net: lwm2m: Allow configuring update period

Add new configuration value CONFIG_LWM2M_UPDATE_PERIOD
that allows calculating update period from last update,
instead of calculating it from the lifetime.

In runtime, server is allowed to change the lifetime of the
registration which causes update perdiod to be effected.

When fixed update period is preferred, UPDATE_PERIOD
config is then used.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>

doc/connectivity/networking/api/lwm2m.rst

@@ -625,6 +625,53 @@ The events are prefixed with ``LWM2M_RD_CLIENT_EVENT_``.
        If the retry counter reaches its limits, this event will be triggered.
      - No actions needed, client will do a re-registrate automatically.
 
+
+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.
+
+.. list-table:: Update period variables
+   :widths: auto
+   :header-rows: 1
+
+   * - 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.
+   * - :kconfig:option:`CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME`
+     - Default lifetime value, unless set by the bootstrap server.
+       Also defines lower limit that client accepts as a lifetime.
+   * - :kconfig:option:`CONFIG_LWM2M_UPDATE_PERIOD`
+     - How long the client can stay idle before sending a next update.
+   * - :kconfig:option:`CONFIG_LWM2M_SECONDS_TO_UPDATE_EARLY`
+     - Minimum time margin to send the update message before the registration lifetime expires.
+
+.. figure:: images/lwm2m_lifetime_seconds_early.png
+    :alt: LwM2M seconds to update early
+
+    Default way of calculating when to update registration.
+
+By default, the client uses :kconfig:option:`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.
+
+.. figure:: images/lwm2m_lifetime_both.png
+    :alt: LwM2M update time when both values are set
+
+    Update time is controlled by UPDATE_PERIOD.
+
+When also the :kconfig:option:`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:
 
 LwM2M shell

samples/net/lwm2m_client/overlay-queue.conf

@@ -1,2 +1,6 @@
 CONFIG_LWM2M_QUEUE_MODE_ENABLED=y
-CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=180
+CONFIG_LWM2M_QUEUE_MODE_UPTIME=20
+# Default lifetime is 1 day
+CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=86400
+# Send update once an hour
+CONFIG_LWM2M_UPDATE_PERIOD=3600

subsys/net/lib/lwm2m/Kconfig

@@ -471,9 +471,19 @@ config LWM2M_ENGINE_DEFAULT_LIFETIME
 	  This is also a minimum lifetime that client accepts. If server sets lifetime
 	  less than this value, client automatically raises it.
 
+config LWM2M_UPDATE_PERIOD
+	int "LwM2M engine update period"
+	default 0
+	range 0 4294967295
+	help
+	  Time period after the previous update or registration when to send the next update message.
+	  This allows modifying lifetime without affecting the update period.
+	  Set to zero, to allow LWM2M_SECONDS_TO_UPDATE_EARLY and lifetime to control when to
+	  send the update. When both values are set, smallest period will be used.
+
 config LWM2M_SECONDS_TO_UPDATE_EARLY
 	int "LWM2M Registration Update transmission time before timeout"
-	default 6
+	default 10
 	range 1 4294967295
 	help
 	  Time in seconds before the registration timeout, when the LWM2M

subsys/net/lib/lwm2m/lwm2m_rd_client.c

@@ -64,13 +64,13 @@ LOG_MODULE_REGISTER(LOG_MODULE_NAME);
 #include "lwm2m_util.h"
 
 #define LWM2M_RD_CLIENT_URI "rd"
-#define SECONDS_TO_UPDATE_EARLY	CONFIG_LWM2M_SECONDS_TO_UPDATE_EARLY
 #define CLIENT_EP_LEN		CONFIG_LWM2M_RD_CLIENT_ENDPOINT_NAME_MAX_LENGTH
 #define CLIENT_BINDING_LEN sizeof("UQ")
 #define CLIENT_QUEUE_LEN sizeof("Q")
 #define DELAY_BEFORE_CLOSING	(1 * MSEC_PER_SEC)
 #define DELAY_FOR_ACK		100U
 #define EXCHANGE_LIFETIME	247U
+#define MINIMUM_PERIOD		15
 
 static void sm_handle_registration_update_failure(void);
 static int sm_send_registration_msg(void);
@@ -1104,11 +1104,21 @@ static int sm_do_registration(void)
 
 static int64_t next_update(void)
 {
-	/*
-	 * check for lifetime seconds - SECONDS_TO_UPDATE_EARLY
-	 * so that we can update early and avoid lifetime timeout
-	 */
-	return client.last_update + (client.lifetime - SECONDS_TO_UPDATE_EARLY) * MSEC_PER_SEC;
+	int64_t next;
+	int64_t period = CONFIG_LWM2M_UPDATE_PERIOD;
+	int64_t early = CONFIG_LWM2M_SECONDS_TO_UPDATE_EARLY;
+
+	if (period == 0) {
+		period = client.lifetime;
+	}
+	if (early > client.lifetime) {
+		early = client.lifetime;
+	}
+
+	next = MIN(period, client.lifetime - early);
+	next = MAX(next, MINIMUM_PERIOD);
+
+	return client.last_update + next * MSEC_PER_SEC;
 }
 
 static int64_t next_rx_off(void)

tests/net/lib/lwm2m/interop/prj.conf

@@ -61,6 +61,8 @@ CONFIG_COAP_EXTENDED_OPTIONS_LEN_VALUE=40
 # Use QUEUE mode by default
 CONFIG_LWM2M_QUEUE_MODE_ENABLED=y
 CONFIG_LWM2M_QUEUE_MODE_UPTIME=20
+CONFIG_LWM2M_UPDATE_PERIOD=30
+CONFIG_LWM2M_SECONDS_TO_UPDATE_EARLY=10
 
 # LwM2M configuration as OMA-ETS-LightweightM2M_INT-V1_1-20190912-D Configuration 3
 CONFIG_LWM2M_ENGINE_DEFAULT_LIFETIME=30

tests/net/lib/lwm2m/lwm2m_rd_client/CMakeLists.txt

@@ -32,3 +32,4 @@ add_compile_definitions(CONFIG_LWM2M_QUEUE_MODE_ENABLED=1)
 add_compile_definitions(CONFIG_LWM2M_TLS_SESSION_CACHING=1)
 add_compile_definitions(CONFIG_LWM2M_RD_CLIENT_LISTEN_AT_IDLE=1)
 add_compile_definitions(CONFIG_LWM2M_RD_CLIENT_SUPPORT_BOOTSTRAP=1)
+add_compile_definitions(CONFIG_LWM2M_UPDATE_PERIOD=0)