Duplicate ACKs
We arrange for TCPs receiving out-of-order packets to respond immediately with one ACK per packet:
- receiver gets: 5, 6, 7, 8, 10, 11, 12, 13
- ACKs: 6, 8, 8, 8, 8, 8 [4 dupacks]
Provides a hint to sender that packet 9 is probably missing at receiver and that 4 packets have arrived after 8 arrived
[think about re-ordering!]