FreeBSD 4.11-RELEASE & SACK

[Kan Cai]

Greetings,

I posted this question on freebsd-question list yesterday, but no replies. 
So I am just trying my luck here. Thanks in advance.

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