CoAP Management Interface (CORECONF)
draft-ietf-core-comi-10

Document Type Active Internet-Draft (core WG)
Authors Michel Veillette  , Peter Van der Stok  , Alexander Pelov  , Andy Bierman  , Ivaylo Petrov 
Last updated 2020-09-26 (latest revision 2020-07-04)
Replaces draft-vanderstok-core-comi
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CoRE                                                   M. Veillette, Ed.
Internet-Draft                                   Trilliant Networks Inc.
Intended status: Standards Track                    P. van der Stok, Ed.
Expires: January 5, 2021                                      consultant
                                                                A. Pelov
                                                                  Acklio
                                                              A. Bierman
                                                               YumaWorks
                                                          I. Petrov, Ed.
                                                                  Acklio
                                                           July 04, 2020

                  CoAP Management Interface (CORECONF)
                        draft-ietf-core-comi-10

Abstract

   This document describes a network management interface for
   constrained devices and networks, called CoAP Management Interface
   (CORECONF).  The Constrained Application Protocol (CoAP) is used to
   access datastore and data node resources specified in YANG, or SMIv2
   converted to YANG.  CORECONF uses the YANG to CBOR mapping and
   converts YANG identifier strings to numeric identifiers for payload
   size reduction.  CORECONF extends the set of YANG based protocols,
   NETCONF and RESTCONF, with the capability to manage constrained
   devices and networks.

Note

   Discussion and suggestions for improvement are requested, and should
   be sent to yot@ietf.org.

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 https://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."

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   This Internet-Draft will expire on January 5, 2021.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  CORECONF Architecture . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Major differences between RESTCONF and CORECONF . . . . .   6
       2.1.1.  Differences due to CoAP and its efficient usage . . .   6
       2.1.2.  Differences due to the use of CBOR  . . . . . . . . .   7
     2.2.  Compression of YANG identifiers . . . . . . . . . . . . .   7
     2.3.  Instance-identifier . . . . . . . . . . . . . . . . . . .   8
     2.4.  Media-Types . . . . . . . . . . . . . . . . . . . . . . .   8
     2.5.  Unified datastore . . . . . . . . . . . . . . . . . . . .  10
   3.  Example syntax  . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  CoAP Interface  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Using the 'k' query parameter . . . . . . . . . . . . . .  13
     4.2.  Data Retrieval  . . . . . . . . . . . . . . . . . . . . .  15
       4.2.1.  Using the 'c' query parameter . . . . . . . . . . . .  15
       4.2.2.  Using the 'd' query parameter . . . . . . . . . . . .  16
       4.2.3.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  16
       4.2.4.  FETCH . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.3.  Data Editing  . . . . . . . . . . . . . . . . . . . . . .  20
       4.3.1.  Data Ordering . . . . . . . . . . . . . . . . . . . .  20
       4.3.2.  POST  . . . . . . . . . . . . . . . . . . . . . . . .  20
       4.3.3.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .  21
       4.3.4.  iPATCH  . . . . . . . . . . . . . . . . . . . . . . .  22
       4.3.5.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  23
     4.4.  Full datastore access . . . . . . . . . . . . . . . . . .  24
       4.4.1.  Full datastore examples . . . . . . . . . . . . . . .  24
     4.5.  Event stream  . . . . . . . . . . . . . . . . . . . . . .  25
       4.5.1.  Notify Examples . . . . . . . . . . . . . . . . . . .  26
       4.5.2.  The 'f' query parameter . . . . . . . . . . . . . . .  27

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     4.6.  RPC statements  . . . . . . . . . . . . . . . . . . . . .  27
       4.6.1.  RPC Example . . . . . . . . . . . . . . . . . . . . .  28
   5.  Use of Block-wise Transfers . . . . . . . . . . . . . . . . .  30
   6.  Application Discovery . . . . . . . . . . . . . . . . . . . .  30
     6.1.  YANG library  . . . . . . . . . . . . . . . . . . . . . .  30
     6.2.  Resource Discovery  . . . . . . . . . . . . . . . . . . .  31
       6.2.1.  Datastore Resource Discovery  . . . . . . . . . . . .  31
       6.2.2.  Data node Resource Discovery  . . . . . . . . . . . .  32
       6.2.3.  Event stream Resource Discovery . . . . . . . . . . .  32
   7.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  33
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
     9.1.  Resource Type (rt=) Link Target Attribute Values Registry  37
     9.2.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  37
     9.3.  Media Types Registry  . . . . . . . . . . . . . . . . . .  38
     9.4.  YANG Namespace Registration . . . . . . . . . . . . . . .  39
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  39
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     11.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Appendix A.  ietf-coreconf YANG module  . . . . . . . . . . . . .  42
   Appendix B.  ietf-coreconf .sid file  . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  51

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is designed for
   Machine to Machine (M2M) applications such as smart energy, smart
   city, and building control.  Constrained devices need to be managed
   in an automatic fashion to handle the large quantities of devices
   that are expected in future installations.  Messages between devices
   need to be as small and infrequent as possible.  The implementation
   complexity and runtime resources need to be as small as possible.

   This draft describes the CoAP Management Interface which uses CoAP
   methods to access structured data defined in YANG [RFC7950].  This
   draft is complementary to [RFC8040] which describes a REST-like
   interface called RESTCONF, which uses HTTP methods to access
   structured data defined in YANG.

   The use of standardized data models specified in a standardized
   language, such as YANG, promotes interoperability between devices and
   applications from different manufacturers.

   CORECONF and RESTCONF are intended to work in a stateless client-
   server fashion.  They use a single round-trip to complete a single
   editing transaction, where NETCONF needs multiple round trips.

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   To promote small messages, CORECONF uses a YANG to CBOR mapping
   [I-D.ietf-core-yang-cbor] and numeric identifiers [I-D.ietf-core-sid]
   to minimize CBOR payloads and URI length.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are defined in the YANG data modeling language
   [RFC7950]: action, anydata, anyxml, client, container, data model,
   data node, identity, instance identifier, leaf, leaf-list, list,
   module, RPC, schema node, server, submodule.

   The following terms are defined in [RFC6241]: configuration data,
   datastore, state data

   The following term is defined in [I-D.ietf-core-sid]: YANG schema
   item identifier (YANG SID, often shorten to simply SID).

   The following terms are defined in the CoAP protocol [RFC7252]:
   Confirmable Message, Content-Format, Endpoint.

   The following terms are defined in this document:

   data node resource:  a CoAP resource that models a YANG data node.

   datastore resource:  a CoAP resource that models a YANG datastore.

   event stream resource:  a CoAP resource used by clients to observe
      YANG notifications.

   notification instance:  An instance of a schema node of type
      notification, specified in a YANG module implemented by the
      server.  The instance is generated in the server at the occurrence
      of the corresponding event and reported by an event stream
      resource.

   list instance identifier:  Handle used to identify a YANG data node
      that is an instance of a YANG "list" specified with the values of
      the key leaves of the list.

   single instance identifier:  Handle used to identify a specific data
      node which can be instantiated only once.  This includes data
      nodes defined at the root of a YANG module and data nodes defined

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      within a container.  This excludes data nodes defined within a
      list or any children of these data nodes.

   instance-identifier:  List instance identifier or single instance
      identifier.

   instance-value:  The value assigned to a data node instance.
      Instance-values are serialized into the payload according to the
      rules defined in section 4 of [I-D.ietf-core-yang-cbor].

2.  CORECONF Architecture

   This section describes the CORECONF architecture to use CoAP for
   reading and modifying the content of datastore(s) used for the
   management of the instrumented node.

   +----------------------------------------------------------------+
   |                SMIv2 specification (optional) (2)              |
   +----------------------------------------------------------------+
                                  |
                                  V
   +----------------------------------------------------------------+
   |                     YANG specification  (1)                    |
   +----------------------------------------------------------------+
            |                                            |
   Client   V                               Server       V
   +----------------+                       +-----------------------+
   |        Request |--> CoAP request(3) -->| Indication            |
   |        Confirm |<-- CoAP response(3)<--| Response          (4) |
   |                |                       |                       |
   |                |<==== Security (7) ===>|+---------------------+|
   +----------------+                       || Datastore(s)    (5) ||
                                            |+---------------------+|
                                            |+---------------------+|
                                            || Event stream(s) (6) ||
                                            |+---------------------+|
                                            +-----------------------+

                 Figure 1: Abstract CORECONF architecture

   Figure 1 is a high-level representation of the main elements of the
   CORECONF management architecture.  The different numbered components
   of Figure 1 are discussed according to the component number.

   (1) YANG specification:  contains a set of named and versioned
      modules.

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   (2) SMIv2 specification:  Optional part that consists of a named
      module which, specifies a set of variables and "conceptual
      tables".  There is an algorithm to translate SMIv2 specifications
      to YANG specifications.

   (3) CoAP request/response messages:  The CORECONF client sends
      request messages to and receives response messages from the
      CORECONF server.

