Currently missing in vanilla 2.4. There was 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 wasn't included in vanilla source.
18.2.2. Support in vanilla Linux kernel 2.6.x
Current versions (as time of writing 2.6.9 and upper) support native IPsec for IPv4 and IPv6.