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core                                                     P. van der Stok
Internet-Draft                                                consultant
Intended status: Standards Track                              A. Bierman
Expires: September 8, 2016                                     YumaWorks
                                                           March 7, 2016


                       CoAP Management Interface
                     draft-vanderstok-core-comi-09

Abstract

   This document describes a network management interface for
   constrained devices, called CoMI.  CoMI is an adaptation of the
   RESTCONF protocol for use in constrained devices and networks.  The
   Constrained Application Protocol (CoAP) is used to access management
   data resources specified in YANG, or SMIv2 converted to YANG.  CoMI
   use the YANG to CBOR mapping and encodes YANG names to reduce payload
   size.

Note

   Discussion and suggestions for improvement are requested, and should
   be sent to core@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 http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 8, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
       1.1.1.  Tree Diagrams . . . . . . . . . . . . . . . . . . . .   5
   2.  CoMI Architecture . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  RESTCONF/YANG Architecture  . . . . . . . . . . . . . . .   9
       2.1.1.  Major differences between RESTCONF and CoMI . . . . .   9
     2.2.  Compression of data-node instance identifier  . . . . . .  10
   3.  CoAP Interface  . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  MG Function Set . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Data Retrieval  . . . . . . . . . . . . . . . . . . . . .  13
       4.1.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  13
       4.1.2.  Using the 'select' Parameter  . . . . . . . . . . . .  14
       4.1.3.  Using the 'content' query parameter . . . . . . . . .  14
       4.1.4.  Retrieval Examples  . . . . . . . . . . . . . . . . .  15
     4.2.  Data Editing  . . . . . . . . . . . . . . . . . . . . . .  22
       4.2.1.  Data Ordering . . . . . . . . . . . . . . . . . . . .  22
       4.2.2.  POST  . . . . . . . . . . . . . . . . . . . . . . . .  22
       4.2.3.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .  23
       4.2.4.  PATCH . . . . . . . . . . . . . . . . . . . . . . . .  23
       4.2.5.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  27
       4.2.6.  Editing Multiple Resources  . . . . . . . . . . . . .  27
     4.3.  Notify functions  . . . . . . . . . . . . . . . . . . . .  28
     4.4.  RPC statements  . . . . . . . . . . . . . . . . . . . . .  30
       4.4.1.  Encoding RPC input parameters . . . . . . . . . . . .  31
       4.4.2.  Encoding RPC output parameters  . . . . . . . . . . .  32
     4.5.  Use of Block  . . . . . . . . . . . . . . . . . . . . . .  32
     4.6.  Resource Discovery  . . . . . . . . . . . . . . . . . . .  33
     4.7.  Error Return Codes  . . . . . . . . . . . . . . . . . . .  35
   5.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  36
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  38
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  39
   9.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  39
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  42
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     10.2.  Informative References . . . . . . . . . . . . . . . . .  44



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   Appendix A.  Payload and Server sizes . . . . . . . . . . . . . .  46
   Appendix B.  Comparison with LWM2M  . . . . . . . . . . . . . . .  47
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  48

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is designed for
   Machine to Machine (M2M) applications such as smart energy 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.  The 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 [RFC6020].  This
   draft is complementary to the draft [I-D.ietf-netconf-restconf] which
   describes a REST-like interface called RESTCONF, which uses HTTP
   methods to access structured data defined in YANG.

   The use of standardized data sets, specified in a standardized
   language such as YANG, promotes interoperability between devices and
   applications from different manufacturers.  A large amount of
   Management Information Base (MIB) [RFC3418] [mibreg] specifications
   already exists for monitoring purposes.  This data can be accessed in
   RESTCONF or CoMI if the server converts the SMIv2 modules to YANG,
   using the mapping rules defined in [RFC6643].

   RESTCONF allows access to data resources contained in NETCONF
   [RFC6241] data-stores.  RESTCONF messages can be encoded in XML [XML]
   or JSON [RFC7159].  The GET method is used to retrieve data resources
   and the POST, PUT, PATCH, and DELETE methods are used to create,
   replace, merge, and delete data resources.

   CoMI supports the methods GET, PUT, PATCH, POST and DELETE.  The
   payload of CoMI is encoded in CBOR [RFC7049] which can be
   automatically generated from JSON [RFC7159].  CBOR has a binary
   format and hence has more coding efficiency than JSON.  RESTCONF
   relies on HTTP with TCP in contrast to CoMI which uses CoAP that is
   optimized for UDP with less overhead for small messages.  RESTCONF
   uses the HTTP methods HEAD, and OPTIONS, which are not used by CoMI.

   CoMI 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 up to 10 round trips.






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   To promote small packets, CoMI uses an additional "data-identifier
   string-to-number conversion" to minimize CBOR payloads and URI
   length.

   Currently, small managed devices need to support at least two
   protocols: CoAP and SNMP [RFC3411].  When the MIB can be accessed
   with the CoMI protocol, the SNMP protocol can be replaced with the
   CoAP protocol.  Although the SNMP server size is not huge (see
   Appendix A), the code for the security aspects of SMIv3 [RFC3414] is
   not negligible.  Using CoAP to access secured management objects
   reduces the code complexity of the stack in the constrained device,
   and harmonizes applications development.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Readers of this specification should be familiar with all the terms
   and concepts discussed in [RFC3410], [RFC3416], and [RFC2578].

   The following terms are defined in the NETCONF protocol [RFC6241]:
   client, configuration data, data-store, and server.

   The following terms are defined in the YANG data modelling language
   [RFC6020]: container, data node, key, key leaf, leaf, leaf-list, and
   list.  The following terms are defined in RESTCONF protocol
   [I-D.ietf-netconf-restconf]: data resource, data-store resource, edit
   operation, query parameter, target resource, and unified data-store.
   The terms YANG hash, and Rehash bit are defined in I-D.yang-hash.

   The following terms are defined in this document:

   Data-node instance:  An instance of a data-node specified in a YANG
      module present in the server.  The instance is stored in the
      memory of the server.

   Notification-node instance:  An instance of a schema node of type
      notification, specified in a YANG module present in the server.
      The instance is generated in the server at the occurrence of the
      corresponding event and appended to a stream.

   The following list contains the abbreviations used in this document.

   XXXX:  TODO, and others to follow.





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1.1.1.  Tree Diagrams

   A simplified graphical representation of the data model is used in
   the YANG modules specified in this document.  The meaning of the
   symbols in these diagrams is as follows:

      Brackets "[" and "]" enclose list keys.

      Abbreviations before data node names: "rw" means configuration
      data (read-write) and "ro" state data (read-only).

      Symbols after data node names: "?" means an optional node, "!"
      means a presence container, and "*" denotes a list and leaf-list.

      Parentheses enclose choice and case nodes, and case nodes are also
      marked with a colon (":").

      Ellipsis ("...") stands for contents of subtrees that are not
      shown.

2.  CoMI Architecture

   This section describes the CoMI architecture to use CoAP for the
   reading and modifying of instrumentation variables used for the
   management of the instrumented node.


























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   Client
   +--------------------------------------------------------------+
   | +----------------+    +----------------+                     |
   | |    SMIv2       | >  |      YANG      |    >     COAP       |
   | |specification(2)|    |specification(1)|       Request(3)    |
   | +----------------+    +----------------+[         *          |
   +-----------------------------*-----------[---------*----------+
                                 *           [         *
                                 *           [    +-----------+
                         mapping *   security[    |  Network  |
                                 *      (8)  [    | packet(4) |
                                 *           [    +-----------+
   Server                        *           [         *
   +-----------------------------*-----------[---------*----------+
   |                             *           [         *          |
   |                             *                 Retrieval,     |
   |                             *               Modification(5)  |
   |                            \*/                    *          |
   | +-------------------------------------------------*--------+ |
   | |                    +--------------+       +------------+ | |
   | |                    |configuration |       |Operational | | |
   | |                    |     (6b)     |       |  state(6a) | | |
   | |                    +--------------+       +------------+ | |
   | |                    variable store (6)           *        | |
   | +-------------------------------------------------*--------+ |
   |                                                   *          |
   |                                                Variable      |
   |                                            Instrumentation(7)|
   +--------------------------------------------------------------+


                   Figure 1: Abstract CoMI architecture

   Figure 1 is a high level representation of the main elements of the
   CoAP management architecture.  A client sends requests as payload in
   packets over the network to a managed constrained node.

   Objectives are:

   o  Equip a constrained node with a management server that provides
      information about the operational characteristics of the code
      running in the constrained node.

   o  The server provides this information in a variable store that
      contains values describing the performance characteristics and the
      code parameter values.





