retransmission of lost TCP segment

Question

What will happen in the following scenario (assume the connection is already established):

1. (stack A) send 10 byte data

2. (stack B) send ACK for 10 byte data

3. (stack B) send 200 byte data

4. (stack B) send 100 byte data

5. (stack B) send 50 byte data

6. (stack A) send ACK for 350 byte data and also send 70 bytes data
   This segment gets lost and does not reach machine B.

7. (stack B) retransmit 200 byte data (step 3)

8. (stack A) send ACK for 200 byte data and the next expected seq number as the one for 70 bytes data

Question: Should 70 bytes data be also transferred with the ACK in step 8? Note that the retransmission timer for step 6 has not expired yet.

Answer 1

No, the 70 bytes should not be included with the immediate ACK for out-of-order segment arrival.

Why? Step 7 is the arrival of an out-of-order segment. TCP must generate an immediate ACK giving the next expected byte (which will be 361 + whatever has been sent before Step 1) on receipt of an out-of-order segment. But TCP doesn't retransmit the segment containing the 70 bytes of data with the piggybacked ACK. That segment is still on the retransmit queue waiting for the timer to expire, or if fast retransmit is enabled, three duplicate ACKs arrive, but neither has happened yet. Out-of-order arrival is an indication of congestion, so it makes sense that TCP should send the smallest ACK possible.