Currently missing in 2.4, perhaps in 2.5 (see below). There is an issue about keeping the Linux kernel source free of export/import-control-laws regarding encryption code. This is also one case why FreeS/WAN project (IPv4 only IPsec) isn't still contained in vanilla source.
The USAGI project has taken over in July 2001 the IPv6 enabled FreeS/WAN code from the IABG / IPv6 Project and included in their kernel extensions, but still work in progress, means that not all IABG features are already working in USAGI extension.