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   o  The client receives the performance characteristics on a regular
      basis or on request.

   o  The client sets the parameter values in the server at bootstrap
      and intermittently when operational conditions change.

   o  The constrained network requires the payload to be as small as
      possible, and the constrained server memory requirements should be
      as small as possible.

   For interoperability it is required that in addition to using the
   Internet Protocol for data transport:

   o  The names, type, and semantics of the instrumentation variables
      are standardized.

   o  The instrumentation variables are described in a standard
      language.

   o  The URI of the CoAP request is standardized.

   o  The format of the packet payload is standardized.

   o  The notification from server to client is standardized.

   The different numbered components of Figure 1 are discussed according
   to component number.

   (1) YANG specification:  contains a set of named and versioned
      modules.  A module specifies a hierarchy of named and typed
      resources.  A resource is uniquely identified by a sequence of its
      name and the names of the enveloping resources following the
      hierarchy order.  The YANG specification serves as input to the
      writers of application and instrumentation code and the humans
      analyzing the returned values (arrow from YANG specification to
      Variable store).  The specification can be used to check the
      correctness of the CoAP request and do the CBOR encoding.

   (2) SMIv2 specification:  A named module specifies a set of variables
      and "conceptual tables".  Named variables have simple types.
      Conceptual tables are composed of typed named columns.  The
      variable name and module name identify the variable uniquely.
      There is an algorithm to translate SMIv2 specifications to YANG
      specifications.

   (3) CoAP request:  The CoAP request needs a Universal Resource
      Identifier (URI) and the payload of the packet to send a request.
      The URI is composed of the schema, server, path and query and



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      looks like coap://entry.example.com/<path>?<query>.  Fragments are
      not supported.  Allowed operations are PUT, PATCH, GET, DELETE,
      and POST.  New variables can be created with POST when they exist
      in the YANG specification.  The Observe option is used to return
      variable values regularly or on event occurrence (notification).

   (3.1) CoAP <path>:  The path identifies the variable in the form
      "/mg/<encoded-name>".

   (3.2) CoAP <query>:  The query parameter is used to specify
      additional (optional) aspects like the container name, list
      instance, and others.  The idea is to keep the path simple and put
      variations on variable specification in the query.

   (3.3) CoAP discovery:  Discovery of the variables is done with
      standard CoAP resource discovery using /.well-known/core with
      ?rt=/core.mg.

   (4) Network packet:  The payload contains the CBOR encoding of JSON
      objects.  This object corresponds with the converted RESTCONF
      message payload.

   (5) Retrieval, modification:  The server needs to parse the CBOR
      encoded message and identify the corresponding instances in the
      Variable store.  In addition, this component includes the code for
      CoAP Observe and block options.

   (6) Variable store:  The store is composed of two parts: Operational
      state and Configuration data-store (see Section 2.1).  CoMI does
      not differentiate between variable store types.  The Variable
      store contains data-node instances.  Values are stored in the
      appropriate instances, and or values are returned from the
      instances into the payload of the packet.

   (7) Variable instrumentation:  This code depends on implementation of
      drivers and other node specific aspects.  The Variable
      instrumentation code stores the values of the parameters into the
      appropriate places in the operational code.  The variable
      instrumentation code reads current execution values from the
      operational code and stores them in the appropriate instances.

   (8) Security:  The server MUST prevent unauthorized users from
      reading or writing any data resources.  CoMI relies on DTLS
      [RFC6347] which is specified to secure CoAP communication.







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2.1.  RESTCONF/YANG Architecture

   CoMI adapts the RESTCONF architecture so data exchange and
   implementation requirements are optimized for constrained devices.

   The RESTCONF protocol uses a unified data-store to edit conceptual
   data structures supported by the server.  The details of transaction
   preparation and non-volatile storage of the data are hidden from the
   RESTCONF client.  CoMI also uses a unified data-store, to allow
   stateless editing of configuration variables and the notification of
   operational variables.

   The child schema nodes of the unified data-store include all the top-
   level YANG data nodes in all the YANG modules supported by the
   server.  The YANG data structures represent a hierarchy of data
   resources.  The client discovers the list of YANG modules, and
   important conformance information such as the module revision dates,
   YANG features supported, and YANG deviations required.  The
   individual data nodes are discovered indirectly by parsing the YANG
   modules supported by the server.

   The YANG data definition statements contain a lot of information that
   can help automation tools, developers, and operators use the data
   model correctly and efficiently.  The YANG definitions and server
   YANG module capability advertisements provide an "API contract" that
   allow a client to determine the detailed server management
   capabilities very quickly

   RESTCONF and CoMI use a simple algorithmic mapping from YANG to URI
   syntax to identify the target resource of a retrieval or edit
   operation.  A client can construct operations or scripts using a
   predictable syntax, based on the YANG data definitions.  The target
   resource URI can reference a data resource instance, or the data-
   store itself (to retrieve the entire data-store or create a top-level
   data resource instance).  A compression algorithm reduces the size of
   the data-node instance identifier (see Section 2.2).

2.1.1.  Major differences between RESTCONF and CoMI

   CoMI uses CoAP/UDP as transport protocol and CBOR as payload format.
   RESTCONF uses HTTP/TCP as transport protocol and JSON or XML as
   payload formats.  CoMI encodes YANG name strings as numbers, where
   RESTCONF does not.

   CoAP servers MUST maintain the order of user-ordered data.  CoMI does
   not support insert-mode (first, last, before, after) and insertion-
   point (before, after) which are supported by RESTCONF.  Many CoAP




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   servers will not support date and time functions.  For that reason
   CoMI does not support the start, stop options for events.

   The CoMI "select" query parameter is equivalent to the RESTCONF
   "fields" query parameter but has a much simpler syntax.  CoMI servers
   only implement the efficient "trim" mode for default values.  CoMI
   servers implement a less rich syntax to specify key values in the URI
   than RESTCONF servers.

   CoMI servers do not support 'filter' query that involves XML parsing,
   'content', 'depth', and 'with-defaults' query parameters.

   CoMI servers do not support the YANG functionality of anyxml,
   anydata, and xpath.

2.2.  Compression of data-node instance identifier

   The JSON/CBOR encoding will include the module name string to specify
   the YANG module.  If a representation of the target resource is
   included in the request or response message, then the data definition
   name string is used to identify each node in the message.  The module
   namespace (or name) may also be present in these identifiers.

   In order to significantly reduce the size of identifiers used in
   CoMI, numeric object identifiers are used instead of these strings.
   The specific encoding of the object identifiers is not hard-wired in
   the protocol.

   YANG Hash is the default encoding for object identifiers
   [I-D.bierman-core-yang-hash].  This encoding in considered to be
   "unstructured" since the particular values for each object are
   determined by a hash algorithm.  It is possible for 2 different
   objects to generate the same hash value.  If this occurs, then the
   client and server will both need to rehash the colliding object
   identifiers to new unused hash values.

   In order to eliminate the need for rehashing, CoMI allows for
   alternate "structured" object identifier encoding formats.
   Structured object identifier MUST be managed such that no object ID
   collisions are possible, and therefore no rehash procedures are
   needed.  Structured object identifiers can also be selected to
   minimize the size of a subset of the object identifiers (e.g., the
   most requested objects).








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3.  CoAP Interface

   In CoAP a group of links can constitute a Function Set. The format of
   the links is specified in [I-D.ietf-core-interfaces].  This note
   specifies a Management Function Set. CoMI end-points that implement
   the CoMI management protocol support at least one discoverable
   management resource of resource type (rt): core.mg, with path: /mg,
   where mg is short-hand for management.  The name /mg is recommended
   but not compulsory (see Section 4.6).

   The path prefix /mg has resources accessible with the following five
   paths:

   /mg:  YANG-based data with path "/mg" and using CBOR content encoding
      format.  This path represents a data-store resource which contains
      YANG data resources as its descendant nodes.  All identifiers
      referring to YANG data nodes within this path are encoded YANG
      names (see for example [I-D.bierman-core-yang-hash].

   /mg/mod.uri:  URI identifying the location of the server module
      information, with path "/mg/mod.uri" and CBOR content format.
      This YANG data is encoded with plain identifier strings, not YANG
      encoded values.  An Entity Tag MUST be maintained for this
      resource by the server, which MUST be changed to a new value when
      the set of YANG modules in use by the server changes.

   /mg/num.typ:  String identifying the object ID numbering scheme used
      by the CoMI server.  Two values are defined in this document: (1)
      'yanghash' to indicate that the YANG Hash numbering scheme is
      used, and (2) 'yangmanag' to indicate that a managed numbering
      scheme is used.  It is possible for other object numbering schemes
      to be defined outside the scope of this document.

