After deleting a rekeyed CHILD_SA we uninstall the outbound SA but don't
destroy the CHILD_SA (and the inbound SA) immediately. We delay it
a few seconds or until the SA expires to allow delayed packets to get
processed. The CHILD_SA remains in state CHILD_DELETING until it finally
gets destroyed.
The responder has all the information needed to install both SAs before
the initiator does. So if the responder immediately installs the outbound
SA it might send packets using the new SA which the initiator is not yet
able to process. This can be avoided by delaying the installation of the
outbound SA until the replaced SA is deleted.
Tobias Brunner