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Versions: 00                                                            
Network Working Group                                         J. Jimenez
Internet-Draft                                                  Ericsson
Intended status: Informational                          October 31, 2016
Expires: May 4, 2017

             CoAP functionality expected in a LWM2M system


   This document provides a strawman summary of information that should
   be used for the LWM2M specification [LWM2M].  LWM2M is based on CoAP,
   on top of which it describes certain management interfaces and data
   models that go beyond the CoAP specifications itself.  However LWM2M
   does not describe all behavior that should be expected from
   implementations of the CoAP specifications.  This document attempts
   to clarify what should be present in a LWM2M system beyond what is
   specified in the LWM2M documents.  Additionally, this document also
   adds information about IPSO Objects [IPSO] and their usage with LWM2M
   as application protocol.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on May 4, 2017.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Interaction Model . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Device and Manager configuration. . . . . . . . . . . . .   3
     3.2.  Device to Device configuration. . . . . . . . . . . . . .   4
     3.3.  Device to Application configuration.  . . . . . . . . . .   4
   4.  Data Model  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Collaboration . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The current LWM2M protocol is probably the main Device Management
   protocol based on CoAP today.  It defines the application layer
   communication protocol between a LWM2M Server and a LWM2M Client,
   which is located in a LWM2M Device.

2.  Terminology

   The LWM2M Specification tends to use its own terminology for client,
   server, etc.  In this document, we use the existing terminology from

   For example the use of LWM2M "Client" and "Server" and the roles they
   play has confused developers that are initiating on the protocol,
   mainly because a CoAP server runs on the device, just like a LWM2M
   client does.  Moreover, most LWM2M devices will often work both as
   client and server depending on the interfaces used, it would be good
   to explore the use of terms like "servients" for devices that
   regularly support both.

   Similarly, the reference to existing drafts of RFCs often can mislead
   the reader to believe that the full RFC has been implemented.  It
   would be better to state the support to an IETF CoRE WG document when

   For example, the Registration interface in LWM2M is based on the CoAP
   Resource Directory.  However, it is not sufficient to implement just

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   the interface described to obtain the benefits provided by the CoAP
   Resource Directory.

3.  Interaction Model

   LWM2M has been created with a strong focus on centralizing control
   and management.  Devices set associations with their manager and all
   traffic is directed to the cloud.  All this is fine but in the
   process some functionalities that could be used locally device to
   device and device to application have not been explicitly described.

   Below we have common configurations that make use of LWM2M.

   o  (1) Device and Manager configuration.

   o  (2) Device to Device configuration.

   o  (3) Device to Application configuration.

      Device               +
      +--------+--------+  |     (1)         +----------+--------------+
      |        |        |  |    LWM2M        |          | LWM2M Server |
      | LWM2M  | IPSO   |  | <-------------> |  Manager +--------------+
      +--------+--------+  |                 |          | BS Server    |
      |                 |  |                 +----------+--------------+
      |      CoAP       |  |     (2)
      +--------+--------+  |    CoAP+IPSO    +----------+
      |        |        |  | <-------------> | Device   |
      | UDP    |  TCP   |  |                 +----------+
      +-----------------+  |     (3)
      |      IPv6       |  |    CoAP+IPSO    +-------------------------+
      |                 |  | <-------------> | User / Application      |
      +-----------------+  |                 +-------------------------+

3.1.  Device and Manager configuration.

   This is covered by common LWM2M compliant implementations we have
   today.  However there are upcoming RFCs and drafts that greatly
   enhance LWM2M with more CoAP features.

   For example TCP support is soon going to be added to CoAP.  The draft
   [I-D.ietf-core-coap-tcp-tls] outlines the changes required to use
   CoAP over TCP, TLS, and WebSockets transports.

   Support for features like PATCH/FETCH [I-D.ietf-core-etch] could be
   greatly beneficial for things like firmware upgrade or observing
   relatively large sets of resources.

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   For systems in which endpoints work behind a gateway or use LWM2M for
   managing the gateways, it might be good to implement other types of
   cryptographic protection than DTLS.  For example some of the setups
   using OSCoAP [I-D.ietf-core-object-security] allow for "smarter"

3.2.  Device to Device configuration.

   Beyond what is described in the LWM2M documentation, devices will
   often talk to each other.  Specially in cases when all devices are
   under the same subnet, this could be pretty common.  For example
   devices could be more resilient if they did not have to contact their
   manager constantly; in case of lack of internet connectivity the
   local IoT network would still function.  Managers could just set
   policies on the devices and they would operate more autonomously.

   For this setup to take place, LWM2M would use more of the device-to-
   device functionality of CoAP.  A more complete Resource Directory
   implementation [I-D.ietf-core-resource-directory] would be needed,
   either on the LWM2M server in addition to the registration interface
   or standalone.  Devices should be able to perform lookup on that RD
   and get the series of links to resources elsewhere.  They should be
   able to find new functionality through /.well-known/core.  If not,
   they should be able to use IP multicast as expressed on [RFC7390].

   Needless to say, it is assumed that devices would be running a CoAP
   Server on them and would support CoAP Observe [RFC7641], so that
   devices can subscribe to updates from one another, thus becoming more

   There are also updates on ACE security framework, that allow for
   securing the communication between two devices via an Authorization
   Server [I-D.ietf-ace-oauth-authz].

   The current LWM2M Data Model needs more expressiveness when it comes
   to data types; More on that in Section 4.  Also Web Linking will be
   dealt at Section 5.

