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Mobile IP Concepts and General Operation
(Page 3 of 3)
Mobile IP Functions
An important difference between Mobile
IP and our mail forwarding example is one that represents the classic
distinction between people and computers: people are smart and computers
are not. When our consultant is traveling in Tokyo, he always knows
he's in Tokyo and that his mail is being forwarded. (Well, assuming
he goes easy on the sake, but that's a different story. J)
He knows to go deal with the Tokyo post office to get his mail. The
post office in London knows what forwarding is all about and how to
do it. The traveler and the post offices all can communicate easily
using the telephone.
In contrast, in the computer world,
when a device travels using Mobile IP, things are more complicated.
Suppose our consultant flies to Tokyo, turns on his notebook and plugs
it in to the network. When the notebook is first turned on, it has no
clue what is going on. It has to figure out that it is in Tokyo. It
needs to find a foreign agent in Tokyo. It needs to know what address
to use while in Tokyo. It needs to communicate back with its home agent
back in London to tell it that it is in Tokyo and to start forwarding
datagrams. Furthermore, it must accomplish its communication without
To this end, Mobile IP includes a
host of special functions that are used to set up and manage datagram
forwarding. To see how these support functions work, we can describe
the general operation of Mobile IP as a simplified series of steps:
- Agent Communication: The mobile
node finds an agent on its local network by engaging in the Agent
Discovery process. It listens for Agent Advertisement messages
sent out by agents and from this can determine where it is located.
If it doesn't hear these messages it can ask for one using an Agent
- Network Location Determination:
The mobile node determines whether it is on its home network or a foreign
one by looking at the information in the Agent Advertisement
If it is on its home network it functions
using regular IP. To show how the rest of the process works, let's say
the device sees that it just moved to a foreign network. The remaining
- Care-Of Address Acquisition:
The device obtains a temporary address called a care-of address.
This either comes from the Agent Advertisement message from the
foreign agent, or through some other means. This address is used only
as the destination point for forwarding datagrams, and for no other
- Agent Registration: The mobile
node informs the home agent on its home network of its presence on the
foreign network and enables datagram forwarding, by registering
with the home agent. This may be done either directly between the node
and the home agent, or indirectly using the foreign agent as a conduit.
- Datagram Forwarding: The home
agent captures datagrams intended for the mobile node and forwards them.
It may send them either directly to the node or indirectly to the foreign
agent for delivery, depending on the type of care-of address in use.
Datagram forwarding continues until
the current agent registration expires. The device can then renew it.
If it moves again, it repeats the process to get a new care-of address
and then registers its new location with the home agent. When the mobile
node returns back to its home network, it deregisters to cancel
datagram forwarding and resumes normal IP operation.
The following topics look in more
detail at the functions summarized in each of the steps above.
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Version 3.0 - Version Date: September 20, 2005
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