   (4) Request, Indication, Response, Confirm:  Processes performed by
      the CORECONF clients and servers.

   (5) Datastore:  A resource used to access configuration data, state
      data, RPCs, and actions.  A CORECONF server may support a single
      unified datastore or multiple datastores as those defined by
      Network Management Datastore Architecture (NMDA) [RFC8342].

   (6) Event stream:  A resource used to get real-time notifications.  A
      CORECONF server may support multiple Event streams serving
      different purposes such as normal monitoring, diagnostic, syslog,
      security monitoring.

   (7) Security:  The server MUST prevent unauthorized users from
      reading or writing any CORECONF resources.  CORECONF relies on
      security protocols such as DTLS [RFC6347] or OSCORE [RFC8613] to
      secure CoAP communications.

2.1.  Major differences between RESTCONF and CORECONF

   CORECONF is a RESTful protocol for small devices where saving bytes
   to transport a message is very important.  Contrary to RESTCONF, many
   design decisions are motivated by the saving of bytes.  Consequently,
   CORECONF is not a RESTCONF over CoAP protocol, but differs more
   significantly from RESTCONF.

2.1.1.  Differences due to CoAP and its efficient usage

   o  CORECONF uses CoAP/UDP as transport protocol and CBOR as payload
      format [I-D.ietf-core-yang-cbor].  RESTCONF uses HTTP/TCP as
      transport protocol and JSON or XML as payload formats.

   o  CORECONF uses the methods FETCH and iPATCH to access multiple data
      nodes.  RESTCONF uses instead the HTTP method PATCH and the HTTP
      method GET with the "fields" Query parameter.

   o  RESTCONF uses the HTTP methods HEAD, and OPTIONS, which are not
      supported by CoAP.

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   o  CORECONF does not support "insert" query parameter (first, last,
      before, after) and the "point" query parameter which are supported
      by RESTCONF.

   o  CORECONF does not support the "start-time" and "stop-time" query
      parameters to retrieve past notifications.

2.1.2.  Differences due to the use of CBOR

   o  CORECONF encodes YANG identifier strings as numbers, where
      RESTCONF does not.

   o  CORECONF also differ in the handling of default values, only
      'report-all' and 'trim' options are supported.

2.2.  Compression of YANG identifiers

   In the YANG specification, items are identified with a name string.
   In order to significantly reduce the size of identifiers used in
   CORECONF, numeric identifiers called YANG Schema Item iDentifier
   (YANG SID or simply SID) are used instead.

   When used in a URI, SIDs are encoded using base64 encoding of the SID
   bytes.  The base64 encoding is using the URL and Filename safe
   alphabet as defined by [RFC4648] section 5, without padding.  The
   last 6 bits encoded is always aligned with the least significant 6
   bits of the SID represented using an unsigned integer.  'A'
   characters (value 0) at the start of the resulting string are
   removed.  See Figure 2 for complete illustration.

   SID in base64 = URLsafeChar[SID >> 60 & 0x3F] |
                    URLsafeChar[SID >> 54 & 0x3F] |
                    URLsafeChar[SID >> 48 & 0x3F] |
                    URLsafeChar[SID >> 42 & 0x3F] |
                    URLsafeChar[SID >> 36 & 0x3F] |
                    URLsafeChar[SID >> 30 & 0x3F] |
                    URLsafeChar[SID >> 24 & 0x3F] |
                    URLsafeChar[SID >> 18 & 0x3F] |
                    URLsafeChar[SID >> 12 & 0x3F] |
                    URLsafeChar[SID >> 6 & 0x3F] |
                    URLsafeChar[SID & 0x3F]

                                 Figure 2

   For example, SID 1721 is encoded as follow.

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   URLsafeChar[1721 >> 60 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 54 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 48 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 42 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 36 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 30 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 24 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 18 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 12 & 0x3F] = URLsafeChar[0] = 'A'
   URLsafeChar[1721 >> 6 & 0x3F]  = URLsafeChar[26] = 'a'
   URLsafeChar[1721 & 0x3F]       = URLsafeChar[57] = '5'

   The resulting base64 representation of SID 1721 is the two-character
   string "a5".

2.3.  Instance-identifier

   Instance-identifiers are used to uniquely identify data node
   instances within a datastore.  This YANG built-in type is defined in
   [RFC7950] section 9.13.  An instance-identifier is composed of the
   data node identifier (i.e. a SID) and for data nodes within list(s)
   the keys used to index within these list(s).

   When part of a payload, instance-identifiers are encoded in CBOR
   based on the rules defined in [I-D.ietf-core-yang-cbor] section
   6.13.1.  When part of a URI, the SID is appended to the URI of the
   targeted datastore, the keys are specified using the 'k' query
   parameter as defined in Section 4.1.

2.4.  Media-Types

   CORECONF uses Media-Types based on the YANG to CBOR mapping specified
   in [I-D.ietf-core-yang-cbor].

   The following Media-Type is used as defined in [I-D.ietf-core-sid].

   o  application/yang-data+cbor; id=sid

   The following new Media-Types are defined in this document:

   application/yang-identifiers+cbor:  This Media-Type represents a CBOR
      YANG document containing a list of instance-identifier used to
      target specific data node instances within a datastore.

      FORMAT: CBOR array of instance-identifier

      The message payload of Media-Type 'application/yang-
      identifiers+cbor' is encoded using a CBOR array.  Each entry of

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      this CBOR array contain an instance-identifier encoded as defined
      in [I-D.ietf-core-yang-cbor] section 6.13.1.

   application/yang-instances+cbor:  This Media-Type represents a CBOR
      YANG document containing a list of data node instances.  Each data
      node instance is identified by its associated instance-identifier.

      FORMAT: CBOR array of CBOR map of instance-identifier, instance-
      value

      The message payload of Media-Type 'application/yang-
      instances+cbor' is encoded using a CBOR array.  Each entry within
      this CBOR array contains a CBOR map carrying an instance-
      identifier and associated instance-value.  Instance-identifiers
      are encoded using the rules defined in [I-D.ietf-core-yang-cbor]
      section 6.13.1, instance-values are encoded using the rules
      defined in [I-D.ietf-core-yang-cbor] section 4.

      When present in an iPATCH request payload, this Media-Type carry a
      list of data node instances to be replaced, created, or deleted.
      For each data node instance D, for which the instance-identifier
      is the same as a data node instance I, in the targeted datastore
      resource: the value of D replaces the value of I.  When the value
      of D is null, the data node instance I is removed.  When the
      targeted datastore resource does not contain a data node instance
      with the same instance-identifier as D, a new instance is created
      with the same instance-identifier and value as D.

   The different Media-Type usages are summarized in the table below:

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   +---------------+--------------+------------------------------------+
   | Method        | Resource     | Media-Type                         |
   +---------------+--------------+------------------------------------+
   | GET response  | data node    | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | PUT request   | data node    | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | POST request  | data node    | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | DELETE        | data node    | n/a                                |
   |               |              |                                    |
   | GET response  | datastore    | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | PUT request   | datastore    | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | POST request  | datastore    | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | FETCH request | datastore    | application/yang-identifiers+cbor  |
   |               |              |                                    |
   | FETCH         | datastore    | application/yang-instances+cbor    |
   | response      |              |                                    |
   |               |              |                                    |
   | iPATCH        | datastore    | application/yang-instances+cbor    |
   | request       |              |                                    |
   |               |              |                                    |
   | GET response  | event stream | application/yang-instances+cbor    |
   |               |              |                                    |
   | POST request  | rpc, action  | application/yang-data+cbor; id=sid |
   |               |              |                                    |
   | POST response | rpc, action  | application/yang-data+cbor; id=sid |
   +---------------+--------------+------------------------------------+

2.5.  Unified datastore

   CORECONF supports a simple datastore model consisting of a single
   unified datastore.  This datastore provides access to both
   configuration and operational data.  Configuration updates performed
   on this datastore are reflected immediately or with a minimal delay
   as operational data.

   Alternatively, CORECONF servers MAY implement a more complex
   datastore model such as the Network Management Datastore Architecture
   (NMDA) as defined by [RFC8342].  Each datastore supported is
   implemented as a datastore resource.

   Characteristics of the unified datastore are summarized in the table
   below:

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   +-------------+-----------------------------------------------------+
   | Name        | Value                                               |
   +-------------+-----------------------------------------------------+
   | Name        | unified                                             |
   |             |                                                     |
   | YANG        | all modules                                         |
   | modules     |                                                     |
   |             |                                                     |
   | YANG nodes  | all data nodes ("config true" and "config false")   |
   |             |                                                     |
   | Access      | read-write                                          |
   |             |                                                     |
   | How applied | changes applied in place immediately or with a      |
   |             | minimal delay                                       |
   |             |                                                     |
   | Protocols   | CORECONF                                            |
   |             |                                                     |
   | Defined in  | "ietf-coreconf"                                     |
   +-------------+-----------------------------------------------------+

3.  Example syntax

   CBOR is used to encode CORECONF request and response payloads.  The
   CBOR syntax of the YANG payloads is specified in [RFC7049].  The
   payload examples are notated in Diagnostic notation (defined in
   section 6 of [RFC7049]) that can be automatically converted to CBOR.

