This page references options(4), where the section documenting TCP_SACK
got deleted in 2017.

This just copies the text that used to be there.

ok?


Index: tcp.4
===================================================================
RCS file: /home/cvs/src/share/man/man4/tcp.4,v
diff -u -p -r1.27 tcp.4
--- tcp.4	11 Aug 2022 14:04:08 -0000	1.27
+++ tcp.4	30 Nov 2024 18:07:14 -0000
@@ -159,8 +159,16 @@ Set the maximum segment size for this co
 The maximum segment size can only be lowered.
 .It Cd TCP_SACK_ENABLE
 Use selective acknowledgements for this connection.
-See
-.Xr options 4 .
+Additional information about
+segments already received can be transmitted back to the sender,
+thus indicating segments that have been lost and allowing for
+a swifter recovery.
+Both communication endpoints need to support
+.Em SACK .
+The fallback behaviour is NewReno fast recovery phase, which allows
+one lost segment to be recovered per round trip time.
+When more than one segment has been dropped per window, the transmission can
+continue without waiting for a retransmission timeout.
 .It Cd TCP_MD5SIG
 Use TCP MD5 signatures per RFC 2385.
 This requires