Network Working Group B. Lengyel
Internet-Draft Ericsson
Intended status: Standards Track B. Claise
Expires: January 3, 2019 Cisco Systems, Inc.
July 2, 2018

YANG Instance Data Files and their use for Documenting Server Capabilities


This document specifies a standard file format for YANG instance data, that is data that could be stored in a datastore and whose syntax and semantics is defined by YANG models. Instance data files can be used to provide information that is defined in design time. There is a need to document Server capabilities (which are often specified in design time). Defining server capabilities is foreseen as the most important use of YANG instance data files.

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Table of Contents

1. Terminology

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.

2. Introduction

A YANG server has a number of server-capabilities that 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. Most server-capabilities are relatively stable but the fact is that some might change. Looking at the change frequency, we have roughly three categories:

  1. only at upgrade, e.g. introduced with a new SW package
  2. rarely e.g. due to licensing or HW inserted
  3. more frequently e.g. a capability might be dependent on the CPU or traffic load, although that would be most unusual

Most capabilities belong to type 1), some to type 2) and a relatively small set to type 3). Many network nodes only have type 1) or type 1+2) capabilities. Stable capabilities are usually defined by a vendor in design time, before the product is released. While these capabilities can be retrieved from the live server in run-time, there is a strong need to provide the same data already during design time. (Often only a part of all the server capabilities can be made available.)

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.)

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

As capabilities are often already known in design time and are relativaly stable, it feasible and advantageous to define/document them early. This document specifies a file format for YANG instance data that may be used to provide server capability information, allowing vendors to specify capabilities early, in design time.

The same instance data file format can be used for other purposes, like providing initial data for any YANG module. E.g. a basic set of access control groups can be provided either by a device vendor or an operator using the network device.

2.1. Use Case 1: Early Documentation of Server Capabilites

An operator wants to integrate his own, in-house built management system with the network node from ACME Systems. The management integration must be ready by the time the first AcmeRouter is installed in the network. To do the integration the operator needs the list of supported YANG modules and features. While this list could be read from the ietf-yang-library via Netconf, in order to allow time for developing the management integration, the operator demands this information early. The operator will value that this information is available in a standard format, that is actually the same format that can be read later from the node via Netconf.

YANG instance data files are used to provide design time information about server capabilities.

2.2. Use Case 2: Preloading Data

There are parts of the configuration that must be fully configurable by the operator, however for which often a semi-standard 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 configurationp.

3. Instance Data File Format

Two standard formats to represent YANG 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 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 Instance data file MUST contain a single instance data set. Instance data MUST conform to the corresponding YANG Modules and follow the XML/JSON encoding rules as defined in [RFC7950] and [RFC7951] and use UTF-8 character encoding. 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:

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

3.1. 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 Files 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. If the information retrieved in run-time via Netconf/Restconf and the YANG Instance Data File for the same YANG module are in conflict, the run-time data MUST be considered authorative. Whether this data changes and if so, when and how, SHOULD be described either in the instance data file description statements or some other implementation specific manner.

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.

4. 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; }
  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 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 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.";                    
      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.";


5. 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.

6. IANA Considerations

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

7. References

7.1. 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.

7.2. Informative References

[I-D.ietf-ccamp-alarm-module] Vallin, S. and M. Bjorklund, "YANG Alarm Module", Internet-Draft draft-ietf-ccamp-alarm-module-01, February 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-06, April 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-17, July 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

v01 - v02

v00 - v01

Authors' Addresses

Balazs Lengyel Ericsson xxx 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: