net: ip: Configurable IPv4 MTU

Allow to configure the initial IPv4 MTU needed to support packets without
fragmentation.

Signed-off-by: Pieter De Gendt <[email protected]>

Author: pdgendt

include/zephyr/net/net_ip.h

@@ -496,7 +496,11 @@ enum net_ip_mtu {
 	/** IPv4 MTU length. We must be able to receive this size IPv4 packet
 	 * without fragmentation.
 	 */
+#if defined(CONFIG_NET_NATIVE_IPV4)
+	NET_IPV4_MTU = CONFIG_NET_IPV4_MTU,
+#else
 	NET_IPV4_MTU = 576,
+#endif
 };
 
 /** @brief Network packet priority settings described in IEEE 802.1Q Annex I.1 */

subsys/net/ip/Kconfig.ipv4

@@ -42,6 +42,14 @@ config NET_IPV4_DEFAULT_NETMASK
 
 if NET_NATIVE_IPV4
 
+config NET_IPV4_MTU
+	int "Initial IPv4 MTU value"
+	default 576
+	range 576 1500
+	help
+	  The value should normally be 576 which is the minimum IPv4 packet
+	  size that implementations need to support without fragmentation.
+
 config NET_INITIAL_TTL
 	int "Initial IPv4 time to live value for unicast packets"
 	default 64