Currently missing in vanilla 2.4. 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. Nowadays they are working on implementing IPsec into 2.5.x series.