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Scalable support for transparent mobile host internetworking
David B. Johnson
Mobile IP is IP with an extra level of indirection.
- Basic Mobile IP: Each mobile host (MH) has a home
agent (HA) that acts as a location registry, maintaining the
binding from the MH's home address and its foreign
care-of-address. Each time the MH moves, this binding is
updated. Correspondent hosts (CH's) address messages with the MH's
home address; the HA forwards this message to the MH via IP tunneling
or encapsulation. MH's gain their care-of-addresses by contacting a
foreign agent; the foreign agent assigns the care-of-address and
updates the MH's binding by contacting the HA. MD-5 checksums of
messages + a secret key + a nonce are used to verify these updates.
- Location caching: Correspondent hosts or intermediate
routers may cache MH's care-of-addresses. Explicit feedback messages
from HA's must be used to both provide and invalidate these
care-of-addresses; care-of-addresses have lifetimes that can help to
purge old cached values.
- Foreign-agent handoff: When a MH changes its care-of-address,
a forwarding pointer can be left at the old foreign agent so that
in-flight messages are not dropped. (This is an extension to the basic
mobile IP protocol; since messages are IP datagrams, it is alright to
- Scalability: The mobile IP specification scales well. The
home network/agent maintains bindings only for its own mobile hosts.
Similarly, foreign networks/agents only need to accept and forward
messages for MH's under their care. The rest of the internet does
not need to maintain extra state, do extra work (except for the
dog-leg delivery route), or be modified to behave correctly.
Mobile IP is a well-thought out specification that has the needed feature
of not needing changes to the internet infrastructure. It can be made
to be secure, and the optimizations under consideration can make
Mobile IP nearly as efficient as regular IP.
- The paper itself has little or no flaws; it accurately and
clearly describes the Mobile IP spec. The basic Mobile IP's spec has
the flaw of the dogleg route and the extra header overhead for IP
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