   /mg/srv.typ:  String identifying the CoMI server type.  The value
      'ro' indicates that the server is a read-only server and no
      editing operations are supported.  A read-only server is not
      required to provide YANG deviation statements for any writable
      YANG data nodes.  The value 'rw' indicates that the server is a
      read-write server and editing operations are supported.  A read-
      write server is required to provide YANG deviation statements for
      any writable YANG data nodes that are not fully implemented.

   /mg/yh.uri:  URI indicating the location of the server YANG hash
      information if any objects needed to be re-hashed by the server.
      It has the path "/mg/yh.uri" and is encoded in CBOR format.  The
      "yang-hash" container within the "ietf-yang-hash" module,
      described in [I-D.bierman-core-yang-hash], is used to define the
      syntax and semantics of this data structure.  This YANG data is



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      encoded with plain identifier strings, not YANG hash values.  The
      server will only have this resource if there are any objects that
      needed to be re-hashed due to a hash collision.

   /mg/stream:  String identifying the default stream resource to which
      YANG notification instances are appended.  Notification support is
      optional, so this resource will not exist if the server does not
      support any notifications.

   /mg/op:  String identifying the resource to which YANG operations are
      appended.

   The mapping of YANG data node instances to CoMI resources is as
   follows: A YANG module describes a set of data trees composed of YANG
   data nodes.  Every root of a data tree in a YANG module loaded in the
   CoMI server represents a resource of the server.  All data root
   descendants represent sub-resources.

   The resource identifiers of the instances of the YANG specifications
   are encoded YANG names.  When multiple instances of a list node
   exist, the instance selection is described in Section 4.1.4.4

   The profile of the management function set, with IF=core.mg, is shown
   in the table below, following the guidelines of
   [I-D.ietf-core-interfaces]:

   +--------------+-------------+-------------------+------------------+
   | name         | path        | rt                | Data Type        |
   +--------------+-------------+-------------------+------------------+
   | Management   | /mg         | core.mg           | n/a              |
   |              |             |                   |                  |
   | Data         | /mg         | core.mg.data      | application/cbor |
   |              |             |                   |                  |
   | Module Set   | /mg/mod.uri | core.mg.moduri    | application/cbor |
   | URI          |             |                   |                  |
   |              |             |                   |                  |
   | Numbering    | /mg/num.typ | core.mg.num-type  | application/cbor |
   | Type         |             |                   |                  |
   |              |             |                   |                  |
   | Server Type  | /mg/srv.typ | core.mg.srv-type  | application/cbor |
   |              |             |                   |                  |
   | YANG Hash    | /mg/yh.uri  | core.mg.yang-hash | application/cbor |
   | Info         |             |                   |                  |
   |              |             |                   |                  |
   | Events       | /mg/stream  | core.mg.stream    | application/cbor |
   |              |             |                   |                  |
   | Operations   | /mg/op      | core.mg.op        | application/cbor |
   +--------------+-------------+-------------------+------------------+



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4.  MG Function Set

   The MG Function Set provides a CoAP interface to perform a subset of
   the functions provided by RESTCONF.

   A subset of the operations defined in RESTCONF are used in CoMI:

   +-----------+------------------------------------------------------+
   | Operation | Description                                          |
   +-----------+------------------------------------------------------+
   | GET       | Retrieve the data-store resource or a data resource  |
   |           |                                                      |
   | POST      | Create a data resource                               |
   |           |                                                      |
   | PUT       | Create or replace a data resource                    |
   |           |                                                      |
   | PATCH     | Replace a data resource partially                    |
   |           |                                                      |
   | DELETE    | Delete a data resource                               |
   +-----------+------------------------------------------------------+

4.1.  Data Retrieval

4.1.1.  GET

   One or more instances of data resources are retrieved by the client
   with the GET method.  The RESTCONF GET operation is supported in
   CoMI.  The same constraints apply as defined in section 3.3 of
   [I-D.ietf-netconf-restconf].  The operation is mapped to the GET
   method defined in section 5.8.1 of [RFC7252].

   It is possible that the size of the payload is too large to fit in a
   single message.  In the case that management data is bigger than the
   maximum supported payload size, the Block mechanism from
   [I-D.ietf-core-block] is used, as explained in more detail in
   Section 4.5.

   There are three query parameters for the GET method.  A CoMI server
   MUST implement the keys parameter and the content parameter, and MAY
   implement the select parameter to allow common data retrieval
   filtering functionality.










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   +---------------+---------------------------------------------------+
   | Query         | Description                                       |
   | Parameter     |                                                   |
   +---------------+---------------------------------------------------+
   | keys          | Request to select instances of a YANG definition  |
   |               |                                                   |
   | select        | Request to select sub-trees from the target       |
   |               | resource                                          |
   |               |                                                   |
   | content       | Request to select configuration and non-          |
   |               | configuration nodes                               |
   +---------------+---------------------------------------------------+

   The "keys" parameter is used to specify a specific instance of the
   list resource.  When keys is not specified, all instances are
   returned.  When no or one instance of the resource exists, the keys
   parameter is ignored.

4.1.2.  Using the 'select' Parameter

   RESTCONF uses the 'filter' parameter next to the 'fields' parameter
   to specify an expression which can represent a subset of all data
   nodes within the target resource [I-D.ietf-netconf-restconf].  The
   'select' parameter is local to CoMI and is useful for filtering sub-
   trees and retrieving only a subset that a managing application is
   interested in.

   Because filtering is a resource intensive task and not all
   constrained devices can be expected to have enough computing
   resources such that they will be able to successfully filter and
   return a subset of a sub-tree.  This is especially likely to be true
   with Class 0 devices that have significantly lesser RAM than 10 KiB
   [RFC7228].  Since CoMI is targeted at constrained devices and
   networks, the 'filter' parameter is not used here.

   The implementation of the 'select' parameter is already optional for
   constrained devices, however, even when implemented it is expected to
   be a best effort feature, rather than a service that nodes must
   provide.  This implies that if a node receives the 'select' parameter
   specifying a set of sub-trees that should be returned, it will only
   return those that it is able to return.

4.1.3.  Using the 'content' query parameter

   The "content" parameter controls how descendant nodes of the
   requested data nodes will be processed in the reply.

   The allowed values are:



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   +------------+------------------------------------------------------+
   | Value      | Description                                          |
   +------------+------------------------------------------------------+
   | config     | Return only configuration descendant data nodes      |
   |            |                                                      |
   | nonconfig  | Return only non-configuration descendant data nodes  |
   |            |                                                      |
   | all        | Return all descendant data nodes                     |
   +------------+------------------------------------------------------+

   This parameter is only allowed for GET methods on datastore and data
   resources.  A 4.00 Bad Request error is returned if used for other
   methods or resource types.

   The default value is determined by the "config" statement value of
   the requested data nodes.  If the "config" value is "false", then the
   default for the "content" parameter is "nonconfig".  If "config" is
   "true" then the default for the "content" parameter is "config".

4.1.4.  Retrieval Examples

   In all examples the path is expressed in readable names and as a
   encoded value of the name (where the encoded value in the payload is
   expressed as a hexadecimal number, and the encoded value in the URL
   as a base64 number).  CoMI payloads use the CBOR format.  The CBOR
   syntax of the YANG payloads is specified in TODO REF.  The examples
   in this section use a JSON payload with extensions to approach the
   permissible CBOR payload, called "diagnostic JSON".

4.1.4.1.  Single instance retrieval

   A request to read the values of instances of a management object or
   the leaf of an object is sent with a confirmable CoAP GET message.  A
   single object is specified in the URI path prefixed with /mg.

   Using for example the clock container from [RFC7317], a request is
   sent to retrieve the value of clock/current-datetime specified in
   module system-state.  The answer to the request returns a
   (identifier, value) pair, transported as a CBOR map with a single
   item.


