Network Working Group                                         B. Lengyel
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               B. Claise
Expires: December 15, 2018                           Cisco Systems, Inc.
                                                           June 13, 2018

     YANG Instance Data Files and their use for Documenting Server


   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), which should be done using instance data

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   to this document.  Code Components extracted from this document must
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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

   o  data defined in ietf-yang-library (YANG modules, submodules,
      features, deviations, schema-mounts)

   o  datastores supported

   o  alarms supported (draft-vallin-ccamp-alarm-module)

   o  data nodes, subtrees that support or do not support on-change
      notifications (draft-ietf-netconf-yang-push)

   o  netconf-capabilities

   o  etc.

   While it is good practice to allow a client to query these
   capabilites from the live YANG server, that is often not enough.
   Many of these server-capabilities are relatively stable.  They may

   1.  only at upgrade, or

   2.  rarely (e.g. due to licensing), or

   3.  more frequently

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   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

   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.

   This document specifies a file format for YANG instance data and
   proposes to use it 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.  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.

   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.

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2.2.  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 9000 is
   installed in the network.  To do the integration the operator needs
   the 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.

2.3.  Use Case 2: Preloading Data

   Defining Access control data is a complex task.  To help with this,
   the device vendor pre-defines some of the data.  Among others a set
   of default groups (/nacm:nacm/nacm:groups) e.g. "read-only",
   "operator", "sys-admin" and rules for these groups to access specific
   parts of common models could be defined.  The operator will often use
   these default groups, but is also free to completely remove them and
   define his own set of groups.

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 defined in
   [RFC7950] while the JSON format is defined in [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 <instance-data> anydata, which is not part of the
   real data itself, SHALL contain all data that would be inside the
   <data> wrapper element.  XML attributes SHOULD NOT be used, 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.

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   A YANG Instance data file MUST contain a single instance data set.
   Instance data MUST conform to the corresponding YANG Modules.
   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.

   Meta data, information about the data set itself SHALL be included in
   the instance data file.  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:

   o  Name of the instance data set

   Meta data SHOULD include:

   o  Revision date of the instance data set (later a semantic version
      MAY also be included)

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

<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

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     "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.",
       "instance-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.  Provide Information about Server Capabilities

   YANG instance data files SHOULD be used to provide design time
   information about server capabilities.  The content of the instance
   data file SHOULD describe the capabilities of the server, however
   there is no general guarantee that the capabilities will not change
   over time.  Whether capabilities change and if so, when and how
   SHOULD be described either in the instance data file description
   statements or some other implementation specific manner.  If the
   information about server capabilities 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

   The set of server capabilities to be documented will be defined by
   other standards and specifications, and is out of scope for this
   document.  Whether and how the instance data files are used by SW
   implementing a YANG server is out of scope for this specification.

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5.  YANG Model

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

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

     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 2018-06-04 {
       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 {

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           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.";

           leaf description { type  string; }

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


6.  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 or handling should
   be applied, that would be needed for the result of a <get> operation
   returning the same data.

7.  IANA Considerations

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

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8.  References

8.1.  Normative References

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

   [RFC7950]  Bjorklund, M., Ed., "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,

8.2.  Informative References

              Bierman, A., Bjorklund, M., Schoenwaelder, J., Watsen, K.,
              and R. Wilton, "YANG Library", draft-ietf-netconf-
              rfc7895bis-06 (work in progress), April 2018.

              Clemm, A., Voit, E., Prieto, A., Tripathy, A., Nilsen-
              Nygaard, E., Bierman, A., and B. Lengyel, "YANG Datastore
              Subscription", draft-ietf-netconf-yang-push-16 (work in
              progress), May 2018.

              Vallin, S. and M. Bjorklund, "YANG Alarm Module", draft-
              vallin-ccamp-alarm-module-01 (work in progress), October

   [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.  Changes between revisions

   v00 - v01

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   o  Redefined using yang-data-ext

   o  Moved meta data into ordinary leafs/leaf-lists

Authors' Addresses

   Balazs Lengyel
   1117 Budapest

   Phone: +36-70-330-7909

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

   Phone: +32 2 704 5622

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