--- a/media/mtransport/transportlayerdtls.cpp
+++ b/media/mtransport/transportlayerdtls.cpp
@@ -969,28 +969,32 @@ void TransportLayerDtls::PacketReceived(
   MOZ_MTLOG(ML_DEBUG, LAYER_INFO << "PacketReceived(" << len << ")");
 
   if (state_ != TS_CONNECTING && state_ != TS_OPEN) {
     MOZ_MTLOG(ML_DEBUG,
               LAYER_INFO << "Discarding packet in inappropriate state");
     return;
   }
 
+  // not DTLS per RFC 7983
+  if (data[0] < 20 || data[0] > 63) {
+    return;
+  }
+
   nspr_io_adapter_->PacketReceived(data, len);
 
   // If we're still connecting, try to handshake
   if (state_ == TS_CONNECTING) {
     Handshake();
   }
 
   // Now try a recv if we're open, since there might be data left
   if (state_ == TS_OPEN) {
     // nICEr uses a 9216 bytes buffer to allow support for jumbo frames
     unsigned char buf[9216];
-
     int32_t rv;
     // One packet might contain several DTLS packets
     do {
       rv = PR_Recv(ssl_fd_, buf, sizeof(buf), 0, PR_INTERVAL_NO_WAIT);
       if (rv > 0) {
         // We have data
         MOZ_MTLOG(ML_DEBUG, LAYER_INFO << "Read " << rv << " bytes from NSS");
         SignalPacketReceived(this, buf, rv);