Lightweight M2M (LWM2M)

Overview

Lightweight Machine to Machine (LwM2M) is an application layer protocol designed with device management, data reporting and device actuation in mind. Based on CoAP/UDP, LwM2M is a standard defined by the Open Mobile Alliance and suitable for constrained devices by its use of CoAP packet-size optimization and a simple, stateless flow that supports a REST API.

One of the key differences between LwM2M and CoAP is that an LwM2M client initiates the connection to an LwM2M server. The server can then use the REST API to manage various interfaces with the client.

LwM2M uses a simple resource model with the core set of objects and resources defined in the specification.

The LwM2M library can be enabled with CONFIG_LWM2M Kconfig option.

Example LwM2M object and resources: Device

Object definition

Object ID	Name	Instance	Mandatory
3	Device	Single	Mandatory

Resource definitions

* R=Read, W=Write, E=Execute

ID	Name	OP*	Instance	Mandatory	Type
0	Manufacturer	R	Single	Optional	String
1	Model	R	Single	Optional	String
2	Serial number	R	Single	Optional	String
3	Firmware version	R	Single	Optional	String
4	Reboot	E	Single	Mandatory
5	Factory Reset	E	Single	Optional
6	Available Power Sources	R	Multiple	Optional	Integer 0-7
7	Power Source Voltage (mV)	R	Multiple	Optional	Integer
8	Power Source Current (mA)	R	Multiple	Optional	Integer
9	Battery Level %	R	Single	Optional	Integer
10	Memory Free (Kb)	R	Single	Optional	Integer
11	Error Code	R	Multiple	Optional	Integer 0-8
12	Reset Error	E	Single	Optional
13	Current Time	RW	Single	Optional	Time
14	UTC Offset	RW	Single	Optional	String
15	Timezone	RW	Single	Optional	String
16	Supported Binding	R	Single	Mandatory	String
17	Device Type	R	Single	Optional	String
18	Hardware Version	R	Single	Optional	String
19	Software Version	R	Single	Optional	String
20	Battery Status	R	Single	Optional	Integer 0-6
21	Memory Total (Kb)	R	Single	Optional	Integer
22	ExtDevInfo	R	Multiple	Optional	ObjLnk

The server could query the Manufacturer resource for Device object instance 0 (the default and only instance) by sending a READ 3/0/0 operation to the client.

The full list of registered objects and resource IDs can be found in the LwM2M registry.

Zephyr’s LwM2M library lives in the subsys/net/lib/lwm2m, with a client sample in samples/net/lwm2m_client. For more information about the provided sample see: LwM2M client. The sample can be configured to use normal unsecure network sockets or sockets secured via DTLS.

The Zephyr LwM2M library implements the following items:

• engine to process networking events and core functions

• RD client which performs BOOTSTRAP and REGISTRATION functions

• SenML CBOR, SenML JSON, CBOR, TLV, JSON, and plain text formatting functions

• LwM2M Technical Specification Enabler objects such as Security, Server, Device, Firmware Update, etc.

• Extended IPSO objects such as Light Control, Temperature Sensor, and Timer

By default, the library implements LwM2M specification 1.0.2 and can be set to LwM2M specification 1.1.1 with a Kconfig option.

For more information about LwM2M visit OMA Specworks LwM2M.

Sample usage

To use the LwM2M library, start by creating an LwM2M client context lwm2m_ctx structure:

/* LwM2M client context */
static struct lwm2m_ctx client;

Create callback functions for LwM2M resource executions:

static int device_reboot_cb(uint16_t obj_inst_id, uint8_t *args,
                            uint16_t args_len)
{
        LOG_INF("Device rebooting.");
        LOG_PANIC();
        sys_reboot(0);
        return 0; /* won't reach this */
}

The LwM2M RD client can send events back to the sample. To receive those events, setup a callback function:

static void rd_client_event(struct lwm2m_ctx *client,
                            enum lwm2m_rd_client_event client_event)
{
        switch (client_event) {

        case LWM2M_RD_CLIENT_EVENT_NONE:
                /* do nothing */
                break;

        case LWM2M_RD_CLIENT_EVENT_BOOTSTRAP_REG_FAILURE:
                LOG_DBG("Bootstrap registration failure!");
                break;

        case LWM2M_RD_CLIENT_EVENT_BOOTSTRAP_REG_COMPLETE:
                LOG_DBG("Bootstrap registration complete");
                break;