   REQ: GET example.com/mg/system-state/clock/current-datetime

   RES: 2.05 Content (Content-Format: application/cbor)
   {
      "current-datetime" : "2014-10-26T12:16:31Z"
   }



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   The specified object can be an entire object.  Accordingly, the
   returned payload is composed of all the leaves associated with the
   object.  The payload is a CBOR map where each leaf is returned as a
   (encoded YANG name, value) pair.  For example, the GET of the clock
   object, sent by the client, results in the following returned payload
   sent by the managed entity, transported as A CBOR map with two items:


   REQ: GET example.com/mg/system-state/clock
      (Content-Format: application/cbor)

   RES: 2.05 Content (Content-Format: application/cbor)
   {
     "clock" : {
         "current-datetime" : "2014-10-26T12:16:51Z",
         "boot-datetime" : "2014-10-21T03:00:00Z"
     }
   }

   For example, the YANG names can be replaced by the hash values for
   'clock', 'current-datetime', and 'boot-datetime'.  The 30 bit murmur3
   hash value of 'clock' equates: CDKSQ (xref target="I-D.bierman-core-
   yang-hash"/>.  The request using hash values is shown below:


   REQ: GET example.com/mg/CDKSQ
      (Content-Format: application/cbor)

   RES: 2.05 Content (Content-Format: application/cbor)
   {
     0x021ca491 : {

         0x047c468b : "2014-10-26T12:16:51Z",
         0x1fb5f4f8 : "2014-10-21T03:00:00Z"
      }
   }

4.1.4.2.  Multiple instance retrieval

   A "list" node can have multiple instances.  Accordingly, the returned
   payload is composed of all the instances associated with the list
   node.  Each instance is returned as a (key, object) pair, where key
   and object are composed of one or more (identifier, value) pairs.

   For example, the GET of the /interfaces/interface/ipv6/neighbor
   results in the following returned payload sent by the managed entity,
   transported as a CBOR map of 3 (key : object) pairs, where key and




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   value are CBOR maps with one entry each.  In this case the key is the
   "ip" attribute and the value is the "link-layer-address" attribute.


   REQ: GET example.com/mg/interfaces/interface/ipv6/neighbor
      (Content-Format: application/cbor)

   RES: 2.05 Content (Content-Format: application/cbor)
   {
      "neighbor" :{
        {"ip" : "fe80::200:f8ff:fe21:67cf"}:
           {"link-layer-address" : "00:00::10:01:23:45"}
        ,
        {"ip" : "fe80::200:f8ff:fe21:6708"}:
           {"link-layer-address" : "00:00::10:54:32:10"}
        ,
        {"ip" : "fe80::200:f8ff:fe21:88ee"}:
           {"link-layer-address" : "00:00::10:98:76:54"}
      }
   }

4.1.4.3.  Access to MIB Data

   The YANG translation of the SMI specifying the
   ipNetToMediaTable [RFC4293] yields:


























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   container IP-MIB {
     container ipNetToPhysicalTable {
       list ipNetToPhysicalEntry {
          key "ipNetToPhysicalIfIndex
               ipNetToPhysicalNetAddressType
               ipNetToPhysicalNetAddress";
          leaf ipNetToMediaIfIndex {
             type: int32;
          }
          leaf ipNetToPhysicalIfIndex {
            type if-mib:InterfaceIndex;
          }
          leaf ipNetToPhysicalNetAddressType {
            type inet-address:InetAddressType;
          }
          leaf ipNetToPhysicalNetAddress {
            type inet-address:InetAddress;
          }
          leaf ipNetToPhysicalPhysAddress {
            type yang:phys-address {
               length "0..65535";
            }
          }
          leaf ipNetToPhysicalLastUpdated {
            type yang:timestamp;
          }
          leaf ipNetToPhysicalType {
            type enumeration { ... }
          }
          leaf ipNetToPhysicalState {
            type enumeration { ... }
          }
          leaf ipNetToPhysicalRowStatus {
            type snmpv2-tc:RowStatus;
          }
       }
    }

   The following example shows an "ipNetToPhysicalTable" with 2
   instances, using JSON encoding as defined in
   [I-D.ietf-netmod-yang-json]:










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   {
     "IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry" : [
           {
             "ipNetToPhysicalIfIndex" : 1,
             "ipNetToPhysicalNetAddressType" : "ipv4",
             "ipNetToPhysicalNetAddress" : "10.0.0.51",
             "ipNetToPhysicalPhysAddress" : "00:00:10:01:23:45",
             "ipNetToPhysicalLastUpdated" : "2333943",
             "ipNetToPhysicalType" : "static",
             "ipNetToPhysicalState" : "reachable",
             "ipNetToPhysicalRowStatus" : "active"
           },
           {
             "ipNetToPhysicalIfIndex" : 1,
             "ipNetToPhysicalNetAddressType" : "ipv4",
             "ipNetToPhysicalNetAddress" : "9.2.3.4",
             "ipNetToPhysicalPhysAddress" : "00:00:10:54:32:10",
             "ipNetToPhysicalLastUpdated" : "2329836",
             "ipNetToPhysicalType" : "dynamic",
             "ipNetToPhysicalState" : "unknown",
             "ipNetToPhysicalRowStatus" : "active"
           }
         ]
       }
     }
   }

4.1.4.4.  The 'keys' Query Parameter

   There is a query parameter that MUST be supported by servers called
   "keys".  This parameter is used to specify the key values for an
   instance of an object identified by an encoded YANG name.  All key
   leaf values of the instance are passed in order.  The first key leaf
   in the top-most list is the first key encoded in the 'keys'
   parameter.

   The key leafs from top to bottom and left to right are encoded as a
   comma-delimited list.  If a key leaf value is missing then all values
   for that key leaf are returned.

   Example: In this example exactly one instance is requested from the
   ipNetToPhysicalEntry (from a previous example).  The CBOR payload,
   here represented with diagnostic JSON, permits to transport the
   selected instance and nothing more.

   TODO refer to the section in YANG to CBOR mapping





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REQ: GET example.com/mg/IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry?keys=1,ipv4,9.2.3.4

RES: 2.05 Content (Content-Format: application/cbor)
{
   "IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry": {
      {
          "ipNetToPhysicalIfIndex" : 1,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "9.2.3.4"}:
        {
          "ipNetToPhysicalPhysAddress" : "00:00:10:54:32:10",
          "ipNetToPhysicalLastUpdated" : "2329836",
          "ipNetToPhysicalType" : "dynamic",
          "ipNetToPhysicalState" : "unknown",
          "ipNetToPhysicalRowStatus" : "active"
        }
    }
}

   An example illustrates the syntax of keys query parameter.  In this
   example the following YANG module is used:


     module foo-mod {
       namespace foo-mod-ns;
       prefix foo;

       list A {
         key "key1 key2";
         leaf key1 { type string; }
         leaf key2 { type int32; }
         list B {
           key "key3";
           leaf key3 { type string; }
           leaf col1 { type uint32; }
         }
       }
     }


   The following string represents the CoMI target resource identifier
   for the instance of the "col1" leaf with key values "top", 17,
   "group":

      /mg/foo-mod:A/B/col1?keys="top",17,"group1"






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4.1.4.5.  The 'select' Query Parameter

   The select parameter is used along with the GET method to provide a
   sub-tree filter mechanism.  A list of encoded YANG names that should
   be filtered is provided along with a list of keys identifying the
   instances that should be returned.  When the keys parameter is used
   together with the select, the key values are added in brackets
   without using the "keys=" text.

   Data may be retrieved using the select query parameter in the
   following way, transported as a CBOR maps of maps:

REQ: GET example.com/mg/IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry?select=(10.0.0.51)

RES: 2.05 Content (Content-Format: application/cbor)
{
    "IP-MIB/ipNetToPhysicalTable/ipNetToPhysicalEntry": {
      {
          "ipNetToPhysicalIfIndex" : 1,
          "ipNetToPhysicalNetAddressType" : "ipv4",
          "ipNetToPhysicalNetAddress" : "10.0.0.51"}:
        {
          "ipNetToPhysicalPhysAddress" : "00:00:10:01:23:45",
          "ipNetToPhysicalLastUpdated" : "2333943",
          "ipNetToPhysicalType" : "static",
          "ipNetToPhysicalState" : "reachable",
          "ipNetToPhysicalRowStatus" : "active"
        }
   }
}

   In this example exactly one instance is returned as response from the
   ipNetToPhysicalTable because only this instance matches the provided
   keys.

   Supposing there were multiple YANG fields with their own sets of keys
   that were to be filtered, the select query parameter can be used to
   retrieve results from these in one go as well.  Using the "foo-mod"
   module of Section 4.1.4.4, the following string represents the CoMI
   target resource identifier when multiple fields, with their own sets
   of key values are queried:

          /mg/foo-mod:A?select=B/col1("top",17,"group"),key2("top")








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4.1.4.6.  Defaults handling

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

   If the target of a GET method is a data node that represents a
   container or list that has any child resources with default values,
   for the child resources that have not been given value yet, the
   server MUST not return the child resource.

4.2.  Data Editing

   CoMI allows data-store contents to be created, modified and deleted
   using CoAP methods.

   Data-editing is an optional feature.  The server will indicate its
   editing capability with the "/core.mg.srv-type resource type.  If the
   value is 'rw' then the server supports editing operations.  If the
   value is 'ro' then the server does not support editing operations.

