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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 and TCP/IP Address Resolution Protocol (ARP) Operation 1 2 Mobile IP Security Considerations

Implications of Reverse Tunneling

To make matters worse, consider what happens if reverse tunneling is used! Here, tunneling is done not just for datagrams sent to the mobile node but sent from it as well. In our “worst case” example, a request/reply pair from the mobile node to another device on the foreign network requires two complete round-trips from Tokyo to London and back. Clearly, this is far from ideal.

There really isn't any solution to this problem within Mobile IP itself; it's just a natural consequence of how the protocol works. The only way to really improve things is to “hack in” a solution that ultimately boils down to one of the two options we always have in IP without mobility support: we either decide to give the mobile device a temporary real IP address on the foreign network, or we use a host-specific route for the mobile device while on the foreign network.

We've already seen that these both have problems, which is why Mobile IP was created in the first place. There may be situations, however, where efficiency is more important than the transparent portability that Mobile IP provides. For a long-term deployment on a foreign network far from the home network, or for applications where efficiency is paramount, it may make sense to employ one of these techniques. For example, a corporation that has a small number of offices in different cities connected using the Internet might set up special routing. This would let mobile devices visiting from other cities talk directly to nodes local to the foreign part of the network without being routed across the Internet.

Mobile IP and TCP/IP Address Resolution Protocol (ARP) Operation 1 2 Mobile IP Security Considerations