gnrc_ipv6_nc: introduce L2 address lookup function

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

@@ -21,8 +21,10 @@
 #ifndef GNRC_IPV6_NC_H_
 #define GNRC_IPV6_NC_H_
 
+#include <assert.h>
 #include <stdbool.h>
 #include <stdint.h>
+#include <string.h>
 
 #include "kernel_types.h"
 #include "net/eui64.h"
@@ -293,6 +295,21 @@ static inline bool gnrc_ipv6_nc_is_reachable(const gnrc_ipv6_nc_t *entry)
  */
 gnrc_ipv6_nc_t *gnrc_ipv6_nc_still_reachable(const ipv6_addr_t *ipv6_addr);
 
+/**
+ * @brief   Gets link-layer address from neighbor cache entry if neighbor is reachable.
+ *
+ * @pre (l2_addr != NULL) && (l2_addr_len != NULL)
+ *
+ * @param[out] l2_addr      The link layer address of @p entry. Must not be NULL.
+ * @param[out] l2_addr_len  Length of @p l2_addr. Must not be NULL.
+ * @param[in] entry         A neighbor cache entry
+ *
+ * @return  PID to the interface where the neighbor is.
+ * @return  KERNEL_PID_UNDEF, if @p entry == NULL or the neighbor is not reachable.
+ */
+kernel_pid_t gnrc_ipv6_nc_get_l2_addr(uint8_t *l2_addr, uint8_t *l2_addr_len,
+                                      const gnrc_ipv6_nc_t *entry);
+
 #ifdef __cplusplus
 }
 #endif

sys/net/gnrc/network_layer/ipv6/nc/gnrc_ipv6_nc.c

@@ -263,4 +263,16 @@ gnrc_ipv6_nc_t *gnrc_ipv6_nc_still_reachable(const ipv6_addr_t *ipv6_addr)
     return entry;
 }
 
+kernel_pid_t gnrc_ipv6_nc_get_l2_addr(uint8_t *l2_addr, uint8_t *l2_addr_len,
+                                      const gnrc_ipv6_nc_t *entry)
+{
+    assert((l2_addr != NULL) && (l2_addr_len != NULL));
+    if ((entry == NULL) || !gnrc_ipv6_nc_is_reachable(entry)) {
+        return KERNEL_PID_UNDEF;
+    }
+    *l2_addr_len = entry->l2_addr_len;
+    memcpy(l2_addr, entry->l2_addr, entry->l2_addr_len);
+    return entry->iface;
+}
+
 /** @} */