   SIDs in URIs are represented as a base64 number, SIDs in the payload
   are represented as decimal numbers.

4.  CoAP Interface

   This note specifies a Management Interface.  CoAP endpoints that
   implement the CORECONF management protocol, support at least one
   discoverable management resource of resource type (rt): core.c.ds.
   The path of the discoverable management resource is left to
   implementers to select (see Section 6).

   The mapping of YANG data node instances to CORECONF resources is as
   follows.  Every data node of the YANG modules loaded in the CORECONF
   server represents a sub-resource of the datastore resource (e.g. /c/
   YANGSID).  When multiple instances of a list exist, instance
   selection is possible as described in Section 4.1, Section 4.2.3.1,
   and Section 4.2.4.

   CORECONF also supports event stream resources used to observe
   notification instances.  Event stream resources can be discovered
   using resource type (rt): core.c.ev.

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   The description of the CORECONF management interface is shown in the
   table below:

   +------------------------------+--------------+-----------+
   | CoAP resource                | Example path | rt        |
   +------------------------------+--------------+-----------+
   | Datastore resource           | /c           | core.c.ds |
   |                              |              |           |
   | Data node resource           | /c/YANGSID   | core.c.dn |
   |                              |              |           |
   | Default event steam resource | /s           | core.c.ev |
   +------------------------------+--------------+-----------+

   The path values in the table are example ones.  On discovery, the
   server makes the actual path values known for these resources.

   The methods used by CORECONF are:

   +-----------+-------------------------------------------------------+
   | Operation | Description                                           |
   +-----------+-------------------------------------------------------+
   | GET       | Retrieve the datastore resource or a data node        |
   |           | resource                                              |
   |           |                                                       |
   | FETCH     | Retrieve specific data nodes within a datastore       |
   |           | resource                                              |
   |           |                                                       |
   | POST      | Create a datastore resource or a data node resource,  |
   |           | invoke an RPC or action                               |
   |           |                                                       |
   | PUT       | Create or replace a datastore resource or a data node |
   |           | resource                                              |
   |           |                                                       |
   | iPATCH    | Idem-potently create, replace, and delete data node   |
   |           | resource(s) within a datastore resource               |
   |           |                                                       |
   | DELETE    | Delete a datastore resource or a data node resource   |
   +-----------+-------------------------------------------------------+

   There is at most one instance of the 'k' query parameter for YANG
   list element selection for the GET, PUT, POST, and DELETE methods.
   Having multiple instances of that query parameter shall be treated as
   an error.

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   +-----------------+----------------------------------------+
   | Query parameter | Description                            |
   +-----------------+----------------------------------------+
   | k               | Select an instance within YANG list(s) |
   +-----------------+----------------------------------------+

   This parameter is not used for FETCH and iPATCH, because their
   request payloads support list instance selection.

4.1.  Using the 'k' query parameter

   The 'k' (key) parameter specifies a specific instance of a data node.
   The SID in the URI is followed by the (?k=key1,key2,...).  Where SID
   identifies a data node, and key1, key2 are the values of the key
   leaves that specify an instance.  Lists can have multiple keys, and
   lists can be part of lists.  The order of key value generation is
   given recursively by:

   o  For a given list, if a parent data node is a list, generate the
      keys for the parent list first.

   o  For a given list, generate key values in the order specified in
      the YANG module.

   Key values are encoded using the rules defined in the following
   table.

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   +-----------------------------+--------------------------------+
   | YANG datatype               | Uri-Query text content         |
   +-----------------------------+--------------------------------+
   | uint8,uint16,unit32, uint64 | int2str(key)                   |
   |                             |                                |
   | int8, int16,int32, int64    | urlSafeBase64(CBORencode(key)) |
   |                             |                                |
   | decimal64                   | urlSafeBase64(CBOR key)        |
   |                             |                                |
   | string                      | key                            |
   |                             |                                |
   | boolean                     | "0" or "1"                     |
   |                             |                                |
   | enumeration                 | int2str(key)                   |
   |                             |                                |
   | bits                        | urlSafeBase64(CBORencode(key)) |
   |                             |                                |
   | binary                      | urlSafeBase64(key)             |
   |                             |                                |
   | identityref                 | int2str(key)                   |
   |                             |                                |
   | union                       | urlSafeBase64(CBORencode(key)) |
   |                             |                                |
   | instance-identifier         | urlSafeBase64(CBORencode(key)) |
   +-----------------------------+--------------------------------+

   In this table:

   o  The method int2str() is used to convert an integer value to a
      decimal string.  For example, int2str(0x0123) return the three-
      character string "291".

   o  The boolean values false and true are represented as the single-
      character strings "0" and "1" respectively.

   o  The method urlSafeBase64() is used to convert a binary string to
      base64 using the URL and Filename safe alphabet as defined by
      [RFC4648] section 5, without padding.  For example,
      urlSafeBase64(0xF956A13C) return the six-character string
      "-VahPA".

   o  The method CBORencode() is used to convert a YANG value to CBOR as
      specified in [I-D.ietf-core-yang-cbor] section 6.

   The resulting key strings are joined using commas between every two
   consecutive key values to produce the value of the 'k' parameter.

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4.2.  Data Retrieval

   One or more data nodes can be retrieved by the client.  The operation
   is mapped to the GET method defined in section 5.8.1 of [RFC7252] and
   to the FETCH method defined in section 2 of [RFC8132].

   There are two additional query parameters for the GET and FETCH
   methods.

   +-------------+-----------------------------------------------------+
   | query       | Description                                         |
   | parameters  |                                                     |
   +-------------+-----------------------------------------------------+
   | c           | Control selection of configuration and non-         |
   |             | configuration data nodes (GET and FETCH)            |
   |             |                                                     |
   | d           | Control retrieval of default values.                |
   +-------------+-----------------------------------------------------+

4.2.1.  Using the 'c' query parameter

   The 'c' (content) option controls how descendant nodes of the
   requested data nodes will be processed in the reply.

   The allowed values are:

   +-------+-----------------------------------------------------+
   | Value | Description                                         |
   +-------+-----------------------------------------------------+
   | c     | Return only configuration descendant data nodes     |
   |       |                                                     |
   | n     | Return only non-configuration descendant data nodes |
   |       |                                                     |
   | a     | Return all descendant data nodes                    |
   +-------+-----------------------------------------------------+

   This option is only allowed for GET and FETCH methods on datastore
   and data node resources.  A 4.02 (Bad Option) error is returned if
   used for other methods or resource types.

   If this query parameter is not present, the default value is "a" (the
   quotes are added for readability, but they are not part of the
   payload).

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4.2.2.  Using the 'd' query parameter

   The 'd' (with-defaults) option controls how the default values of the
   descendant nodes of the requested data nodes will be processed.

   The allowed values are:

   +-------+-----------------------------------------------------------+
   | Value | Description                                               |
   +-------+-----------------------------------------------------------+
   | a     | All data nodes are reported. Defined as 'report-all' in   |
   |       | section 3.1 of [RFC6243].                                 |
   |       |                                                           |
   | t     | Data nodes set to the YANG default are not reported.      |
   |       | Defined as 'trim' in section 3.2 of [RFC6243].            |
   +-------+-----------------------------------------------------------+

   If the target of a GET or FETCH method is a data node that represents
   a leaf that has a default value, and the leaf has not been given a
   value by any client yet, the server MUST return the default value of
   the leaf.

   If the target of a GET method is a data node that represents a
   container or list that has child resources with default values, and
   these have not been given value yet,

      The server MUST NOT return the child resource if d=t

      The server MUST return the child resource if d=a.

   If this query parameter is not present, the default value is "t" (the
   quotes are added for readability, but they are not part of the
   payload).

4.2.3.  GET

   A request to read the value of a data node instance is sent with a
   CoAP GET message.  The URI is set to the data node resource
   requested, the 'k' query parameter is added if any of the parents of
   the requested data node is a list node.

   FORMAT:
     GET <data node resource> ['k' Uri-Query option]

     2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

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   The returned payload contains the CBOR encoding of the requested
   instance-value.

4.2.3.1.  GET Examples

   Using, for example, the current-datetime leaf from module ietf-system
   [RFC7317], a request is sent to retrieve the value of 'system-
   state/clock/current-datetime'.  The SID of 'system-state/clock/
   current-datetime' is 1723, encoded in base64 according to
   Section 2.2, yields a7.  The response to the request returns the CBOR
   map with the key set to the SID of the requested data node (i.e.
   1723) and the value encoded using a 'text string' as defined in
   [I-D.ietf-core-yang-cbor] section 6.4.  The datastore resource path
   /c is an example location discovered with a request similar to
   Figure 4.

