The Selective Acknowledgment (SACK) is a mechanism that includes a retransmission algorithm which helps overcome weak links on the TCP/IP stack.
The selective acknowledgment extension uses two TCP options. The first is an enabling option, SACK-permitted, which can be sent in a SYN segment to indicate that the SACK option can be used once the connection is established. The SACK-permitted option is a two-byte option.
The second option is the SACK option itself, which can be sent over an established connection once both the sender and the receiver have successfully negotiated the SACK-permit option. Whenever there is loss of data, the data receiver can send the SACK option to acknowledge the out-of- order segments.
For more information on this, see Selective Acknowledgement.