        case LWM2M_RD_CLIENT_EVENT_BOOTSTRAP_TRANSFER_COMPLETE:
                LOG_DBG("Bootstrap transfer complete");
                break;

        case LWM2M_RD_CLIENT_EVENT_REGISTRATION_FAILURE:
                LOG_DBG("Registration failure!");
                break;

        case LWM2M_RD_CLIENT_EVENT_REGISTRATION_COMPLETE:
                LOG_DBG("Registration complete");
                break;

        case LWM2M_RD_CLIENT_EVENT_REG_TIMEOUT:
                LOG_DBG("Registration timeout!");
                break;

        case LWM2M_RD_CLIENT_EVENT_REG_UPDATE_COMPLETE:
                LOG_DBG("Registration update complete");
                break;

        case LWM2M_RD_CLIENT_EVENT_DEREGISTER_FAILURE:
                LOG_DBG("Deregister failure!");
                break;

        case LWM2M_RD_CLIENT_EVENT_DISCONNECT:
                LOG_DBG("Disconnected");
                break;

        case LWM2M_RD_CLIENT_EVENT_REG_UPDATE:
                LOG_DBG("Registration update");
                break;

        case LWM2M_RD_CLIENT_EVENT_DEREGISTER:
                LOG_DBG("Deregistration client");
                break;

        case LWM2M_RD_CLIENT_EVENT_SERVER_DISABLED:
                LOG_DBG("LwM2M server disabled");
          break;
        }
}

Next we assign Security resource values to let the client know where and how to connect as well as set the Manufacturer and Reboot resources in the Device object with some data and the callback we defined above:

/*
 * Server URL of default Security object = 0/0/0
 * Use leshan.eclipse.org server IP (5.39.83.206) for connection
 */
lwm2m_set_string(&LWM2M_OBJ(0, 0, 0), "coap://5.39.83.206");

/*
 * Security Mode of default Security object = 0/0/2
 * 3 = NoSec mode (no security beware!)
 */
lwm2m_set_u8(&LWM2M_OBJ(0, 0, 2), 3);

#define CLIENT_MANUFACTURER "Zephyr Manufacturer"

/*
 * Manufacturer resource of Device object = 3/0/0
 * We use lwm2m_set_res_data() function to set a pointer to the
 * CLIENT_MANUFACTURER string.
 * Note the LWM2M_RES_DATA_FLAG_RO flag which stops the engine from
 * trying to assign a new value to the buffer.
 */
lwm2m_set_res_data(&LWM2M_OBJ(3, 0, 0), CLIENT_MANUFACTURER,
                   sizeof(CLIENT_MANUFACTURER),
                   LWM2M_RES_DATA_FLAG_RO);

/* Reboot resource of Device object = 3/0/4 */
lwm2m_register_exec_callback(&LWM2M_OBJ(3, 0, 4), device_reboot_cb);

Lastly, we start the LwM2M RD client (which in turn starts the LwM2M engine). The second parameter of lwm2m_rd_client_start() is the client endpoint name. This is important as it needs to be unique per LwM2M server:

(void)memset(&client, 0x0, sizeof(client));
lwm2m_rd_client_start(&client, "unique-endpoint-name", 0, rd_client_event);

LwM2M security modes

The Zephyr LwM2M library can be used either without security or use DTLS to secure the communication channel. When using DTLS with the LwM2M engine, PSK (Pre-Shared Key) and X.509 certificates are the security modes that can be used to secure the communication. The engine uses LwM2M Security object (Id 0) to read the stored credentials and feed keys from the security object into the TLS credential subsystem, see secure sockets documentation. Enable the CONFIG_LWM2M_DTLS_SUPPORT Kconfig option to use the security.

Depending on the selected mode, the security object must contain following data:

PSK

Security Mode (Resource ID 2) set to zero (Pre-Shared Key mode). Identity (Resource ID 3) contains PSK ID in binary form. Secret key (Resource ID 5) contains the PSK key in binary form. If the key or identity is provided as a hex string, it must be converted to binary before storing into the security object.

X509

When X509 certificates are used, set Security Mode (ID 2) to 2 (Certificate mode). Identity (ID 3) is used to store the client certificate and Secret key (ID 5) must have a private key associated with the certificate. Server Public Key resource (ID 4) must contain a server certificate or CA certificate used to sign the certificate chain. If the CONFIG_MBEDTLS_PEM_CERTIFICATE_FORMAT Kconfig option is enabled, certificates and private key can be entered in PEM format. Otherwise, they must be in binary DER format.

