Which TCP Optimization feature is used by WAN Edge to prevent unnecessary retransmissions and large initial TCP window sizes to maximize throughput and achieve a better quality?

SEQ refers to TCP sequence numbers used for ordering segments - it is a fundamental TCP field, not a specific optimization feature for preventing retransmissions.

SYN is the TCP flag used during the three-way handshake to initiate connections - while TCP Optimization intercepts SYN packets to proxy connections, SYN itself is not the feature that prevents retransmissions.

RTT (Round-Trip Time) is a measurement metric used by TCP to calculate retransmission timers, but it is not an optimization feature that actively prevents unnecessary retransmissions.

SACK (Selective Acknowledgment) extends standard TCP acknowledgment to allow a receiver to report multiple non-contiguous blocks of received data. This prevents the sender from blindly retransmitting all data after a lost packet and also allows better control of initial window sizes. Cisco SD-WAN's TCP Optimization uses SACK to maximize throughput and improve quality by minimizing redundant retransmissions across WAN links.