Answer

(iv) GBN discards packets that are received out of order.

Explanation

In Go-Back-N (GBN) ARQ, the receiver accepts only the next packet in sequence. If a packet arrives out of order, the receiver does not buffer it as an accepted packet; it discards it and typically sends a duplicate cumulative ACK for the last correctly received in-order packet.

Why the other options are wrong

• (i) False: GBN does not require the sequence-number space to be at least twice the window size. A common condition is $W \le 2^k-1$, where $W$ is the sender window and $k$ is the number of sequence-number bits.
• (ii) False: Standard GBN uses one timer, associated with the oldest unacknowledged packet.
• (iii) False: Duplicate ACKs inform the sender about the missing packet, but standard GBN retransmission is primarily triggered by the timer expiring, after which the outstanding packets are retransmitted from the missing one onward.
• (iv) True: Out-of-order packets are discarded by the GBN receiver.

Final answer: (iv) Discards packets received out of order.