code: def pubrec(self, mid):
          
          if self.sock == NC.INVALID_SOCKET:
              return NC.ERR_NO_CONN
  
          self.logger.info("Send PUBREC (msgid=%s)", mid)
          pkt = MqttPkt()
  
          pkt.command = NC.CMD_PUBREC
          pkt.remaining_length = 2
  
          ret = pkt.alloc()
          if ret != NC.ERR_SUCCESS:
              return ret
  
          #variable header: acknowledged message id
          pkt.write_uint16(mid)
  
          return self.packet_queue(pkt)
docs: Send PUBREC response to server.