TCP Selective Acknowledgement (SACK) is a TCP option that works as an alternative segment recovery/retransmit to the normal cumulative acknowledgements and 3 duplicate ACKs rule. If both client/server support the option then one will see SACK Permitted in a packet capture during the initial three way handshake.
When a segment is lost during a TCP connection the sender will continue transmitting data in line with the allowed sender window. When SACK is enabled the receiver will append a normal ACK of what’s received with the SACK option, telling the receiver what blocks of data have been received beyond the missing segment. The receiver sends the SACK with what’s called the Left Edge of the data block and the Right Edge of the data block. This tells the TCP sender that the missing segment is the block of data (segment number) before the left edge of the SACK Option block. The SACK option is more efficient over the wire because it does not require the sender to retransmit segments that have already reached the receiver. The 3 duplicate ACKs requires the sender to retransmit from the missing segment to the point at which the 3 duplicate ACKs were received.