+#define GRE_F_IN_CSUM (1 << 0) /* Require incoming packets to have checksums. */
+#define GRE_F_OUT_CSUM (1 << 1) /* Checksum outgoing packets. */
+#define GRE_F_IN_KEY_MATCH (1 << 2) /* Store the key in tun_id to match in flow table. */
+#define GRE_F_OUT_KEY_ACTION (1 << 3) /* Get the key from a SET_TUNNEL action. */
+#define GRE_F_TOS_INHERIT (1 << 4) /* Inherit the ToS from the inner packet. */
+#define GRE_F_TTL_INHERIT (1 << 5) /* Inherit the TTL from the inner packet. */
+#define GRE_F_PMTUD (1 << 6) /* Enable path MTU discovery. */