright Notice

   Copyright (c) 2018 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.  High Level Principles . . . . . . . . . . . . . . . . . .   4
   3.  Instance Data File Format . . . . . . . . . . . . . . . . . .   4
     3.1.  Specifying the Target YANG Modules: target-ptr  . . . . .   6
       3.1.1.  IN-LINE Method  . . . . . . . . . . . . . . . . . . .   7
       3.1.2.  URI Method  . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   8
   4.  Data Life cycle . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Delivery of Instance Data . . . . . . . . . . . . . . . . . .  12
   6.  YANG Model  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  URI Registration  . . . . . . . . . . . . . . . . . . . .  15
     8.2.  YANG Module Name Registration . . . . . . . . . . . . . .  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     10.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Appendix A.  Open Issues  . . . . . . . . . . . . . . . . . . . .  17
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  17
   Appendix C.  Detailed Use Cases - Non-Normative . . . . . . . . .  19
     C.1.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  19
       C.1.1.  Use Case 1: Early Documentation of Server
               Capabilities  . . . . . . . . . . . . . . . . . . . .  19
       C.1.2.  Use Case 2: Preloading Data . . . . . . . . . . . . .  20
       C.1.3.  Use Case 3: Documenting Factory Default Settings  . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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.

   Design time: A time during which a YANG model and the implementation
   behind it is created.  Sometimes in other documents this period is
   divided into design and implementation time.





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   Instance Data Set: A named set of data items 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.

   Target YANG Module: A YANG module for which the instance data set
   contains instance data, like ietf-yang-library in the examples.

   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.

2.  Introduction

   There is a need to document data defined in YANG models when a live
   YANG server is not available.  Data is often needed already in design
   time or needed by groups that do not have a live running YANG 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

   UC9  Providing data examples in RFCs or internet drafts

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





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   There are already 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
   outside the instance data set itself will define specific use cases.
   Use cases are listed here only to indicate the need for this work.

2.1.  High Level 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 from the <get> operation/request

   P3  Add metadata about the instance data set

   P4  A YANG instance data file shall contain only a single YANG
       instance data set

   P5  A YANG instance data set may contain data for many target YANG
       modules

   P6  Instance data may include configuration data, state data or a mix
       of the two

   P7  Partial data sets are allowed

   P8  YANG instance data format may be used for any data for which
       target YANG module(s) are defined and available to the reader,
       independent of whether the module is actually implemented by a
       YANG server

3.  Instance Data File Format

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

   The instance data set is placed in a top level auxiliary container
   named "instance-data-set".  An instance data set is made up of a
   header part and content-data.  The initial header part carries
   metadata for the instance data set.  It is defined by the ietf-yang-
   instance-data YANG module.  The content-data is all data inside the
   anydata datanode, this carries the "real data" that we want to
   document/provide.  The syntax and semantics of content-data is
   defined by the target YANG modules.




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

   The content-data part of the XML format SHALL follow the format
   returned for a NETCONF GET operation.  The <content-data> anydata
   node SHALL contain all elements that would be inside the <data>
   wrapper element of a reply to the <get> operation.  Some XML
   attributes (e.g. metadata like origin) MAY be absent.  SW handling
   YANG instance data MUST ignore XML attributes unknown to it, allowing
   them to be used later for other purposes.

   The content-data part of the JSON format SHALL follow the format of
   the payload of the reply returned for a RESTCONF GET request directed
   at the datastore resource: {+restconf}/data or
   {+restconf}/ds/<datastore>.

   Instance data MUST conform to the corresponding target YANG Modules
   and follow the XML/JSON encoding rules as defined in [RFC7950] and
   [RFC7951] and MUST use UTF-8 character encoding.  A single instance
   data set MAY contain data for any number of target YANG modules; if
   needed it MAY carry the complete configuration and state data set for
   a YANG 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 or require-instance=true constrains MAY be
   violated.

   The name of the file SHALL be of the form:

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

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

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

   Metadata, information about the data set itself SHALL be included in
   the instance data set.  This data will be children of the top level
   instance-data-set container as defined in the ietf-instance-data YANG
   module.  Metadata MUST include:

   o  name of the instance data set




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   Metadata SHOULD include:

   o  target-ptr: A pointer to the list of target YANG modules their
      revision, supported features and deviations.

   o  Revision date of the instance data set.  If both this date and and
      the date in the instance data file name are present they MUST have
      the same value.

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

   Metadata MAY include:

   o  Organization responsible for the instance data set

   o  Contact information

   o  Information about the datastore associated with the instance data
      set 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.  This information is optional, as often a single
      datastore can not be specified.

   o  Timestamp: The date and time when the instance data set was last
      modified.

   o  It is anticipated that different organizations will have the need
      to augment the metadata with various other data nodes.