4.2.1.  Data Ordering

   A CoMI server is not required to support entry insertion of lists and
   leaf-lists that are ordered by the user (i.e., YANG statement
   "ordered-by user").  The 'insert' and 'point' query parameters from
   RESTCONF are not used in CoMI.

   A CoMI server SHOULD 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 arrays so
   messages will preserve their order.

4.2.2.  POST

   Data resource instances are created with the POST method.  The
   RESTCONF POST operation is supported in CoMI for creation of data
   resources and the invocation operation resources.  Refer to
   Section 4.4 for details on operation resources.  The same constraints
   apply as defined in section 4.4.1 of [I-D.ietf-netconf-restconf].
   The operation is mapped to the POST method defined in section 5.8.2
   of [RFC7252].

   There are no query parameters for the POST method.







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4.2.3.  PUT

   Data resource instances are created or replaced with the PUT method.
   The PUT operation is supported in CoMI.  A request to set the values
   of instances of an object/leaf is sent with a confirmable CoAP PUT
   message.  The Response is piggybacked to the CoAP ACK message
   corresponding with the Request.  The same constraints apply as
   defined in section 3.5 of [I-D.ietf-netconf-restconf].  The operation
   is mapped to the PUT method defined in section 5.8.3 of [RFC7252].

   There are no query parameters for the PUT method.

4.2.4.  PATCH

   Data resource instances are partially replaced with the PATCH method
   [I-D.vanderstok-core-patch].  The PATCH operation is supported in
   CoMI.  A request to set the values of instances of a subset of the
   values of the resource is sent with a confirmable CoAP PATCH message.
   The Response is piggybacked to the CoAP ACK message corresponding
   with the Request.  The same constraints apply as defined in section
   3.5 of [I-D.ietf-netconf-restconf].  The operation is mapped to the
   PATCH method defined in [I-D.vanderstok-core-patch].

   The processing of the PATCH command is specified by the CBOR payload.
   The CBOR patch payload describes the changes to be made to target
   YANG data nodes.  It follows closely the rules described in
   [RFC7396].  If the CBOR patch payload contains objects that are not
   present in the target, these objects are added.  If the target
   contains the specified object, the contents of the objects are
   replaced with the values of the payload.  Null values indicate the
   removal of existing values.  The CBOR patch extends [RFC7396] by
   specifying rules for list elements.

   TODO, review text after publication of YANG/CBOR and CBOR-merge
   drafts.

   For example consider the following YANG specification:














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   module foo {
     namespace "http://example.com/book";
     prefix "bo";
     revision 2015-06-07;

     list B {
         key key1;
         key key2;
         leaf key1 { type string; }
         leaf key2 {type string; }
         leaf col1 { type int32; }
         leaf counter1 { type uint32; }
     }

     container book {
       leaf title { type string; }
       container author {
         leaf  givenName {type string; }
         leaf  familyName {type string; }
       }
       leaf-list tags {type string; }
       leaf content{type string;}
       leaf phoneNumber {type string;}
     }


   Consider the following target data nodes described with the JSON
   encoding of [I-D.ietf-netmod-yang-json].























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    "B": [
        {
         "key1" : "author1",
         "key2" : "book2",
         "col1" : 25,
         "counter1" : 4321
        },
       {
         "key1" : "author5",
         "key2" : "book6",
         "col1" : 2,
         "counter1" : 1234
        }
     ]


    "book": {
     "title" : "mytitle",
     "author": {
       "givenName" : "John",
       "familyName" : "Doe"
       }
     "tags" : [ "example", "sample"],
     "content" : "This will be unchanged"
   }

   The following changes are requested for the document (following the
   example from [RFC7396]: the title changes from "mytitle" to
   "favoured", the phoneNumber is added to the book container, the
   familyName is deleted, and "sample" is removed from the tags leaf-
   list.  In addition author1, book1 item is removed, author5 counter1
   is upgraded, and a new author is added in B list.  The following CBOR
   Patch payload, represented in JSON is sent.

   TODO: edit after publication of CBOR-merge draft.
















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    {
       "B":  {
        { "key1" : "author1",
         "key2" : "book2"}:
         { null : null},
        { "key1" : "author5"} :
          {"counter1" : 4444},
        { "key1" : "newauthor",
         "key2" : "newbook"}:
        { "col1" : 1,
          "counter1" : 1}
       },
     "book" : {
        "title" : "favoured",
        "author": {"familyName" : null},
        "tags" : [ "example"],
        "phoneNumber" : "+01-123-456-7890"
      }
   }

   In his example, the value "author5" specifies the entry uniquely.
   However, when several entries exist with the "author5" value for
   "key1", the outcome of the example Patch is undefined.

   The processing of the Patch payload results in the following new
   target data nodes.

























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    "B": [
        {
         "key1" : "newauthor",
         "key2" : "newbook",
         "col1" : 1,
         "counter1" : 1
        },
       {
         "key1" : "author5",
         "key2" : "book6",
         "col1" : 2,
         "counter1" : 4444
        }
     ]

   "book": {
     "title" : "favoured",
     "author": {
       "givenName" : "John"
       }
     "tags" : [ "example"],
     "content" : "This will be unchanged",
     "phoneNumber" : +01-123-456-7890"
   }

   There are no query parameters for the PATCH method.

4.2.5.  DELETE

   Data resource instances are deleted with the DELETE method.  The
   RESTCONF DELETE operation is supported in CoMI.  The same constraints
   apply as defined in section 3.7 of [I-D.ietf-netconf-restconf].  The
   operation is mapped to the DELETE method defined in section 5.8.4 of
   [RFC7252].

   There are no optional query parameters for the DELETE method.

4.2.6.  Editing Multiple Resources

   Editing multiple data resources at once can allow a client to use
   fewer messages to make a configuration change.  It also allows
   multiple edits to all be applied or none applied, which is not
   possible if the data resources are edited one at a time.

   It is easy to add multiple entries at once.  The "PATCH" method can
   be used to simply patch the parent node(s) of the data resources to
   be added.  If multiple top-level data resources need to be added,
   then the data-store itself ('/mg') can be patched.



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   If other operations need to be performed, or multiple operations need
   to be performed at once, then the YANG Patch
   [I-D.ietf-netconf-yang-patch] media type can be used with the PATCH
   method.  A YANG patch is an ordered list of edits on the target
   resource, which can be a specific data node instance, or the data-
   store itself.  The resource type used by YANG Patch is 'application/
   yang.patch'.  A status message is returned in the response, using
   resource type 'application/yang.patch.status'.

   The following YANG tree diagram describes the YANG Patch structure,
   Each 'edit' list entry has its own operation, sub-resource target,
   and new value (if needed).


     +--rw yang-patch
         +--rw patch-id?   string
         +--rw comment?    string
         +--rw edit* [edit-id]
            +--rw edit-id      string
            +--rw operation    enumeration
            +--rw target       target-resource-offset
            +--rw point?       target-resource-offset
            +--rw where?       enumeration
            +--rw value


   Refer to [I-D.ietf-netconf-yang-patch] for more details on the YANG
   Patch request and response contents.

4.3.  Notify functions

   Notification by the server to a selection of clients when an event
   occurs in the server is an essential function for the management of
   servers.  CoMI allows events specified in YANG [RFC5277] to be
   notified to a selection of requesting clients.  The server appends
   newly generated events to a stream.  There is one, so-called
   "default", stream in a CoMI server.  The /mg/stream resource
   identifies the default stream.  The server MAY create additional
   streams.  When a CoMI server generates an internal event, it is
   appended to the chosen stream, and the contents of a notification
   instance is ready to be sent to all CoMI clients which observe the
   chosen stream resource.

   Reception of generated notification instances is enabled with the
   CoAP Observe [I-D.ietf-core-observe] function.  The client subscribes
   to the notifications by sending a GET request with an "Observe"
   option, specifying the /mg/stream resource when the default stream is
   selected.



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   Every time an event is generated, the chosen stream is cleared, and
   the generated notification instance is appended to the chosen stream.
   After appending the instance, the contents of the instance is sent to
   all clients observing the modified stream.

   Suppose the server generates the event specified with:


   module example-port {
     ...
     prefix ep;
     ...
     notification example-port-fault {
       description
         "Event generated if a hardware fault on a
          line card port is detected";
       leaf port-name {
         type string;
         description "Port name";
       }
       leaf port-fault {
         type string;
         description "Error condition detected";
       }
     }
   }

   }

   By executing a GET on the /mg/stream resource the client receives the
   following response:


   REQ: GET example.com/mg/stream
       (observe option register)

   RES: 2.05 Content (Content-Format: application/cbor)
   {
         "example-port-fault":{
               "port-name" : "0/4/21",
               "port-fault" : "Open pin 2"
         }
   }

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




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   To check that the client is still alive, the server MUST send
   confirmable notifications once in a while.  When the client does not
   confirm the notification from the server, the server will remove the
   client from the list of observers [I-D.ietf-core-observe].

