Selective Acknowledgements (SACK)

Originally, the data after the error (either 8-10 or 4-10 in the examples above) would be dropped.

RFCs 1017 and 2018 introduced/refined selective acknowledgements. It allows all segments received properly to be ACKed, resulting in only the missing/wrong data being re-sent.

To facilitate this, a SACK TCP Option header must be created and appended to the TCP header. Negotiation for the use SACK is done at the beginning of the TCP connection.

It uses a "left edge" and "right edge" to pinpoint the missing data.

The ACKno will be the same as a non-SACK connection. The appended header will also include the SACK option for TCP indicating the other data it received. The sender may then retransmit the missing data.