  REQ: GET </c/a7>

  RES: 2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
  {
    1723 : "2014-10-26T12:16:31Z"
  }

   The next example represents the retrieval of a YANG container.  In
   this case, the CORECONF client performs a GET request on the clock
   container (SID = 1721; base64: a5).  The container returned is
   encoded using a CBOR map as specified by [I-D.ietf-core-yang-cbor]
   section 4.2.

  REQ: GET </c/a5>

  RES: 2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
  {
    1721 : {
      2 : "2014-10-26T12:16:51Z",    / current-datetime (SID 1723) /
      1 : "2014-10-21T03:00:00Z"     / boot-datetime (SID 1722) /
    }
  }

                                 Figure 3

   This example shows the retrieval of the /interfaces/interface YANG
   list accessed using SID 1533 (base64: X9).  The return payload is
   encoded using a CBOR array as specified by [I-D.ietf-core-yang-cbor]
   section 4.4.1 containing 2 instances.

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  REQ: GET </c/X9>

  RES: 2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
  {
    1533 : [
      {
        4 : "eth0",                 / name  (SID 1537) /
        1 : "Ethernet adaptor",     / description (SID 1534) /
        5 : 1880,                   / type, (SID 1538) identity /
                                    / ethernetCsmacd (SID 1880) /
        2 : true                    / enabled (SID 1535) /
      },
      {
        4 : "eth1",                 / name (SID 1537) /
        1 : "Ethernet adaptor",     / description (SID 1534) /
        5 : 1880,                   / type, (SID 1538) identity /
                                    / ethernetCsmacd (SID 1880) /
        2 : false                   / enabled (SID 1535) /
      }
    ]
  }

   To retrieve a specific instance within the /interfaces/interface YANG
   list, the CORECONF client adds the key of the targeted instance in
   its CoAP request using the 'k' query parameter.  The return payload
   containing the instance requested is encoded using a CBOR array as
   specified by [I-D.ietf-core-yang-cbor] section 4.4.1 containing the
   requested instance.

  REQ: GET </c/X9?k=eth0>

  RES: 2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
  {
    1533 : [
      {
        4 : "eth0",                 / name  (SID 1537) /
        1 : "Ethernet adaptor",     / description (SID 1534) /
        5 : 1880,                   / type, (SID 1538) identity /
                                    / ethernetCsmacd (SID 1880) /
        2 : true                    / enabled (SID 1535) /
      }
    ]
  }

   It is equally possible to select a leaf of a specific instance of a
   list.  The example below requests the description leaf (SID 1534,
   base64: X-) within the interface list corresponding to the interface

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   name "eth0".  The returned value is encoded in CBOR based on the
   rules specified by [I-D.ietf-core-yang-cbor] section 6.4.

  REQ: GET </c/X-?k=eth0>

  RES: 2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
  {
    1534 : "Ethernet adaptor"
  }

4.2.4.  FETCH

   The FETCH is used to retrieve multiple instance-values.  The FETCH
   request payload contains the list of instance-identifier of the data
   node instances requested.

   The return response payload contains a list of data node instance-
   values in the same order as requested.  A CBOR null is returned for
   each data node requested by the client, not supported by the server
   or not currently instantiated.

   For compactness, indexes of the list instance identifiers returned by
   the FETCH response SHOULD be elided, only the SID is provided.  This
   approach may also help reducing implementations complexity since the
   format of each entry within the CBOR array of the FETCH response is
   identical to the format of the corresponding GET response.

   FORMAT:
     FETCH <datastore resource>
           (Content-Format: application/yang-identifiers+cbor)
     CBOR array of instance-identifier

     2.05 Content (Content-Format: application/yang-instances+cbor)
     CBOR array of CBOR map of SID, instance-value

4.2.4.1.  FETCH examples

   This example uses the current-datetime leaf from module ietf-system
   [RFC7317] and the interface list from module ietf-interfaces
   [RFC7223].  In this example the value of current-datetime (SID 1723)
   and the interface list (SID 1533) instance identified with
   name="eth0" are queried.

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   REQ: FETCH </c>
        (Content-Format: application/yang-identifiers+cbor)
   [
     1723,            / current-datetime (SID 1723) /
     [1533, "eth0"]   / interface (SID 1533) with name = "eth0" /
   ]

   RES: 2.05 Content (Content-Format: application/yang-instances+cbor)
   [
     {
       1723 : "2014-10-26T12:16:31Z" / current-datetime (SID 1723) /
     },
     {
       1533 : {
         4 : "eth0",                 / name (SID 1537) /
         1 : "Ethernet adaptor",     / description (SID 1534) /
         5 : 1880,                   / type (SID 1538), identity /
                                     / ethernetCsmacd (SID 1880) /
         2 : true                    / enabled (SID 1535) /
       }
     }
   ]

4.3.  Data Editing

   CORECONF allows datastore contents to be created, modified and
   deleted using CoAP methods.

4.3.1.  Data Ordering

   A CORECONF server MUST preserve the relative order of all user-
   ordered list and leaf-list entries that are received in a single edit
   request.  These YANG data node types are encoded as CBOR arrays so
   messages will preserve their order.

4.3.2.  POST

   The CoAP POST operation is used in CORECONF for the creation of data
   node resources and the invocation of "ACTION" and "RPC" resources.
   Refer to Section 4.6 for details on "ACTION" and "RPC" resources.

   A request to create a data node instance is sent with a CoAP POST
   message.  The URI specifies the data node resource of the instance to
   be created.  In the case of a list instance, keys MUST be present in
   the payload.

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   FORMAT:
     POST <data node resource>
          (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

     2.01 Created

   If the data node instance already exists, then the POST request MUST
   fail and a "4.09 Conflict" response code MUST be returned

4.3.2.1.  Post example

   The example uses the interface list from module ietf-interfaces
   [RFC7223].  This example creates a new list instance within the
   interface list (SID = 1533), while assuming the datastore resource is
   hosted on the CoAP server with DNS name example.com and with path
   /ds.  The path /ds is an example location that is assumed to have
   been discovered using request similar to Figure 4.

   REQ: POST <coap://example.com/ds/X9>
        (Content-Format: application/yang-data+cbor; id=sid)
   {
     1533 : [
       {
         4 : "eth5",              / name (SID 1537) /
         1 : "Ethernet adaptor",  / description (SID 1534) /
         5 : 1880,                / type (SID 1538), identity /
                                  / ethernetCsmacd (SID 1880) /
         2 : true                 / enabled (SID 1535) /
       }
     ]
   }

   RES: 2.01 Created

4.3.3.  PUT

   A data node resource instance is created or replaced with the PUT
   method.  A request to set the value of a data node instance is sent
   with a CoAP PUT message.

   FORMAT:
     PUT <data node resource> ['k' Uri-Query option]
         (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

     2.01 Created

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4.3.3.1.  PUT example

   The example uses the interface list from module ietf-interfaces
   [RFC7223].  This example updates the instance of the list interface
   (SID = 1533) with key name="eth0".  The example location /c is an
   example location that is discovered using a request similar to
   Figure 4.

   REQ: PUT </c/X9?k=eth0>
        (Content-Format: application/yang-data+cbor; id=sid)
   {
     1533 : [
       {
         4 : "eth0",              / name (SID 1537) /
         1 : "Ethernet adaptor",  / description (SID 1534) /
         5 : 1880,                / type (SID 1538), identity /
                                  / ethernetCsmacd (SID 1880) /
         2 : true                 / enabled (SID 1535) /
       }
     ]
   }

   RES:  2.04 Changed

4.3.4.  iPATCH

   One or multiple data node instances are replaced with the idempotent
   CoAP iPATCH method [RFC8132].

   There are no query parameters for the iPATCH method.

   The processing of the iPATCH command is specified by Media-Type
   'application/yang-instances+cbor'.  In summary, if the CBOR patch
   payload contains a data node instance that is not present in the
   target, this instance is added.  If the target contains the specified
   instance, the content of this instance is replaced with the value of
   the payload.  A null value indicates the removal of an existing data
   node instance.

   FORMAT:
     iPATCH <datastore resource>
            (Content-Format: application/yang-instances+cbor)
     CBOR array of CBOR map of instance-identifier, instance-value

     2.04 Changed

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4.3.4.1.  iPATCH example

   In this example, a CORECONF client requests the following operations:

   o  Set "/system/ntp/enabled" (SID 1755) to true.

   o  Remove the server "tac.nrc.ca" from the "/system/ntp/server" (SID
      1756) list.

   o  Add/set the server "NTP Pool server 2" to the list "/system/ntp/
      server" (SID 1756).

   REQ: iPATCH </c>
        (Content-Format: application/yang-instances+cbor)
   [
     {
       1755 : true                   / enabled (SID 1755) /
     },
     {
       [1756, "tac.nrc.ca"] : null   / server (SID 1756) /
     },
     {
       1756 : {                      / server (SID 1756) /
         3 : "tic.nrc.ca",           / name (SID 1759) /
         4 : true,                   / prefer (SID 1760) /
         5 : {                       / udp (SID 1761) /
           1 : "132.246.11.231"      / address (SID 1762) /
         }
       }
     }
   ]

   RES: 2.04 Changed

4.3.5.  DELETE

   A data node resource is deleted with the DELETE method.