3.1.  Specifying the Target YANG Modules: target-ptr

   To properly understand and use an instance data set the user needs to
   know the list of target YANG modules their revision, supported
   features and deviations.  The metadata "target-ptr" is used to
   specify the YANG target module list.  One of the following 3 options
   SHALL be used:

      IN-LINE method: Include the needed information as part of instance
      data as defined by ietf-yang-library

      URI method: Include a URI that points to the target module set.
      (if you don't want to repeat the info again and again)

      EXTERNAL Method: Do not include the target-ptr as the target YANG
      module set is already known, or the information is available
      through external documents.



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   Note, the specified target YANG modules only indicate the set of
   modules that were used to define this YANG instance data set.
   Sometimes instance data may be used for a YANG 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 YANG
   server are updated, an unchanged instance data set may still be
   usable.  Whether the instance data set is usable for a possibly
   different real-life target YANG module set depends on the
   compatibility between the specified target and the real-life target
   YANG module set (considering modules, revisions, features,
   deviations).

3.1.1.  IN-LINE Method

   Target-ptr MUST bet set to:

      'inline:ietf-yang-library@' revision-date '.yang'

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

   The revision date in the inline target-ptr is mandatory, it specifies
   the revision of the ietf-yang-library used.  The first group of data
   inside the "anydata data" element MUST be instance data targeted at
   the ietf-yang-library.  This data SHALL specify the target YANG
   modules, revisions, supported features and deviations for this and
   all the other target YANG modules.

3.1.2.  URI Method

   Target-ptr MUST bet set to a URI that references another YANG
   instance data file.  The current instance data file will use the same
   set of target YANG modules, revisions, supported features and
   deviations as the other referenced YANG instance data file.

   The referenced instance data file will usually contain data only for
   ietf-yang-library to specify the target YANG modules for the original
   instance data file.

   The URI method is advantageous when the user wants to avoid the
   overhead of specifying the target YANG modules in the instance data
   file: E.g.  In Use Case 6, when the system creates a diagnostic file
   every 10 minutes to document the state of the YANG server.

   The referenced YANG instance data file might use the in-line method
   or might use the URI method to reference further instance data
   file(s).  However at the end of this reference chain there MUST be an
   instance data file using the in-line method.




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

3.2.  Examples

   The following example is based on UC1, documenting server
   capabilities.  It provides (a shortened) list of supported YANG
   modules for a YANG server.  It uses the inline method for the target-
   ptr.

<?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>
  <target-ptr>inline:ietf-yang-library@2016-06-21.yang</target-ptr>
  <revision>
    <date>2108-01-25</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 YANG server, potentially many
      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>
        <feature>sys:ntp</feature>
        <deviation>
          <name>acme-system-ext</name>
          <revision>2018-08-06</revision>
        </deviation>
        <conformance-type>implement</conformance-type>
      </module>
      <module>



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        <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>
  </content-data>
</instance-data-set>

     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 the target-
   ptr.





























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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>
  <target-ptr>inline:ietf-yang-library@2016-06-21.yang</target-ptr>
  <revision>
    <date>2018-01-25</date>
    <description>Initial version</description>
  </revision>
  <description>Access control rules for a read-only role.</description>
  <contact>info@acme.com</contact>
  <content-data>
    <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-acm</name>
        <revision>2012-02-22</revision>
      </module>
    </module-state>
    <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 YANG server every 15 minutes
   that contains statistics about netconf.  As a new set is produced
   automatically 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",
       "target-ptr": "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

   Data defined or documented in YANG instance data sets may be used for
   preloading a YANG server with this data, but the server may populate
   the data without using the actual file in which case the instance
   data file is only used as documentation.

   While such data will usually not change, data documented by instance
   data sets MAY be changed by the YANG server itself or by management
   operations.  It is out of scope for this document to specify a method
   to prevent this.  Whether such data changes and if so, when and how,
   SHOULD be described either in the instance data file description
   statement or in some other implementation specific manner.

   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






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   Notifications about the change of data documented by instance data
   sets may be supplied by e.g. the Yang-Push mechanism, but it is out
   of scope for this document.

