nrf_modem_lib: lte_net_if: query actual link MTU

doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst

@@ -492,6 +492,8 @@ Modem libraries
 
   * Fixed an issue with the :c:func:`modem_key_mgmt_clear` function where it returned ``-ENOENT`` when the credential was cleared.
 
+* Updated the :ref:`nrf_modem_lib_lte_net_if` to automatically set the actual link :term:`Maximum Transmission Unit (MTU)` on the network interface when PDN connectivity is gained.
+
 Multiprotocol Service Layer libraries
 -------------------------------------
 

lib/nrf_modem_lib/lte_net_if/lte_net_if.c

@@ -60,13 +60,29 @@ void lte_net_if_modem_fault_handler(void)
 	net_if_dormant_on(iface_bound);
 }
 
+static uint16_t pdn_mtu(void)
+{
+	struct pdn_dynamic_info info;
+	int rc;
+
+	rc = pdn_dynamic_info_get(0, &info);
+	if ((rc != 0) || (info.ipv4_mtu == 0)) {
+		LOG_WRN("MTU query failed, error: %d, MTU: %d", rc, info.ipv4_mtu);
+		/* Fallback to the minimum value that IPv4 is required to support */
+		info.ipv4_mtu = NET_IPV4_MTU;
+	}
+	LOG_DBG("Network MTU: %d", info.ipv4_mtu);
+	return info.ipv4_mtu;
+}
+
 /* Called when we detect LTE connectivity has been gained.
  *
- * Marks the iface as active and cancels any pending timeout.
+ * Queries the network MTU, marks the iface as active, and cancels any pending timeout.
  */
 static void become_active(void)
 {
 	LOG_DBG("Becoming active");
+	net_if_set_mtu(iface_bound, pdn_mtu());
 	net_if_dormant_off(iface_bound);
 	k_work_cancel_delayable(&connection_timeout_work);
 }

lib/nrf_modem_lib/nrf9x_sockets.c

@@ -1168,7 +1168,7 @@ static struct offloaded_if_api nrf9x_iface_offload_api = {
 	.enable = nrf9x_iface_enable,
 };
 
-/* TODO Get the actual MTU for the nRF9x LTE link. */
+/* Actual MTU for the nRF9x LTE link is handled by `lte_net_if.c` */
 NET_DEVICE_OFFLOAD_INIT(nrf9x_socket, "nrf9x_socket",
 			nrf9x_socket_offload_init,
 			NULL,

tests/lib/nrf_modem_lib/lte_net_if/src/main.c

@@ -44,6 +44,14 @@ static void lte_lc_register_handler_stub(lte_lc_evt_handler_t cb, int num_of_cal
 	lte_lc_event_handler_callback = cb;
 }
 
+/* Stub used to report a non-default link MTU */
+static int pdn_dynamic_info_get_1000_stub(uint32_t cid,  struct pdn_dynamic_info *pdn_info,
+										  int num_of_calls)
+{
+	pdn_info->ipv4_mtu = 1000;
+	return 0;
+}
+
 static void bring_network_interface_up(void)
 {
 	struct net_if *net_if = net_if_get_default();
@@ -442,11 +450,17 @@ void test_pdn_act_with_cereg_should_activate_iface(void)
 			"AT+CGPADDR=0", "+CGPADDR: %*d,\"%46[.:0-9A-F]\",\"%46[:0-9A-F]\"", 1);
 	__mock_nrf_modem_at_scanf_ReturnVarg_string("192.9.201.39");
 
+	/* Netowrk MTU is 1000 */
+	__cmock_pdn_dynamic_info_get_Stub(&pdn_dynamic_info_get_1000_stub);
+
 	/* Fire PDN active */
 	pdn_event_handler_callback(0, PDN_EVENT_ACTIVATED, 0);
 
 	/* Check that iface becomes active */
 	TEST_ASSERT_FALSE(net_if_is_dormant(net_if));
+
+	/* MTU should have been set to reported value */
+	TEST_ASSERT_EQUAL(1000, net_if_get_mtu(net_if));
 }
 
 
@@ -504,6 +518,9 @@ void test_cereg_registered_home_with_pdn_should_activate_iface(void)
 			"AT+CGPADDR=0", "+CGPADDR: %*d,\"%46[.:0-9A-F]\",\"%46[:0-9A-F]\"", 1);
 	__mock_nrf_modem_at_scanf_ReturnVarg_string("192.9.201.39");
 
+	/* MTU query fails */
+	__cmock_pdn_dynamic_info_get_IgnoreAndReturn(-1);
+
 	/* Fire PDN active */
 	pdn_event_handler_callback(0, PDN_EVENT_ACTIVATED, 0);
 
@@ -516,6 +533,9 @@ void test_cereg_registered_home_with_pdn_should_activate_iface(void)
 
 	/* Check that iface becomes active */
 	TEST_ASSERT_FALSE(net_if_is_dormant(net_if));
+
+	/* MTU should have reverted to default minimum */
+	TEST_ASSERT_EQUAL(NET_IPV4_MTU, net_if_get_mtu(net_if));
 }
 
 /* Verify that CEREG registration (roaming) does activate iface if PDN is active */
@@ -542,6 +562,9 @@ void test_cereg_registered_roaming_with_pdn_should_activate_iface(void)
 			"AT+CGPADDR=0", "+CGPADDR: %*d,\"%46[.:0-9A-F]\",\"%46[:0-9A-F]\"", 1);
 	__mock_nrf_modem_at_scanf_ReturnVarg_string("192.9.201.39");
 
+	/* MTU not set by function */
+	__cmock_pdn_dynamic_info_get_IgnoreAndReturn(0);
+
 	/* Fire PDN active */
 	pdn_event_handler_callback(0, PDN_EVENT_ACTIVATED, 0);
 
@@ -554,6 +577,9 @@ void test_cereg_registered_roaming_with_pdn_should_activate_iface(void)
 
 	/* Check that iface becomes active */
 	TEST_ASSERT_FALSE(net_if_is_dormant(net_if));
+
+	/* MTU should have reverted to default minimum */
+	TEST_ASSERT_EQUAL(NET_IPV4_MTU, net_if_get_mtu(net_if));
 }
 
 /* Verify that CEREG searching does not affect iface activation (from inactive to active) */
@@ -602,6 +628,7 @@ void test_cereg_searching_should_not_deactivate_iface(void)
 	 */
 	__cmock_nrf_modem_is_initialized_IgnoreAndReturn(1);
 	__cmock_lte_lc_func_mode_set_IgnoreAndReturn(0);
+	__cmock_pdn_dynamic_info_get_IgnoreAndReturn(0);
 
 	/* Take the iface admin-up */
 	net_if_up(net_if);
@@ -644,6 +671,7 @@ void test_cereg_unregistered_should_deactivate_iface(void)
 	 */
 	__cmock_nrf_modem_is_initialized_IgnoreAndReturn(1);
 	__cmock_lte_lc_func_mode_set_IgnoreAndReturn(0);
+	__cmock_pdn_dynamic_info_get_IgnoreAndReturn(0);
 
 	/* Take the iface admin-up */
 	net_if_up(net_if);
@@ -686,6 +714,7 @@ void test_pdn_deact_should_deactivate_iface(void)
 	 */
 	__cmock_nrf_modem_is_initialized_IgnoreAndReturn(1);
 	__cmock_lte_lc_func_mode_set_IgnoreAndReturn(0);
+	__cmock_pdn_dynamic_info_get_IgnoreAndReturn(0);
 
 	/* Take the iface admin-up */
 	net_if_up(net_if);