   FORMAT:
     Delete <data node resource> ['k' Uri-Query option]

     2.02 Deleted

4.3.5.1.  DELETE example

   This example uses the interface list from module ietf-interfaces
   [RFC7223].  This example deletes an instance of the interface list
   (SID = 1533):

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   REQ:   DELETE </c/X9?k=eth0>

   RES:   2.02 Deleted

4.4.  Full datastore access

   The methods GET, PUT, POST, and DELETE can be used to request,
   replace, create, and delete a whole datastore respectively.

   FORMAT:
     GET <datastore resource>

     2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

   FORMAT:
     PUT <datastore resource>
         (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

     2.04 Changed

   FORMAT:
     POST <datastore resource>
          (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

     2.01 Created

   FORMAT:
     DELETE <datastore resource>

     2.02 Deleted

   The content of the CBOR map represents the complete datastore of the
   server at the GET indication of after a successful processing of a
   PUT or POST request.

4.4.1.  Full datastore examples

   The example uses the interface list from module ietf-interfaces
   [RFC7223] and the clock container from module ietf-system [RFC7317].
   We assume that the datastore contains two modules ietf-system (SID
   1700) and ietf-interfaces (SID 1500); they contain the 'interface'
   list (SID 1533) with one instance and the 'clock' container (SID
   1721).  After invocation of GET, a CBOR map with data nodes from
   these two modules is returned:

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  REQ:  GET </c>

  RES: 2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
  {
    1721 : {                       / Clock (SID 1721) /
      2: "2016-10-26T12:16:31Z",   / current-datetime (SID 1723) /
      1: "2014-10-05T09:00:00Z"    / boot-datetime (SID 1722) /
    },
    1533 : [
      {                              / interface (SID 1533) /
        4 : "eth0",                  / name (SID 1537) /
        1 : "Ethernet adaptor",      / description (SID 1534) /
        5 : 1880,                    / type (SID 1538), identity: /
                                     / ethernetCsmacd (SID 1880) /
        2 : true                     / enabled (SID 1535) /
      }
    ]
  }

4.5.  Event stream

   Event notification is an essential function for the management of
   servers.  CORECONF allows notifications specified in YANG [RFC5277]
   to be reported to a list of clients.  The path for the default event
   stream can be discovered as described in Section 4.  The server MAY
   support additional event stream resources to address different
   notification needs.

   Reception of notification instances is enabled with the CoAP Observe
   [RFC7641] function.  Clients subscribe to the notifications by
   sending a GET request with an "Observe" option to the stream
   resource.

   Each response payload carries one or multiple notifications.  The
   number of notifications reported, and the conditions used to remove
   notifications from the reported list are left to implementers.  When
   multiple notifications are reported, they MUST be ordered starting
   from the newest notification at index zero.  Note that this could
   lead to notifications being sent multiple times, which increases the
   probability for the client to receive them, but it might potentially
   lead to messages that exceed the MTU of a single CoAP packet.  If
   such cases could arise, implementers should make sure appropriate
   fragmentation is available - for example the one described in
   Section 5.

   The format of notification without any content is a null value.  The
   format of single notification is defined in [I-D.ietf-core-yang-cbor]
   section 4.2.1.  For multiple notifications the format is an array

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   where each element is a single notification as described in
   [I-D.ietf-core-yang-cbor] section 4.2.1.

   FORMAT:
     GET <stream-resource> Observe(0)

     2.05 Content (Content-Format: application/yang-instances+cbor)
     CBOR array of CBOR map of instance-identifier, instance-value

   The array of data node instances may contain identical entries which
   have been generated at different times.

   An example implementation is:

      Every time an event is generated, the generated notification
      instance is appended to the chosen stream(s).  After an
      aggregation period, which may be limited by the maximum number of
      notifications supported, the content of the instance is sent to
      all clients observing the modified stream.

4.5.1.  Notify Examples

   Let suppose the server generates the example-port-fault event as
   defined below.

   module example-port {
     ...
     notification example-port-fault {   // SID 60010
       description
         "Event generated if a hardware fault is detected";
       leaf port-name {                  // SID 60011
         type string;
       }
       leaf port-fault {                 // SID 60012
         type string;
       }
     }
   }

   In this example the default event stream resource path /s is an
   example location discovered with a request similar to Figure 5.  By
   executing a GET with Observe 0 on the default event stream resource
   the client receives the following response:

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   REQ:  GET </s> Observe(0)

   RES:  2.05 Content (Content-Format: application/yang-tree+cbor)
         Observe(12)
   [
     {
       60010 : {             / example-port-fault (SID 60010) /
         1 : "0/4/21",       / port-name (SID 60011) /
         2 : "Open pin 2"    / port-fault (SID 60012) /
       }
     },
     {
       60010 : {             / example-port-fault (SID 60010) /
         1 : "1/4/21",       / port-name (SID 60011) /
         2 : "Open pin 5"    / port-fault (SID 60012) /
       }
     }
   ]

   In the example, the request returns a success response with the
   contents of the last two generated events.  Consecutively the server
   will regularly notify the client when a new event is generated.

4.5.2.  The 'f' query parameter

   The 'f' (filter) option is used to indicate which subset of all
   possible notifications is of interest.  If not present, all
   notifications supported by the event stream are reported.

   When not supported by a CORECONF server, this option shall be
   ignored, all events notifications are reported independently of the
   presence and content of the 'f' (filter) option.

   When present, this option contains a comma-separated list of
   notification SIDs.  For example, the following request returns
   notifications 60010 and 60020.

   REQ:  GET </s?f=60010,60020> Observe(0)

4.6.  RPC statements

   The YANG "action" and "RPC" statements specify the execution of a
   Remote procedure Call (RPC) in the server.  It is invoked using a
   POST method to an "Action" or "RPC" resource instance.

   The request payload contains the values assigned to the input
   container when specified.  The response payload contains the values
   of the output container when specified.  Both the input and output

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   containers are encoded in CBOR using the rules defined in
   [I-D.ietf-core-yang-cbor] section 4.2.1.

   The returned success response code is 2.05 Content.

   FORMAT:
     POST <data node resource> ['k' Uri-Query option]
          (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

     2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
     CBOR map of SID, instance-value

4.6.1.  RPC Example

   The example is based on the YANG action "reset" as defined in
   [RFC7950] section 7.15.3 and annotated below with SIDs.

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   module example-server-farm {
     yang-version 1.1;
     namespace "urn:example:server-farm";
     prefix "sfarm";

     import ietf-yang-types {
       prefix "yang";
     }

     list server {                        // SID 60000
       key name;
       leaf name {                        // SID 60001
         type string;
       }
       action reset {                     // SID 60002
         input {
           leaf reset-at {                // SID 60003
             type yang:date-and-time;
             mandatory true;
            }
          }
          output {
            leaf reset-finished-at {      // SID 60004
              type yang:date-and-time;
              mandatory true;
            }
          }
        }
      }
    }

   This example invokes the 'reset' action (SID 60002, base64: Opq), of
   the server instance with name equal to "myserver".

 REQ:  POST </c/Opq?k=myserver>
       (Content-Format: application/yang-data+cbor; id=sid)
 {
   60002 : {
     1 : "2016-02-08T14:10:08Z09:00" / reset-at (SID 60003) /
   }
 }

 RES:  2.05 Content (Content-Format: application/yang-data+cbor; id=sid)
 {
   60002 : {
     2 : "2016-02-08T14:10:08Z09:18" / reset-finished-at (SID 60004)/
   }
 }

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5.  Use of Block-wise Transfers

   The CoAP protocol provides reliability by acknowledging the UDP
   datagrams.  However, when large pieces of data need to be
   transported, datagrams get fragmented, thus creating constraints on
   the resources in the client, server and intermediate routers.  The
   block option [RFC7959] allows the transport of the total payload in
   individual blocks of which the size can be adapted to the underlying
   transport sizes such as: (UDP datagram size ~64KiB, IPv6 MTU of 1280,
   IEEE 802.15.4 payload of 60-80 bytes).  Each block is individually
   acknowledged to guarantee reliability.

   Notice that the Block mechanism splits the data at fixed positions,
   such that individual data fields may become fragmented.  Therefore,
   assembly of multiple blocks may be required to process complete data
   fields.

   Beware of race conditions.  In case blocks are filled one at a time,
   care should be taken that the whole and consistent data
   representation is sent in multiple blocks sequentially without
   interruption.  On the server, values might change, lists might get
   re-ordered, extended or reduced.  When these actions happen during
   the serialization of the contents of the resource, the transported
   results do not correspond with a state having occurred in the server;
   or worse the returned values are inconsistent.  For example: array
   length does not correspond with the actual number of items.  It may
   be advisable to use Indefinite-length CBOR arrays and maps, which are
   foreseen for data streaming purposes.

