nrf_modem_lib: lte_net_if: query actual link MTU

Update the network interface MTU before raising the connectivity event, so that the MTU as queried by downstream users is correct, instead of an assumed value based on IPv6 required minimums. Signed-off-by: Jordan Yates <[email protected]>
nrfconnect · Dec 19, 2024 · aa3153b · aa3153b
1 parent ef172cd
commit aa3153b
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Showing 3 changed files with 20 additions and 2 deletions.
diff --git a/doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst b/doc/nrf/releases_and_maturity/releases/release-notes-changelog.rst
@@ -455,6 +455,8 @@ Modem libraries
 
   * Added the :c:func:`pdn_dynamic_params_get_v6` function to get PDN parameters for IPv6-only.
 
+* 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
 -------------------------------------
 

diff --git a/lib/nrf_modem_lib/lte_net_if/lte_net_if.c b/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)
+{
+	unsigned int mtu = 0;
+	int rc;
+
+	rc = pdn_dynamic_params_get(0, NULL, NULL, &mtu);
+	if ((rc != 0) || (mtu == 0)) {
+		LOG_WRN("MTU query failed, error: %d, MTU: %d", rc, mtu);
+		/* Fallback to the minimum value that IPv4 is required to support */
+		mtu = NET_IPV4_MTU;
+	}
+	LOG_DBG("Network MTU: %d", mtu);
+	return 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);
 }

diff --git a/lib/nrf_modem_lib/nrf9x_sockets.c b/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,
-Original file line number
+Diff line change
@@ Expand Up / @@ -455,6 +455,8 @@ Modem libraries @@
       * Added the :c:func:`pdn_dynamic_params_get_v6` function to get PDN parameters for IPv6-only.
+    * 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
     -------------------------------------
@@ Expand Down @@