This paper uses simulations to explore the benefits of adding selective acknowledgments (SACK) and selective repeat to TCP. It compares Tahoe and Reno TCP, the two most common TCP implementations, with two modified versions of Reno TCP: New-Reno TCP and SACK TCP. New-Reno TCP avoids some of Reno's performance problems when multiple packets are dropped from a window of data without requiring SACK. SACK TCP, which uses the SACK option, is a conservative extension of Reno TCP that allows the sender to better understand which packets have been successfully delivered, leading to improved throughput. The paper shows that without SACK, TCP implementations are constrained to retransmit at most one dropped packet per round-trip time or retransmit packets that might have already been successfully delivered. SACK TCP allows the sender to retransmit only the packets that have been lost, improving performance. The paper also shows that SACK TCP recovers more quickly from packet losses than Reno or New-Reno TCP. The simulations demonstrate that SACK TCP performs better than Reno and New-Reno TCP in scenarios with multiple packet drops. The paper concludes that adding SACK to TCP is one of the most important changes that should be made to improve its performance.This paper uses simulations to explore the benefits of adding selective acknowledgments (SACK) and selective repeat to TCP. It compares Tahoe and Reno TCP, the two most common TCP implementations, with two modified versions of Reno TCP: New-Reno TCP and SACK TCP. New-Reno TCP avoids some of Reno's performance problems when multiple packets are dropped from a window of data without requiring SACK. SACK TCP, which uses the SACK option, is a conservative extension of Reno TCP that allows the sender to better understand which packets have been successfully delivered, leading to improved throughput. The paper shows that without SACK, TCP implementations are constrained to retransmit at most one dropped packet per round-trip time or retransmit packets that might have already been successfully delivered. SACK TCP allows the sender to retransmit only the packets that have been lost, improving performance. The paper also shows that SACK TCP recovers more quickly from packet losses than Reno or New-Reno TCP. The simulations demonstrate that SACK TCP performs better than Reno and New-Reno TCP in scenarios with multiple packet drops. The paper concludes that adding SACK to TCP is one of the most important changes that should be made to improve its performance.