Mobile IP With Reverse Tunneling
The section How Mobile IP Works assumes that the routing within the Internet is independent of
the source address of the datagram. However, intermediate routers might check for a
topologically correct source address. If an intermediate router does check, the mobile node
needs to set up a reverse tunnel. By setting up a reverse tunnel
from the care-of address to the home agent, you ensure a topologically correct
source address for the IP data packet. Reverse tunnel support is advertised by
foreign agents and home agents. A mobile node can request a reverse tunnel
between the foreign agent and the home agent when the mobile node
registers. A reverse tunnel is a tunnel that starts at the care-of address
of the mobile node and terminates at the home agent. The following figure
shows the Mobile IP topology that uses a reverse tunnel.
Figure 27-4 Mobile IP With a Reverse Tunnel
Limited Private Addresses Support
Mobile nodes that have private addresses that are not globally routeable through the
Internet require reverse tunnels. Solaris Mobile IP supports mobile nodes that are privately
addressed. See Overview of the Solaris Mobile IP Implementation for the functions that Solaris Mobile IP does not support.
Enterprises employ private addresses when external connectivity is not required. Private addresses are
not routeable through the Internet. When a mobile node has a private address,
the mobile node can only communicate with a correspondent node by having its
datagrams reverse-tunneled to its home agent. The home agent then delivers the datagram
to the correspondent node in whatever manner the datagram is normally delivered when
the mobile node is at home. The following figure shows a network
topology with two mobile nodes that are privately addressed. The two mobile nodes
use the same care-of address when they are registered to the same foreign
agent.
Figure 27-5 Privately Addressed Mobile Nodes Residing on the Same Foreign Network
The care-of address and the home agent address must be globally routeable
addresses if these addresses belong to different domains that are connected by a
public Internet.
The same foreign network can include two mobile nodes that are privately
addressed with the same IP address. However, each mobile node must have a
different home agent. Also, each mobile node must be on different advertising subnets
of a single foreign agent. The following figure shows a network topology that
depicts this situation.
Figure 27-6 Privately Addressed Mobile Nodes Residing on Different Foreign Networks
