drop HELLO if session already established

draft-ietf-lsvr-l3dl.xml

@@ -729,6 +729,9 @@ uint32_t sbox_checksum_32(const uint8_t *b, const size_t n)
     (plus VID).  The two devices establish an L3DL session by exchanging
     OPEN PDUs.</t>
 
+    <t>If a HELLO is received from a MAC address with which there is an
+    established session, it should be dropped.</t>
+
     <t>The Payload Length is zero as there is no payload.</t>
 
     <t>HELLO PDUs can not be signed as keying material has yet to be