Mobile IP Applet

Mobile IP

Theoretical Background

How to use the applet

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Basic knowledge about Mobile IP

Today portable computers have become very popular. The users of them of course want to be able to receive mail and have the same services as if they were working at the same place all the time. The problem is that when they swap locations on the Internet, other users are not automatically updated with their new addresses. This is a problem for data packages, which keep on being sent to the old address.

It is not a good idea to give the mobile host (the portable computer) a new address every time it changes location. This would require that all the other users need to be updated with the new information. It is desirable that the mobile hosts always keep the same address. 

We could model the Internet as a lot of LANs (Local Area Networks) connected to each other by a WAN (Wide Area Network). Every LAN is controlled by an agent. If a LAN is the mobile host's home location, the agent is called the home agent. Otherwise it is called the foreign agent.

The task of the home agent is to be updated with the current location of all the mobile hosts. The hosts have their home location at the home agent's LAN. The task of a foreign agent is to be updated with all mobile hosts, who belong to other LANs, but are currently visiting the foreign agent's LAN.

When a new mobile host enters a LAN it must register itself with the agent in that network. The registration procedure works like this:

1. All foreign agents periodically broadcast a message in their network giving their address. A newly arrived mobile host may wait for this package or if it takes too long, it may announce its presence by sending a package itself, asking for a foreign agent.
2. The mobile host registers itself with the foreign agent by sending itshome address, its current location in the new network and some security information to the foreign    agent.
3. The foreign agent now sends its network address and the security information which the mobile host supplied in step 2 to the home agent. The security info is to     gurantee the home agent that the mobile host really is at the foreign agent.
4. If the security information is in order, the home agent registers the foreign agent's address. It will now be used as a "care of" address for the mobile host.
5. When the foreign agent receives conformation from the home agent, it informs the mobile host that the registration is now complete.

A package now sent to the mobile host is of course first sent to the home agent because that is where the mobile host is expected to be. The home agent now encapsulates the message in a package and sends it to the foreign agent. The foreign agent removes the message from the encapsulated package and sends it to the mobile host. This procedure is called tunnelling. The home agent also informs the sender that it shall send all future messages directly to the foreign agent.

Terminology

• Home Agent (HA):
  A router on the mobile host's home network which delivers data to departed mobile host, and maintains current location information for each.
• Foreign Agent (FA):
  A router on a mobile host's visited network which cooperates with the home agent to complete the delivery of data to the mobile host while it is away from home.
• Mobile Host (MH):
  A host that changes its point of attachment from one network or subnetwork to another, without changing its IP address.
• Home address:
  IP address assigned for a mobile host. It remains unchanged regardless of where the node is attached to the internet.
• Care of address:
  The address of the foreign agent with which the mobile host is connected. When the mobile host is away from home data is encapsulated and forwarded to this address.
• Tunnel:
  The path followed by data while encapsulted.
• Tunneling:
  In order fir data to be delivered to the mobile host whileaway from home, the home agent (or another host knowing the care of address) has to tunnel the data to thecare of address.

How to use the applet
 

To illustrate the procedure with Mobile IP we have made an applet which explains the basic procedure of Mobile IP. To understand how it works you are recommended to read the following user's description.

What is on the screen?
In our applet we have one Mobile Host (MH), with the respective Home Agent (HA) and three Foreign Agents (FA). A circle around each agent symbolizes the range of that agent's network. When the MH is within the range of an agent's network it is considered connected to that network. Messages sent between agents are symbolized by a letter. When a message is to be tunnelled (as described above), we show the tunnelling process by putting the message in a package and then sending it to its destination. When arriving at the destination the letter is unpacked.

When a letter or package is moving across the screen you are able to stop it by pressing the STOP button. You may then click on the letter or package to get information about its contents.  To continue tou must press the CONTINUE button. Every event is described, step by step, in the text area to the right.

What can you do?
You may move the MH by dragging it with the mouse. Try to move it between the networks of different agents! You may also get a dialogue frame with the status of an agent by doubleclicking on the corresponding agent. In such a dialogue frame you are able to send a message to the MH and change the network range.

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