   In the registration request, the client MAY include a "Response-To-
   Uri-Host" and optionally "Response-To-Uri-Port" option as defined in
   [I-D.becker-core-coap-sms-gprs].  In this case, the observations
   SHOULD be sent to the address and port indicated in these options.
   This can be useful when the client wants the managed device to send
   the trap information to a multicast address.

4.4.  RPC statements

   An operation resource represents a protocol operation defined with
   the YANG "rpc" statement.  It is invoked using a POST method on the
   operation resource with a Token value as specified in section 5.3
   "Request/Resonse matching" of [RFC7252] to match the operation
   request sent by the RPC requester to the Operation request sent by
   the RPC executor.

         POST mg/op/<operation>

   The <operation> field identifies the module name and rpc identifier
   string for the desired operation.

   For example, if "module-A" defined a "reset" operation, then invoking
   the operation from "module-A" would be requested as follows:


         POST example.com/mg/op/module-A:reset

   If the "rpc" statement has input parameters, then a message-body MAY
   be sent by the client in the request, otherwise the request message
   MUST NOT include a message-body.

   If the operation is successfully invoked the server MUST send a 2.04
   Changed status code.  If the operation is not successfully invoked,
   then a message-body SHOULD be sent by the server, containing an
   error, as defined in Section 4.7.

   If the "rpc" statement has return parameters, then the server invokes
   a POST method with the same Token value as used in the request from
   the client.  The payload contains the values of the return
   parameters.






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4.4.1.  Encoding RPC input parameters

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

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


        module example-ops {
         namespace "https://example.com/ns/example-ops";
         prefix "ops";

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

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

   The client might send the following POST request message:












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         POST example.com/restconf/operations/example-ops:reboot
         Token:0x56
         Content-Type:
          { "example-ops:input":
            {
              "delay": 600,
              "message": "Going down for system maintenance"
              "language": "en-US"
            }
          }

   The server may respond with a 2.04 Changed.

4.4.2.  Encoding RPC output parameters

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

   The "example-ops" YANG module defined Section 4.4 is used for the
   examples in this section.

   The client might send the following POST request message:


         POST example.com/mg/op/example-ops:get-reboot-info
   Token: 0x56

   The server might respond with a POST request that is related to the
   original RPC invocation by the Token value:


         POST [ip:port]/mg/op/example-ops:output
   Token: 0x56
         {"example-ops:output" :
           {
             "reboot-time" : 30,
             "message" : "Going down for system maintenance",
             "language" : "en-US"
           }
         }

4.5.  Use of Block

   The CoAP protocol provides reliability by acknowledging the UDP
   datagrams.  However, when large pieces of text need to be transported
   the datagrams get fragmented, thus creating constraints on the
   resources in the client, server and intermediate routers.  The block



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   option [I-D.ietf-core-block] allows the transport of the total
   payload in individual blocks of which the size can be adapted to the
   underlying fragment 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.

   The block size is specified as exponents of the power 2.  The SZX
   exponent value can have 7 values ranging from 0 to 6 with associated
   block sizes given by 2**(SZX+4); for example SZX=0 specifies block
   size 16, and SZX=3 specifies block size 128.

   The block number of the block to transmit can be specified.  There
   are two block options: Block1 option for the request payload
   transported with PUT, POST or PATCH, and the block2 option for the
   response payload with GET.  Block1 and block2 can be combined.
   Examples showing the use of block option in conjunction with observer
   options are provided in [I-D.ietf-core-block].

   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 the complete
   data field.

   Beware of race conditions.  Blocks are filled one at a time and care
   should be taken that the whole data representation is sent in
   multiple blocks sequentially without interruption.  In the server,
   values are changed, lists are re-ordered, extended or reduced.  When
   these actions happen during the serialization of the contents of the
   variables, 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
   actual number of items.  It may be advisable to use CBOR maps or CBOR
   arrays of undefined length which are foreseen for data streaming
   purposes.

4.6.  Resource Discovery

   The presence and location of (path to) the management data are
   discovered by sending a GET request to "/.well-known/core" including
   a resource type (RT) parameter with the value "core.mg" [RFC6690].
   Upon success, the return payload will contain the root resource of
   the management data.  It is up to the implementation to choose its
   root resource, but it is recommended that the value "/mg" is used,
   where possible.  The example below shows the discovery of the
   presence and location of management data.






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     REQ: GET /.well-known/core?rt=core.mg

     RES: 2.05 Content </mg>; rt="core.mg"


   Management objects MAY be discovered with the standard CoAP resource
   discovery.  The implementation can add the encoded values of the
   object identifiers to /.well-known/core with rt="core.mg.data".  The
   available objects identified by the encoded values can be discovered
   by sending a GET request to "/.well-known/core" including a resource
   type (RT) parameter with the value "core.mg.data".  Upon success, the
   return payload will contain the registered encoded values and their
   location.  The example below shows the discovery of the presence and
   location of management data.


     REQ: GET /.well-known/core?rt=core.mg.data

     RES: 2.05 Content </mg/BaAiN>; rt="core.mg.data",
     </mg/CF_fA>; rt="core.mg.data"


   Lists of encoded values may become prohibitively long.  It is
   discouraged to provide long lists of objects on discovery.
   Therefore, it is recommended that details about management objects
   are discovered by reading the YANG module information stored in the
   "ietf-yang-library" module [I-D.ietf-netconf-restconf].  The resource
   "/mg/mod.uri" is used to retrieve the location of the YANG module
   library.

   The module list can be stored locally on each server, or remotely on
   a different server.  The latter is advised when the deployment of
   many servers are identical.


















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     Local in example.com server:

     REQ: GET example.com/mg/mod.uri

     RES: 2.05 Content (Content-Format: application/cbor)
     {
       "mod.uri" : "example.com/mg/modules"
     }


     Remote in example-remote-server:

     REQ: GET example.com/mg/mod.uri

     RES: 2.05 Content (Content-Format: application/cbor)
     {
       "moduri" : "example-remote-server.com/mg/group17/modules"
     }


   Within the YANG module library all information about the module is
   stored such as: module identifier, identifier hierarchy, grouping,
   features and revision numbers.

   The hash identifier is obtained as specified in
   [I-D.bierman-core-yang-hash].  When a collision occurred in the name
   space of the target server, a rehash is executed.

4.7.  Error Return Codes

   The RESTCONF return status codes defined in section 6 of the RESTCONF
   draft are used in CoMI error responses, except they are converted to
   CoAP error codes.


















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           +-------------------------------+------------------+
           | RESTCONF Status Line          | CoAP Status Code |
           +-------------------------------+------------------+
           | 100 Continue                  | none?            |
           |                               |                  |
           | 200 OK                        | 2.05             |
           |                               |                  |
           | 201 Created                   | 2.01             |
           |                               |                  |
           | 202 Accepted                  | none?            |
           |                               |                  |
           | 204 No Content                | 2.04 Changed     |
           |                               |                  |
           | 304 Not Modified              | 2.03             |
           |                               |                  |
           | 400 Bad Request               | 4.00             |
           |                               |                  |
           | 403 Forbidden                 | 4.03             |
           |                               |                  |
           | 404 Not Found                 | 4.04             |
           |                               |                  |
           | 405 Method Not Allowed        | 4.05             |
           |                               |                  |
           | 409 Conflict                  | none?            |
           |                               |                  |
           | 412 Precondition Failed       | 4.12             |
           |                               |                  |
           | 413 Request Entity Too Large  | 4.13             |
           |                               |                  |
           | 414 Request-URI Too Large     | 4.00             |
           |                               |                  |
           | 415 Unsupported Media Type    | 4.15             |
           |                               |                  |
           | 500 Internal Server Error     | 5.00             |
           |                               |                  |
           | 501 Not Implemented           | 5.01             |
           |                               |                  |
           | 503 Service Unavailable       | 5.03             |
           +-------------------------------+------------------+

5.  Error Handling

   In case a request is received which cannot be processed properly, the
   managed entity MUST return an error message.  This error message MUST
   contain a CoAP 4.xx or 5.xx response code, and SHOULD include
   additional information in the payload.





