LwM2M¶
LwM2M (Lightweight Machine to Machine) is a device management protocol from the Open Mobile Alliance, now maintained by OMA SpecWorks. It runs on CoAP and gives a standard way for a server to provision, configure, monitor and update constrained devices. Where MQTT moves messages and leaves their meaning to you, LwM2M defines both the transport and a data model for common device functions.
Versions 1.0 (2017), 1.1 (2018) and 1.2 (2020) are in use. 1.1 added non-IP transports and LwM2M over TCP; 1.2 added gateway support and MQTT and HTTP transports.
The parts¶
Bootstrap and registration.
- Client: runs on the device and exposes its resources.
- Bootstrap server: gives a new device the address and credentials of the management server it should use. This lets the same factory image work for any customer or region.
- LwM2M server: manages the device once registered.
The object model¶
Everything on the device is addressed as /object/instance/resource. Objects are standardised and numbered:
| Object | ID | Example resource |
|---|---|---|
| Security | 0 | Server URI and keys |
| Server | 1 | Registration lifetime |
| Device | 3 | /3/0/0 manufacturer, /3/0/9 battery level, /3/0/4 reboot |
| Connectivity monitoring | 4 | /4/0/2 signal strength |
| Firmware update | 5 | /5/0/1 package URI, /5/0/2 update |
| Location | 6 | Latitude and longitude |
| Temperature (IPSO) | 3303 | /3303/0/5700 sensor value |
Some standard LwM2M objects.
Hundreds of further objects are registered by OMA and other bodies, including the IPSO smart objects for common sensors. Vendors can define their own.
Operations¶
- Register and update: the device announces itself and its objects, then refreshes its registration before its lifetime expires. A sleepy device uses queue mode, and the server holds requests until the device next checks in.
- Read, write, execute: the server reads values, changes configuration, or triggers actions such as a reboot.
- Observe and notify: the server subscribes to resources, and the device reports changes, with attributes such as minimum period or “only if it changes by more than 0.5”.
- Send (1.1 and later): the device pushes data without being asked.
Payloads use compact formats: TLV, SenML in JSON or CBOR, and LwM2M CBOR in 1.2.
Firmware updates¶
Object 5 defines a standard update state machine. The server gives the device a package, either by pushing it with block-wise transfer or by giving a URI to pull from. The device downloads and verifies it, the server triggers the update, and the device reports the result. Every compliant server and client agrees on this flow, which is one of LwM2M’s main attractions. See firmware over the air.
Security¶
LwM2M uses DTLS with pre-shared keys, raw public keys or certificates, or OSCORE. Credentials are delivered at bootstrap, so devices can be re-keyed later without visiting them. See TLS and DTLS.
When to choose it¶
LwM2M suits large fleets of constrained, often cellular, devices that need standard management, especially where devices and servers come from different suppliers. Open-source implementations include Eclipse Leshan (server), Eclipse Wakaama and Anjay (clients), and the client in Zephyr.
Where next¶
- CoAP covers the transport underneath.
- The Handbook covers device management architecture, and the Glossary defines the terms.
- OMA SpecWorks publishes the specifications and the object registry at openmobilealliance.org.