Hw_offload option in ipsec

Hi, i'm trying to do ipsec offloading.
by adding

hw_offload '1' in /etc/config/ipsec 

and on /etc/init.d/swanctl reload it gives

ERROR: hw_offload value 1 invalid

if i add hw_offload 'yes' then if give

/etc/rc.common: eval: line 334: swanctl_append4: not found

what is the issue? how to do hw_offloading?