Netconf B. Lengyel
Internet-Draft Ericsson
Intended status: Standards Track B. Claise
Expires: April 22, 2019 Cisco Systems, Inc.
October 19, 2018

YANG Based Instance Data Files Format


There is a need to document data defined in YANG models without the need to fetch it from a live YANG server. 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 Based Instance data, that is data that could be stored in a datastore and whose syntax and semantics is defined by YANG models. Most important use cases foreseen include documenting server capabilities, factory-default settings, or vendor provided default configurations.

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

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 April 22, 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 ( 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.

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.

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.

2. Introduction

There is a need to provide instance data defined in YANG models without the need to fetch it from a live YANG server. Data is often needed already in design time before the YANG server is implemented 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 file format for YANG Based Instance data, that is data that could be stored in a datastore and whose syntax and semantics is defined by YANG models.

2.1. Use Cases

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

2.1.1. Use Case 1: Early Documentation of Server Capabilites

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

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

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 capabilites.

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 Based Instance Data File.

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

2.1.2. 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 Based Instance data files are used to document and/or preload the default configurationp.

2.1.3. Use Case 3: Dcoumenting 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 do this for the startup configuration. There are ongoing efforts to introduce a new, more generic reset 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 Based Instance data can be used to document the factory default configuration.

3. Instance Data File Format

Two standard formats to represent YANG Based Instance Data are specified based on the XML and JSON encoding. The XML format is based on [RFC7950] while the JSON format is based on [RFC7951]. Later as other YANG encodings (e.g. CBOR) are defined further Instance Data formats may be specified.

For both formats data is placed in a top level auxiliary container named "instance-data-set". The purpose of the container, which is not part of the real data itself, is to carry meta-data for the complete instance-data-set.

The XML format SHALL follow the format returned for a NETCONF GET operation. The <data> anydata (which is not part of the real data itself) SHALL contain all data that would be inside the <data> wrapper element of a reply to the <get> operation. XML attributes SHOULD NOT be present, however if a SW receiving a YANG Based Instance data file encounters XML attributes unknown to it, it MUST ignore them, allowing them to be used later for other purposes.

The JSON format SHALL follow the format of the reply returmed for a RESTCONF GET request directed at the datastore resource: {+restconf}/data. ETags and Timestamps SHOULD NOT be included, but if present SHOULD be ignored.

A YANG Based Instance data file MUST contain a single instance data set. Instance data MUST conform to the corresponding target YANG Modules and follow the XML/JSON encoding rules as defined in [RFC7950] and [RFC7951] and 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 configuraton and state data set for a YANG server. Default values SHOULD NOT but MAY 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 SHOULD be of the form:

The revision date is optional. It SHOULD NOT be used if the file is stored in a version control system (e.g. git) because the change of file names will break the connection between the different revisions of the file.

Meta data, 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. Meta data SHALL include:

Meta data SHOULD include:

<?xml version="1.0" encoding="UTF-8"?>
<instance-data-set xmlns=
  <description>Defines the minimal set of modules that any acme-router
       will contain. These modules will always be present.</description> 
    <yang-library xmlns="urn:ietf:params:xml:ns:yang:ietf-yang-library">
          <!-- description "A later revision may be used."; -->

Figure 1: XML Instance Data File example

  "ietf-yang-instance-data:instance-data-set": {
    "name": "acme-router-modules", 
    "revision": "2108-01-25", 
    "contact": "", 
      "Defines the set of modules that an acme-router will contain.", 
    "data": {
      "ietf-yang-library:yang-library": {
        "module-set": [
          "name": "basic",
          "module": [
              "name": "ietf-system",
              "revision": "2014-08-06",
              "namespace": "urn:ietf:params:xml:ns:yang:ietf-system",
              "feature": ["authentication", "radius-authentication"]

Figure 2: JSON Instance Data File example

4. Data Life cycle

Data defined or documented in YANG Based 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 Based 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

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 files SHOULD be available without the need for a live YANG server e.g. via download from the vendor's website, or any other way together with other product documentation.

6. YANG Model

<CODE BEGINS> file "ietf-yang-instance-data.yang"

module ietf-yang-instance-data {
  yang-version 1.1;
  prefix yid ;

  import ietf-yang-data-ext { prefix yd; }
  import ietf-datastores { prefix ds; }
  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.";
  revision 2018-06-30 {
    description  "Initial revision.";
    reference "RFC XXXX: YANG Based Instance Data";
  yd:yang-data instance-data-format {
    container instance-data-set {               
      description "Auxiliary container to carry meta-data for 
        the complete instance data set.";
      leaf name {
        type string;
        mandatory true;
        description "Name of a YANG Based Instance data set.";
      leaf description { type string; }
      leaf contact {
        type string;
        description "Contains the same information the contact 
            statement carries for a YANG module.";
      leaf organization {
        type string;
        description "Contains the same information the 
            organization statement carries for a YANG module.";                    
      leaf datastore {
        type ds:datastore-ref;
        description  "The identity of the datastore for which 
          the instance data is documented for config=true data nodes. 
          The leaf MAY be absent in which case the running dtastore or
          if thats not writable, the candidate datastore is implied.
          For config=false data nodes always the operational 
          data store is implied.";
      list revision {
        key date;
        description "An instance-data-set 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.";
        leaf date { 
          type string {
            pattern '\d{4}-\d{2}-\d{2}';
          description "Specifies the data the revision 
              was last modified. Formated as YYYY-MM-DD";
        leaf description { type  string; }  
      anydata data {
        mandatory true;
        description "Contains the real instance data.
            The data MUST conform to the relevant YANG Modules.";


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

To be completed, all the usual requests for a new YANG module

9. References

9.1. Normative References

[I-D.ietf-netmod-yang-data-ext] Bierman, A., Bjorklund, M. and K. Watsen, "YANG Data Extensions", Internet-Draft draft-ietf-netmod-yang-data-ext-01, March 2018.
[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.

9.2. Informative References

[I-D.ietf-ccamp-alarm-module] Vallin, S. and M. Bjorklund, "YANG Alarm Module", Internet-Draft draft-ietf-ccamp-alarm-module-04, October 2018.
[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., Voit, E., Prieto, A., Tripathy, A., Nilsen-Nygaard, E., Bierman, A. and B. Lengyel, "YANG Datastore Subscription", Internet-Draft draft-ietf-netconf-yang-push-19, September 2018.
[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

v04 - v05

v03 - v04

v02 - v03

v01 - v02

v00 - v01

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: