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Versions: 00 01

Network Working Group                                         A. Bierman
Internet-Draft                                                 YumaWorks
Intended status: Standards Track                            M. Bjorklund
Expires: June 3, 2013                                     Tail-f Systems
                                                       November 30, 2012


                           YANG-API Protocol
                   draft-bierman-netconf-yang-api-01

Abstract

   This document describes a RESTful protocol that provides a
   programmatic interface over HTTP for accessing data defined in YANG,
   using the datastores defined in NETCONF.

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 http://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 3, 2013.

Copyright Notice

   Copyright (c) 2012 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
   (http://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
   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.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Simple Subset of NETCONF Functionality . . . . . . . . . .  4
     1.2.  Data Model Driven API  . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
       1.3.1.  NETCONF  . . . . . . . . . . . . . . . . . . . . . . .  6
       1.3.2.  HTTP . . . . . . . . . . . . . . . . . . . . . . . . .  7
       1.3.3.  YANG . . . . . . . . . . . . . . . . . . . . . . . . .  7
       1.3.4.  Terms  . . . . . . . . . . . . . . . . . . . . . . . .  8
     1.4.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  8
       1.4.1.  Resource URI Map . . . . . . . . . . . . . . . . . . .  9
       1.4.2.  YANG-API Message Examples  . . . . . . . . . . . . . .  9
   2.  Framework  . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     2.1.  Message Model  . . . . . . . . . . . . . . . . . . . . . . 15
     2.2.  Resource Model . . . . . . . . . . . . . . . . . . . . . . 15
       2.2.1.  YANG-API Resource Types  . . . . . . . . . . . . . . . 15
       2.2.2.  Resource Discovery . . . . . . . . . . . . . . . . . . 16
     2.3.  Datastore Model  . . . . . . . . . . . . . . . . . . . . . 16
       2.3.1.  Content Model  . . . . . . . . . . . . . . . . . . . . 17
       2.3.2.  Editing Model  . . . . . . . . . . . . . . . . . . . . 17
       2.3.3.  Locking Model  . . . . . . . . . . . . . . . . . . . . 19
       2.3.4.  Persistence Model  . . . . . . . . . . . . . . . . . . 19
       2.3.5.  Defaults Model . . . . . . . . . . . . . . . . . . . . 19
     2.4.  Transaction Model  . . . . . . . . . . . . . . . . . . . . 20
     2.5.  Extensibility Model  . . . . . . . . . . . . . . . . . . . 20
     2.6.  Versioning Model . . . . . . . . . . . . . . . . . . . . . 20
     2.7.  Retrieval Filtering Model  . . . . . . . . . . . . . . . . 21
     2.8.  Access Control Model . . . . . . . . . . . . . . . . . . . 21
   3.  Operations . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     3.1.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . . . . 22
     3.2.  HEAD . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     3.3.  GET  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     3.4.  POST . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     3.5.  PUT  . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     3.6.  PATCH  . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     3.7.  DELETE . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     3.8.  Query Parameters . . . . . . . . . . . . . . . . . . . . . 28
       3.8.1.  "config" Parameter . . . . . . . . . . . . . . . . . . 28
       3.8.2.  "depth" Parameter  . . . . . . . . . . . . . . . . . . 29
       3.8.3.  "format" Parameter . . . . . . . . . . . . . . . . . . 30
       3.8.4.  "insert" Parameter . . . . . . . . . . . . . . . . . . 30
       3.8.5.  "point" Parameter  . . . . . . . . . . . . . . . . . . 31
       3.8.6.  "select" Parameter . . . . . . . . . . . . . . . . . . 32
     3.9.  Protocol Operations  . . . . . . . . . . . . . . . . . . . 32
   4.  Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
     4.1.  Request URI Structure  . . . . . . . . . . . . . . . . . . 34
     4.2.  Message Headers  . . . . . . . . . . . . . . . . . . . . . 35



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     4.3.  Message Encoding . . . . . . . . . . . . . . . . . . . . . 36
     4.4.  Return Status  . . . . . . . . . . . . . . . . . . . . . . 36
     4.5.  Message Caching  . . . . . . . . . . . . . . . . . . . . . 37
   5.  Resources  . . . . . . . . . . . . . . . . . . . . . . . . . . 38
     5.1.  API Resource (/yang-api) . . . . . . . . . . . . . . . . . 38
       5.1.1.  /yang-api/datastore  . . . . . . . . . . . . . . . . . 38
       5.1.2.  /yang-api/modules  . . . . . . . . . . . . . . . . . . 38
       5.1.3.  /yang-api/operations . . . . . . . . . . . . . . . . . 38
       5.1.4.  /yang-api/version  . . . . . . . . . . . . . . . . . . 40
     5.2.  Datastore Resource . . . . . . . . . . . . . . . . . . . . 41
     5.3.  Data Resource  . . . . . . . . . . . . . . . . . . . . . . 41
       5.3.1.  Encoding YANG Instance Identifiers in the Request
               URI  . . . . . . . . . . . . . . . . . . . . . . . . . 42
       5.3.2.  Identifying YANG-defined Data Resources  . . . . . . . 44
       5.3.3.  Identifying Optional Keys  . . . . . . . . . . . . . . 45
       5.3.4.  Data Resource Retrieval  . . . . . . . . . . . . . . . 45
     5.4.  Operation Resource . . . . . . . . . . . . . . . . . . . . 47
       5.4.1.  Encoding Operation Input Parameters  . . . . . . . . . 48
       5.4.2.  Encoding Operation Output Parameters . . . . . . . . . 49
       5.4.3.  Identifying YANG-defined Operation Resources . . . . . 50
   6.  Error Reporting  . . . . . . . . . . . . . . . . . . . . . . . 51
     6.1.  Error Response Message . . . . . . . . . . . . . . . . . . 52
   7.  RelaxNG Grammar  . . . . . . . . . . . . . . . . . . . . . . . 55
   8.  YANG-API module  . . . . . . . . . . . . . . . . . . . . . . . 56
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 58
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 59
   11. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 60
     11.1. 00-01  . . . . . . . . . . . . . . . . . . . . . . . . . . 60
   12. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 61
   13. Example YANG Module  . . . . . . . . . . . . . . . . . . . . . 63
   14. Normative References . . . . . . . . . . . . . . . . . . . . . 67
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 68



















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1.  Introduction

   There is a need for standard mechanisms to allow WEB applications to
   access the configuration data, operational data, and data-model
   specific protocol operations within a networking device, in a modular
   and extensible manner.

   This document describes a RESTful protocol called YANG-API, running
   over HTTP [RFC2616], for accessing data defined in YANG [RFC6020],
   using datastores defined in NETCONF [RFC6241].

   The NETCONF protocol defines configuration datastores and a set of
   Create, Retrieve, Update, Delete (CRUD) operations that can be used
   to access these datastores.  The YANG language defines the syntax and
   semantics of datastore content and operational data.  RESTful
   operations are used to access the hierarchical data within a
   datastore.

   A RESTful API can be created that provides CRUD operations on a
   NETCONF datastore containing YANG-defined data.  This can be done in
   a simplified manner, compatible with HTTP and RESTful design
   principles.  Since NETCONF protocol operations are not relevant, the
   user should not need any prior knowledge of NETCONF in order to use
   the RESTful API.

   Configuration data and state data are exposed as resources that can
   be retrieved with the GET method.  Resources representing
   configuration data can be modified with the DELETE, PATCH, POST, and
   PUT methods.  Data-model specific protocol operations defined with
   the YANG "rpc" statement can be invoked with the POST method.

1.1.  Simple Subset of NETCONF Functionality

   The framework and meta-model used for a RESTful API does not need to
   mirror those used by the NETCONF protocol.  It just needs to be
   compatible with NETCONF.  A simplified framework and protocol is
   needed that utilizes the three NETCONF datastores (candidate,
   running, startup), but hides the complexity of multiple datastores
   from the client.

   A simplified transaction model is needed that allows basic CRUD
   operations on a hierarchy of conceptual resources.  This represents a
   limited subset of the transaction capabilities of the NETCONF
   protocol.

   Applications that require more complex transaction capabilities might
   consider NETCONF instead of YANG-API.  The following transaction
   features are not provided in YANG-API:



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   o  datastore locking (full or partial)

   o  candidate datastore

   o  validate operation

   o  confirmed-commit procedure

   The RESTful API is not intended to replace NETCONF, but rather
   provide an additional simplified interface that follows RESTful
   principles and is compatible with a resource-oriented device
   abstraction.  It is expected that applications that need the full
   feature set of NETCONF such as notifications will continue to use
   NETCONF.

   The following figure shows the system components:

         +-----------+           +-----------------+
         |  WEB app  | <-------> |                 |
         +-----------+   HTTP    | network device  |
                                 |                 |
         +-----------+           |   +-----------+ |
         |  NMS app  | <-------> |   | datastore | |
         +-----------+  NETCONF  |   +-----------+ |
                                 +-----------------+

1.2.  Data Model Driven API

   YANG-API combines the simplicity of a RESTful API over HTTP with the
   predictability and automation potential of a schema-driven API.

   A RESTful client using YANG-API will not use any data modelling
   language to define the application-specific content of the API.  The
   client would discover each new child resource as it traverses the
   URIs return as Location IDs to discover the server capabilities.

   This approach has 3 significant weaknesses wrt/ control of complex
   networking devices:

   o  inefficient performance: configuration APIs will be quite complex
      and may require thousands of protocol messages to discover all the
      schema information.  Typically the data type information has to be
      passed in the protocol messages, which is also wasteful overhead.

   o  no data model richness: without a data model, the schema-level
      semantics and validation constraints are not available to the
      application.  Data model modules such as YANG modules serve as an
      "API contract" that will be honored by the server.  An application



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      designer can code to the data model, knowing in advance important
      details about the exact protocol operations and datastore content
      a conforming server implementation will support.

   o  no tool automation: API automation tools need some sort of content
      schema to function.  Such tools can automate various programming
      and documentation tasks related to specific data models.

   YANG-API provides the YANG module capability information supported by
   the server, in case the client wants to use it.  The URIs for custom
   protocol operations and datastore content are predictable, based on
   the YANG module definitions.  Note that the YANG modules and
   predictable URIs are optional to use by the client.  They can be
   completely ignored without any loss of protocol functionality.

   Operational experience with CLI and SNMP indicates that operators
   learn the 'location' of specific service or device related data and
   do not expect such information to be arbitrary and discovered each
   time the client opens a management session to a server.

1.3.  Terminology

   The keywords "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, [RFC2119].

1.3.1.  NETCONF

   The following terms are defined in [RFC6241]:

   o  candidate configuration datastore

   o  client

   o  configuration data

   o  datastore

   o  configuration datastore

   o  protocol operation

   o  running configuration datastore

   o  server





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   o  startup configuration datastore

   o  state data

   o  user

1.3.2.  HTTP

   The following terms are defined in [RFC2616]:

   o  entity tag

   o  fragment

   o  header line

   o  message body

   o  method

   o  path

   o  query

   o  request URI

   o  response body

1.3.3.  YANG

   The following terms are defined in [RFC6020]:

   o  container

   o  data node

   o  key leaf

   o  leaf

   o  leaf-list

   o  list

   o  presence container (or P-container)

   o  RPC operation (now called protocol operation)




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   o  non-presence container (or NP-container)

   o  ordered-by system

   o  ordered-by user

1.3.4.  Terms

   The following terms are used within this document:

   o  API resource: a resource with the media type "application/
      vnd.yang.api+xml" or ""application/vnd.yang.api+json".

   o  data resource: a resource with the media type "application/
      vnd.yang.data+xml" or "application/vnd.yang.data+json".

   o  datastore resource: a resource with the media type "application/
      vnd.yang.datastore+xml" or "application/vnd.yang.datastore+json"

   o  edit operation: a YANG-API operation on a data resource using the
      POST, PUT, PATCH, or DELETE method.

   o  operation: the conceptual YANG-API operation for a message,
      derived from the method, request URI, headers, and message body.

   o  operation resource: a resource with the media type
      "vnd.yang.operation+xml" or "vnd.yang.operation+json"

   o  optional key: a key leaf for a YANG list data node, which MAY be
      omitted by the client when an instance of the list is created.

   o  query parameter: a parameter (and its value if any), encoded
      within the query portion of the request URI.

   o  resource: a conceptual object representing a manageable component
      within a device.

   o  retrieval request: an operation using the GET or HEAD methods.

   o  target resource: the resource that is associated with a particular
      message, identified by the "path" component of the request URI.

1.4.  Overview

   This document defines the YANG-API protocol, a RESTful API for
   accessing conceptual datastores containing data defined with YANG
   language.  YANG-API provides an application framework and meta-model,
   using HTTP operations.



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   The YANG-API resources are accessed via a set of URIs defined in this
   document.  The set of YANG modules supported by the server will
   determine the additional data model specific operations and top-level
   data node resources available on the server.

1.4.1.  Resource URI Map

   The URI hierarchy for the YANG-API resources consists of an entry
   point and up to 4 top-level resources and/or fields.  Refer to
   Section 5 for details on each URI.

     /yang-api
        /datastore
           /<top-level-data-nodes> (config=true or false)
        /modules
           /module
        /operations
           /<custom protocol operations>
        /version

1.4.2.  YANG-API Message Examples

   The examples within this document use the non-normative example YANG
   module defined in Section 13.

   This section shows some typical YANG-API message exchanges.

1.4.2.1.  Retrieve the Top-level API Resource

   By default, when a resource is retrieved, all of its fields are
   returned, but none (if any) of the nested resources are returned.
   Also, the default encoding is JSON.  Data resources are encoded
   according to the encoding rules in [I-D.lhotka-netmod-json].

   The client starts by retrieving the top-level API resource, using the
   entry point URI "/yang-api".

      GET /yang-api HTTP/1.1
      Host: example.com

   The server might respond as follows.  The "module" lines below are
   split for display purposes only:









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      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:01:00 GMT
      Server: example-server
      Content-Type: application/vnd.yang.api+json

      {
        "yang-api": {
          "modules": {
            "module": [
              "urn:ietf:params:xml:ns:yang:ietf-yang-api
                 ?module=ietf-yang-api&revision=2012-05-27",
              "example.com?module=example-jukebox
                 &revision=2012-05-30"
            ]
          },
          "version": "1.0"
        }
      }

   To request that the response content to be encoded in XML, the
   "Accept" header can be used, as in this example request:

      GET /yang-api HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.api+xml

   An alternate approach is provided using the "format" query parameter,
   as in this example request:

      GET /yang-api?format=xml HTTP/1.1
      Host: example.com

   The server will return the same response either way, which might be
   as follows :

      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:01:00 GMT
      Server: example-server
      Cache-Control: no-cache
      Pragma: no-cache
      Content-Type: application/vnd.yang.api+xml










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      <yang-api>
        <modules> <!-- wrapped for display only -->
          <module>urn:ietf:params:xml:ns:yang:ietf-yang-api
             ?module=ietf-yang-api
             &amp;revision=2012-05-27</module>
          <module>example.com?module=example-jukebox
              &amp;revision=2012-05-30</module>
        </modules>
        <version>1.0</version>
      </yang-api>

   Refer to Section 3.3 for details on the GET operation.

1.4.2.2.  Create New Data Resources

   To create a new "jukebox" resource, the client might send:

      POST /yang-api/datastore/jukebox HTTP/1.1
      Host: example.com

   If the resource is created, the server might respond:

      HTTP/1.1 201 Created
      Date: Mon, 23 Apr 2012 17:01:00 GMT
      Server: example-server
      Location: http://example.com/yang-api/datastore/jukebox
      Last-Modified: Mon, 23 Apr 2012 17:01:00 GMT
      ETag: b3a3e673be2

   To create a new "artist" resource within the "jukebox" resource, the
   client might send the following request, Note that the arbitrary
   integer "index" is not provided, since it is an optional key:

      POST /yang-api/datastore/jukebox/artist HTTP/1.1
      Host: example.com
      Content-Type: application/vnd.yang.data+json

      {
        "artist" : {
          "name" : "The Foo Fighters"
        }
      }

   If the resource is created, the server might respond:







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      HTTP/1.1 201 Created
      Date: Mon, 23 Apr 2012 17:02:00 GMT
      Server: example-server
      Location: http://example.com/yang-api/datastore/jukebox/artist/1
      Last-Modified: Mon, 23 Apr 2012 17:02:00 GMT
      ETag: b3830f23a4c

   To create a new "album" resource for this artist within the "jukebox"
   resource, the client might send the following request,

      POST /yang-api/datastore/jukebox/artist/1/album HTTP/1.1
      Host: example.com
      Content-Type: application/vnd.yang.data+json

      {
        "album" : {
          "name" : "Wasting Light",
          "genre" : "example-jukebox:Alternative",
          "year" : 2012
        }
      }

   If the resource is created, the server might respond as follows.
   Note that the "Location" header line is wrapped for display purposes
   only:

      HTTP/1.1 201 Created
      Date: Mon, 23 Apr 2012 17:03:00 GMT
      Server: example-server
      Location: http://example.com/yang-api/datastore/
        jukebox/artist/1/album/Wasting%20Light
      Last-Modified: Mon, 23 Apr 2012 17:03:00 GMT
      ETag: b8389233a4c

   Refer to Section 3.4 for details on the POST operation.

1.4.2.3.  Replace an Existing Data Resource

   Note: replacing a resource is a fairly drastic operation.  The PATCH
   operation is often more appropriate.

   The album sub-resource is re-added here for example purposes only.
   To replace the "artist" resource contents, the client might send:








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      PUT /yang-api/datastore/jukebox/artist/1 HTTP/1.1
      Host: example.com
      If-Match: b3830f23a4c
      Content-Type: application/vnd.yang.data+json

      {
        "artist" : {
          "name" : "Foo Fighters",
          "album" : {
            "name" : "Wasting Light",
            "genre" : "example-jukebox:Alternative",
            "year" : 2012
          }
        }
      }

   If the resource is updated, the server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:04:00 GMT
      Server: example-server
      Last-Modified: Mon, 23 Apr 2012 17:04:00 GMT
      ETag: b27480aeda4c

   Refer to Section 3.5 for details on the PUT operation.

1.4.2.4.  Patch an Existing Data Resource

   To replace just the "year" field in the "album" resource, the client
   might send:

      PATCH /yang-api/datastore/jukebox/artist/1/album/
         Wasting%20Light/year HTTP/1.1
      Host: example.com
      If-Match: b8389233a4c
      Content-Type: application/vnd.yang.data+json

      { "year" : 2011 }

   If the resource is updated, the server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:49:30 GMT
      Server: example-server
      Last-Modified: Mon, 23 Apr 2012 17:49:30 GMT
      ETag: b2788923da4c

   Refer to Section 3.6 for details on the PATCH operation.



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1.4.2.5.  Delete an Existing Data Resource

   To delete a resource such as the "album" resource, the client might
   send:

      DELETE /yang-api/datastore/jukebox/artist/1/album/
         Wasting%20Light HTTP/1.1
      Host: example.com

   If the resource is deleted, the server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:49:40 GMT
      Server: example-server

   Refer to Section 3.7 for details on the DELETE operation.

1.4.2.6.  Invoke a Data Model Specific Operation

   To invoke a data-model specific operation via an operation resource,
   the POST operation is used.  A client might send a "backup-datastore"
   request as follows:

      POST /yang-api/operations/backup-datastore HTTP/1.1
      Host: example.com

   The server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:50:00 GMT
      Server: example-server

   Refer to Section 3.9 for details on using the POST operation with
   operation resources.

















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2.  Framework

   The YANG-API protocol defines a framework that can be used to
   implement a common API for configuration management.  This section
   describes the components of the YANG-API framework.

2.1.  Message Model

   The YANG-API protocol uses HTTP entities for messages.  A single HTTP
   message corresponds to a single protocol operation.  A message can
   perform a single task on a single resource, such as retrieving a
   resource or editing a resource.  It cannot be used to combine
   multiple tasks.  The client cannot provide multiple (possibly
   unrelated) edit operations within a single request, like the NETCONF
   <edit-config> protocol operation.

2.2.  Resource Model

   The YANG-API protocol operates on a hierarchy of resources, starting
   with the top-level API resource itself.  Each resource represents a
   manageable component within the device.

   A resource can be considered a collection of conceptual data and the
   set of allowed operations on that data.  It can contain child nodes
   that are either "fields" or other resources.  The child resource
   types and operations allowed on them are data-model specific.

   A resource has its own media type identifier, represented by the
   "Content-Type" header in the HTTP response message.  A resource can
   contain zero or more fields and zero or more resources.  A resource
   can be created and deleted independently of its parent resource, as
   long as the parent resource exist.

   A field is a child node defined within a resource.  A field can
   contain zero or more fields and zero or more resources.  A field
   cannot be created and deleted independently of its parent resource.

   All YANG-API resources and fields are defined in this document except
   datastore contents and protocol operations.  These resource types are
   defined with YANG data definition statements and the "rpc" statement.
   A default mapping is defined to differentiate sub-resources from
   fields within data resources.

2.2.1.  YANG-API Resource Types

   The YANG-API protocol defines some application specific media types
   to identify each of the available resource types.  The following
   table summarizes the purpose of each resource.



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              +-----------+--------------------------------+
              | Resource  | Media Type                     |
              +-----------+--------------------------------+
              | API       | application/vnd.yang.api       |
              | Datastore | application/vnd.yang.datastore |
              | Data      | application/vnd.yang.data      |
              | Operation | application/vnd.yang.operation |
              +-----------+--------------------------------+

                           YANG-API Media Types

   These resources are described in Section 5.

2.2.2.  Resource Discovery

   A client SHOULD start by retrieving the top-level API resource, using
   the entry point URI "/yang-api".

   The YANG-API protocol does not include a resource discovery
   mechanism.  Instead, the definitions within the YANG modules
   advertised by the server are used to construct a predictable
   operation or data resource identifier.

   The "depth" query parameter can be used to control how many
   descendant levels should be included when retrieving sub-resources.
   This parameter can be used with the GET operation to discover sub-
   resources within a particular resource.

   Refer to Section 3.8.2 for more details on the "depth" parameter.

2.3.  Datastore Model

   A conceptual "unified datastore" is used to simplify resource
   management for the client.  The YANG-API datastore is a combination
   of the running configuration and any non-configuration data supported
   by the device.  By default only configuration data is returned by a
   GET operation on the datastore contents.

   The underlying NETCONF datastores can be used to implement the
   unified datastore, but the server design is not limited to the exact
   datastore procedures defined in NETCONF.

   The "candidate" and "startup" datastores are not visible in the YANG-
   API protocol.  Transaction management and configuration persistence
   are handled by the server and not controlled by the client.






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2.3.1.  Content Model

   The YANG-API protocol operates on a conceptual datastore defined with
   the YANG data modeling language.  The server lists each YANG module
   it supports in the "/yang-api/modules/module" field in the top-level
   API resource type, using the YANG module capability URI format
   defined in RFC 6020.

   The conceptual datastore contents and data-model-specific operations
   are identified by the set of YANG module capability URIs.  All YANG-
   API content identified as either a data resource or an operation
   resource is defined with the YANG language.

   The classification of data as configuration or non-configuration is
   derived from the YANG "config" statement.  Data retrieval with the
   GET operation can be filtered in several ways, including the "config"
   parameter to retrieve configuration or non-configuration data.

   The classification of data as a resource or field within a resource
   is derived from the rules specified in Section 5.3.2.

   Data ordering behavior is derived from the YANG "ordered-by"
   statement.  Editing mechanisms are provided to allow list or leaf-
   list resources to be inserted or moved in the same manner as NETCONF,
   and defined in YANG.

   The server is not required to maintain system ordered data in any
   particular persistent order.  The server SHOULD maintain the same
   data ordering for system ordered data until the next reboot or
   termination of the server.

2.3.2.  Editing Model

   The YANG-API datastore editing model is simple and direct, similar to
   the behavior of the ":writable-running" capability in NETCONF.

   Each YANG-API edit of a datastore resource is activated upon
   successful completion of the transaction.  It is an implementation-
   specific matter how the server accomplishes a YANG-API edit request.
   For example, a server which only accepts edits through a candidate
   datastore may internally edit this datastore and perform the "commit"
   operation automatically.

   Applications which need more control over the editing model might
   consider using NETCONF instead of YANG-API.






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2.3.2.1.  Edit Operation Discovery

   Sometimes a server does not implement every operation for every
   resource.  Sometimes data model requirements cause a node to
   implement a subset of the edit operations.  For example, a server may
   not allow modification of a particular configuration data node after
   the parent resource has been created.

   The OPTIONS operation can be used to identify which operations are
   supported by the server for a particular resource.  For example, if
   the server will allow a data resource node to be created then the
   POST operation will be returned in the response.

2.3.2.2.  Edit Collision Detection

   Two "edit collision detection" mechanisms are provided in YANG-API,
   for datastore and data resources.

   o  timestamp: the last change time is maintained and the
      "Last-Modified" and "Date" headers are returned in the response
      for a retrieval request.  The "If-Unmodified-Since" header can be
      used in edit operation requests to cause the server to reject the
      request if the resource has been modified since the specified
      timestamp.

   o  entity tag: a unique opaque string is maintained and the "ETag"
      header is returned in the response for a retrieval request.  The
      "If-Match" header can be used in edit operation requests to cause
      the server to reject the request if the resource entity tag does
      not match the specified value.

   Note that the server is only required to maintain these fields for a
   datastore resource, not for individual data resources.

   Example:

   In this example, the server just supports the mandatory datastore
   last-changed timestamp.  The client has previously retrieved the
   "Last-Modified" header and has some value cached to provide in the
   following request to replace a list entry with key value "11":

      PATCH /yang-api/datastore/jukebox/artist/1/album/
        Wasting%20Light/year HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.data+json
      If-Unmodified-Since: Mon, 23 Apr 2012 17:01:00 GMT
      Content-Type: application/vnd.yang.data+json




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      { "year" : "2011" }

   In this example the datastore resource has changed since the time
   specified in the "If-Unmodified-Since" header.  The server might
   respond:

      HTTP/1.1 304 Not Modified
      Date: Mon, 23 Apr 2012 19:01:00 GMT
      Server: example-server
      Last-Modified: Mon, 23 Apr 2012 17:45:00 GMT
      ETag: b34aed893a4c

2.3.3.  Locking Model

   Datastore locking is not provided by YANG-API.  An application that
   needs to make several changes to the running configuration datastore
   contents in sequence, without disturbance from other clients might
   consider using the NETCONF protocol instead of YANG-API.

2.3.4.  Persistence Model

   Each YANG-API edit of a datastore resource is saved to non-volatile
   storage in an implementation-specific matter by the server.  There is
   no guarantee that configuration changes are saved immediately, or
   that the saved configuration is always a mirror of the running
   configuration.

   Applications which need more control over the persistence model might
   consider using NETCONF instead of YANG-API.

2.3.5.  Defaults Model

   NETCONF has a rather complex defaults handling model for leafs.
   YANG-API attempts to avoid this complexity by restricting the
   operations that can be applied to a resource and fields within that
   resource.

   The GET method returns only nodes that exist, which will be
   determined by the server.  There is no mechanism for the client to
   ask the server for the default values that would be used for any
   nodes not present, but some default value is in use by the server.
   (There is no

    retrieval mode like "with-defaults=report-all" in NETCONF.)

   If a leaf definition has a default value, and the leaf has not been
   given a value yet, the server SHOULD NOT return any value for the
   leaf in the response for a GET operation.



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   Applications which need more control over the defaults model might
   consider using NETCONF instead of YANG-API.

2.4.  Transaction Model

   The YANG-API protocol does not provide a complex transaction model
   that allows for multiple protocol operations, or even operations on
   multiple resources in one protocol operation.  A very simple "one
   operation one one resource" per transaction model is used instead.

   Applications which need more control over the transaction model might
   consider using NETCONF instead of YANG-API.

2.5.  Extensibility Model

   The YANG-API protocol is designed to be extensible for datastore
   content and data-model specific protocol operations.  New protocol
   operations can be added without changing the entry point if they are
   optional and do not alter any existing operations.

   Separate namespaces for each YANG module are used.  Content encoded
   in XML will indicate the module using the "namespace" URI value in
   the YANG module.  Content encoded in JSON will indicate the module
   using the module name specified in the YANG module.  JSON encoding
   rules for module namespaces are specified in
   [I-D.lhotka-netmod-json].

2.6.  Versioning Model

   The version of a resource instance is identified with an entity tag,
   as defined by HTTP.  The version identifiers in this section apply to
   the version of the schema definition of a resource.  There are two
   types of schema versioning information used in the YANG-API protocol:

   o  the YANG-API protocol version

   o  data and operation resource definition versions

   The protocol version is identified by the string used for the well-
   known URI entry point "/yang-api".  This would be changed (e.g.,
   "/yang-api2") if non-backward compatible changes are ever needed.
   Minor version changes that do not break backward-compatibility will
   not cause the entry point to change.

   The API "yang-api/version" field can be used by the client to
   identify the exact version of the YANG-API protocol implemented by
   the server.  This value will include the complete YANG-API protocol
   version.  The "/yang-api" entry point will only change (e.g.,



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   "/yang-api2") if non-backward compatible changes are made to the
   protocol.  The "/yang-api/version" field MUST be updated every time
   the protocol specification is republished.

   The resource definition version for a data or operation resource is a
   date string, which is the revision date of the YANG module that
   defines the resource.  The resource version for all other resource
   types is a numeric string, defined by the "/yang-api/version" field.

2.7.  Retrieval Filtering Model

   There are four types of filtering for retrieval of data resources in
   the YANG-API protocol.

   o  conditional all-or-nothing: use some conditional test mechanism in
      the request headers and retrieve either a complete "200 OK"
      response if the condition is met, or a "304 Not Modified" Status-
      Line if the condition is not met.

   o  data classification: request configuration or non-configuration
      data.

   o  subset: request a subset of all possible instances of a list or
      leaf-list data resource.

   o  filter: request a subset of all possible descendant nodes within
      the target resource.  The "select" query parameter can be used for
      this purpose.

   Refer to Section 5.3.4 for details on data retrieval filtering.

2.8.  Access Control Model

   The YANG-API protocol provides no granular access control for any
   content except for operation and data resources.  The NETCONF Access
   Control Model (NACM) is defined in [RFC6536].  There is a specific
   mapping between YANG-API operations and NETCONF edit operations,
   defined in Table 1.  The resource path also needs to be converted
   internally by the server to the corresponding YANG instance-
   identifier.  Using this information, the server can apply the NACM
   access control rules to YANG-API messages.

   The server MUST NOT allow any operation to any resources that the
   client is not authorized to access.







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3.  Operations

   The YANG-API protocol uses HTTP methods to identify the CRUD
   operation requested for a particular resource or field within a
   resource.  The following table shows how the YANG-API operations
   relate to NETCONF protocol operations:

            +----------+-------------------------------------+
            | YANG-API | NETCONF                             |
            +----------+-------------------------------------+
            | OPTIONS  | none                                |
            | HEAD     | none                                |
            | GET      | <get-config>, <get>                 |
            | POST     | <edit-config> (operation="create")  |
            | PUT      | <edit-config> (operation="replace") |
            | PATCH    | <edit-config> (operation="merge")   |
            | DELETE   | <edit-config> (operation="delete")  |
            +----------+-------------------------------------+

                   Table 1: CRUD Operations in YANG-API

   The NETCONF "remove" operation attribute is not supported by the HTTP
   DELETE method.  The resource must exist or the DELETE operation will
   fail.

   This section defines the YANG-API protocol usage for each HTTP
   method.

3.1.  OPTIONS

   The OPTIONS method is sent by the client to discover which methods
   are supported by the server for a specific resource, or field within
   a resource.  It is supported for all media types.  Note that
   implementation of this operation is part of HTTP, and this section
   does not introduce any additional requirements.

   The request MUST contain a request URI that contains at least the
   entry point component.

   The server will return a "Status-Line" header containing "204 No
   Content". and include the "Allow" header in the response.  This
   header will be filled in, based on the target resource media type.
   Other headers MAY also be included in the response.

   Example 1:

   A client might request the methods supported for a data resource
   called "library"



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      OPTIONS /yang-api/datastore/jukebox/library HTTP/1.1
      Host: example.com

   The server might respond (for a config=true list):

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:01:00 GMT
      Server: example-server
      Allow: OPTIONS,HEAD,GET,POST,PUT,PATCH,DELETE

   Example 2:

   A client might request the methods supported for a non-configuration
   leaf within a data resource:

      OPTIONS /yang-api/datastore/jukebox/library/
          song-count HTTP/1.1
      Host: example.com

   The server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:02:00 GMT
      Server: example-server
      Allow: OPTIONS,HEAD,GET

   Example 3:

   A client might request the methods supported for an operation
   resource called "play":

      OPTIONS /yang-api/operations/play HTTP/1.1
      Host: example.com

   The server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 17:02:00 GMT
      Server: example-server
      Allow: POST

3.2.  HEAD

   The HEAD operation is sent by the client to retrieve just the headers
   that would be returned for the comparable GET operation, without the
   response body.  The HTTP HEAD method is used for this operation.  It
   is supported for all resource types, except operation resources.




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   The request MUST contain a request URI that contains at least the
   entry point component.

   The same query parameters supported by the GET operation are
   supported by the HEAD operation.  For example, the "select" query
   parameter can be used to specify a field within the target resource.

   The access control behavior is enforced as if the method was GET
   instead of HEAD.  The server MUST respond the same as if the method
   was GET instead of HEAD, except that no response body is included.

   Example:

   The client might request the response headers for the default (JSON)
   representation of the "library" resource:

      HEAD /yang-api/datastore/jukebox/library HTTP/1.1
      Host: example.com

   The server might respond:

      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:02:40 GMT
      Server: example-server
      Content-Type: application/vnd.yang.data+json
      Cache-Control: no-cache
      Pragma: no-cache
      ETag: a74eefc993a2b
      Last-Modified: Mon, 23 Apr 2012 11:02:14 GMT

3.3.  GET

   The GET operation is sent by the client to retrieve data and meta-
   data for a resource or field within a resource.  The HTTP GET method
   is used for this operation.  It is supported for all resource types,
   except operation resources.  The request MUST contain a request URI
   that contains at least the entry point component.

   The following query parameters are supported by the GET operation:

   +--------+---------+------------------------------------------------+
   | Name   | Section | Description                                    |
   +--------+---------+------------------------------------------------+
   | config | 3.8.1   | Request either configuration or                |
   |        |         | non-configuration data                         |
   | depth  | 3.8.2   | Control the depth of a retrieval request       |
   | format | 3.8.3   | Request either JSON or XML content in the      |
   |        |         | response                                       |



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   | select | 3.8.6   | Specify a field within the target resource     |
   +--------+---------+------------------------------------------------+

                           GET Query Parameters

   The server MUST NOT return any data resources or fields within any
   data resources for which the user does not have read privileges.

   If the user is not authorized to read any portion of the target
   resource, an error response containing a "403 Forbidden" Status-Line
   is returned to the client.

   If the user is authorized to read some but not all of the target
   resource, the unauthorized content is omitted from the response
   message body, and the authorized content is returned to the client.

   Example:

   The client might request the response headers for a JSON
   representation of the "library" resource:

      GET /yang-api/datastore/jukebox/library/artist/
        1/album HTTP/1.1
      Host: example.com

   The server might respond:

      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:02:40 GMT
      Server: example-server
      Content-Type: application/vnd.yang.data+json
      Cache-Control: no-cache
      Pragma: no-cache
      ETag: a74eefc993a2b
      Last-Modified: Mon, 23 Apr 2012 11:02:14 GMT

      {
        "album" : {
          "name" : "Wasting Light",
          "genre" : "example-jukebox:Alternative",
          "year" : 2011
        }
      }








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3.4.  POST

   The POST operation is sent by the client for various reasons.  The
   HTTP POST method is used for this purpose.  The request MUST contain
   a request URI that contains a target resource that identifies one of
   the following resource types:

          +-------------+--------------------------------------+
          | Type        | Description                          |
          +-------------+--------------------------------------+
          | Data        | Create a configuration data resource |
          | Operation   | Invoke protocol operation            |
          | Transaction | Create a new transaction             |
          +-------------+--------------------------------------+

                     Resource Types that Support POST

   The following query parameters are supported by the POST operation:

      +--------+---------+-----------------------------------------+
      | Name   | Section | Description                             |
      +--------+---------+-----------------------------------------+
      | insert | 3.8.4   | Specify where to insert a resource      |
      | point  | 3.8.5   | Specify the insert point for a resource |
      +--------+---------+-----------------------------------------+

                           POST Query Parameters

   If the POST operation succeeds, a "200 OK" Status-Line is returned if
   there is no response message body, and a "204 No Content" Status-Line
   is returned if there is a response message body.

   If the user is not authorized to invoke the target (operation)
   resource, or create the target resource, an error response containing
   a "403 Forbidden" Status-Line is returned to the client.  All other
   error responses are handled according to the procedures defined in
   Section 6.

3.5.  PUT

   The PUT operation is sent by the client to replace the target
   resource.

   The HTTP PUT method is used for this purpose.  The request MUST
   contain a request URI that contains a target resource that identifies
   the data resource to replace.

   The following query parameters are supported by the PUT operation:



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       +--------+---------+---------------------------------------+
       | Name   | Section | Description                           |
       +--------+---------+---------------------------------------+
       | insert | 3.8.4   | Specify where to move a resource      |
       | point  | 3.8.5   | Specify the move point for a resource |
       +--------+---------+---------------------------------------+

                           PUT Query Parameters

   If the PUT operation succeeds, a "200 OK" Status-Line is returned,
   and there is no response message body.

   If the user is not authorized to replace the target resource an error
   response containing a "403 Forbidden" Status-Line is returned to the
   client.  All other error responses are handled according to the
   procedures defined in Section 6.

3.6.  PATCH

   The PATCH operation uses the HTTP PATCH method defined in [RFC5789]
   to provide a "merge" editing mode for data resources.  Instead of
   replacing all or part of the target resource, the supplied values are
   merged into the target resource.

   If the PATCH operation succeeds, a "200 OK" Status-Line is returned,
   and there is no response message body.

   If the user is not authorized to alter the target resource an error
   response containing a "403 Forbidden" Status-Line is returned to the
   client.  All other error responses are handled according to the
   procedures defined in Section 6.

3.7.  DELETE

   The DELETE operation uses the HTTP DELETE method to delete the target
   resource.

   If the DELETE operation succeeds, a "200 OK" Status-Line is returned,
   and there is no response message body.

   If the user is not authorized to delete the target resource then an
   error response containing a "403 Forbidden" Status-Line is returned
   to the client.  All other error responses are handled according to
   the procedures defined in Section 6.







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3.8.  Query Parameters

   Each YANG-API operation allows zero or more query parameters to be
   present in the request URI.  Refer to Section 3 for details on the
   query parameters used in the definition of each operation.

   Query parameters can be given in any order.  Each parameter can
   appear zero or one time.  A default value may apply if the parameter
   is missing.

   This section defines all the YANG-API query parameters.

3.8.1.  "config" Parameter

   The "config" parameter is used to specify whether configuration or
   non-configuration data is requested.

   This parameter is only supported for the GET and HEAD methods.  It is
   also only supported if the target resource is a data resource.

        syntax: config= true | false
        default: true

   Example:

   This example request by the client would retrieve only the non-
   configuration data nodes that exist within the second-level "library"
   resource.

      GET /yang-api/datastore/jukebox/library?config=false HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.data+xml

   The server might respond:

      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:01:30 GMT
      Server: example-server
      Cache-Control: no-cache
      Pragma: no-cache
      Content-Type: application/vnd.yang.data+json

      {
        "library" : {
           "artist-count" : 42,
           "album-count" : 59,
           "song-count" : 374
        }



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      }

3.8.2.  "depth" Parameter

   The "depth" parameter is used to specify the number of nest levels
   returned in a response for a GET operation.  A nest-level consists of
   the target resource and any child nodes which are optional data nodes
   (anyxml, leaf, or leaf-list).  A non-presence container is
   transparent when determining the nest level.  A child node (which is
   not a non-presence container) within a non-presence container is used
   to determine the nest-level.

   The start level is determined by the target resource for the
   operation.

        syntax: depth=<range: 1..max> | unbounded
        default: 1

   Example:

   This example operation would retrieve 2 levels of configuration data
   nodes that exist within the top-level "jukebox" resource.

      GET /yang-api/datastore/jukebox?depth=2 HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.data+json

   The server might respond:

      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:11:30 GMT
      Server: example-server
      Cache-Control: no-cache
      Pragma: no-cache
      Content-Type: application/vnd.yang.data+json

      {
        "jukebox" : {
          "library" : {
            "artist" : {
              "index" : 1,
              "name" : "Foo Fighters"
            }
          },
          "player" : {
            "gap" : 0.5
          }
        }



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      }

3.8.3.  "format" Parameter

   The "format" parameter is used to specify the format of any content
   returned in the response.  Note that the "Accept" header MAY be used
   instead of this parameter to identify the format desired in the
   response.  For example:

      GET /yang-api/datastore/routing HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.data+xml

   This example request would retrieve only the configuration data nodes
   that exist within the top-level "routing" resource, and retrieve them
   in XML encoding instead of JSON encoding.

   The "format" parameter is only supported for the GET and HEAD
   methods.  It is supported for all YANG-API media types.

        syntax: format= xml | json
        default: json

   Example:

      GET /yang-api/datastore/routing?format=xml HTTP/1.1
      Host: example.com

   This example URI would retrieve only the configuration data nodes
   that exist within the top-level "routing" resource, and retrieve them
   in XML encoding instead of JSON encoding.

3.8.4.  "insert" Parameter

   The "insert" parameter is used to specify how a resource should be
   inserted (or moved) within the user-ordered list or leaf-list data
   resource.

   This parameter is only supported for the POST and PUT methods.  It is
   also only supported if the target resource is a data resource, and
   that data represents a YANG list or leaf-list that is ordered by the
   user, not the system.

   If the values "before" or "after" are used, then a "point" parameter
   for the insertion parameter MUST also be present.

        syntax: insert= first | last | before | after
        default: last



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   Example:

    Request from client:

      POST /yang-api/datastore/jukebox/library/artist/1/album
         /Wasting%20Light/song?insert=first HTTP/1.1
      Host: example.com
      Content-Type: application/vnd.yang.data+json

      {
        "song" : {
           "name" : "Bridge Burning",
           "location" : "/media/bridge_burning.mp3",
           "format" : "MP3",
           "length" : 286
        }
      }

    Response from server:  201 status

      HTTP/1.1 201 Created
      Date: Mon, 23 Apr 2012 13:01:20 GMT
      Server: example-server
      Last-Modified: Mon, 23 Apr 2012 13:01:20 GMT

      Location: http://example.com/yang-api/datastore/jukebox
         /library/artist/1/album?Wasting%20Light/song/1
      ETag: eeeada438af

3.8.5.  "point" Parameter

   The "point" parameter is used to specify the insertion point for a
   data resource that is being created or moved within a user ordered
   list or leaf-list.  It is ignored unless the "insert" query parameter
   is also present, and has the value "before" or "after".

   This parameter contains the instance identifier of the resource, or
   field within a resource, to be used as the insertion point for a POST
   or PUT operation.  It is encoded according to the rules defined in
   Section 5.3.1.  There is no default for this parameter.

      syntax: point= <instance-identifier of insertion point node>

   Example:

   In this example, the client is moving an existing "song" resource
   within an "album" resource after another song.  The request URI is
   split for display purposes only.



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    Request from client:

      PUT /yang-api/datastore/jukebox/library/artist/1/album/
        Wasting%20Light/song/2?insert=after
        &point=/yang-api/datastore/jukebox/library/artist/1/
        album/Wasting%20Light/song/4   HTTP/1.1
      Host: example.com

    Response from server:

      HTTP/1.1 204 No Content
      Date: Mon, 23 Apr 2012 13:01:20 GMT
      Server: example-server
      Last-Modified: Mon, 23 Apr 2012 13:01:20 GMT
      ETag: abcada438af

3.8.6.  "select" Parameter

   The "select" query parameter is used to specify an expression which
   can represent a subset of all data nodes within the target resource.
   It contains a relative path expression, using the target resource as
   the context node.

   It is supported for all resource types except operation resources.
   The contents are encoded according to the "api-select" rule defined
   in Section 5.3.1.  This parameter is only allowed for GET and HEAD
   operations.

   [FIXME: the syntax of the select string is still TBD; XPath, schema-
   identifier, regular expressions, something else]

   Refer to Section 1.4.2 for example request messages using the
   "select" parameter.

3.9.  Protocol Operations

   The YANG-API also allows data-model specific protocol operations to
   be invoked using the POST method.  The media type
   "vnd.yang.operation+xml" or "vnd.yang.operation+json" MUST be used in
   the "Content-Type" field in the message header.

   Data model specific operations are supported.  The syntax and
   semantics of these operations exactly correspond to the YANG "rpc"
   statement definition for the operation.

   Any input for a protocol operation is encoded in an element called
   "input", which corresponds to the <input> element in a NETCONF
   message.  The child nodes of the "input" element are encoded



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   according to the data definition statements in the input section of
   the "rpc" statement.

   Any output for a protocol operation is encoded in an element called
   "output", which corresponds to the <rpc-reply> element in a NETCONF
   message.  The child nodes of the "output" element are encoded
   according to the data definition statements in the output section of
   the "rpc" statement.











































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4.  Messages

   This section describes the messages that are used in the YANG-API
   protocol.

4.1.  Request URI Structure

   Resources are represented with URIs following the structure for
   generic URIs in [RFC3986].

   A YANG-API operation is derived from the HTTP method and the request
   URI, using the following conceptual fields:

        <OP> /yang-api/<path>?<query>#<fragment>


         ^      ^       ^        ^         ^
         |      |       |        |         |
       method entry  resource  query    fragment

         M       M        O        O         I


       M=mandatory, O=optional, I=ignored


       <text> replaced by client with real values

   o  method: the HTTP method identifying the YANG-API operation
      requested by the client, to act upon the target resource specified
      in the request URI.  YANG-API operation details are described in
      Section 3.

   o  entry: the well-known YANG-API entry point ("/yang-api").

   o  resource: the path expression identifying the resource that is
      being accessed by the operation.  If this field is not present,
      then the target resource is the API itself, represented by the
      media type "vnd.yang.api".

   o  query: the set of parameters associated with the YANG-API message.
      These have the familiar form of "name=value" pairs.  There is a
      specific set of parameters defined, although the server MAY choose
      to support additional parameters not defined in this document.

   o  fragment: This field is not used by the YANG-API protocol.

   The client SHOULD NOT assume the final structure of a URI path for a



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   resource.  Instead, existing resources can be discovered with the GET
   operation.  When new resources are created by the client, a
   "Location" header is returned, which identifies the path of the newly
   created resource.  The client MUST use this exact path identifier to
   access the resource once it has been created.

   The "target" of an operation is a resource.  The "path" field in the
   request URI represents the target resource for the operation.

4.2.  Message Headers

   There are several HTTP header lines utilized in YANG-API messages.
   Messages are not limited to the HTTP headers listed in this section.

   HTTP defines which header lines are required for particular
   circumstances.  Refer to each operation definition section in
   Section 3 for examples on how particular headers are used.

   There are some request headers that are used within YANG-API, usually
   applied to data resources.  The following tables summarize the
   headers most relevant in YANG-API message requests:

   +---------------------+---------------------------------------------+
   | Name                | Description                                 |
   +---------------------+---------------------------------------------+
   | Accept              | Response Content-Types that are acceptable  |
   | Content-Type        | The media type of the request body          |
   | Host                | The host address of the server              |
   | If-Match            | Only perform the action if the entity       |
   |                     | matches ETag                                |
   | If-Modified-Since   | Only perform the action if modified since   |
   |                     | time                                        |
   | If-Range            | Only retrieve range if resource unchanged   |
   | If-Unmodified-Since | Only perform the action if un-modified      |
   |                     | since time                                  |
   | Range               | Specify a range of data resource entries    |
   +---------------------+---------------------------------------------+

                         YANG-API Request Headers

   The following tables summarize the headers most relevant in YANG-API
   message responses:









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   +---------------+---------------------------------------------------+
   | Name          | Description                                       |
   +---------------+---------------------------------------------------+
   | Allow         | Valid actions when 405 error returned             |
   | Content-Type  | The media type of the response body               |
   | Date          | The date and time the message was sent            |
   | ETag          | An identifier for a specific version of a         |
   |               | resource                                          |
   | Last-Modified | The last modified date and time of a resource     |
   | Location      | The resource identifier for a newly created       |
   |               | resource                                          |
   +---------------+---------------------------------------------------+

                         YANG-API Response Headers

4.3.  Message Encoding

   YANG-API messages are encoded in HTTP according to RFC 2616.  The
   "utf-8" character set is used for all messages.  YANG-API message
   content is sent in the HTTP message body.

   Content is encoded in either JSON or XML format.

   XML encoding rules for data nodes are defined in [RFC6020].  The same
   encoding rules are used for all XML content.  XML attributes are not
   used and will be ignored if present in an XML-encoded message.

   JSON encoding rules are defined in [I-D.lhotka-netmod-json].  Special
   encoding rules are needed to handle multiple module namespaces and
   provide consistent data type processing.

   Request input content encoding format is identified with the Content-
   Type header.  This field MUST be present if message input is sent by
   the client.

   Response output content encoding format is identified with the Accept
   header, the "format" query parameter, or if neither is specified, the
   request input encoding format is used.  If there was no request
   input, then the default output encoding is JSON.  File extensions
   encoded in the request are not used to identify format encoding.

4.4.  Return Status

   Each message represents some sort of resource access.  An HTTP
   "Status-Line" header line is returned for each request.  If a 4xx or
   5xx range status code is returned in the Status-Line, then the error
   information will be returned in the response, according to the format
   defined in Section 6.1.



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4.5.  Message Caching

   Since the datastore contents change at unpredictable times, responses
   from a YANG-API server generally SHOULD NOT be cached.

   The server SHOULD include a "Cache-Control" header in every response
   that specifies whether the response should be cached.  A "Pragma"
   header specifying "no-cache" MAY also be sent in case the
   "Cache-Control" header is not supported.

   Instead of using HTTP caching, the client SHOULD track the "ETag"
   and/or "Last-Modified" headers returned by the server for the
   datastore resource (or data resource if the server supports it).

   A retrieval request for a resource can include headers such as
   "If-None-Match" or "If-Modified-Since" which will cause the server to
   return a "304 Not Modified" Status-Line if the resource has not
   changed.

   The client MAY use the HEAD operation to retrieve just the message
   headers, which SHOULD include the "ETag" and "Last-Modified" headers,
   if this meta-data is maintained for the target resource.





























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5.  Resources

   The resources used in the YANG-API protocol are identified by the
   "path" component in the request URI.  Each operation is performed on
   a target resource.

5.1.  API Resource (/yang-api)

   The API resource contains the state and access points for the YANG-
   API features.

   It is the top-level resource and has the media type "application/
   vnd.yang.api+xml" or "application/vnd.yang.api+json".  It is
   accessible through the well-known URI "/yang-api".

   This resource has the following fields:

              +------------+--------------------------------+
              | Field Name | Description                    |
              +------------+--------------------------------+
              | datastore  | Link to "datastore" resource   |
              | modules    | YANG module capability URIs    |
              | operations | Data-model specific operations |
              +------------+--------------------------------+

                         YANG-API Resource Fields

5.1.1.  /yang-api/datastore

   This mandatory resource represents the running configuration
   datastore and any non-configuration data available.  It may be
   retrieved and edited directly.  It cannot be created or deleted by
   the client.  This resource type is defined in Section 5.2.

5.1.2.  /yang-api/modules

   This mandatory field contains the identifiers for the YANG data model
   modules supported by the server.  There MUST be exactly one instance
   of this field.

   The server MUST maintain a last-modified timestamp for this field,
   and return the "Last-Modified" header when this field is retrieved
   with the GET or HEAD methods.

5.1.3.  /yang-api/operations

   This optional field provides access to the data-model specific
   protocol operations supported by the server.  The server MAY omit



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   this field if no data-model specific operations are advertised.

   Any data-model specific operations defined in the YANG modules
   advertised by the server SHOULD be available as child nodes of this
   field.

5.1.3.1.  /yang-api/modules/module

   This mandatory field contains one URI string for each YANG data model
   module supported by the server.  There MUST be an instance of this
   field for every YANG module that is accessible via an operation
   resource or a data resource.

   The server MAY maintain a last-modified timestamp for each instance
   of this resource, and return the "Last-Modified" header when this
   resource is retrieved with the GET or HEAD methods.  If not supported
   then the timestamp for the parent "modules" field MUST NOT be used
   instead.

   The contents of this field are encoded with the "uri" derived type
   from the "ietf-iana-types" modules in [RFC6021].

   There are additional encoding requirements for this field.  The URI
   MUST follow the YANG module capability URI formatting defined in
   section 5.6.4 of [RFC6020].

5.1.3.2.  Retrieval Example

   In this example the client is retrieving the modules field from the
   server in the default JSON format:

      GET /yang-api?select=modules HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.api+json

   The server might respond as follows.  Note that the content below is
   split across multiple lines for display purposes only:














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      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 17:01:00 GMT
      Server: example-server
      Cache-Control: no-cache
      Pragma: no-cache
      Last-Modified: Sun, 22 Apr 2012 01:00:14 GMT
      Content-Type: application/vnd.yang.api+json

      {
        "yang-api": {
          "modules": {
            "module": [
              "example.com?module=foo&revision=2012-01-02",
              "example.com?module=bar&revision=2011-10-10"
              "example.com?module=itf&revision=2011-10-10
                  &feature=restore"
            ]
          }
        }
      }

5.1.4.  /yang-api/version

   This mandatory field identifies the specific version of the YANG-API
   protocol implemented by the server.

   The same server-wide response MUST be returned each time this field
   is retrieved.  It is assigned by the server when the server is
   started.  The server MUST return the value "1.0" for this version of
   the YANG-API protocol.

   This field is encoded with the rules for an "enumeration" data type,
   using the following leaf definition:

      leaf version {
        config false;
        type enum {
          enum "1.0" {
            description
              "Version 1.0 of the YANG-API protocol.";
          }
        }
      }








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5.2.  Datastore Resource

   A datastore resource represents the conceptual root of a tree of data
   resources.

   The server MUST maintain a last-modified timestamp for this resource,
   and return the "Last-Modified" header when this resource is retrieved
   with the GET or HEAD methods.  Only changes to configuration data
   resources within the datastore affect this timestamp.

   The server SHOULD maintain a resource entity tag for this resource,
   and return the "ETag" header when this resource is retrieved with the
   GET or HEAD methods.  The resource entity tag SHOULD be changed to a
   new previously unused value if changes to any configuration data
   resources within the datastore are made.

   A datastore resource can be retrieved with the GET operation, to
   retrieve either configuration data resources or non-configuration
   data resources within the datastore.  The "config" query parameter is
   used to choose between them.  Refer to Section 3.8.1 for more
   details.

   The depth of the subtrees returned in retrieval operations can be
   controlled with the "depth" query parameter.  The number of nest
   levels, starting at the target resource, can be specified, or an
   unlimited number can be returned.  Refer to Section 3.8.2 for more
   details.

   A datastore resource cannot be written directly with any edit
   operation.  Only the configuration data resources within the
   datastore resource can be edited.

5.3.  Data Resource

   A data resource represents a YANG data node that is a descendant node
   of a datastore resource.

   For configuration data resources, the server MAY maintain a last-
   modified timestamp for the resource, and return the "Last-Modified"
   header when it is retrieved with the GET or HEAD methods.

   For configuration data resources, the server MAY maintain a resource
   entity tag for the resource, and return the "ETag" header when it is
   retrieved as the target resource with the GET or HEAD methods.  The
   resource entity tag SHOULD be changed to a new previously unused
   value if changes to the resource or any configuration field within
   the resource is altered.




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   A data resource can be retrieved with the GET operation, to retrieve
   either configuration data resources or non-configuration data
   resources within the target resource.  The "config" query parameter
   is used to choose between them.  Refer to Section 3.8.1 for more
   details.

   The depth of the subtrees returned in retrieval operations can be
   controlled with the "depth" query parameter.  The number of nest
   levels, starting at the target resource, can be specified, or an
   unlimited number can be returned.  Refer to Section 3.8.2 for more
   details.

   A configuration data resource can be altered by the client with some
   of all of the edit operations, depending on the target resource and
   the specific operation.  Refer to Section 3 for more details on edit
   operations.

5.3.1.  Encoding YANG Instance Identifiers in the Request URI

   In YANG, data nodes are named with an absolute XPath expression, from
   the document root to the target resource.  In YANG-API, URL friendly
   path expressions are used instead.

   The YANG "instance-identifier" (i-i) data type is represented in
   YANG-API with the path expression format defined in this section.

           +-------+-------------------------------------------+
           | Name  | Comments                                  |
           +-------+-------------------------------------------+
           | point | Insertion point is always a full i-i      |
           | path  | Request URI path is a full or partial i-i |
           +-------+-------------------------------------------+

               YANG-API instance-identifier Type Conversion

   The "path" component of the request URI contains the absolute path
   expression that identifies the target resource.  The "select" query
   parameter is used to optionally identify the requested data nodes
   within the target resource to be retrieved in a GET operation.

   A predictable location for a data resource is important, since
   applications will code to the YANG data model module, which uses
   static naming and defines an absolute path location for all data
   nodes.

   A YANG-API data resource identifier is not an XPath expression.  It
   is encoded from left to right, starting with the top-level data node,
   according to the "api-path" rule in Section 5.3.1.1.  The node name



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   of each ancestor of the target resource node is encoded in order,
   ending with the node name for the target resource.

   If the "select" is present, it is encoded, starting with a child node
   of the target resource, according to the "api-select" rule defined in
   Section 5.3.1.1.

   If a data node in the path expression is a YANG list node, then the
   key values for the list (if any) are encoded according to the
   "key-value" rule.  If the list node is the target resource, then the
   key values MAY be omitted, according to the operation.  For example,
   the POST operation to create a new data resource for a list node does
   not allow the key values to be present in the request URI.

   The key leaf values for a data resource representing a YANG list MUST
   be encoded as follows:

   o  The value of each leaf identified in the "key" statement is
      encoded in order.

   o  All the components in the "key" statement MUST be encoded.
      Partial instance identifiers are not supported.

   o  Each value is encoded using the "key-value" rule in
      Section 5.3.1.1, according to the encoding rules for the data type
      of the key leaf.

   o  An empty string can be a valid key value (e.g., "/top/list/key1//
      key3").

   o  The "/" character MUST be URL-encoded (i.e., "%2F").

   o  All whitespace MUST be URL-encoded.

   o  A "null" value is not allowed since the "empty" data type is not
      allowed for key leafs.

   o  The XML encoding is defined in [RFC6020].

   o  The JSON encoding is defined in [I-D.lhotka-netmod-json].

   o  The entire "key-value" MUST be properly URL-encoded, according to
      the rules defined in [RFC3986].

   Notifications are not supported by YANG-API because they are not
   supported by HTTP.  YANG notification statements are ignored by a
   YANG-API server.




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   Examples:

     /yang-api/datastore/jukebox/library/artist/17&select=name

     /yang-api/datastore/newlist/17&select=nextlist/22/44/myleaf

     /yang-api/datastore/somelist/fred%20and%20wilma

     /yang-api/datastore/somelist/fred%20and%20wilma/address

5.3.1.1.  ABNF For Data Resource Identifiers

   The following ABNF syntax is used to construct YANG-API path
   identifiers:

       api-path = "/" api-identifier
                       0*("/" (api-identifier | key-value ))

       [FIXME: the syntax for the select string is still TBD]
       api-select = api-identifier
                       0*("/" (api-identifier | key-value ))

       api-identifier = [module-name ":"] identifier

       module-name = identifier

       key-value = string

       ;; An identifier MUST NOT start with
       ;; (('X'|'x') ('M'|'m') ('L'|'l'))
       identifier  = (ALPHA / "_")
                     *(ALPHA / DIGIT / "_" / "-" / ".")

       string = <an unquoted string>

5.3.2.  Identifying YANG-defined Data Resources

   The data resources used in YANG-API are defined with YANG data
   definition statements.

   Not every data node defined in a YANG module should be treated as a
   resource.  The YANG-API needs to know which YANG data nodes are
   resources, and which are fields within a resource.

   For data resources, YANG-API uses a simple algorithm for defining
   resource boundaries, within the conceptual sub-trees described by
   YANG data definition statements.




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   All top-level data nodes are considered to be resources.  For nodes
   within a top-level resource:

   o  a presence container starts a new resource

   o  a list starts a new resource

   o  an optional terminal node (anyxml, leaf, or leaf-list) starts a
      new resource

   o  a data node of type "anyxml" cannot have any sub-resources

   A non-configuration data node cannot be a separate resource from its
   parent.  Only top-level data nodes are considered to be resources
   (which only support retrieval methods).

5.3.3.  Identifying Optional Keys

   It is sometimes useful to have the server assign the key(s) for a new
   resource.  The "Location" header will indicate the key value(s) that
   the server selected, so the client does not need to provide all the
   key leaf values.

   It is useful to identify in the YANG data model module which key
   leafs are optional to provide, and which are not.  The YANG extension
   statement "optional-key" is provided to indicate that the leaf
   definition represents an optional key.

   The client MAY provide a value for a key leaf in a POST operation.
   Refer to Section 8 for details on the "optional-key" extension.
   Refer to Section 13 for usage examples of this YANG extension
   statement.

5.3.4.  Data Resource Retrieval

   There are four types of filtering for retrieval of data resources.
   This section defines each mode.

5.3.4.1.  Conditional Retrieval

   The HTTP headers (such as "If-Modified-Since" and "If-Match") can by
   used in for a request message for a GET operation to check a
   condition within the server state, such as the last time the
   datastore resource was modified, or the resource entity tag of the
   target resource.

   If the condition is met according to the header definition, a "200
   OK" Status-Line and the data requested is returned in the response



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   message.  If the condition is not met, a "304 Not Modified" Status-
   Line is returned in response message instead.

5.3.4.2.  Data Classification Retrieval

   The "config" query parameter can be used with the GET operation to
   specify whether configuration or non-configuration data is requested.
   Refer to Section 3.8.1 for more details on the "config" query
   parameter.

5.3.4.3.  Subset Retrieval

   The "Range" header is used to request a specific subset of the
   instances of a list or leaf-list data resource that are returned by
   the server for a retrieval operation.  Normally, if the target
   resource in a request message does not specify an instance, then all
   instances are returned.

   The YANG-API protocol uses the token "entries" instead of "bytes" as
   the range units.

   The entries are numbered starting from "0".  A list or leaf-list can
   change order between requests so the client needs to be aware of the
   data model semantics, and whether the list contents are stable enough
   to use the subset retrieval mechanism.

   If the requested range cannot be returned because the range
   specification includes index values for entries that do not exist,
   then an error occurs, and the server MUST return a "416 Requested
   range not satisfiable" Status-Line.

   If the range request can be satisfied, then a "200 OK" Status-Line is
   returned, and the response MUST include a "Content-Range" header
   indicating which entries are returned.  The response message body
   contains the data for the requested range of entries.

   Example:

   In this example, the client is requesting 5 "artist" resource
   entries, starting with the 10th entry:











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    Request from client:

      GET /yang-api/datastore/jukebox/library/artist HTTP/1.1
      Host: example.com
      Accept: application/vnd.yang.data+json
      Range: entries 10-14

    Response from server:

      HTTP/1.1 200 OK
      Date: Mon, 23 Apr 2012 13:01:20 GMT
      Cache-Control: no-cache
      Pragma: no-cache
      Content-Type: application/vnd.yang.data+json
      Content-Range: entries 10-14
      Server: example-server
      Last-Modified: Mon, 23 Apr 2012 02:12:20 GMT
      ETag: abcada438af

      {
        "artist" : {
            // content removed for brevity
        }
      }

5.3.4.4.  Filtered Retrieval

   The "select" query parameter is used to specify a filter that should
   be applied to the target resource to request a subset of all possible
   descendant nodes within the target resource.

   The format of the "select" parameter string is defined in
   Section 3.8.6.  The set of nodes selected by the filter expression is
   applied to each context node identified by the target resource.

5.4.  Operation Resource

   An operation resource represents an protocol operation defined with
   the YANG "rpc" statement.

   All operation resources share the same module namespace as any top-
   level data resources, so the name of an operation resource cannot
   conflict with the name of a top-level data resource defined within
   the same module.

   If 2 different YANG modules define the same "rpc" identifier, then
   the module name MUST be used in the request URI.  For example, if
   "module-A" and "module-B" both defined a "reset" operation, then



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   invoking the operation from "module-A" would be requested as follows:

      POST /yang-api/operations/module-A:reset HTTP/1.1
      Server example.com

   Any usage of an operation resource from the same module, with the
   same name, refers to the same "rpc" statement definition.  This
   behavior can be used to design protocol operations that perform the
   same general function on different resource types.

   If the "rpc" statement has an "input" section, then a message body
   MAY be sent by the client in the request, otherwise the request
   message MUST NOT include a message body.  If the "rpc" statement has
   an "output" section, then a message body MAY be sent by the server in
   the response.  Otherwise the server MUST NOT include a message body
   in the response message, and MUST send a "204 No Content" Status-Line
   instead.

5.4.1.  Encoding Operation Input Parameters

   If the "rpc" statement has an "input" section, then the "input" node
   is provided in the message body, corresponding to the YANG data
   definition statements within the "input" section.

   Example:

   The following YANG definition is used for the examples in this
   section.

       rpc reboot {
         input {
           leaf delay {
             units seconds;
             type uint32;
             default 0;
           }
           leaf message { type string; }
           leaf language { type string; }
         }
       }

   The client might send the following POST request message:









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      POST /yang-api/datastore/operations/reboot HTTP/1.1
      Host: example.com
      Content-Type: application/vnd.yang.data+json

      {
        "input" : {
          "delay" : 600,
          "message" : "Going down for system maintenance",
          "language" : "en-US"
        }
      }

   The server might respond:

      HTTP/1.1 204 No Content
      Date: Mon, 25 Apr 2012 11:01:00 GMT
      Server: example-server

5.4.2.  Encoding Operation Output Parameters

   If the "rpc" statement has an "output" section, then the "output"
   node is provided in the message body, corresponding to the YANG data
   definition statements within the "output" section.

   Example:

   The following YANG definition is used for the examples in this
   section.

       rpc get-reboot-info {
         input {
           leaf reboot-time {
             units seconds;
             type uint32;
           }
           leaf message { type string; }
           leaf language { type string; }
         }
       }

   The client might send the following POST request message:

      POST /yang-api/datastore/operations/get-reboot-info HTTP/1.1
      Host: example.com

   The server might respond:





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      HTTP/1.1 200 OK
      Date: Mon, 25 Apr 2012 11:10:30 GMT
      Server: example-server
      Content-Type: application/vnd.yang.data+json

      {
        "output" : {
          "reboot-time" : 30,
          "message" : "Going down for system maintenance",
          "language" : "en-US"
        }
      }

5.4.3.  Identifying YANG-defined Operation Resources

   The operation resources used in YANG-API are defined with YANG "rpc"
   statements.  All "rpc" statements within a YANG module that are
   supported by the server are available as operation resources.

































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6.  Error Reporting

   HTTP Status-Lines are used to report success or failure for YANG-API
   operations.  The <rpc-error> element returned in NETCONF error
   responses contains some useful information.  This error information
   is adapted for use in YANG-API, and error information is returned for
   "4xx" class of status codes.

   The following table summarizes the return status codes used
   specifically by YANG-API operations:

   +-------------------------------+-----------------------------------+
   | Status-Line                   | Description                       |
   +-------------------------------+-----------------------------------+
   | 100 Continue                  | POST accepted, 201 should follow  |
   | 200 OK                        | Success with response body        |
   | 201 Created                   | POST to create a resource success |
   | 202 Accepted                  | POST to create a resource         |
   |                               | accepted                          |
   | 204 No Content                | Success without response body     |
   | 304 Not Modified              | Conditional operation not done    |
   | 400 Bad Request               | Invalid request message           |
   | 403 Forbidden                 | Access to resource denied         |
   | 404 Not Found                 | Resource target or resource node  |
   |                               | not found                         |
   | 405 Method Not Allowed        | Method not allowed for target     |
   |                               | resource                          |
   | 409 Conflict                  | Resource or lock in use           |
   | 413 Request Entity Too Large  | too-big error                     |
   | 414 Request-URI Too Large     | too-big error                     |
   | 415 Unsupported Media Type    | non YANG-API media type           |
   | 416 Requested range not       | If-Range error                    |
   | satisfiable                   |                                   |
   | 500 Internal Server Error     | operation-failed                  |
   | 501 Not Implemented           | unknown-operation                 |
   | 503 Service Unavailable       | Recoverable server error          |
   +-------------------------------+-----------------------------------+

                    HTTP Status Codes used in YANG-API

   Since an operation resource is defined with a YANG "rpc" statement, a
   mapping between the NETCONF <error-tag> value and the HTTP status
   code is needed.  The specific error condition and response code to
   use are data-model specific and might be contained in the YANG
   "description" statement for the "rpc" statement.






