def remaining_flight_space(self) -> int:
        """
        Returns the remaining number of bytes which can be used in
        the current packet.
        """
        return (
            self._flight_capacity
            - self._buffer.tell()
            - self._packet_crypto.aead_tag_size
        )

        Returns the remaining number of bytes which can be used in
        the current packet.
        
src/aioquic/quic/packet_builder.py