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 YANG 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.  YANG Model

   <CODE BEGINS> file "ietf-yang-instance-data.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-data-ext { prefix yd; }
   import ietf-datastores { prefix ds; }
   import ietf-inet-types { prefix inet; }
   import ietf-yang-types { prefix yang; }

   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.";

   revision 2018-11-30 {
     description  "Initial revision.";
     reference "RFC XXXX: YANG Instance Data Format";
   }

   yd:yang-data instance-data-format {
     container instance-data-set {
       description "Auxiliary container to carry meta-data for



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         the complete instance data set.";

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

       leaf target-ptr {
         type union {
           type string {
             pattern 'inline:ietf-yang-library@\d{4}-\d{2}-\d{2}\.yang';
           }
           type inet:uri;
         }
         description "A pointer to the list of target YANG modules
           their revisions, supported features and deviations.
           target-ptr SHALL use one of the following formats:

           IN-LINE format: target-ptr should bet set to:
           'inline:ietf-yang-library@' revision-date '.yang'
           E.g. inline:ietf-yang-library@2016-06-21.yang
           The revision date is mandatory. When using the in-line
           format the first group of data inside the content-data
           node MUST be instance data targeted at the
           ietf-yang-library. This data SHALL specify the target YANG
           modules, revisions, supported features and deviations for
           this and all the other target YANG modules of the set.

           URI format. target-ptr MUST be a URI that references
           another YANG instance data file.
           This instance data file will use the same set of target
           YANG modules, revisions, supported features and deviations
           as this other referenced YANG instance data file.";
       }

       leaf description { type string; }

       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



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           data set.";
       }

       leaf datastore {
         type ds:datastore-ref;
         description  "The identity of the datastore with which the
           instance data set is associated. 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 the YANG server or otherwise
           subject to frequent updates or changes, revision
           SHOULD NOT be present";

         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; }
       }

       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 the YANG server or otherwise subject to frequent
           updates or changes, timestamp SHOULD be present";
       }




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       anydata content-data {
         mandatory true;
         description "Contains the real instance data.
             The data MUST conform to the relevant YANG Modules.";
       }
     }
   }
 }

   <CODE ENDS>

7.  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.

8.  IANA Considerations

   This document registers one URI and one YANG module.

8.1.  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.

8.2.  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








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9.  Acknowledgments

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

10.  References

10.1.  Normative References

   [I-D.ietf-netmod-yang-data-ext]
              Bierman, A., Bjorklund, M., and K. Watsen, "YANG Data
              Extensions", draft-ietf-netmod-yang-data-ext-01 (work in
              progress), March 2018.

   [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>.

   [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>.

10.2.  Informative References

   [I-D.ietf-ccamp-alarm-module]
              Vallin, S. and M. Bjorklund, "YANG Alarm Module", draft-
              ietf-ccamp-alarm-module-06 (work in progress), November
              2018.

   [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.







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   [I-D.ietf-netconf-yang-push]
              Clemm, A., Voit, E., Prieto, A., Tripathy, A., Nilsen-
              Nygaard, E., Bierman, A., and B. Lengyel, "Subscription to
              YANG Datastores", draft-ietf-netconf-yang-push-20 (work in
              progress), October 2018.

   [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  Augmenting metadata must be possible.  As of now it looks like
      yang-data-ext will solve that.  If not, define instance data as
      regular YANG instead of yd:yang-data.

Appendix B.  Changes between revisions

   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

   v05 - v00-netmod

   o  New name for the draft following Netmod workgroup adoption.  No
      other changes



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   v04 - v05

   o  Changed title and introduction to clarify that this draft is only
      about the file format and documenting server capabilities is just
      a use case.

   o  Added reference to draft-wu-netconf-restconf-factory-restore

   o  Added new open issues.

   v03 - v04

   o  Updated changelog for v02-v03

   v02 - v03

   o  Updated the document according to comments received at IETF102

   o  Added parameter to specify datastore

   o  Rearranged chapters

   o  Added new use case: Documenting Factory Default Settings

   o  Added "Target YANG Module" to terminology

   o  Clarified that instance data is a snapshot valid at the time of
      creation, so it does not contain any later changes.

   o  Removed topics from Open Issues according to comments received at
      IETF102

   v01 - v02

   o  The recommendation to document server capabilities was changed to
      be just the primary use-case.  (Merged chapter 4 into the use case
      chapter.)

   o  Stated that RFC7950/7951 encoding must be followed which also
      defines (dis)allowed whitespace rules.

   o  Added UTF-8 encoding as it is not specified in t950 for instance
      data

   o  added XML declaration

   v00 - v01




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   o  Redefined using yang-data-ext

   o  Moved metadata into ordinary leafs/leaf-lists

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 YANG 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.  YANG server capabilities include

   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 YANG 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 YANG 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 Yang server's capabilities.




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   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 in 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.

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 YANG 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.



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Authors' Addresses

   Balazs Lengyel
   Ericsson
   Magyar Tudosok korutja 11
   1117 Budapest
   Hungary

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


   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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