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   Such an error message payload is encoded in CBOR, using the following
   structure:

   TODO: Adapt RESTCONF <errors> data structure for use in CoMI.  Need
   to select the most important fields like <error-path>.


   errorMsg     : ErrorMsg;

   *ErrorMsg {
     errorCode  : uint;
     ?errorText : tstr;
   }

   The variable "errorCode" has one of the values from the table below,
   and the OPTIONAL "errorText" field contains a human readable
   explanation of the error.

   +----------------+----------------+---------------------------------+
   | CoMI Error     | CoAP Error     | Description                     |
   | Code           | Code           |                                 |
   +----------------+----------------+---------------------------------+
   | 0              | 4.00           | General error                   |
   |                |                |                                 |
   | 1              | 4.00           | Malformed CBOR data             |
   |                |                |                                 |
   | 2              | 4.00           | Incorrect CBOR datatype         |
   |                |                |                                 |
   | 3              | 4.00           | Unknown MIB variable            |
   |                |                |                                 |
   | 4              | 4.00           | Unknown conversion table        |
   |                |                |                                 |
   | 5              | 4.05           | Attempt to write read-only      |
   |                |                | variable                        |
   |                |                |                                 |
   | 0..2           | 5.01           | Access exceptions               |
   |                |                |                                 |
   | 0..18          | 5.00           | SMI error status                |
   +----------------+----------------+---------------------------------+

   The CoAP error code 5.01 is associated with the exceptions defined in
   [RFC3416] and CoAP error code 5.00 is associated with the error-
   status defined in [RFC3416].








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6.  Security Considerations

   For secure network management, it is important to restrict access to
   configuration variables only to authorized parties.  This requires
   integrity protection of both requests and responses, and depending on
   the application encryption.

   CoMI re-uses the security mechanisms already available to CoAP as
   much as possible.  This includes DTLS [RFC6347] for protected access
   to resources, as well suitable authentication and authorization
   mechanisms.

   Among the security decisions that need to be made are selecting
   security modes and encryption mechanisms (see [RFC7252]).  This
   requires a trade-off, as the NoKey mode gives no protection at all,
   but is easy to implement, whereas the X.509 mode is quite secure, but
   may be too complex for constrained devices.

   In addition, mechanisms for authentication and authorization may need
   to be selected.

   CoMI avoids defining new security mechanisms as much as possible.
   However some adaptations may still be required, to cater for CoMI's
   specific requirements.

7.  IANA Considerations

   'rt="core.mg.data"' needs registration with IANA.

   'rt="core.mg.moduri"' needs registration with IANA.

   'rt="core.mg.num-type"' needs registration with IANA.

   'rt="core.mg.srv-type"' needs registration with IANA.

   'rt="core.mg.yang-hash"' needs registration with IANA.

   'rt="core.mg.stream"' needs registration with IANA.

   'rt="core.mg.op"' needs registration with IANA.

   Content types to be registered:

   o  application/comi+cbor







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8.  Acknowledgements

   We are very grateful to Bert Greevenbosch who was one of the original
   authors of the CoMI specification and specified CBOR encoding and use
   of hashes.  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 Somaraju Abhinav, Rodney Cummings, Dee
   Denteneer, Esko Dijk, Michael van Hartskamp, Alexander Pelov, Juergen
   Schoenwaelder, Anuj Sehgal, Zach Shelby, Hannes Tschofenig, Michel
   Veillette, Michael Verschoor, and Thomas Watteyne.

9.  Changelog

   Changes from version 00 to version 01

   o  Focus on MIB only

   o  Introduced CBOR, JSON, removed BER

   o  defined mappings from SMI to xx

   o  Introduced the concept of addressable table rows

   Changes from version 01 to version 02

   o  Focus on CBOR, used JSON for examples, removed XML and EXI

   o  added uri-query attributes mod and con to specify modules and
      contexts

   o  Definition of CBOR string conversion tables for data reduction

   o  use of Block for multiple fragments

   o  Error returns generalized

   o  SMI - YANG - CBOR conversion

   Changes from version 02 to version 03

   o  Added security considerations

   Changes from version 03 to version 04

   o  Added design considerations section

   o  Extended comparison of management protocols in introduction



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   o  Added automatic generation of CBOR tables

   o  Moved lowpan table to Appendix

   Changes from version 04 to version 05

   o  Merged SNMP access with RESTCONF access to management objects in
      small devices

   o  Added CoMI architecture section

   o  Added RESTCONf NETMOD description

   o  Rewrote section 5 with YANG examples

   o  Added server and payload size appendix

   o  Removed Appendix C for now.  It will be replaced with a YANG
      example.

   Changes from version 04 to version 05

   o  Extended examples with hash representation

   o  Added keys query parameter text

   o  Added select query parameter text

   o  Better separation between specification and instance

   o  Section on discovery updated

   o  Text on rehashing introduced

   o  Elaborated SMI MIB example

   o  Yang library use described

   o  use of BigEndian/LittleEndian in Hash generation specified

   Changes from version 05 to version 06

   o  Hash values in payload as hexadecimal and in URL in base64 numbers

   o  Streamlined CoMI architecture text

   o  Added select query parameter text




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   o  Data editing optional

   o  Text on Notify added

   o  Text on rehashing improved with example

   Changes from version 06 to version 07

   o  reduced payload size by removing JSON hierarchy

   o  changed rehash handling to support small clients

   o  added LWM2M comparison

   o  Notification handling as specified in YANG

   o  Added Patch function

   o  Rehashing completely reviewed

   o  Discover type of YANG name encoding

   o  Added new resource types

   o  Read-only servers introduced

   o  Multiple updates explained

   Changes from version 07 to version 08

   o  Changed YANG Hash algorithm to use module name instead of prefix

   o  Added rehash bit to allow return values to identify rehashed nodes
      in the response

   o  Removed /mg/mod.set resource since this is not needed

   o  Clarified that YANG Hash is done even for unimplemented objects

   o  YANG lists transported as CBOR maps of maps

   o  Adapted examples with more CBOR explanation

   o  Added CBOR code examples in new appendix

   o  Possibility to use other than default stream

   o  Added text and examples for Patch payload



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   o  Repaired some examples

   o  Added appendices on hash clash probability and hash clash storage
      overhead

   Changes from version 08 to version 09

   o  Removed hash and YANG to CBOR sections

   o  removed hashes from examples.

   o  Added RPC

   o  Added content query parameter.

   o  Added default handling.

   o  Listed differences with RESTCONF

10.  References

10.1.  Normative References

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

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

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <http://www.rfc-editor.org/info/rfc6020>.

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

   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <http://www.rfc-editor.org/info/rfc7159>.







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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,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7396]  Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
              DOI 10.17487/RFC7396, October 2014,
              <http://www.rfc-editor.org/info/rfc7396>.

   [I-D.becker-core-coap-sms-gprs]
              Becker, M., Li, K., Kuladinithi, K., and T. Poetsch,
              "Transport of CoAP over SMS", draft-becker-core-coap-sms-
              gprs-05 (work in progress), August 2014.

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",
              draft-ietf-core-block-18 (work in progress), September
              2015.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP", draft-ietf-
              core-observe-16 (work in progress), December 2014.

   [I-D.ietf-netmod-yang-json]
              Lhotka, L., "JSON Encoding of Data Modeled with YANG",
              draft-ietf-netmod-yang-json-08 (work in progress),
              February 2016.

   [I-D.ietf-netconf-restconf]
              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", draft-ietf-netconf-restconf-09 (work in
              progress), December 2015.

   [I-D.vanderstok-core-patch]
              Stok, P. and A. Sehgal, "Patch Method for Constrained
              Application Protocol (CoAP)", draft-vanderstok-core-
              patch-02 (work in progress), October 2015.

   [I-D.ietf-netconf-yang-patch]
              Bierman, A., Bjorklund, M., and K. Watsen, "YANG Patch
              Media Type", draft-ietf-netconf-yang-patch-07 (work in
              progress), December 2015.

   [I-D.bierman-core-yang-hash]
              Bierman, A. and P. Stok, "YANG Hash", draft-bierman-core-
              yang-hash-00 (work in progress), February 2016.





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10.2.  Informative References

   [RFC2578]  McCloghrie, K., Ed., Perkins, D., Ed., and J.
              Schoenwaelder, Ed., "Structure of Management Information
              Version 2 (SMIv2)", STD 58, RFC 2578,
              DOI 10.17487/RFC2578, April 1999,
              <http://www.rfc-editor.org/info/rfc2578>.

   [RFC3410]  Case, J., Mundy, R., Partain, D., and B. Stewart,
              "Introduction and Applicability Statements for Internet-
              Standard Management Framework", RFC 3410,
              DOI 10.17487/RFC3410, December 2002,
              <http://www.rfc-editor.org/info/rfc3410>.