NoSec

When no security is used, set Security Mode (Resource ID 2) to 3 (NoSec).

In all modes, Server URI resource (ID 0) must contain the full URI for the target server. When DNS names are used, the DNS resolver must be enabled.

When DTLS is used, following options are recommended to reduce DTLS handshake traffic when connection is re-established:

• CONFIG_LWM2M_DTLS_CID enables DTLS Connection Identifier support. When server supports it, this completely removes the handshake when device resumes operation after long idle period. Greatly helps when NAT mappings have timed out.

• CONFIG_LWM2M_TLS_SESSION_CACHING uses session cache when before falling back to full DTLS handshake. Reduces few packets from handshake, when session is still cached on server side. Most significant effect is to avoid full registration.

LwM2M stack provides callbacks in the lwm2m_ctx structure. They are used to feed keys from the LwM2M security object into the TLS credential subsystem. By default, these callbacks can be left as NULL pointers, in which case default callbacks are used. When an external TLS stack, or non-default socket options are required, you can overwrite the lwm2m_ctx.load_credentials() or lwm2m_ctx.set_socketoptions() callbacks.

An example of setting up the security object for PSK mode:

/* "000102030405060708090a0b0c0d0e0f" */
static unsigned char client_psk[] = {
        0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
        0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f
};

static const char client_identity[] = "Client_identity";

lwm2m_set_string(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 0), "coaps://lwm2m.example.com");
lwm2m_set_u8(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 2), LWM2M_SECURITY_PSK);
/* Set the client identity as a string, but this could be binary as well */
lwm2m_set_string(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 3), client_identity);
/* Set the client pre-shared key (PSK) */
lwm2m_set_opaque(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 5), client_psk, sizeof(client_psk));

An example of setting up the security object for X509 certificate mode:

static const char certificate[] = "-----BEGIN CERTIFICATE-----\nMIIB6jCCAY+gAw...";
static const char key[] = "-----BEGIN EC PRIVATE KEY-----\nMHcCAQ...";
static const char root_ca[] = "-----BEGIN CERTIFICATE-----\nMIIBaz...";

lwm2m_set_string(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 0), "coaps://lwm2m.example.com");
lwm2m_set_u8(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 2), LWM2M_SECURITY_CERT);
lwm2m_set_string(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 3), certificate);
lwm2m_set_string(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 5), key);
lwm2m_set_string(&LWM2M_OBJ(LWM2M_OBJECT_SECURITY_ID, 0, 4), root_ca);

Before calling lwm2m_rd_client_start() assign the tls_tag # where the LwM2M library should store the DTLS information prior to connection (normally a value of 1 is ok here).

(void)memset(&client, 0x0, sizeof(client));
client.tls_tag = 1; /* <---- */
lwm2m_rd_client_start(&client, "endpoint-name", 0, rd_client_event);

For a more detailed LwM2M client sample see: LwM2M client.

Multi-thread usage

Writing a value to a resource can be done using functions like lwm2m_set_u8. When writing to multiple resources, the function lwm2m_registry_lock will ensure that the client halts until all writing operations are finished:

lwm2m_registry_lock();
lwm2m_set_u32(&LWM2M_OBJ(1, 0, 1), 60);
lwm2m_set_u8(&LWM2M_OBJ(5, 0, 3), 0);
lwm2m_set_f64(&LWM2M_OBJ(3303, 0, 5700), value);
lwm2m_registry_unlock();

This is especially useful if the server is composite-observing the resources being written to. Locking will then ensure that the client only updates and sends notifications to the server after all operations are done, resulting in fewer messages in general.

Support for time series data

LwM2M version 1.1 adds support for SenML CBOR and SenML JSON data formats. These data formats add support for time series data. Time series formats can be used for READ, NOTIFY and SEND operations. When data cache is enabled for a resource, each write will create a timestamped entry in a cache, and its content is then returned as a content in READ, NOTIFY or SEND operation for a given resource.

Data cache is only supported for resources with a fixed data size.

Supported resource types:

• Signed and unsigned 8-64-bit integers

• Float

• Boolean

Enabling and configuring

Enable data cache by selecting CONFIG_LWM2M_RESOURCE_DATA_CACHE_SUPPORT. Application needs to allocate an array of lwm2m_time_series_elem structures and then enable the cache by calling lwm2m_engine_enable_cache() for a given resource. Earch resource must be enabled separately and each resource needs their own storage.

