code: def unwrap_rtx(rtx, payload_type, ssrc):
      
      packet = RtpPacket(
          payload_type=payload_type,
          marker=rtx.marker,
          sequence_number=unpack('!H', rtx.payload[0:2])[0],
          timestamp=rtx.timestamp,
          ssrc=ssrc,
          payload=rtx.payload[2:])
      packet.csrc = rtx.csrc
      packet.extensions = rtx.extensions
      return packet
docs: Recover initial packet from a retransmission packet.