Send an RTCP packet to report missing RTP packets.

async def _send_rtcp_nack(self, media_ssrc, lost):
        """
        Send an RTCP packet to report missing RTP packets.
        """
        if self.__rtcp_ssrc is not None:
            packet = RtcpRtpfbPacket(
                fmt=RTCP_RTPFB_NACK, ssrc=self.__rtcp_ssrc, media_ssrc=media_ssrc)
            packet.lost = lost
            await self._send_rtcp(packet)