Netmod B. Lengyel
Internet-Draft Ericsson
Intended status: Standards Track B. Claise
Expires: January 6, 2020 Cisco Systems, Inc.
July 5, 2019

YANG Instance Data File Format


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

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This Internet-Draft will expire on January 6, 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 ( in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1. 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 RFC 8174 when, and only when, they appear in all capitals, as shown here.

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.

Documentation of server capabilities
Preloading default configuration data
Documenting Factory Default Settings
Instance data used as backup
Storing the configuration of a device, e.g. for archive or audit purposes
Storing diagnostics data
Allowing YANG instance data to potentially be carried within other IPC message formats
Default instance data used as part of a templating solution
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:

Two standard formats are based on the XML and the JSON encoding
Re-use existing formats similar to the response to a <get> operation/request
Add metadata about the instance data set (Section 3, Paragraph 9)
A YANG instance data set may contain data for many YANG modules
Instance data may include configuration data, state data or a mix of the two
Partial data sets are allowed
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.

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:

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:

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:

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:

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.

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. 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 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=
    <module-state xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
    <description>Initial version</description>
  <description>Defines the minimal set of modules that any acme-router
      will contain.</description>
    <!-- The example lists only 4 modules, but it could list the 
      full set of supported modules for a server, potentially many 
      dozens of modules --> 
    <module-state xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">

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.

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=
    <yang-library xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
    <description>Initial version</description>
  <description>Access control rules for a read-only role.</description>
    <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">

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.

  "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",
      "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.

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.

7. Yang Instance Data Model

7.1. Tree Diagram

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

module: ietf-yang-instance-data
  structure instance-data-set:
     +--rw name?                         string
     +--rw (content-schema-spec)?
     |  +--:(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;
  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";
   "WG Web:   <>
    WG List:  <>

    Author:  Balazs Lengyel

  description "The module defines the structure and content of YANG
     instance data sets.
     The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
     '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

     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";

  md:annotation entity-tag {
    type string;
    description "Used to encode the entity-tag defined in
      RFC8040 for the annotated instance.";
    reference "RESTCONF Protocol RFC8040";

  md:annotation last-modified {
    type yang:date-and-time;
    description "Contains the date and time when the annotated
      instance was last modified (or created).";
    reference "RESTCONF Protocol RFC8040";

  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 inline {
        leaf-list inline-spec {
          type string {
            pattern '.+@\d{4}-\d{2}-\d{2}\.yang';
          min-elements 1;
          ordered-by user;
            "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
            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

            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

      case uri {
        leaf schema-uri {
          type inet:uri;
            "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.";

    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";

      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 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.";

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.

9.1. URI Registration

This document registers one URI in the IETF XML registry. 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", Internet-Draft draft-ietf-netmod-yang-data-ext-03, April 2019.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010.
[RFC6243] Bierman, A. and B. Lengyel, "With-defaults Capability for NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016.
[RFC7951] Lhotka, L., "JSON Encoding of Data Modeled with YANG", RFC 7951, DOI 10.17487/RFC7951, August 2016.
[RFC7952] Lhotka, L., "Defining and Using Metadata with YANG", RFC 7952, DOI 10.17487/RFC7952, August 2016.
[RFC8040] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017.
[RFC8340] Bjorklund, M. and L. Berger, "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018.
[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.
[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.
[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.

11.2. Informative References

[I-D.ietf-ccamp-alarm-module] Vallin, S. and M. Bjorklund, "YANG Alarm Module", Internet-Draft draft-ietf-ccamp-alarm-module-09, April 2019.
[I-D.ietf-netconf-rfc7895bis] Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K. and R. Wilton, "YANG Library", Internet-Draft draft-ietf-netconf-rfc7895bis-07, October 2018.
[I-D.ietf-netconf-yang-push] Clemm, A. and E. Voit, "Subscription to YANG Datastores", Internet-Draft draft-ietf-netconf-yang-push-25, May 2019.
[I-D.wu-netconf-restconf-factory-restore] Wu, Q., Lengyel, B. and Y. Niu, "Factory default Setting", Internet-Draft draft-wu-netconf-restconf-factory-restore-03, 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.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

Appendix A. Open Issues

Appendix B. Changes between revisions

v02 - v03

v01 - v02

v00 - v01

v05 - v00-netmod

v04 - v05

v03 - v04

v02 - v03

v01 - v02

v00 - v01

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

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.

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:
Benoit Claise Cisco Systems, Inc. De Kleetlaan 6a b1 1831 Diegem, Belgium Phone: +32 2 704 5622 EMail: