Windows 2000 introduces support for an important performance feature known as Selective Acknowledgement (SACK). SACK is especially important for connections using large TCP window sizes. Prior to SACK, a receiver could only acknowledge the latest sequence number of contiguous data that had been received, or the left edge of the receive window. When SACK is enabled, the receiver continues to use the ACK number to acknowledge the left edge of the receive window, but it can also acknowledge other non-contiguous blocks of received data individually. SACK uses TCP header options, as shown below. This text was taken directly from RFC 2018:
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