Add filter to handle multicast downlinks exceptions.

The Multicast frames share most of the unicast frame format with a few exceptions:
  • They are not allowed to carry MAC commands, neither in the FOpt field, nor in the
    payload on port 0 because a multicast downlink does not have the same authentication
    robustness as a unicast frame.
  • The ACK and ADRACKReq bits MUST be zero. The MType field MUST carry the
    value for Unconfirmed Data Down.

Author: mluis1

src/mac/LoRaMac.c

@@ -1117,6 +1117,16 @@ static void ProcessRadioRxDone( void )
                 }
             }
 
+            // Filter messages according to multicast downlink exceptions
+            if( ( multicast == 1 ) && ( ( fType != FRAME_TYPE_D ) ||
+                                        ( macMsgData.FHDR.FCtrl.Bits.Ack == true ) ||
+                                        ( macMsgData.FHDR.FCtrl.Bits.AdrAckReq == true ) ) )
+            {
+                MacCtx.McpsIndication.Status = LORAMAC_EVENT_INFO_STATUS_ERROR;
+                PrepareRxDoneAbort( );
+                return;
+            }
+
             // Get maximum allowed counter difference
             getPhy.Attribute = PHY_MAX_FCNT_GAP;
             phyParam = RegionGetPhyParam( MacCtx.NvmCtx->Region, &getPhy );