3.3.  Device to Application configuration.

   In some other cases applications would be running on the phone
   connecting locally to sensors and/or control actuators.  A smartphone
   can access directly a CoAP home sensor using a mutually authenticated
   'https' request, provided its home router runs a HTTP to CoAP (HC)
   proxy and is configured with the appropriate certificate.  For this
   scenario to happen, the GW should implement a HC proxy.  It is highly
   recommended then that they make use of [I-D.ietf-core-http-mapping]
   to properly do the URI mapping and specific ABNF queries.

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   Just like other devices, smartphone applications should be able to
   discover devices using standard methods, thus they would need access
   to the RD as well.

4.  Data Model

   The LWM2M Object Model is specified in [LWM2M].  Other models that
   build on it like IPSOs or OneM2M have spawned out of it.  They
   normally introduce incremental features.  They usually allow for
   performing any set of operations on a device through a CoAP
   interface.  Resources are exposed as Objects using the same data
   model used for management.

   For example, in the case of a temperature sensor we can access and
   subscribe to the readings of the device (using [IPSO]).

   Req: GET /3303/0/5700 Observe_Option=1
   Res: 2.05 Content (25 C)
   Res (Notify): 2.05 Content (26 C)

   There has also been much work on different serialization and
   compression mechanisms that LWM2M could consider adopting.  A
   serialized JSON file like the one below could be greatly compressed
   (about 46% max, depending on the case) using CBOR representation
   format [RFC7049] instead.

   {       "e": [{
           "bn": "/3303/0",
           "e": [{
               "n": "5700",
               "v": 20.0 }, {
               "n": "5701",
               "v": "c" }, {
               "n": "5603",
               "v": 10 }, {
               "n": "5604",
               "v": 40
           }],    }, {
           "bn": "/3302/0",
           "e": [{
               "n": "5500",
               "v": true }, {
               "n": "5501",
               "v": 23
   }       ]}]}

   LWM2M ResourceIDs at the moment have no specific semantic meaning
   like ObjectIDs do.  Adding a similar registry for ResourceIDs could

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   be useful.  Specially to those using LWM2M for their applications.
   For example IPSO uses such ResourceIDs to register resources
   univocally, so that the string _5701_ consistently represents units.

5.  Web Linking

   One thing that that could be very useful in the future is some form
   of Web Link resource type.  ObjectLinks are not sufficient to
   represent links between devices or applications.  There has been much
   work on web linking on [RFC6690] that could be used in the LWM2M
   spec.  For example a new Data Type named "Web Link" could be a
   simple, yet useful addition.  Instead of the current
   _ObjectID:InstanceID_ expressed now, a full WebLink would be used.
   That would take advantage of other features like
   [I-D.ietf-core-links-json] or even newer Object Models.

   Other use cases contemplate some form of Object Redirection to help
   decouple management and applications.  LWM2M expects that the
   management servers will observe resources and collect telemetry on
   the management server itself.  If LWM2M is to be used as application
   protocol as well as management, it should provide a way for
   applications or CoAP Clients to observe resources on the devices,
   together with their required credentials.  Such credentials should be
   stored on the device in some way, maybe a new Object.

6.  Collaboration

   To further develop the relationship between the LWM2M specification
   and other specifications based on CoAP, it would also be advisable to
   foster collaboration between organizations developing CoAP-based
   standard implementations.  At the moment there is no forum for inter
   group communication nor discussion.  That should change.

   The IETF CoRE WG has quite some people also interested in device
   management.  Communication would be mutually beneficial.  Example of
   that work is on COMI [I-D.ietf-core-yang-cbor] or data model

   OMA LWM2M already has benefited from workshops that gather most of
   the industry, such as [IOTSI] and [IOTSU].  Similarly, specifications
   can be developed in the IETF with a view to be directly usable in

7.  Informative References

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              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE)", draft-ietf-ace-oauth-
              authz-04 (work in progress), October 2016.

              Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              draft-ietf-core-coap-tcp-tls-05 (work in progress),
              October 2016.

              Stok, P., Bormann, C., and A. Sehgal, "Patch and Fetch
              Methods for Constrained Application Protocol (CoAP)",
              draft-ietf-core-etch-03 (work in progress), October 2016.

              Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Guidelines for HTTP-to-CoAP Mapping
              Implementations", draft-ietf-core-http-mapping-16 (work in
              progress), October 2016.

              Li, K., Rahman, A., and C. Bormann, "Representing CoRE
              Formats in JSON and CBOR", draft-ietf-core-links-json-06
              (work in progress), July 2016.

              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-ietf-core-
              object-security-00 (work in progress), October 2016.

              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", draft-ietf-core-resource-directory-08
              (work in progress), July 2016.

              Veillette, M., Pelov, A., Somaraju, A., Turner, R., and A.
              Minaburo, "CBOR Encoding of Data Modeled with YANG",
              draft-ietf-core-yang-cbor-02 (work in progress), July

   [IOTSI]    IAB, "IoT Workshop for Semantic Interoperability (IOTSI)",
              2016, <https://www.iab.org/activities/workshops/iotsi/>.

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   [IOTSU]    IAB, "Internet of Things Software Update Workshop
              (IoTSU)", 2016, <https://www.iab.org/activities/workshops/

   [IPSO]     IPSO, "IPSO Object Model", n.d.,

   [LWM2M]    OMA, "LWM2M specification", n.d.,

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,

   [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>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
              RFC7252, June 2014,

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641, DOI 10.17487/
              RFC7641, September 2015,

Author's Address

   Jaime Jimenez

   Phone: +358-442-992-827
   Email: jaime.jimenez@ericsson.com

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