2. OMA LwM2M - Brief description

Lightweight Machine to Machine is a protocol developed by OMA SpecWorks for remote device management in the Internet of Things and other Machine-to-Machine applications.

Initially LwM2M was designed to be transported either over UDP, or over SMS directly in cellular phone networks. The next iteration of the standard, LwM2M 1.1, added support for another IP transport, TCP, and two more non-IP transports, 3GPP CIoT and LoRaWAN. All of these transports can be additionally secured with use of (D)TLS.

The application data is encapsulated using the Constrained Application Protocol (CoAP). CoAP is an application layer protocol similar to HTTP in philosophy and general semantics, but designed specifically with being lightweight in mind. CoAP makes it possible to transmit messages with low overhead - the minimal header size is just 4 bytes, which makes it feasible to send meaningful content within single, non-fragmented UDP datagrams, even over links with low MTU, such as SMS.

Since LwM2M 1.1, CoAP messages can be additionally secured using OSCORE, an application layer protocol with which it’s possible to not only establish a secure communication channel between LwM2M Clients and Servers, but also between the Client and external applications. It’s independent of security protocols used on other layers.

Anjay is designed to hide most of the protocol details from application developers - however, a rudimentary understanding of the protocol is necessary to understand the general philosophy and semantics of the parts that remain to be implemented. You are encouraged to read the full specification, but this article provides a quick summary of the most important concepts.

2.1. Clients and servers

A network environment using the LwM2M protocol consists of three types of entities:

  • LwM2M Clients are located on end devices. They communicate with server(s), allowing them to manage and monitor the devices’ resources, which are exposed via a standardized data model.

    • A LwM2M Client is uniquely identified independently of its network address by an Endpoint Client Name - a URN uniquely assigned to a device by its manufacturer. OMA recommends the Endpoint Client Name to be in one of the following formats:

      • urn:uuid:########-####-####-############ (UUID = Universally Unique Identifier)

      • urn:dev:ops:{OUI}-{ProductClass}-{SerialNumber} (OUI = Organizationally Unique Identifier - as in e.g. MAC addresses)

      • urn:dev:os:{OUI}-{SerialNumber}

      • urn:imei:############### (IMEI = International Mobile Equipment Identity)

      • urn:esn:######## (ESN = Electronic Serial Number)

      • urn:meid:############## (MEID = Mobile Equipment Identifier)

      • urn:imei-msisdn:###############-############### (MSISDN = Mobile Station International Subscriber Directory Number, i.e. a standardized phone number)

  • LwM2M Bootstrap Server is a specific server that may be contacted by the Client during its first or every boot-up. Its only purpose is to initialize the data model, including connections to regular LwM2M Servers, before first contact to such. The Bootstrap Server communicates with the Client using a different set of commands, so it cannot be considered a “LwM2M Server” in the ordinary sense.

  • LwM2M Servers maintain connections with the clients and have the ability to read from and write to the data model exposed by the clients. Any given client may be concurrently connected to more than one LwM2M Server, and each of them may have access only to a part of the whole data model.

Anjay is a framework for implementing LwM2M Clients. For this reason, the rest of this article will be written from the Client perspective.

2.2. Data model

Each LwM2M Client presents a data model - standardized, symbolic representation of its configuration and state that is accessible for reading and modifying by LwM2M Servers. It can be thought of as a combination of a hierarchical configuration file, and a view on statistical information about the device and its environment.

The LwM2M data model organized as a tree up to four levels deep. Entities on each of those levels are identified with numerical identifiers, in range 0-65534, inclusive. A reserved value of 65535 (called MAX_ID in protocol’s specification) carries a special meaning in many contexts. Those levels are:

  • Object - each Object represent some different concept of data accessible via the LwM2M Client. For example, separate Objects are defined for managing connections with LwM2M Servers, for managing network connections, for accessing data from various types of sensors, etc.

    Each Object is assigned a unique numerical identifier. OMA manages a registry of known Object IDs. Each Object defines a set of Resources whose meanings are common for each Object Instance.

