This indicates whether TCP Selective Acknowledgement is enabled on this FCIP entity, a TCP extension that lets the receiving end acknowledge multiple non-contiguous lost segments in a single ACK for faster loss recovery. SACK matters especially on long-haul, higher-latency FCIP links where a lost segment without SACK would otherwise force a slow, whole-window retransmission wait, directly impacting SAN replication throughput across distance. A transcontinental FCIP link carrying synchronous storage replication would typically want this set to enabled(1) so that occasional packet loss on the long-haul path does not stall the replication stream waiting for a full retransmission timeout.