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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Mobility Support (Mobile IP)

Mobile IP Home Agent Registration and Registration Messages 1 2 3 Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation

Mobile IP Reverse Tunneling

There may be situations where it is not feasible or desired to have the mobile node send datagrams directly to the internetwork using a router on the foreign network as we just saw. In this case, an optional feature called reverse tunneling may be deployed, if it is supported by the mobile node, the home agent and if relevant, the foreign agent.

When this is done, a reverse tunnel to complement the normal one is set up between the mobile node and the home agent, or between the foreign agent and the home agent, depending on care-of address type. All transmissions from the mobile node are tunneled back to the home network where the home agent transmits them over the internetwork, resulting in a more symmetric operation rather than the “triangle” just described. This is basically what I described earlier as being “needlessly inefficient”, because it means each communication requires four steps. Thus, it is used only when necessary.

One situation where reverse tunneling may be required is if the network where the mobile node is located has implemented certain security measures that prohibit the node from sending datagrams using its normal IP address. In particular, a network may be set up to disallow outgoing datagrams with a source address that doesn’t match its network prefix. This is often done to prevent “spoofing” (impersonating another’s IP address.)

Note that everything I've just discussed is applicable to “normal”—meaning unicast—datagrams sent to and from the mobile node. Broadcast datagrams on the home network, which would normally be intended for the mobile node if it were at home, are not forwarded unless the node specifically asks for this service during registration. Multicast operation on the foreign network is also supported, but extra work is required by the mobile node to set it up.

Mobile IP Home Agent Registration and Registration Messages 1 2 3 Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation