Index: sys/net/if_gre.c
===================================================================
--- sys/net/if_gre.c
+++ sys/net/if_gre.c
@@ -715,6 +715,10 @@
 		opts++;
 	}
 	if (flags & GRE_FLAGS_KP) {
+        /* XXX: The current implementation uses the key only for outgoing
+         * packets. But we can check the key value here, or even in the
+         * encapcheck function.
+         */
 		key = ntohl(*opts);
 		hlen += sizeof(uint32_t);
 		opts++;