Answer: ACK= 3489
Explanation:
When sending an ACK for a segment received, Host B returns as an ACK, the number of the next byte that it expects to receive, so, if it received all bytes till 3488, the ACK includes as next sequence number 3489.
Now, as the last window size advertised is 2000 bytes, Host A can send many segments without receiving ACKs, till reaching to the 2000 bytes limit.
So, in this case, it sends in sequence, three data streams, P, Q and R, adding to 846 bytes.
If packet P is lost, before receiving this packet, Host B will ACK any packet with the sequence number that expects, i.e., 3489, so when it receives packet R, will ACK with sequence number 3489 as ACK, which will inform to sender that packet P has been probably lost, and in this way, asking to host A to retransmit the segment.
