regarding payment of RCM on freight charged on invoice - Goods and Services Tax - GST

respected sir, supplier of goods has paid advance Rs-30000/- freight to the transporter of goods, against the freight Rs-36000/(no GST charged by the transporter on freight)-mention in the document issued by the transporter to the recipient of goods and the supplier mention the advance freight rs-30000 paid to the transporter in its Tax invoice issued to the recipient of goods, but does not charged any GST on the advance freight paid to the transporter. remaining balance of Rs-6000/- has been paid by the recipient of the goods.my query is whom does the liability of RCM arise, recipient or supplier.