DragonFly now has a optimized scoreboard for SACK, thanks to Sepherosa Ziehau. What’s that mean? SACK is a way to make sure only the needed parts of a TCP transmission get retransmitted, when multiple packets are lost. The scoreboard is where the packets needing retransmission are tracked. So, the result of these improvements is better performance in packet-lossy situations.
(Please correct me if your understanding is better than mine; my explanation is based on stumbling around the Internet for a few minutes of reading.)