Technical Report Document Number


Introduction to OMA LightweightM2M (LWM2M)



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5.3 Introduction to OMA LightweightM2M (LWM2M)

5.3.1 Description


OMA Lightweight M2M is a protocol for device and service management for M2M. The main purpose of this technology is to address service and management needs for constrained M2M devices, over UDP and SMS bearers. The crucial aspects in this work are the:

  • Target devices for this protocol are resource constraint devices (e.g., 8-16bit MCU, RAM is in tens of KB and flash is in hundreds of KB)

  • Ability to perform Data collection and remote control of devices without the need for complex computing and UI operations

  • Optimization of network resources to allow a large numbers of devices may be connected to the communication network simultaneously

  • Fusion of device functionalities management and service manipulation into a single protocol

From the implementation view LWM2M has the following features:

  • Suitable for resource constraint devices

  • Usage of compact binary packets

  • Support for multiple data encoding formats that include Binary , JSON, plain text and opaque data formats

  • Easy to be implemented though the reuse of existing implementation of IETF technologies (e.g., CoAP)

One of typical use cases of using LWM2M technology is the firmware upgrade of streetlights [i.18].

  1. A Streetlights supervisor is responsible for managing the streetlights system. (There are thousands of streetlights in the city and low-cost LWM2M devices embedded in the streetlights.)

  2. The supervisor needs to remotely upgrade of the firmware of a specific streetlight or a group of streetlights.



Figure 5.3.1: Firmware Upgrade of Streetlight of Use Case using LWM2M

5.3.2 Architecture




Figure 5.3.2: LWM2M Architecture

As shown in the Figure 5.3.2, the layout is the architecture of LWM2M [i.19]. The Components specified by OMA LWM2M compose the LWM2M enabler which specifies the LWM2M Server / LWM2M Client interface. The LWM2M Server and LWM2M Client are typically instantiated in a M2M Server and a M2M Device.



Based on the deployment scenario, the LWM2M Server has the bootstrapping capability itself, or the LWM2M Bootstrap Server exists separately for security reasons.

5.3.3 Reference Points


This section introduces the interfaces carried over the reference point consisting of two main components LWM2M Server and the LWM2M Client.

5.3.3.1 Functional Components

5.3.3.1.1 LWM2M Server

The LWM2M Server is a logical component which serves as an endpoint of the LWM2M protocol.
5.3.3.1.2 LWM2M Client

The LWM2M Client is a logical component. This LWM2M Client serves as an endpoint of the LWM2M protocol and communicates with the LWM2M Server to execute the device management and service enablement operations from the LWM2M Server and reporting results of the operations.

5.3.3.2 Interfaces


There are four interfaces supported by the reference point between LWM2M server and LWM2M Client. The logical operation of each interface is defined as follows:.

  • Bootstrap

This interface is used to provision essential information into the LWM2M Client so that the LWM2M Client can register to the LWM2M Server(s) after bootstrap procedure has completed.

  • Client Registration

This interface allows the LWM2M Client register to the LWM2M Server. This procedure lets the Server know the existence and information (e.g., address, capabilities) of the LWM2M Client so that LWM2M Server can perform M2M services and device management on the LWM2M Client.

  • Device Management and Service Enablement

This interface allows the LWM2M Server to perform the device management and M2M service enablement operations. Over this interface, the LWM2M Server can send operations to the LWM2M Client and gets response of the operations from the LWM2M Client.

  • Information Reporting

This interface allows the LWM2M Client to report resource information to the LWM2M Server. This Information Reporting can be triggered periodically or by events (e.g., resource information is changed and configured conditions are met).

5.3.4 Protocols

5.3.4.1 Protocol Stack


The LWM2M has the protocol stack defined as below.



Figure 5.3.3: LWM2M Protocol Stack

  • LWM2M Objects: LWM2M Objects are designed for the functionality provided by the LWM2M enabler. The LWM2M specification [i.20] defines a set of Standard Objects. Other Objects may also be added by OMA, external SDOs (e.g., the IPSO alliance) or vendors to enable certain M2M Services.

  • LWM2M Protocol: LWM2M protocol defines the logical operations and mechanisms per each interface.

  • CoAP: The LWM2M utilizes the IETF Constrained Application Protocol [i.21] as an underlying transfer protocol across UDP and SMS bearers. This protocol defines the message header, request/response codes, message options and retransmission mechanisms. The LWM2M only uses the subset of features defined in CoAP.

  • DTLS: DTLS [i.22] is used to provide secure UDP channel between the LWM2M Server and the LWM2M Client for all the messages interchanged.

  • UDP Binding with CoAP (Mandatory): Reliability over the UDP transport is provided by the built-in retransmission mechanisms of CoAP.

  • SMS Binding with CoAP (Optional): CoAP is used over SMS by placing a CoAP message in the SMS payload using 8-bit encoding.

