|SackOpts||SackOpts (Selective Acknowledgements - RFC 2018). SACKs allow a receiver to acknowledge non-consecutive data. This is particularly helpful on paths with large TCP Windows (large RWIN / BDP). Selective acknowledgements also allow TCP to recover from IP packet loss without resending packets that were already received by the client.
While SACK is now supported by most operating systems, it may have to be explicitly turned on by the system administrator.