Mobile IPv4 White Paper

Reverse tunneling is useful for installations with ingress filtering
(filters that check inbound packets for topologically correct
source addresses). With reverse tunneling, the packet leaving the
foreign network will have a topologically correct source address,
because the source address in the outermost IP header will be an
address from the foreign network (the Care-of Address) instead
of the Mobile Node’s home address.
Route Optimization
HP-UX Mobile IPv4 facilitates the routing of datagrams directly
from the Correspondent Node to the Mobile Node’s Care-of
Address without going through the Mobile Node’s home
network. This improves the data transmission rates between the
Correspondent Node and Mobile Node and reduces congestion
in the home network.
Normally, packets from the Correspondent Node to the Mobile
Node are sent through the Home Agent (and through the Mobile
Node’s home network). When Route Optimization is used, the
Home Agent sends an authenticated message to the
Correspondent Node with the Mobile Node’s current Care-of
Address. This message informs the Correspondent Node to form
a route optimization tunnel to the Mobile Node's Care-of
Address.
7 HP-UX Mobile IPv4 White Paper