TCP Selective Acknowledgment (SACK) and Fast Retransmission

TCP Selection Acknowledgment (SACK)

In the above scenario, there are total of 5 segments to be sent from 10.0.0.1 to 172.16.0.1:
1) 1 – 1400, 2) 1401 – 2800, 3) 2801 – 4200, 4) 4201 – 5600, and 5) 5601 – 7000.

Below describes the steps involved in the scenario:
i) Steps 1, 2, and 3 show the TCP three-way handshake connection establishment phase. Both end systems agree to use SACK. The agreed MSS is 1460 1400 bytes.
ii) 10.10.10.1 transmits the 1st and 2nd segments to 172.16.0.1 (Steps 5 and 6). 172.16.0.1 acknowledges the segments (Step 7).
iii) 10.10.10.1 transmits the 4th and 5th segments to 172.16.0.1 (Step 8 and 10). The 3rd segment is lost. With selective acknowledgment, 172.16.0.1 selectively acknowledges the out-of-order segments, instead of cumulatively acknowledging the last in-order segment received (Step 9 and 11).
iv) With fast retransmission, which skips the retransmission of selectively acknowledged segments, 10.10.10.1 retransmits the 3rd segment to 172.16.0.1 (Step 12).
v) Finally, 172.16.0.1 cumulatively acknowledges the receipt of all segments (Step 13).