09-08-2012, 03:50 PM
IP version 4 assumes
IP version .docx (Size: 102.62 KB / Downloads: 27)
Introduction
IP version 4 assumes that a node’s IP address uniquely identifies its physical attachment to the Internet. Therefore, when a corespondent host (CH) tries to send a packet to a mobile node (MN), that packet is routed to the MN’s home network, independently of the current attachment of that MN (this is because CHs do not have any knowledge of mobility).
When the MN is on its home network, and a CH sends packets to the mobile node, the Mobile Node obtains those packets and answers them as a normal host (this is one important requirement in Mobile IP), but if the MN is away from its home network, it needs an agent to work on behalf of it. That agent is called Home Agent (HA). This agent must be able to communicate with the MN all the time that it is “on-line”, independently of the current position of the MN. So, HA must know where the physical location of the MN is.
In order to do that, when the MN is away from home, it must get a temporary address (which is called care-of address), which will be communicated to the HA to tell its current point of attachment. This care-of address can be obtained by several ways, but the most typical one is that the MN gets that address from an agent. In this case, this agent is called Foreign Agent (FA).
Theoretical Background
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 its home 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 guarantee 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.
When a user leaves a LAN he should first deregister but since it is easy just to turn off the computer this is often forgotten.
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 tunneling. The home agent also informs the sender that it shall send all future messages directly to the foreign agent.
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 sub network 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.