Go Back N ARQ Question
1-2-3 is correctly sent and acknowledged so the sender's window is now over 4-5-6.

4 is received correctly and so it is acknowledged. This makes the window (currently over 4-5-6) slide to 5-6-7. However, since 5 was lost, 5 will not be acknowledged. Therefore, despite 6 being well received, 6 will not be acknowledged as well.  Meanwhile the sender sends 7 because his window allows him to (it looks like you think it doesn't trigger the sender to send 7). It is received well but the recipient doesn't acknowledge it because it's still waiting for 5 to arrive. The sender's timer goes off and therefore it sends 5-6-7 again... (etc).

If you haven't seen it already, this animation is really helpful.