5.3.4.2 Resource Model


In the LWM2M Enabler technical specification [i.20], a simple resource model is described. Basically, a resource made available by resource model of the LWM2M Client is a Resource, and Resources are logically organized into Objects. Figure 5.3.4 illustrates this structure, and the relationship between Resources, Objects, and the LWM2M Client. The LWM2M Client may have any number of Resources, each of which belongs to an Object.



Figure 5.3.4: LWM2M Resource Model [i.20]

Resources are defined per Object, and each resource is given a unique identifier within that Object. Each Resource is designed to have one or more Operations that it supports. A Resource may contain multiple instances dependent on the Resource definition in Object specification.

An Object defines a grouping of Resources, for example the Firmware Object contains all the Resources used for firmware update purposes. The LWM2M enabler defines standard Objects and Resources and other Objects may be added to enable a certain M2M Services.

Object is instantiated either by the LWM2M Server or the LWM2M Client, which is called Object Instance before using the functionality of an Object. After Object Instance is created, the LWM2M Server can access that Object Instance and Resources in the Object Instance.


5.3.4.3 Interface Descriptions

5.3.4.3.1 Bootstrap

The Bootstrap interface is used to provision essential information into the LWM2M Client in order to allow the LWM2M Client to be able to register to a certain LWM2M Server. There are four modes for bootstrapping:

  • Factory Bootstrap: the LWM2M Client is already provisioned at the time of the device manufacture. The pre-configured data is stored in the LWM2M Client.

  • Bootstrap from Smartcard: When the Device supports a Smartcard and retrieval of bootstrap message from Smartcard is successful, the LWM2M Client processes the bootstrap message from the Smartcard and applies it to the LWM2M Client.

  • Client initiated Bootstrap: the LWM2M Client retrieves the bootstrap message from a LWM2M Bootstrap Server. In this case the LWM2M Client needs to be pre-provisioned with the LWM2M Bootstrap Information before bootstrapping.

  • Server initiated Bootstrap: the LWM2M Server provisions the bootstrap message into the LWM2M Client after recognizing the existence of the LWM2M Device. In this case the LWM2M Client needs to be pre-provisioned with the LWM2M Bootstrap Information before bootstrapping.



Figure 5.3.5: Bootstrap Modes

5.3.4.3.2 Client Registration



The Client Registration interface is used by the LWM2M Client to register with one or more LWM2M Servers, maintain each registration, and de-register from the LWM2M Server(s). When registering, the LWM2M Client indicates its Endpoint Name, MSISDN, supporting binding modes, lifetime of registration, the list of Objects the Client supports and available Object Instances. The registration is a soft state, with a lifetime indicated by the registration lifetime. The LWM2M Client periodically performs an update of its registration information to the registered Server(s). If the lifetime of a registration expires without receiving an update from the Client, the Server removes the registration information. Finally, when shutting down or discontinuing use of a Server, the Client performs de-registration.



Figure 5.3.6: Example of Registration Procedure
5.3.4.3.3 Device Management and Service Enablement

This interface is used by the LWM2M Server to access Resources available from a LWM2M Client using Create, Read, Write, Delete, or Execute operations. The operations that a Resource supports are defined in the definition of its Object.



Figure 5.3.7: Example of Device Management and Service Enablement Interface
5.3.4.3.4 Information Reporting

This interface is used by the LWM2M Server to observe any changes in a Resource on the LWM2M Client, receiving notifications when new values are available. The LWM2M Server needs to configure observation related parameters by sending “Write Attribute” operation before observing Resources in the LWM2M Client. This observation relationship is initiated by sending an “Observe” operation to the L2M2M Client for an Object Instance or Resource. An observation ends when a “Cancel Observation” operation is performed or “Write Attribute” with cancel parameter operation is performed.



Figure 5.3.8: Example of Information Reporting Interface

5.3.5 Functions


A first set of standard Objects for the LWM2M 1.0 enabler have been developed. The Standard Objects are intended to support a variety of functionalities to manage LWM2M Devices. OMA may create further objects in future. Furthermore, other organizations and companies may define additional LWM2M Objects for their own M2M services using according to LWM2M Object Template and Guideline Annex in [i.20].

  • Server Security: security data related to the LWM2M server(s) and/or the LWM2M Bootstrap Server

  • Server: data, configuration, functions related to the LWM2M Server

  • Access Control: to check whether the LWM2M server has access right for performing an operation on Resources in the LWM2M Client

  • Device: provision of a range of device related information, device reboot and factory reset function

  • Connectivity Monitoring: to monitor parameters related to underlying network connectivity

  • Firmware: provision of firmware management, installing and updating new firmware

  • Location: provides location information of the LWM2M Devices

  • Connectivity Statistics: to statistical information of network connection (e.g., SMS counter, UDP data size)


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