/* Allocate data cache storage */
static struct lwm2m_time_series_elem temperature_cache[10];
/* Enable data cache */
lwm2m_engine_enable_cache(LWM2M_PATH(IPSO_OBJECT_TEMP_SENSOR_ID, 0, SENSOR_VALUE_RID),
        temperature_cache, ARRAY_SIZE(temperature_cache));

LwM2M engine have room for four resources that have cache enabled. Limit can be increased by changing CONFIG_LWM2M_MAX_CACHED_RESOURCES. This affects a static memory usage of engine.

Data caches depends on one of the SenML data formats CONFIG_LWM2M_RW_SENML_CBOR_SUPPORT or CONFIG_LWM2M_RW_SENML_JSON_SUPPORT and needs CONFIG_POSIX_CLOCK so it can request a timestamp from the system and CONFIG_RING_BUFFER for ring buffer.

Read and Write operations

Full content of data cache is written into a payload when any READ, SEND or NOTIFY operation internally reads the content of a given resource. This has a side effect that any read callbacks registered for a that resource are ignored when cache is enabled. Data is written into a cache when any of the lwm2m_set_* functions are called. To filter the data entering the cache, application may register a validation callback using lwm2m_register_validate_callback().

Limitations

Cache size should be manually set so small that the content can fit normal packets sizes. When cache is full, new values are dropped.

LwM2M engine and application events

The Zephyr LwM2M engine defines events that can be sent back to the application through callback functions. The engine state machine shows when the events are spawned. Events depicted in the diagram are listed in the table. The events are prefixed with LWM2M_RD_CLIENT_EVENT_.

State machine for the LwM2M engine

LwM2M RD Client events

Event ID	Event Name	Description
0	NONE	No event
1	BOOTSTRAP_REG_FAILURE	Bootstrap registration failed. Occurs if there is a timeout or failure in bootstrap registration.
2	BOOTSTRAP_REG_COMPLETE	Bootstrap registration complete. Occurs after successful bootstrap registration.
3	BOOTSTRAP_TRANSFER_COMPLETE	Bootstrap finish command received from the server.
4	REGISTRATION_FAILURE	Registration to LwM2M server failed. Occurs if there is a failure in the registration.
5	REGISTRATION_COMPLETE	Registration to LwM2M server successful. Occurs after a successful registration reply from the LwM2M server or when session resumption is used.
6	REG_TIMEOUT	Registration or registration update timeout. Occurs if there is a timeout during registration. Client have lost connection to the server.
7	REG_UPDATE_COMPLETE	Registration update completed. Occurs after successful registration update reply from the LwM2M server.
8	DEREGISTER_FAILURE	Deregistration to LwM2M server failed. Occurs if there is a timeout or failure in the deregistration.
9	DISCONNECT	LwM2M client have de-registered from server and is now stopped. Triggered only if the application have requested the client to stop.
10	QUEUE_MODE_RX_OFF	Used only in queue mode, not actively listening for incoming packets. In queue mode the client is not required to actively listen for the incoming packets after a configured time period.
11	ENGINE_SUSPENDED	Indicate that client has now paused as a result of calling lwm2m_engine_pause(). State machine is no longer running and the handler thread is suspended. All timers are stopped so notifications are not triggered.
12	SERVER_DISABLED	Server have executed the disable command. Client will deregister and stay idle for the disable period.
13	NETWORK_ERROR	Sending messages to the network failed too many times. Client cannot reach any servers or fallback to bootstrap. LwM2M engine cannot recover and have stopped.

The LwM2M client engine handles most of the state transitions automatically. The application needs to handle only the events that indicate that the client have stopped or is in a state where it cannot recover.

How application should react to events

Event Name	How application should react
NONE	Ignore the event.
BOOTSTRAP_REG_FAILURE	Try to recover network connection. Then restart the client by calling lwm2m_rd_client_start(). This might also indicate configuration issue.
BOOTSTRAP_REG_COMPLETE	No actions needed
BOOTSTRAP_TRANSFER_COMPLETE	No actions needed
REGISTRATION_FAILURE	No actions needed
REGISTRATION_COMPLETE	No actions needed. Application can send or receive data.
REG_TIMEOUT	No actions needed. Client proceeds to re-registration automatically. Cannot send or receive data.
REG_UPDATE_COMPLETE	No actions needed Application can send or receive data.
DEREGISTER_FAILURE	No actions needed, client proceeds to idle state automatically. Cannot send or receive data.
DISCONNECT	Engine have stopped as a result of calling lwm2m_rd_client_stop(). If connection is required, the application should restart the client by calling lwm2m_rd_client_start().
QUEUE_MODE_RX_OFF	No actions needed. Application can send but cannot receive data. Any data transmission will trigger a registration update.
ENGINE_SUSPENDED	Engine can be resumed by calling lwm2m_engine_resume(). Cannot send or receive data.
SERVER_DISABLED	No actions needed, client will re-register once the disable period is over. Cannot send or receive data.
NETWORK_ERROR	Try to recover network connection. Then restart the client by calling lwm2m_rd_client_start(). This might also indicate configuration issue.

