שאלה | What is the purpose of the buffer in Selective Repeat protocol?

What is the purpose of the buffer in Selective Repeat protocol?

		To store all transmitted packets until the receiver acknowledges them
		To maintain a record of all packets received by the sender for retransmission
		To hold packets received out of order until they can be delivered to the upper layer
		To provide temporary storage for packets while the sender waits for acknowledgment from the receiver