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_startnetkey brings up the Openswan kernel component. This involves loading any required modules.
ip(8), ipsec_tncfg(8).
The ip xfrm commands are not documented in the man page. Ask linux-net people, but do not hold your breath. See ip xfrm state help and ip xfrm policy help for the magic words.
Man page written for the Linux Openswan project <http://www.openswan.org/> by Michael Richardson.