FreeBSD 4.11-RELEASE & SACK

[Kan Cai]

Greetings,

I've installed the standard FreeBSD 4.11-RELEASE and have realized that 
the sysctl option for enabling SACK in TCP is not available 
(net.inet.tcp.do_sack).  Additionally, the tcp_sack.c file is not in the 
/usr/src/sys/netinet so I'm guessing this indicates that I need a patch. 
Hoping to be able to use SACK in FreeBSD 4.11-RELEASE, I wanted to ask :

1) Is my guess correct (Do I need a patch)?  Or is my kernel configuration 
file missing an option?
2) Assuming I need a patch, what patch is generally recommended for using 
SACK under TCP in FreeBSD 4.11-RELEASE?

Any help would be most appreciated.  Thanks,

ken