This splits the SA installation also on the initiator, so we can avoid
installing the outbound SA if we lost a rekey collision, which might
have caused traffic loss depending on the timing of the DELETEs that are
sent in both directions.
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.
This happens if the peer deletes the redundant SA before we are able to
handle the response. The deleted SA will be in state CHILD_INSTALLED but
we don't want to trigger the child_updown() event for it or recreate it.
Generally, we will not find the CHILD_SA by searching for it with the
outbound SPI (the initiator of the DELETE sent its inbound SPI) - and if
we found a CHILD_SA it would most likely be the wrong one (one in which
we used the same inbound SPI as the peer used for the one it deletes).
And we don't actually want to destroy the CHILD_SA at this point as we
know we already initiated a DELETE ourselves, which means that task
still has a reference to it and will destroy the CHILD_SA when it
receives the response from the other peer.
Tobias Brunner