code: func (*protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.TransportEndpointID, vv buffer.VectorisedView) bool {
  	s := newSegment(r, id, vv)
  	defer s.decRef()
  
  	if !s.parse() || !s.csumValid {
  		return false
  	}
  
  	// There's nothing to do if this is already a reset packet.
  	if s.flagIsSet(header.TCPFlagRst) {
  		return true
  	}
  
  	replyWithReset(s)
  	return true
  }
docs: // HandleUnknownDestinationPacket handles packets targeted at this protocol but
  // that don't match any existing endpoint.
  //
  // RFC 793, page 36, states that "If the connection does not exist (CLOSED) then
  // a reset is sent in response to any incoming segment except another reset. In
  // particular, SYNs addressed to a non-existent connection are rejected by this
  // means."