To alleviate this issue TCP employs the "selective acknowledgment (SACK)" option, defined in 1996 in RFC 2018, which allows the receiver to acknowledge discontinuous blocks of packets which were received correctly, in addition to the sequence number immediately following the last sequence number of the last contiguous byte received successively, as in the basic TCP acknowledgment. The acknowledgement can specify a number of "SACK blocks", where each SACK block is conveyed by the "Left Edge of Block" (the first sequence number of the block) and the "Right Edge of Block" (the sequence number immediately following the last sequence number of the block), with a "Block" being a contiguous range that the receiver correctly received. In the example above, the receiver would send an ACK segment with a cumulative ACK value of 2,000 and a SACK option header with sequence numbers 3,000 and 11,000. The sender would accordingly retransmit only the second segment with sequence numbers 2,000 to 2,999.