Mobile IPv4 White Paper

How a Mobile Node Receives Packets
When the Mobile Node is not attached to its home network, the
Home Agent receives all packets destined for the Mobile Node's
home address. The Home Agent encapsulates the original IP
packet and directs it to the Mobile Node's Care-of Address.
When the packet arrives at the Care-of Address, the original IP
packet is extracted and delivered to the Mobile Node. This
encapsulation is also called tunneling.
How a Mobile Node Sends Packets
Tunneling is generally not required when the Mobile Node
sends a packet. The Mobile Node transmits an IP packet with its
Home Address as the source IP address. The packet is routed
through the Mobile Node's default router in the foreign network.
In networks that do source IP address checking, reverse
tunneling may be desirable. With reverse tunneling, packets
from the Mobile Node are encapsulated by the Care-of Address
and sent to the Home Agent. The HA decapsulates these packets
and routes them to the original destination.
Figure 1.1 shows the data path of an IP packet from a
Correspondent Node to a Mobile Node.
Figure 1.1 Data path of an IP packet sent from a
Correspondent Node to a Mobile Node
3 HP-UX Mobile IPv4 White Paper