gnrc_ipv6_netif: initialize MTU from device, if possible

sys/include/net/gnrc/ipv6/netif.h

@@ -28,6 +28,7 @@
 #include "kernel_macros.h"
 #include "kernel_types.h"
 #include "mutex.h"
+#include "net/ipv6.h"
 #include "net/ipv6/addr.h"
 #include "vtimer.h"
 
@@ -51,11 +52,19 @@ extern "C" {
 /**
  * @brief   Default MTU
  *
- * @see <a href="https://tools.ietf.org/html/rfc2460#section-5">
- *          RFC 2460, section 5
- *      </a>
+ * An interface will choose this MTU if the link-layer's maximum packet size
+ * (see @ref NETOPT_MAX_PACKET_SIZE) is lesser than the @ref IPV6_MIN_MTU or if it just not
+ * provide it. For RFC-compatible communication it must be at least @ref IPV6_MIN_MTU.
+ *
+ * @note    If the scenario the node is used in allows for it and the packet size is predictable,
+ *          a user might choose to set @ref GNRC_IPV6_NETIF_DEFAULT_MTU to a lesser value than
+ *          @ref IPV6_MIN_MTU to optimize for code size (e.g. because it is then possible to omit
+ *          @ref net_gnrc_sixlowpan_frag) and memory usage (e.g. because @ref GNRC_PKTBUF_SIZE
+ *          can be much smaller).
  */
-#define GNRC_IPV6_NETIF_DEFAULT_MTU (1280)
+#ifndef GNRC_IPV6_NETIF_DEFAULT_MTU
+#define GNRC_IPV6_NETIF_DEFAULT_MTU (IPV6_MIN_MTU)
+#endif
 
 /**
  * @brief   Default hop limit

sys/include/net/ipv6.h

@@ -32,6 +32,15 @@
 extern "C" {
 #endif
 
+/**
+ * @brief   minimum **M**aximum **T**ransition **U**nit
+ *
+ * @see <a href="https://tools.ietf.org/html/rfc4944#section-5.3">
+ *          RFC 2460, section 5.3
+ *      </a>
+ */
+#define IPV6_MIN_MTU    (1280)
+
 #ifdef __cplusplus
 }
 #endif

sys/net/gnrc/network_layer/ipv6/netif/gnrc_ipv6_netif.c

@@ -695,6 +695,7 @@ void gnrc_ipv6_netif_init_by_dev(void)
     for (size_t i = 0; i < ifnum; i++) {
         ipv6_addr_t addr;
         eui64_t iid;
+        uint16_t mtu;
         gnrc_ipv6_netif_t *ipv6_if = gnrc_ipv6_netif_get(ifs[i]);
 
         if (ipv6_if == NULL) {
@@ -730,6 +731,7 @@ void gnrc_ipv6_netif_init_by_dev(void)
         }
 #endif
 
+        /* set link-local address */
         if ((gnrc_netapi_get(ifs[i], NETOPT_IPV6_IID, 0, &iid,
                              sizeof(eui64_t)) < 0)) {
             mutex_unlock(&ipv6_if->mutex);
@@ -740,6 +742,16 @@ void gnrc_ipv6_netif_init_by_dev(void)
         ipv6_addr_set_link_local_prefix(&addr);
         _add_addr_to_entry(ipv6_if, &addr, 64, 0);
 
+        /* set link MTU */
+        if ((gnrc_netapi_get(ifs[i], NETOPT_MAX_PACKET_SIZE, 0, &mtu,
+                             sizeof(uint16_t)) >= 0)) {
+            if (mtu >= IPV6_MIN_MTU) {
+                ipv6_if->mtu = mtu;
+            }
+            /* otherwise leave at GNRC_IPV6_NETIF_DEFAULT_MTU as initialized in
+             * gnrc_ipv6_netif_add() */
+        }
+
         mutex_unlock(&ipv6_if->mutex);
     }
 }