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Mobile IP Efficiency Issues
(Page 2 of 2)
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.
Inefficiency is an Inherent Part of Mobile IP
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.
 Key Concept: Since datagrams are sent to a mobile node at its home address, each datagram sent to the mobile device must first go back to its home network and then be forwarded to its current location. The level of inefficiency that results depends on how far the sender is from the mobile’s home network. The worst case actually occurs if the sender and mobile are on the same foreign network, in which case each transmission must make a round-trip to the mobile’s home network and then back again. This is an inherent part of how Mobile IP and cannot readily be rectified.
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