Sending of data in the table above refers to calling lwm2m_send_cb() or by writing into of of the observed resources where observation would trigger a notify message. Receiving of data refers to receiving read, write or execute operations from the server. Application can register callbacks for these operations.

Configuring lifetime and activity period

In LwM2M engine, there are three Kconfig options and one runtime value that configures how often the client will send LwM2M Update message.

Update period variables

Variable	Effect
LwM2M registration lifetime	The lifetime parameter in LwM2M specifies how long a device’s registration with an LwM2M server remains valid. Device is expected to send LwM2M Update message before the lifetime exprires.
CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME	Default lifetime value, unless set by the bootstrap server. Also defines lower limit that client accepts as a lifetime.
CONFIG_LWM2M_UPDATE_PERIOD	How long the client can stay idle before sending a next update.
CONFIG_LWM2M_SECONDS_TO_UPDATE_EARLY	Minimum time margin to send the update message before the registration lifetime expires.

Default way of calculating when to update registration.

By default, the client uses CONFIG_LWM2M_SECONDS_TO_UPDATE_EARLY to calculate how many seconds before the expiration of lifetime it is going to send the registration update. The problem with default mode is when the server changes the lifetime of the registration. This is then affecting the period of updates the client is doing. If this is used with the QUEUE mode, which is typical in IPv4 networks, it is also affecting the period of when the device is reachable from the server.

Update time is controlled by UPDATE_PERIOD.

When also the CONFIG_LWM2M_UPDATE_PERIOD is set, time to send the update message is the earliest when any of these values expire. This allows setting long lifetime for the registration and configure the period accurately, even if server changes the lifetime parameter.

In runtime, the update frequency is limited to once in 15 seconds to avoid flooding.

LwM2M shell

For testing the client it is possible to enable Zephyr’s shell and LwM2M specific commands which support changing the state of the client. Operations supported are read, write and execute resources. Client start, stop, pause and resume are also available. The feature is enabled by selecting CONFIG_LWM2M_SHELL. The shell is meant for testing so productions systems should not enable it.

One imaginable scenario, where to use the shell, would be executing client side actions over UART when a server side tests would require those. It is assumed that not all tests are able to trigger required actions from the server side.

uart:~$ lwm2m
lwm2m - LwM2M commands
Subcommands:
  send    :send PATHS
          LwM2M SEND operation

  exec    :exec PATH [PARAM]
          Execute a resource

  read    :read PATH [OPTIONS]
          Read value from LwM2M resource
          -x   Read value as hex stream (default)
          -s   Read value as string
          -b   Read value as bool (1/0)
          -uX  Read value as uintX_t
          -sX  Read value as intX_t
          -f   Read value as float
          -t   Read value as time_t

  write   :write PATH [OPTIONS] VALUE
          Write into LwM2M resource
          -s   Write value as string (default)
          -b   Write value as bool
          -uX  Write value as uintX_t
          -sX  Write value as intX_t
          -f   Write value as float
          -t   Write value as time_t

  create  :create PATH
          Create object or resource instance

  delete  :delete PATH
          Delete object or resource instance

  cache   :cache PATH NUM
          Enable data cache for resource
          PATH is LwM2M path
          NUM how many elements to cache

  start   :start EP_NAME [BOOTSTRAP FLAG]
          Start the LwM2M RD (Registration / Discovery) Client
          -b   Set the bootstrap flag (default 0)

  stop    :stop [OPTIONS]
          Stop the LwM2M RD (De-register) Client
          -f   Force close the connection

  update  :Trigger Registration Update of the LwM2M RD Client

  pause   :LwM2M engine thread pause
  resume  :LwM2M engine thread resume
  lock    :Lock the LwM2M registry
  unlock  :Unlock the LwM2M registry

API Reference

LwM2M high-level API