  • Object Instance - some Objects are described as “single-instance” - such Objects always have exactly one Instance with identifier 0. Examples of such Objects include the Device object which describes the device itself, and the Firmware Update object which is used to perform firmware upgrades.

    Other Objects may have multiple Instances; sometimes the number of Instances may be variable and the Instances themselves may be creatable via LwM2M. Examples of such Objects include the Object that manages connections to LwM2M Servers, Object that represents optional software packages installed on the device, and Objects representing sensors (whose instances are, however, not creatable).

  • Resource - each Object Instance of a given Object supports the same set of Resources, as defined by the Object definition. Within a given Object, each Resource ID has a well-defined meaning, and represent the same concept. However, some Resources may not be present in some Object Instances, and, obviously, their values and mapping onto real-world entities may be different.

  • Resource Instance - some Resources may have multiple instances (they’re sometimes called Multiple-Resources), effectively causing the type of that resource to be an associative array with an integer index. Since LwM2M 1.1, instances of these resources may be addressed individually.

The numerical identifiers on each of these levels form a path, which is used as the path portion of CoAP URLs. For example, a path /9/2/17/3 refers to Resource Instance ID=3 of Resource ID=17 in Object Instance ID=2 of Object ID=9. Whole Resources (/9/2/17), Object Instances (/9/2) or even Objects (/9) may be referred to using this syntax as well.

2.2.1. Objects

Each Object definition, which may be found in the LwM2M specification, features the following information:

  • Name - description of the object; it is not used in the actual on-wire protocol.

  • Object ID - numerical identifier of the Object

  • Instances - Single (always has one Instance with ID=0) or Multiple (may have arbitrary number of Instances depending on current configuration)

  • Mandatory - Mandatory (must be supported by all LwM2M Client implementations) or Optional (may not be supported)

  • Object URN

  • Resource definitions

The current set of Mandatory Objects consists of:

  • /0 - LwM2M Security - contains confidential part of information about connections to the LwM2M Servers configured in the Client. From the on-wire protocol perspective, it is write-only and accessible only via the Bootstrap Interface. Implementation of this object is readily available in Anjay’s security module.

  • /1 - LwM2M Server - contains non-confidential part of information about connections to the LwM2M Servers configured in the Client. Implementation of this object is readily available in Anjay’s server module.

  • /3 - Device - contains basic information about the device, such as e.g. serial number.

Additionally, Object /2 (Access Control) needs to be supported and present if the Client supports more than one LwM2M Server connection at once. Implementation of this object is readily available in Anjay’s access_control module.

2.2.2. Resources

Each of the Resource definitions, contained in each Object definition, features the following information:

  • ID - numerical identifier of the Resource.

  • Name - short description of the resource; it is not used in the actual on-wire protocol.

  • Operations - one of:

  • Instances - Single or Multiple; “Multiple” means that the type of data in the resource is actually an “array” - called such in the Anjay API, but actually more similar to an associative data structure. It is a list of pairs, each of which containing a unique Resource Instance ID (range 0-65535, inclusive) and instance value, of the type referred in the Resource definition.

  • Mandatory - Mandatory or Optional; Mandatory resources need to be present in all Instances on all devices. Optional resources may not be present in all Instances, and may even be not supported at all on some Clients.

  • Type - data type of the Resource value (or its instances in case of Multiple Resources).

  • Range or Enumeration - specification of valid values for the Resource, within the given data type.

  • Units - units in which a numerical value is given.

  • Description - detailed description of the resource.

2.2.3. Attributes

Each entity in the data model (Object, Object Instance, Resource or Resource Instance) can have various “attributes” attached. Most of these attributes are handled automatically by Anjay. There are two types of attributes currently defined in the LwM2M specification:

  • <PROPERTIES> Class Attributes are read-only metadata that may be read by Servers without accessing the data itself, possibly allowing it to operate more effectively. These include:

    • Dimension (dim) - in case of Multiple Resources, it is the number of Resource Instances.

    • Object Version (ver) - provides a way for versioning Object definitions. This attribute, if not present, implies 1.0 version of the Object, although the user is free to adjust it in anjay_dm_object_def_t structure.

    • additional properties used only in Bootstrap-Discover operation: Short Server ID (ssid) and Server URI (uri).

  • <NOTIFICATION> Class Attributes are writeable by LwM2M Servers and affect the way changes in observed resources are notified over the Information Reporting Interface.

    By default, Notify messages are sent each time there is some change to the value of the queried path (which may be a Resource, or all Resources within a given Object Instance or Object, if the Observe request was called on such higher-level path).

    This behavior can be modified using the following available attributes:

    • Minimum Period (pmin) - if set to a non-zero value, notifications will never be sent more often than once every pmin seconds.

    • Maximum Period (pmax) - if set, notifications will always be sent at least once every pmax seconds, even if the value did not change.

    • Greater Than (gt) and Less Than (lt) - applicable only to numeric Resources - if set, notifications will only be sent when the value changes from below to above or from above to below the specified threshold. Contrary to what the names of these Attributes might suggest, there is no semantic difference between the two - both behave as equivalent bi-directional thresholds.

    • Step (st) - applicable only to numeric Resources - if set, notifications will only be sent if the numerical value is different from the previously notified value by at least st.

    • Minimum Evaluation Period (epmin) - if set, the notification criteria won’t be evaluated more often than every epmin seconds.

    • Maximum Evaluation Period (epmax) - if set, the notification criteria will be evaluated at least every epmax seconds (although this attribute is ignored in Anjay, as the automatic evaluation happens when pmax seconds pass).

    When several Attributes are specified at the same time, the relations between them are as follows:

    • pmin and pmax have higher priority - even if the requirements for gt, lt and st are not met, a notification will always be sent at least once every pmax seconds - and conversely, even when the requirements for gt, lt and st are met, a notification will never be sent more often than once every pmin seconds.

    • Requirements for just at least one of gt, lt or st need to be met if they are set at the same time. For example, if the new value differs by at least st from the previously sent one, it does not need to cross either of the lt/gt thresholds - the st condition alone is enough to trigger sending notification.

2.3. Interfaces

LwM2M currently consists of four interfaces through which the Clients, Servers and Bootstrap Servers communicate:

2.3.1. Bootstrap Interface

Bootstrap Interface defines the set of commands that the Bootstrap Server may use to provision the initial configuration onto the client. In this interface, both the LwM2M Client and the LwM2M Bootstrap Server act as both a CoAP server and a CoAP client. The messages that may be exchanged between those include:

  • POST /bs?ep={Endpoint Client Name} request sent from the Client to the Bootstrap Server signifies a Bootstrap Request command. It informs the Bootstrap Server that a new client has appeared on the network and is requesting bootstrap information. However, the protocol also allows the Bootstrap Server to start issuing Bootstrap commands on its own, without receiving a Bootstrap Request message.

  • GET Accept: application/link-format request sent from the Bootstrap Server to the Client is interpreted as Bootstrap Discover. It allows the Bootstrap Server to get information about the data model supported by and present on the client device.

  • GET with Content Format option sent from the Bootstrap Server to the Client is a Bootstrap Read. It can be used to read LwM2M Server and Access Control Objects during the Bootstrap phase to query for already configured Servers on the Client.

  • PUT requests sent from the Bootstrap Server to the Client are interpreted as Bootstrap Write commands. These allow creating and writing to Object Instances and Resources in order to initialize the data model to a state appropriate for communication with regular LwM2M Servers.

  • Bootstrap Delete command, represented as DELETE requests from the Bootstrap Server to the Client, allows the Bootstrap Server to delete existing Object Instances.

  • Finally, the Bootstrap Server sends a Bootstrap Finish command, represented as a POST /bs CoAP request send to the Client. Upon receiving it, the Client validates the data model, and in case of success, connects to regular LwM2M Servers, according to the configured stored within the data model.

As you can see, the Bootstrap Interface is mostly write-only. The Bootstrap Server is not able to do any actual management or monitoring of the Client. It can only prepare it for communication with regular LwM2M Servers. Nevertheless, nothing prevents Bootstrap Server and regular Server applications from coexisting on the same host.

The Bootstrap Server is the only entity that can manage connections to LwM2M Servers on a Client via the LwM2M protocol itself. For this reason, an association with a Bootstrap Server may be maintained indefinitely - however, the protocol also provides an option to permanently disconnect from the Bootstrap Server after a successful bootstrap.

Bootstrap information may also be provided by means other than the Bootstrap Server. The protocol also allows the bootstrap information to be pre-provisioned at the factory, or read from a smart-card. In those cases, an attempt to contact a Bootstrap Server may not even be made.

2.3.2. Registration Interface

The Registration Interface defines the protocol the Client uses to inform an LwM2M Server about its presence and availability. In this interface, the LwM2M Server acts as a CoAP server, and the LwM2M Client is a CoAP client. The requests that may be sent from the Client to the Server include:

  • Register, represented as a CoAP POST /rd?… request, is initially sent by the Client when it goes online. It informs the Server that the Client is available for receiving commands on the Device Management and Service Enablement Interface and the Information Reporting Interface, and presents it with basic metadata describing its data model. It also gives the server the IP address and port (or phone number, in case of SMS transport) on which the Client is accessible - these are taken directly from the source fields in IP and UDP layer headers.

  • Update, which is a CoAP POST request on an URL previously returned in a response to Register. Update is sent in following situations:

    • periodically - to ensure the Server that the device is still online,

    • whenever any of the information previously given in a Register message changes - so that the Server always has up-to-date information about the Client’s state.

  • De-register (CoAP DELETE) may be sent by the Client if it can determine that it is shutting down. It terminates the association between the Client and Server. Sending it is, however, not required, as the Server will also consider the association terminated if the Client does not report with a Register or Update message for a configured period of time.

2.3.3. Device Management and Service Enablement Interface

This is the main interface on which the actual device management occurs. In this interface, most of the requests are made by the LwM2M Server, that is, it acts as a CoAP client, sending requests to the LwM2M Client, which acts as a server on the CoAP layer. The exception is the Send request (available since LwM2M 1.1), which is issued by the LwM2M Client. Please note that the IP addresses and port numbers are exactly the same as previously established via the Registration Interface - it means that for given two endpoints, the client/server relationship on the CoAP layer is reversible at any time.

Following operations can be issued by the LwM2M Server:

  • Discover (CoAP GET Accept: application/link-format) allows the Server to get a list of all supported and present Objects, Object Instances and Resources, and to read Attributes attached to them. Data stored in Resources is not returned.

  • Read (CoAP GET other than the above) reads data - either from a single Resource Instance, entire Resource, Object Instance, or even a whole Object at once.

  • Write allows the Server to modify the data model. It comes in two flavors:

    • PUT /{Object ID}/{Instance ID}[/{Resource ID}[/{Resource Instance ID}]] request signifies the Replace method. It can be called on either a single Resource to replace its value, or on a whole Object Instance - in that case all existing contents of that Instance are erased and replaced with the supplied data.

    • POST /{Object ID}/{Instance ID} request means Partial Update. It can only be called on a whole Object Instance and only replaces the Resources present in the request payload, retaining other previously existing data.

    Both methods require the Content-Format option to be included in the request.

    Anjay attempts to abstract away the difference between the two. All such bulk writes are translated to series of writes on single values. However, to properly support the Replace semantics, an additional virtual operation called Reset is introduced, called before the series of writes during a Replace and intended to revert the Object Instance to its initial, empty state.

  • While most entities in the data model are designed to be read and written, a given entity may alternatively be specified as supporting the Execute operation, represented by a POST /{Object ID}/{Instance ID}/{Resource ID} request. Execute operation is introduced in the data model wherever a way is necessary to instruct the device to perform some non-idempotent operation such as a reboot or a firmware upgrade.

  • A PUT request without a Content-Format option is interpreted as Write Attributes. The Attributes are passed as query string elements in the target URL. These Attributes mostly alter the way the Client behaves in relation to the Information Reporting Interface and are explained in detail in the Attributes section.

  • A POST request targeting one of the root paths in the data model (called “Objects”, see Data model) represents the Create operation. It creates a new Object Instance, which gives a way to manage configuration entities that might have a variable and configurable number of similar but distinct entries - for example, software packages or APN connections.

  • Composite counterparts of Read and Write operations - Read-Composite (CoAP FETCH) and Write-Composite (CoAP iPATCH), which can target many Object Instances and Resources of different Objects. These operations were added in LwM2M 1.1.

  • Finally, the Delete operation (CoAP DELETE) is the reverse of Create, allowing to remove previously created Object Instances.

Following operations can be issued by the LwM2M Client:

  • A POST /dp request represents the Send operation. It’s used by the Client to send data to the Server without an explicit request, which is in some circumstances a more flexible option compared to the standard Information Reporting Interface described further.

2.3.4. Information Reporting Interface

This interface can be thought of as an extension to the Device Management and Service Enablement Interface, allowing the Server to automatically receive periodic updates about some values in the data model it is particularly interested in. It is based on the OBSERVE extension to CoAP, applying its semantics mostly unchanged onto the LwM2M mapping of CoAP concepts.

  • A Read operation (CoAP GET), after adding the Observe option = 0, becomes Observe. Upon receiving such request, in addition to returning the current value, the Client will start sending Notify messages when appropriate.

  • Cancel Observation command can be issued either by performing a Read with Observe option = 1 or by responding to the Notify message with a CoAP RESET.

  • Composite counterparts of Observe and Cancel Observation operations - Observe-Composite (CoAP FETCH with Observe option = 0) and Cancel Observation-Composite (CoAP FETCH with Observe option = 1), which can target many entities at once.

  • Notify is an asynchronous CoAP response as described in RFC 7641. It is essentially a repeated reply to a Read, sent whenever the observed value changes, and/or periodically, according to relevant Attributes.

    It may be sent as a Non-confirmable or as a Confirmable message at discretion of the Client. Anjay currently sends almost all notifications as Non-confirmable messages; Confirmable notifications are sent once every 24 hours, to comply with RFC 7641 section 4.5.

2.4. Queue mode

The Register command includes a “Queue Mode” parameter which indicates if the client device is running in Queue Mode. It’s a special mode of operation in which the client device is not required to actively listen for incoming packets. The client is only required to listen for such packets for a limited period of time after each exchange of messages with the Server - typically after the Update command.

The specification recommends to use CoAP’s MAX_TRANSMIT_WAIT value (93 seconds by default) as that aforementioned limited period of time, and this recommendation is respected in Anjay.

Anjay automatically handles the queue mode by hiding connections which are not required to actively listen from the library user. In particular, if the anjay_get_sockets() function returns an empty list, it likely means that all active connections are in queue mode and the listening period has passed. In that case, it is safe to passively sleep for the period returned by anjay_sched_time_to_next() (or one of its convenience wrappers).

In LwM2M 1.0 the use of Queue Mode was handled by Binding parameter, included in Register and Update commands.

2.5. Trigger mode

SMS messages can be used not only just a sole communication method for LwM2M-enabled devices, but they can be also used in coordination with any other binding to wake the device up from sleep in Queue Mode and send the Update message to the Server. This mode of operation is called “Trigger mode”.

To wake the device up, the LwM2M Server sends the Execute operation on 1/x/8 Resource (Registration Update Trigger), expecting a response to that message on the other communication channel, like TCP or UDP.

You can find more information about Trigger mode in SMS Binding feature description.

In LwM2M 1.0, similar mode of operation could be achieved with use of “UQS” Binding Mode.