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                 +-------------------------+-------------+
                 | <error-tag>             | status code |
                 +-------------------------+-------------+
                 | in-use                  | 409         |
                 | invalid-value           | 400         |
                 | too-big                 | 413         |
                 | missing-attribute       | 400         |
                 | bad-attribute           | 400         |
                 | unknown-attribute       | 400         |
                 | bad-element             | 400         |
                 | unknown-element         | 400         |
                 | unknown-namespace       | 400         |
                 | access-denied           | 403         |
                 | lock-denied             | 409         |
                 | resource-denied         | 409         |
                 | rollback-failed         | 500         |
                 | data-exists             | 409         |
                 | data-missing            | 409         |
                 | operation-not-supported | 501         |
                 | operation-failed        | 500         |
                 | partial-operation       | 500         |
                 | malformed-message       | 400         |
                 +-------------------------+-------------+

                   Mapping from error-tag to status code

6.1.  Error Response Message

   When an error occurs for a request message on a data resource or an
   operation resource, and a "4xx" class of status codes (except for
   status code "403"), then the server SHOULD send a response body
   containing the information described by the following YANG data
   definition statement:


















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       container errors {
         config false;

         list error {
           reference "RFC 6241, Section 4.3";
           leaf error-type {
             mandatory true;
             type enumeration {
               enum transport;
               enum rpc;
               enum protocol;
               enum application;
             }
           }
           leaf error-tag {
             mandatory true;
             type string;
           }
           leaf error-app-tag {
             type string;
           }
           leaf error-path {
             type string;  // YANG-API encoded instance-identifier
           }
           leaf error-message {
             type string;
           }
           container error-info {
              // anyxml content here
           }
         }
       }

   Example:

   The following example shows an error returned for an "lock-denied"
   error on a datastore resource.

      POST /yang-api/operations/lock-datastore HTTP/1.1
      Host: example.com

   The server might respond:









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      HTTP/1.1 409 Conflict
      Date: Mon, 23 Apr 2012 17:11:00 GMT
      Server: example-server
      Content-Type: application/vnd.yang.api+json

      {
        "errors": {
          "error": {
            "error-type": "protocol",
            "error-tag": "lock-denied",
            "error-message": "Lock failed, lock is already held",
          }
        }
      }





































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7.  RelaxNG Grammar

   TBD
















































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8.  YANG-API module

   RFC Ed.: update the date below with the date of RFC publication and
   remove this note.

   <CODE BEGINS> file "ietf-yang-api@2012-11-30.yang"

   module ietf-yang-api {
     namespace "urn:ietf:params:xml:ns:yang:ietf-yang-api";
     prefix "api";

     organization
       "IETF NETCONF (Network Configuration) Working Group";

     contact
       "Editor:   Andy Bierman
                  <mailto:andy@yumaworks.com>

        Editor:   Martin Bjorklund
                  <mailto:mbj@tail-f.com>";

     description
       "This module contains a collection of YANG language extensions
        to describe REST API Resources using YANG data definition
        statements.

        Copyright (c) 2012 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
        (http://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; see
        the RFC itself for full legal notices.";

     // RFC Ed.: replace XXXX with actual RFC number and remove this
     // note.

     // RFC Ed.: remove this note
     // Note: extracted from draft-bierman-netconf-yang-api-01.txt

     // RFC Ed.: update the date below with the date of RFC publication
     // and remove this note.
     revision 2012-11-30 {



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       description
         "Initial revision.";
       reference
         "RFC XXXX: YANG-API Protocol.";
     }

     /*
      * Extensions
      */

     extension optional-key {
       description
         "This extension is used to allow the client to create
          a new instance of a resource without providing a
          value for the key leaf containing this statement.
          This extension is ignored for NETCONF, and only
          applies to YANG-API resources and fields.
          This extension is ignored unless it appears
          directly within a 'leaf' data definition statement.";
     }

   }


   <CODE ENDS>


























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9.  IANA Considerations

   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-api
        Registrant Contact: The NETMOD WG of the IETF.
        XML: N/A, the requested URI is an XML namespace.

   This document registers one YANG module in the YANG Module Names
   registry [RFC6020].

     name:         ietf-yang-api
     namespace:    urn:ietf:params:xml:ns:yang:ietf-yang-api
     prefix:       api
     reference:    RFC XXXX


































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10.  Security Considerations

   TBD
















































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11.  Change Log

       -- RFC Ed.: remove this section before publication.

11.1.  00-01

   o  expanded introduction

   o  removed transactions

   o  removed capabilities

   o  simplified editing model

   o  removed global protocol operations from ietf-yang-api.yang

   o  changed RPC operation terminology to protocol operation

   o  updated JSON draft reference

   o  updated open issues section

   o  updated IANA section




























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12.  Open Issues

   o  Which WG should do this work?  NETCONF?  NETMOD?  It is not clear
      since YANG-API builds on concepts and standards from documents
      owned by both working groups.

   o  Resource creation order and other dependencies between resources
      are not well identified in YANG.  YANG has leafrefs and instance-
      identifiers, which can be used to identify some order
      dependencies.  Are any new mechanisms needed in YANG-API needed to
      identify resource creation order and other dependency
      requirements?

   o  There is no "message-id" field in a YANG-API message.  Is a
      message identifier needed?  If so, should either the "Message-ID"
      or "Content-ID" header from RFC 2392 be used for this purpose?

   o  Should sessions be used or not?  Should "reusable sessions" be
      used?  Better for auditing?  How does locking of the /yang-api/
      datastore resource work for multiple edits if a session is 1
      operation?  When does the server release the lock and decide it
      has been abandoned or client was disconnected?

   o  What syntax should be used for the "select" query parameter?

   o  Should the "/yang-api/modules" field within the API resource be a
      separate resource, with its own timestamp?  Currently the API
      timestamp is coupled to any changes to the list of loaded modules.
      Should the API resource be static and cacheable?

   o  What to do about no REMOVE operation, just DELETE?  The effect is
      local to the request; in a NETCONF edit-config it is worse, since
      the netconf request might create/delete/modify many nodes

   o  Should every YANG data node be a data resource and every YANG RPC
      statement an operation resource?  Is a YANG extension needed to
      allow data modeler control of resource boundaries?

   o  Encoding of leafrefs?  Is there some additional meta-data needed?
      Do leafref nodes need to be identified in responses (RFC 5988) or
      is the YANG module definition sufficient to provide this meta-
      data?

   o  What should the default algorithm be for defining data resources?
      Should the default for an augment from another namespace be to
      start a new resource?  Top-level data node defaults as a resource
      OK?




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   o  Is the token "entries" legal in the YANG-API usage of Range?  What
      units should be used? "bytes" is the only token defined by HTTP.

   o  Are all header lines used by YANG-API supported by common
      application frameworks, such as FastCGI and WSGI?  If not, then
      should query parameters be used instead, since the QUERY_STRING is
      widely available to WEB applications?

   o  Should the <errors> element returned in error responses be a
      separate media type?

   o  How should additional datastores be supported, which may be added
      to the NETCONF/NETMOD framework in the future?






































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13.  Example YANG Module

   module example-jukebox {

      namespace "http://example.com/ns/example-jukebox";
      prefix "jbox";

      import ietf-yang-api { prefix api; }

      organization "Example, Inc.";
      description "Example Jukebox Data Model Module";
      revision "2012-05-30";


      identity genre {
        description "Base for all genre types";
      }

      // abbreviated list of genre classifications
      identity Alternative {
        base genre;
      }
      identity Blues {
        base genre;
      }
      identity Country {
        base genre;
      }
      identity Jazz {
        base genre;
      }
      identity Pop {
        base genre;
      }
      identity Rock {
        base genre;
      }

      container jukebox {
        presence
          "An empty container indicates that the jukebox
           service is available";

        container library {
          list artist {
            key index;
            unique name;




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            leaf index {
              api:optional-key;
              type uint32;
              description
                "Optional key used instead of natural key for
                 example. Also rare but possible artists with
                 the same name are really different entities.";
            }
            leaf name {
              type string;
            }

            list album {
              key name;
              leaf name {
                type string {
                  length "1 .. max";
                }
              }
              leaf genre {
                type identityref { base genre; }
              }
              leaf year {
                type uint16 {
                  range "1900 .. max";
                }
              }
              list song {
                api:optional-key;
                key index;
                ordered-by user;
                leaf index {
                  type uint32;
                }
                leaf name {
                  mandatory true;
                  type string;
                }
                leaf location {
                  mandatory true;
                  type string;
                }
                leaf format {
                  type string;
                }
                leaf length {
                  units "seconds";
                  type uint32;



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                }
              }
            }
          }
          leaf artist-count {
             config false;
             type uint32;
             units "songs";
             description "Number of artists in the library";
          }
          leaf album-count {
             config false;
             type uint32;
             units "albums";
             description "Number of albums in the library";
          }
          leaf song-count {
             type uint32;
             units "songs";
             description "Number of songs in the library";
          }
        }

        list playlist {
          description
            "Example configuration data resource";
          key name;
          leaf name {
            type string;
          }
          leaf description {
            type string;
          }
          list song {
            description
              "Example nested configuration data resource";
            ordered-by user;
            key index;
            leaf index {
              api:optional-key;
              type uint32;
            }
            leaf id {
              mandatory true;
              type instance-identifier;
              description
                "Song identifier. Must identify an instance of
                 /jukebox/library/artist/album/song.



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                 The id is not the key to allow duplicates
                 in a playlist";
            }
          }
        }

        container player {
          leaf gap {
            description "Time gap between each song";
            units "tenths of seconds";
            type decimal64 {
              fraction-digits 1;
              range "0.0 .. 2.0";
            }
          }
        }
      }

      rpc play {
        description "Control function for the jukebox player";
        input {
          leaf playlist {
            type string;
            mandatory true;
            description "playlist name";
          }
          leaf song-number {
            type uint32;
            mandatory true;
            description "Song number in playlist to play";
          }
        }
      }
   }

















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14.  Normative References

   [I-D.lhotka-netmod-json]
              Lhotka, L., "Modeling JSON Text with YANG",
              draft-lhotka-netmod-yang-json-00 (work in progress),
              October 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, March 2010.

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6021]  Schoenwaelder, J., "Common YANG Data Types", RFC 6021,
              October 2010.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, June 2011.

   [RFC6536]  Bierman, A. and M. Bjorklund, "Network Configuration
              Protocol (NETCONF) Access Control Model", RFC 6536,
              March 2012.













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Authors' Addresses

   Andy Bierman
   YumaWorks

   Email: andy@yumaworks.com


   Martin Bjorklund
   Tail-f Systems

   Email: mbj@tail-f.com







































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