syncache(4) on FreeBSD
The text d74b6347fa24, as FreeBSD 5.5-RELEASE, FreeBSD 5.4-RELEASE, FreeBSD 5.3-RELEASE, FreeBSD 5.2.1-RELEASE, FreeBSD 5.2-RELEASE, FreeBSD 5.1-RELEASE, FreeBSD 5.0-RELEASE shipped it.
sysctl(8) MIBs for controlling TCP SYN caching