HostA has established a TCP connection with HostB in a remote network. HostA is sending packets to HostB. Assume we have configured TCP, somehow, to ACK every segment (no ACKing every other segment). Assume that the timeout is the same for all packets. HostB’s "window size" is 20000 bytes. HostB has already received and acknowledged everything sent by HostA’s application up to and including byte #3,488. HostA now sends segments of the same application data stream in order: P: 212 bytes Q: 206 bytes R: 438 bytes Suppose that packet P is lost, but packets Q and R are received. What is the acknowledgement number in the ACK for packet R.

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.