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              DOI 10.17487/RFC3411, December 2002,
              <http://www.rfc-editor.org/info/rfc3411>.

   [RFC3414]  Blumenthal, U. and B. Wijnen, "User-based Security Model
              (USM) for version 3 of the Simple Network Management
              Protocol (SNMPv3)", STD 62, RFC 3414,
              DOI 10.17487/RFC3414, December 2002,
              <http://www.rfc-editor.org/info/rfc3414>.

   [RFC3416]  Presuhn, R., Ed., "Version 2 of the Protocol Operations
              for the Simple Network Management Protocol (SNMP)",
              STD 62, RFC 3416, DOI 10.17487/RFC3416, December 2002,
              <http://www.rfc-editor.org/info/rfc3416>.

   [RFC3418]  Presuhn, R., Ed., "Management Information Base (MIB) for
              the Simple Network Management Protocol (SNMP)", STD 62,
              RFC 3418, DOI 10.17487/RFC3418, December 2002,
              <http://www.rfc-editor.org/info/rfc3418>.

   [RFC4293]  Routhier, S., Ed., "Management Information Base for the
              Internet Protocol (IP)", RFC 4293, DOI 10.17487/RFC4293,
              April 2006, <http://www.rfc-editor.org/info/rfc4293>.

   [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,
              <http://www.rfc-editor.org/info/rfc6241>.

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



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   [RFC6643]  Schoenwaelder, J., "Translation of Structure of Management
              Information Version 2 (SMIv2) MIB Modules to YANG
              Modules", RFC 6643, DOI 10.17487/RFC6643, July 2012,
              <http://www.rfc-editor.org/info/rfc6643>.

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

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

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

   [I-D.ietf-core-interfaces]
              Shelby, Z., Vial, M., and M. Koster, "Reusable Interface
              Definitions for Constrained RESTful Environments", draft-
              ietf-core-interfaces-04 (work in progress), October 2015.

   [I-D.ietf-lwig-coap]
              Kovatsch, M., Bergmann, O., and C. Bormann, "CoAP
              Implementation Guidance", draft-ietf-lwig-coap-03 (work in
              progress), July 2015.

   [XML]      "Extensible Markup Language (XML)",
              Web http://www.w3.org/xml.

   [OMA]      "OMA-TS-LightweightM2M-V1_0-20131210-C", Web
              http://technical.openmobilealliance.org/Technical/
              current_releases.aspx.

   [DTLS-size]
              Hummen, R., Shafagh, H., Raza, S., Voigt, T., and K.
              Wehrle, "Delegation-based Authentication and Authorization
              for the IP-based Internet of Things", Web
              http://www.vs.inf.ethz.ch/publ/papers/
              mshafagh_secon14.pdf.

   [mibreg]   "Structure of Management Information (SMI) Numbers (MIB
              Module Registrations)", Web
              http://www.iana.org/assignments/smi-numbers/
              smi-numbers.xhtml/.





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   [management]
              Schoenwalder, J. and A. Sehgal, "Management of the
              Internet of Things", Web http://cnds.eecs.jacobs-
              university.de/slides/2013-im-iot-management.pdf, 2013.

   [dcaf]     Bormann, C., Bergmann, O., and S. Gerdes, "Delegated
              Authenticated Authorization for Constrained Environments",
              Private Information .

   [openwsn]  Watteijne, T., "Coap size in Openwsn",
              Web http://builder.openwsn.org/.

   [Erbium]   Kovatsch, M., "Erbium Memory footprint for coap-18",
              Private Communication .

Appendix A.  Payload and Server sizes

   This section provides information on code sizes and payload sizes for
   a set of management servers.  Approximate code sizes are:

   +---------------+------------+-------+-------+----------------------+
   | Code          | processor  | Text  | Data  | reference            |
   +---------------+------------+-------+-------+----------------------+
   | Observe agent | erbium     | 800   | n/a   | [Erbium]             |
   |               |            |       |       |                      |
   | CoAP server   | MSP430     | 1K    | 6     | [openwsn]            |
   |               |            |       |       |                      |
   | SNMP server   | ATmega128  | 9K    | 700   | [management]         |
   |               |            |       |       |                      |
   | Secure SNMP   | ATmega128  | 30K   | 1.5K  | [management]         |
   |               |            |       |       |                      |
   | DTLS server   | ATmega128  | 37K   | 2K    | [management]         |
   |               |            |       |       |                      |
   | NETCONF       | ATmega128  | 23K   | 627   | [management]         |
   |               |            |       |       |                      |
   | JSON parser   | CC2538     | 4.6K  | 8     | [dcaf]               |
   |               |            |       |       |                      |
   | CBOR parser   | CC2538     | 1.5K  | 2.6K  | [dcaf]               |
   |               |            |       |       |                      |
   | DTLS server   | ARM7       | 15K   | 4     | [I-D.ietf-lwig-coap] |
   |               |            |       |       |                      |
   | DTLS server   | MSP430     | 15K   | 4     | [DTLS-size]          |
   |               |            |       |       |                      |
   | Certificate   | MSP430     | 23K   |       | [DTLS-size]          |
   |               |            |       |       |                      |
   | Crypto        | MSP430     | 2-8K  |       | [DTLS-size]          |
   +---------------+------------+-------+-------+----------------------+




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   Thomas says that the size of the CoAP server is rather arbitrary, as
   its size depends mostly on the implementation of the underlying
   library modules and interfaces.

   Payload sizes are compared for the following request payloads, where
   each attribute value is null (N.B. these sizes are educated guesses,
   will be replaced with generated data).  The identifier are assumed to
   be a string representation of the OID.  Sizes for SysUpTime differ
   due to preambles of payload.  "CBOR opt" stands for CBOR payload
   where the strings are replaced by table numbers.

     +-------------------------+-----------+------+------+----------+
     | Request                 | BERR SNMP | JSON | CBOR | CBOR opt |
     +-------------------------+-----------+------+------+----------+
     | IPnetTOMediaTable       | 205       | 327  | ~327 | ~51      |
     |                         |           |      |      |          |
     | lowpanIfStatsTable      |           | 710  | 614  | 121      |
     |                         |           |      |      |          |
     | sysUpTime               | 29        | 13   | ~13  | 20       |
     |                         |           |      |      |          |
     | RESTCONF example        |           |      |      |          |
     +-------------------------+-----------+------+------+----------+

Appendix B.  Comparison with LWM2M

   CoMI and LWM2M, both, provide RESTful device management services over
   CoAP.  Differences between the designs are highlighted in this
   section.

   LWM2M [OMA] objects are defined by standardized numbers.  When new
   types are needed, new numbers need to be defined.  This is the major
   difference with CoMI and YANG, where new modules can incorporate any
   type that is required without going through a standardization
   process, but may lead to rehashing.  On the one hand LWM2M is static
   with very small numbered objects, where CoMI with YANG is more
   dynamic, with a number conversion overhead.

   Unlike CoMI, which enables the use of SMIv2 and YANG data models for
   device management, LWM2M defines a new object resource model.  This
   means that data models need to be redefined in order to use LWM2M.
   In contrast, CoMI provides access to a large variety of SMIv2 and
   YANG data modules that can be used immediately.

   Objects and resources within CoMI are identified with a YANG hash
   value, however, each object is described as a link in the CoRE Link
   Format by LWM2M.  This approach by LWM2M can lead to larger complex
   URIs and more importantly payloads can grow large in size.  Using a
   hash value to represent the objects and resources allows URIs and



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   payloads to be smaller in size, which is important for constrained
   devices that may not have enough resources to process large messages.

   LWM2M encodes payload data in Type-length-value (TLV), JSON or plain
   text formats.  While the TLV encoding is binary and can result in
   reduced message sizes, JSON and plain text are likely to result in
   large message sizes when lots of resources are being monitored or
   configured.  Furthermore, CoMI's use of CBOR gives it an advantage
   over the LWM2M's TLV encoding as well since this too is more
   efficient [citation needed].

   CoMI is aligned with RESTCONF for constrained devices and uses YANG
   data models that have objects containing resources organized in a
   tree-like structure.  On the other hand, LWM2M uses a very flat data
   model that follows the "object/instance/resource" format, with no
   possibility to have sub-resources.  Complex data models are, as such,
   harder to model with LWM2M.

   In situations where resources need to be modified, CoMI uses the CoAP
   PATCH operation when resources are modified partially.  However,
   LWM2M uses the CoAP PUT and POST operations, even when a subset of
   the resource needs modifications.

Authors' Addresses

   Peter van der Stok
   consultant

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


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

   Email: andy@yumaworks.com










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