6.  Application Discovery

   Two application discovery mechanisms are supported by CORECONF, the
   YANG library data model as defined by [I-D.ietf-core-yang-library]
   and the CORE resource discovery [RFC6690].  Implementers may choose
   to implement one or the other or both.

6.1.  YANG library

   The YANG library data model [I-D.ietf-core-yang-library] provides a
   high-level description of the resources available.  The YANG library
   contains the list of modules, features, and deviations supported by
   the CORECONF server.  From this information, CORECONF clients can
   infer the list of data nodes supported and the interaction model to
   be used to access them.  This module also contains the list of
   datastores implemented.

   As described in [RFC6690], the location of the YANG library can be
   found by sending a GET request to "/.well-known/core" including a

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   resource type (RT) parameter with the value "core.c.yl".  Upon
   success, the return payload will contain the root resource of the
   YANG library module.

   The following example assumes that the SID of the YANG library is
   2351 (kv encoded as specified in Section 2.2) and that the server
   uses /c as datastore resource path.

   REQ: GET </.well-known/core?rt=core.c.yl>

   RES: 2.05 Content (Content-Format: application/link-format)
   </c/kv>;rt="core.c.yl"

6.2.  Resource Discovery

   As some CoAP interfaces and services might not support the YANG
   library interface and still be interested to discover resources that
   are available, implementations MAY choose to support discovery of all
   available resources using "/.well-known/core" as defined by
   [RFC6690].

6.2.1.  Datastore Resource Discovery

   The presence and location of (path to) each datastore implemented by
   the CORECONF server can be discovered by sending a GET request to
   "/.well-known/core" including a resource type (RT) parameter with the
   value "core.c.ds".

   Upon success, the return payload contains the list of datastore
   resources.

   Each datastore returned is further qualified using the "ds" Link-
   Format attribute.  This attribute is set to the SID assigned to the
   datastore identity.  When a unified datastore is implemented, the ds
   attribute is set to 1029 as specified in Appendix B.  For other
   examples of datastores, see the Network Management Datastore
   Architecture (NMDA) [RFC7950].

   link-extension    = ( "ds" "=" sid ) )
                       ; SID assigned to the datastore identity
   sid               = 1*DIGIT

   The following example assumes that the server uses /c as datastore
   resource path.

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   REQ: GET </.well-known/core?rt=core.c.ds>

   RES: 2.05 Content (Content-Format: application/link-format)
   </c>; rt="core.c.ds";ds=1029

                                 Figure 4

6.2.2.  Data node Resource Discovery

   If implemented, the presence and location of (path to) each data node
   implemented by the CORECONF server are discovered by sending a GET
   request to "/.well-known/core" including a resource type (RT)
   parameter with the value "core.c.dn".

   Upon success, the return payload contains the SID assigned to each
   data node and their location.

   The example below shows the discovery of the presence and location of
   data nodes.  Data nodes '/ietf-system:system-state/clock/boot-
   datetime' (SID 1722) and '/ietf-system:system-state/clock/current-
   datetime' (SID 1723) are returned.  The example assumes that the
   server uses /c as datastore resource path.

   REQ: GET </.well-known/core?rt=core.c.dn>

   RES: 2.05 Content (Content-Format: application/link-format)
   </c/a6>;rt="core.c.dn",
   </c/a7>;rt="core.c.dn"

   Without additional filtering, the list of data nodes may become
   prohibitively long.  If this is the case implementations SHOULD
   support a way to obtain all links using multiple GET requests (for
   example through some form of pagination).

6.2.3.  Event stream Resource Discovery

   The presence and location of (path to) each event stream implemented
   by the CORECONF server are discovered by sending a GET request to
   "/.well-known/core" including a resource type (RT) parameter with the
   value "core.c.es".

   Upon success, the return payload contains the list of event stream
   resources.

   The following example assumes that the server uses /s as the default
   event stream resource.

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   REQ: GET </.well-known/core?rt=core.c.es>

   RES: 2.05 Content (Content-Format: application/link-format)
   </s>;rt="core.c.es"

                                 Figure 5

7.  Error Handling

   In case a request is received which cannot be processed properly, the
   CORECONF server MUST return an error response.  This error response
   MUST contain a CoAP 4.xx or 5.xx response code.

   Errors returned by a CORECONF server can be broken into two
   categories, those associated with the CoAP protocol itself and those
   generated during the validation of the YANG data model constrains as
   described in [RFC7950] section 8.

   The following list of common CoAP errors should be implemented by
   CORECONF servers.  This list is not exhaustive, other errors defined
   by CoAP and associated RFCs may be applicable.

   o  Error 4.01 (Unauthorized) is returned by the CORECONF server when
      the CORECONF client is not authorized to perform the requested
      action on the targeted resource (i.e. data node, datastore, rpc,
      action or event stream).

   o  Error 4.02 (Bad Option) is returned by the CORECONF server when
      one or more CoAP options are unknown or malformed.

   o  Error 4.04 (Not Found) is returned by the CORECONF server when the
      CORECONF client is requesting a non-instantiated resource (i.e.
      data node, datastore, rpc, action or event stream).

   o  Error 4.05 (Method Not Allowed) is returned by the CORECONF server
      when the CORECONF client is requesting a method not supported on
      the targeted resource. (e.g.  GET on an rpc, PUT or POST on a data
      node with "config" set to false).

   o  Error 4.08 (Request Entity Incomplete) is returned by the CORECONF
      server if one or multiple blocks of a block transfer request is
      missing, see [RFC7959] for more details.

   o  Error 4.13 (Request Entity Too Large) may be returned by the
      CORECONF server during a block transfer request, see [RFC7959] for
      more details.

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   o  Error 4.15 (Unsupported Content-Format) is returned by the
      CORECONF server when the Content-Format used in the request does
      not match those specified in section Section 2.4.

   The CORECONF server MUST also enforce the different constraints
   associated with the YANG data models implemented.  These constraints
   are described in [RFC7950] section 8.  These errors are reported
   using the CoAP error code 4.00 (Bad Request) and may have the
   following error container as payload.  The YANG definition and
   associated .sid file are available in Appendix A and Appendix B.  The
   error container is encoded using the encoding rules of a YANG data
   template as defined in [I-D.ietf-core-yang-cbor] section 5.

   +--rw error!
      +--rw error-tag             identityref
      +--rw error-app-tag?        identityref
      +--rw error-data-node?      instance-identifier
      +--rw error-message?        string

   The following 'error-tag' and 'error-app-tag' are defined by the
   ietf-coreconf YANG module, these tags are implemented as YANG
   identity and can be extended as needed.

   o  error-tag 'operation-failed' is returned by the CORECONF server
      when the operation request cannot be processed successfully.

      *  error-app-tag 'malformed-message' is returned by the CORECONF
         server when the payload received from the CORECONF client does
         not contain a well-formed CBOR content as defined in [RFC7049]
         section 3.3 or does not comply with the CBOR structure defined
         within this document.

      *  error-app-tag 'data-not-unique' is returned by the CORECONF
         server when the validation of the 'unique' constraint of a list
         or leaf-list fails.

      *  error-app-tag 'too-many-elements' is returned by the CORECONF
         server when the validation of the 'max-elements' constraint of
         a list or leaf-list fails.

      *  error-app-tag 'too-few-elements' is returned by the CORECONF
         server when the validation of the 'min-elements' constraint of
         a list or leaf-list fails.

      *  error-app-tag 'must-violation' is returned by the CORECONF
         server when the restrictions imposed by a 'must' statement are
         violated.

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      *  error-app-tag 'duplicate' is returned by the CORECONF server
         when a client tries to create a duplicate list or leaf-list
         entry.

   o  error-tag 'invalid-value' is returned by the CORECONF server when
      the CORECONF client tries to update or create a leaf with a value
      encoded using an invalid CBOR datatype or if the 'range',
      'length', 'pattern' or 'require-instance' constrain is not
      fulfilled.

      *  error-app-tag 'invalid-datatype' is returned by the CORECONF
         server when CBOR encoding does not follow the rules set by the
         YANG Build-In type or when the value is incompatible with it
         (e.g. a value greater than 127 for an int8, undefined
         enumeration).

      *  error-app-tag 'not-in-range' is returned by the CORECONF server
         when the validation of the 'range' property fails.

      *  error-app-tag 'invalid-length' is returned by the CORECONF
         server when the validation of the 'length' property fails.

      *  error-app-tag 'pattern-test-failed' is returned by the CORECONF
         server when the validation of the 'pattern' property fails.

   o  error-tag 'missing-element' is returned by the CORECONF server
      when the operation requested by a CORECONF client fails to comply
      with the 'mandatory' constraint defined.  The 'mandatory'
      constraint is enforced for leafs and choices, unless the node or
      any of its ancestors have a 'when' condition or 'if-feature'
      expression that evaluates to 'false'.

      *  error-app-tag 'missing-key' is returned by the CORECONF server
         to further qualify a missing-element error.  This error is
         returned when the CORECONF client tries to create or list
         instance, without all the 'key' specified or when the CORECONF
         client tries to delete a leaf listed as a 'key'.

      *  error-app-tag 'missing-input-parameter' is returned by the
         CORECONF server when the input parameters of an RPC or action
         are incomplete.

   o  error-tag 'unknown-element' is returned by the CORECONF server
      when the CORECONF client tries to access a data node of a YANG
      module not supported, of a data node associated with an 'if-
      feature' expression evaluated to 'false' or to a 'when' condition
      evaluated to 'false'.

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   o  error-tag 'bad-element' is returned by the CORECONF server when
      the CORECONF client tries to create data nodes for more than one
      case in a choice.

   o  error-tag 'data-missing' is returned by the CORECONF server when a
      data node required to accept the request is not present.

      *  error-app-tag 'instance-required' is returned by the CORECONF
         server when a leaf of type 'instance-identifier' or 'leafref'
         marked with require-instance set to 'true' refers to an
         instance that does not exist.

      *  error-app-tag 'missing-choice' is returned by the CORECONF
         server when no nodes exist in a mandatory choice.

   o  error-tag 'error' is returned by the CORECONF server when an
      unspecified error has occurred.

   For example, the CORECONF server might return the following error.

RES:  4.00 Bad Request (Content-Format: application/yang-data+cbor; id=sid)
{
  1024 : {
    4 : 1011,        / error-tag (SID 1028) /
                     /   = invalid-value (SID 1011) /
    1 : 1018,        / error-app-tag (SID 1025) /
                     /   = not-in-range (SID 1018) /
    2 : 1740,        / error-data-node (SID 1026) /
                     /   = timezone-utc-offset (SID 1740) /
    3 : "maximum value exceeded" / error-message (SID 1027) /
  }
}

8.  Security Considerations

   For secure network management, it is important to restrict access to
   configuration variables only to authorized parties.  CORECONF re-uses
   the security mechanisms already available to CoAP, this includes DTLS
   [RFC6347] and OSCORE [RFC8613] for protected access to resources, as
   well as suitable authentication and authorization mechanisms, for
   example those defined in ACE OAuth [I-D.ietf-ace-oauth-authz].

   All the security considerations of [RFC7252], [RFC7959], [RFC8132]
   and [RFC7641] apply to this document as well.  The use of NoSec DTLS,
   when OSCORE is not used, is NOT RECOMMENDED.

   In addition, mechanisms for authentication and authorization may need
   to be selected if not provided with the CoAP security mode.

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   As [I-D.ietf-core-yang-cbor] and [RFC4648] are used for payload and
   SID encoding, the security considerations of those documents also
   need to be well-understood.

9.  IANA Considerations

9.1.  Resource Type (rt=) Link Target Attribute Values Registry

   This document adds the following resource type to the "Resource Type
   (rt=) Link Target Attribute Values", within the "Constrained RESTful
   Environments (CoRE) Parameters" registry.

   +-----------+---------------------+-----------+
   | Value     | Description         | Reference |
   +-----------+---------------------+-----------+
   | core.c.ds | YANG datastore      | RFC XXXX  |
   |           |                     |           |
   | core.c.dn | YANG data node      | RFC XXXX  |
   |           |                     |           |
   | core.c.yl | YANG module library | RFC XXXX  |
   |           |                     |           |
   | core.c.es | YANG event stream   | RFC XXXX  |
   +-----------+---------------------+-----------+

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

9.2.  CoAP Content-Formats Registry

   This document adds the following Content-Format to the "CoAP Content-
   Formats", within the "Constrained RESTful Environments (CoRE)
   Parameters" registry.

   +-----------------------------------+------------+------+-----------+
   | Media Type                        | Content    | ID   | Reference |
   |                                   | Coding     |      |           |
   +-----------------------------------+------------+------+-----------+
   | application/yang-identifiers+cbor |            | TBD2 | RFC XXXX  |
   |                                   |            |      |           |
   | application/yang-instances+cbor   |            | TBD3 | RFC XXXX  |
   +-----------------------------------+------------+------+-----------+

   // RFC Ed.: replace TBD1, TBD2 and TBD3 with assigned IDs and remove
   this note.  // RFC Ed.: replace RFC XXXX with this RFC number and
   remove this note.

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9.3.  Media Types Registry

   This document adds the following media types to the "Media Types"
   registry.

   +-----------------------+-----------------------+-----------+
   | Name                  | Template              | Reference |
   +-----------------------+-----------------------+-----------+
   | yang-identifiers+cbor | application/          | RFC XXXX  |
   |                       |                       |           |
   |                       | yang-identifiers+cbor |           |
   |                       |                       |           |
   | yang-instances+cbor   | application/          | RFC XXXX  |
   |                       |                       |           |
   |                       | yang-instances+cbor   |           |
   +-----------------------+-----------------------+-----------+

   Each of these media types share the following information:

   o  Subtype name: <as listed in table>

   o  Required parameters: N/A

   o  Optional parameters: N/A

   o  Encoding considerations: binary

   o  Security considerations: See the Security Considerations section
      of RFC XXXX

   o  Interoperability considerations: N/A

   o  Published specification: RFC XXXX

   o  Applications that use this media type: CORECONF

   o  Fragment identifier considerations: N/A

   o  Additional information:

   *  Deprecated alias names for this type: N/A

   *  Magic number(s): N/A

   *  File extension(s): N/A

   *  Macintosh file type code(s): N/A

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   o  Person & email address to contact for further information:
      iesg&ietf.org

   o  Intended usage: COMMON

   o  Restrictions on usage: N/A

   o  Author: Michel Veillette, ietf&augustcellars.com

   o  Change Controller: IESG

   o  Provisional registration?  No

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

9.4.  YANG Namespace Registration

   This document registers the following XML namespace URN in the "IETF
   XML Registry", following the format defined in [RFC3688]:

   URI: please assign urn:ietf:params:xml:ns:yang:ietf-coreconf

   Registrant Contact: The IESG.

   XML: N/A, the requested URI is an XML namespace.

   Reference: RFC XXXX

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

10.  Acknowledgments

   We are very grateful to Bert Greevenbosch who was one of the original
   authors of the CORECONF specification.

   Mehmet Ersue and Bert Wijnen explained the encoding aspects of PDUs
   transported under SNMP.  Carsten Bormann has given feedback on the
   use of CBOR.

   The draft has benefited from comments (alphabetical order) by Rodney
   Cummings, Dee Denteneer, Esko Dijk, Klaus Hartke, Michael van
   Hartskamp, Tanguy Ropitault, Juergen Schoenwaelder, Anuj Sehgal, Zach
   Shelby, Hannes Tschofenig, Michael Verschoor, and Thomas Watteyne.

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11.  References

11.1.  Normative References

   [I-D.ietf-core-sid]
              Veillette, M., Pelov, A., and I. Petrov, "YANG Schema Item
              iDentifier (SID)", draft-ietf-core-sid-13 (work in
              progress), June 2020.

   [I-D.ietf-core-yang-cbor]
              Veillette, M., Petrov, I., and A. Pelov, "CBOR Encoding of
              Data Modeled with YANG", draft-ietf-core-yang-cbor-12
              (work in progress), March 2020.

   [I-D.ietf-core-yang-library]
              Veillette, M. and I. Petrov, "Constrained YANG Module
              Library", draft-ietf-core-yang-library-01 (work in
              progress), January 2020.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
              <https://www.rfc-editor.org/info/rfc5277>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6243]  Bierman, A. and B. Lengyel, "With-defaults Capability for
              NETCONF", RFC 6243, DOI 10.17487/RFC6243, June 2011,
              <https://www.rfc-editor.org/info/rfc6243>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

11.2.  Informative References

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-35
              (work in progress), June 2020.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

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   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
              <https://www.rfc-editor.org/info/rfc7223>.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <https://www.rfc-editor.org/info/rfc7317>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

Appendix A.  ietf-coreconf YANG module

<CODE BEGINS> file "ietf-coreconf@2019-03-28.yang"
module ietf-coreconf {
  yang-version 1.1;

  namespace "urn:ietf:params:xml:ns:yang:ietf-coreconf";
  prefix coreconf;

  import ietf-datastores {
    prefix ds;
  }

  import ietf-restconf {
    prefix rc;
    description
      "This import statement is required to access
       the yang-data extension defined in RFC 8040.";
    reference "RFC 8040: RESTCONF Protocol";
  }

  organization
    "IETF Core Working Group";

  contact
    "Michel Veillette
     <mailto:michel.veillette@trilliantinc.com>

     Alexander Pelov
     <mailto:alexander@ackl.io>

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     Peter van der Stok
     <mailto:consultancy@vanderstok.org>

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

  description
    "This module contains the different definitions required
     by the CORECONF protocol.

     Copyright (c) 2019 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject to
     the license terms contained in, the Simplified BSD License set
     forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents
     (https://trustee.ietf.org/license-info).

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

  revision 2019-03-28 {
     description
      "Initial revision.";
    reference
      "[I-D.ietf-core-comi] CoAP Management Interface";
  }

  identity unified {
    base ds:datastore;
    description
      "Identifier of the unified configuration and operational
       state datastore.";
  }

  identity error-tag {
    description
      "Base identity for error-tag.";
  }

  identity operation-failed {
    base error-tag;
    description
      "Returned by the CORECONF server when the operation request
       can't be processed successfully.";
  }

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  identity invalid-value {
    base error-tag;
    description
      "Returned by the CORECONF server when the CORECONF client tries to
       update or create a leaf with a value encoded using an
       invalid CBOR datatype or if the 'range', 'length',
       'pattern' or 'require-instance' constrain is not
       fulfilled.";
  }

  identity missing-element {
    base error-tag;
    description
      "Returned by the CORECONF server when the operation requested
       by a CORECONF client fails to comply with the 'mandatory'
       constraint defined. The 'mandatory' constraint is
       enforced for leafs and choices, unless the node or any of
       its ancestors have a 'when' condition or 'if-feature'
       expression that evaluates to 'false'.";
  }

  identity unknown-element {
    base error-tag;
    description
      "Returned by the CORECONF server when the CORECONF client tries to
       access a data node of a YANG module not supported, of a
       data node associated with an 'if-feature' expression
       evaluated to 'false' or to a 'when' condition evaluated
       to 'false'.";
  }

  identity bad-element {
    base error-tag;
    description
      "Returned by the CORECONF server when the CORECONF client tries to
       create data nodes for more than one case in a choice.";
  }

  identity data-missing {
    base error-tag;
    description
      "Returned by the CORECONF server when a data node required to
       accept the request is not present.";
  }

  identity error {
    base error-tag;
    description

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      "Returned by the CORECONF server when an unspecified error has
      occurred.";
  }

  identity error-app-tag {
    description
      "Base identity for error-app-tag.";
  }

  identity malformed-message {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the payload received
       from the CORECONF client don't contain a well-formed CBOR
       content as defined in [RFC7049] section 3.3 or don't
       comply with the CBOR structure defined within this
       document.";
  }

  identity data-not-unique {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the validation of the
       'unique' constraint of a list or leaf-list fails.";
  }

  identity too-many-elements {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the validation of the
       'max-elements' constraint of a list or leaf-list fails.";
  }

  identity too-few-elements {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the validation of the
       'min-elements' constraint of a list or leaf-list fails.";
  }

  identity must-violation {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the restrictions
       imposed by a 'must' statement are violated.";
  }

  identity duplicate {

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    base error-app-tag;
    description
      "Returned by the CORECONF server when a client tries to create
       a duplicate list or leaf-list entry.";
  }

  identity invalid-datatype {
    base error-app-tag;
    description
      "Returned by the CORECONF server when CBOR encoding is
       incorect or when the value encoded is incompatible with
       the YANG Built-In type. (e.g. value greater than 127
       for an int8, undefined enumeration).";
  }

  identity not-in-range {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the validation of the
       'range' property fails.";
  }

  identity invalid-length {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the validation of the
       'length' property fails.";
  }

  identity pattern-test-failed {
    base error-app-tag;
    description
      "Returned by the CORECONF server when the validation of the
       'pattern' property fails.";
  }

  identity missing-key {
    base error-app-tag;
    description
      "Returned by the CORECONF server to further qualify a
       missing-element error. This error is returned when the
       CORECONF client tries to create or list instance, without all
       the 'key' specified or when the CORECONF client tries to
       delete a leaf listed as a 'key'.";
  }

  identity missing-input-parameter {
    base error-app-tag;

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    description
      "Returned by the CORECONF server when the input parameters
       of a RPC or action are incomplete.";
  }

  identity instance-required {
    base error-app-tag;
    description
      "Returned by the CORECONF server when a leaf of type
       'instance-identifier' or 'leafref' marked with
       require-instance set to 'true' refers to an instance
       that does not exist.";
  }

  identity missing-choice {
    base error-app-tag;
    description
      "Returned by the CORECONF server when no nodes exist in a
       mandatory choice.";
  }

  rc:yang-data coreconf-error {
    container error {
      description
        "Optional payload of a 4.00 Bad Request CoAP error.";

      leaf error-tag {
        type identityref {
          base error-tag;
        }
        mandatory true;
        description
          "The enumerated error-tag.";
      }

      leaf error-app-tag {
        type identityref {
          base error-app-tag;
        }
        description
          "The application-specific error-tag.";
      }

      leaf error-data-node {
        type instance-identifier;
        description
          "When the error reported is caused by a specific data node,
           this leaf identifies the data node in error.";

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      }

      leaf error-message {
        type string;
        description
          "A message describing the error.";
      }
    }
  }
}
<CODE ENDS>

Appendix B.  ietf-coreconf .sid file

   {
     "assignment-ranges": [
       {
         "entry-point": 1000,
         "size": 100
       }
     ],
     "module-name": "ietf-coreconf",
     "module-revision": "2019-03-28",
     "items": [
       {
         "namespace": "module",
         "identifier": "ietf-coreconf",
         "sid": 1000
       },
       {
         "namespace": "identity",
         "identifier": "bad-element",
         "sid": 1001
       },
       {
         "namespace": "identity",
         "identifier": "data-missing",
         "sid": 1002
       },
       {
         "namespace": "identity",
         "identifier": "data-not-unique",
         "sid": 1003
       },
       {
         "namespace": "identity",
         "identifier": "duplicate",
         "sid": 1004

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       },
       {
         "namespace": "identity",
         "identifier": "error",
         "sid": 1005
       },
       {
         "namespace": "identity",
         "identifier": "error-app-tag",
         "sid": 1006
       },
       {
         "namespace": "identity",
         "identifier": "error-tag",
         "sid": 1007
       },
       {
         "namespace": "identity",
         "identifier": "instance-required",
         "sid": 1008
       },
       {
         "namespace": "identity",
         "identifier": "invalid-datatype",
         "sid": 1009
       },
       {
         "namespace": "identity",
         "identifier": "invalid-length",
         "sid": 1010
       },
       {
         "namespace": "identity",
         "identifier": "invalid-value",
         "sid": 1011
       },
       {
         "namespace": "identity",
         "identifier": "malformed-message",
         "sid": 1012
       },
       {
         "namespace": "identity",
         "identifier": "missing-choice",
         "sid": 1013
       },
       {
         "namespace": "identity",

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         "identifier": "missing-element",
         "sid": 1014
       },
       {
         "namespace": "identity",
         "identifier": "missing-input-parameter",
         "sid": 1015
       },
       {
         "namespace": "identity",
         "identifier": "missing-key",
         "sid": 1016
       },
       {
         "namespace": "identity",
         "identifier": "must-violation",
         "sid": 1017
       },
       {
         "namespace": "identity",
         "identifier": "not-in-range",
         "sid": 1018
       },
       {
         "namespace": "identity",
         "identifier": "operation-failed",
         "sid": 1019
       },
       {
         "namespace": "identity",
         "identifier": "pattern-test-failed",
         "sid": 1020
       },
       {
         "namespace": "identity",
         "identifier": "too-few-elements",
         "sid": 1021
       },
       {
         "namespace": "identity",
         "identifier": "too-many-elements",
         "sid": 1022
       },
       {
         "namespace": "identity",
         "identifier": "unified",
         "sid": 1029
       },

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       {
         "namespace": "identity",
         "identifier": "unknown-element",
         "sid": 1023
       },
       {
         "namespace": "data",
         "identifier": "/ietf-coreconf:error",
         "sid": 1024
       },
       {
         "namespace": "data",
         "identifier": "/ietf-coreconf:error/error-app-tag",
         "sid": 1025
       },
       {
         "namespace": "data",
         "identifier": "/ietf-coreconf:error/error-data-node",
         "sid": 1026
       },
       {
         "namespace": "data",
         "identifier": "/ietf-coreconf:error/error-message",
         "sid": 1027
       },
       {
         "namespace": "data",
         "identifier": "/ietf-coreconf:error/error-tag",
         "sid": 1028
       }
     ]
   }

Authors' Addresses

   Michel Veillette (editor)
   Trilliant Networks Inc.
   610 Rue du Luxembourg
   Granby, Quebec  J2J 2V2
   Canada

   Email: michel.veillette@trilliant.com

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   Peter van der Stok (editor)
   consultant

   Phone: +31-492474673 (Netherlands), +33-966015248 (France)
   Email: consultancy@vanderstok.org
   URI:   www.vanderstok.org

   Alexander Pelov
   Acklio
   2bis rue de la Chataigneraie
   Cesson-Sevigne, Bretagne  35510
   France

   Email: a@ackl.io

   Andy Bierman
   YumaWorks
   685 Cochran St.
   Suite #160
   Simi Valley, CA  93065
   USA

   Email: andy@yumaworks.com

   Ivaylo Petrov (editor)
   Acklio
   1137A avenue des Champs Blancs
   Cesson-Sevigne, Bretagne  35510
   France

   Email: ivaylo@ackl.io

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