Introduction to Mobile IP

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). 

Therefore, when a MN is away from home, and it’s connected to a foreign network, it detects is on a different network and sends a registration request through the FA to the HA requesting mobile capabilities for a period of time. The HA sends a registration reply back to the MN (through the FA) allowing or denying that registration. This is true when the Mobile Node is using a Foreign Agent for the registration. If the Mobile Node obtains the care-of address by other meanings, that step (registration through the FA) is not necessary. 


If the HA allows that registration, it will work as a proxy of the MN. When MN’s home network receives packets addressed to the MN, HA intercepts those packets (using Proxy ARP), encapsulates them, and sends them to the care-of address, which is one of the addresses of the FA. The FA will decapsulate those packets, and it will forward them to the MN (because it knows exactly, where the MN is). 

Encapsulation is the method used by the HA to deliver information to the MN putting an extra IP header on top of the packet and tunnelling that packet to the MN (when it's on a foreign network). Tunneling and encapsulation are defined in  IP in IP tunneling and IP encapsulation within IP. 

So, when the MN is on a foreign network, it uses its home agent to tunnel encapsulated packets to itself via FA. This occurs until the lifetime expires (or the MN moves away). When this happens (time out) MN must register again with its HA through the FA (if the MN obtains its care-of address for other meanings, it acts as its own FA). 

When the MN moves to another network and it detects so, it sends a new registration request through (one more time) the new FA. In this case, HA will change MN’s care-of address and it will forward encapsulated packets to that new care-of address (which, usually, belongs to the FA). Some extensions of Mobile IP allows to a MN to have several care-of addresses. Then, HA will send the same information to all the care-of addresses. This is particularly useful when the MN is at the edges of cells on a wireless environment, and it is moving constantly. 

MN bases its movement detection basically looking at periodic adverts of the FA (and HA), which sends to its localnet. Those messages are one extension of the ICMP router discovery messages and they are called Agent Advertisement (because they advertises a valid agent for Mobile Nodes). 

There are two different methods to detect network movement: 

a) The first method is based on network prefixes. For further information look at Mobile IP RFC 2002  (section, page 22). This method is not included in our current implementation. 

b) The second method is based upon the Lifetime field within the main body of the ICMP Router Advertisement portion of the Agent Advertisement. Mobile nodes keep track of that Lifetime and if it expires, it sends an Agent Solicitation (asking for a new Agent Advertisement) and it presumes that it has been moved. 

When the MN returns to its home network, it does not require mobility capabilities, so it sends a deregistration request to the HA, telling it that it’s at home (just to deactivate tunneling and to remove previous care-of address(es)). 

At this point, MN does not have to (de)register again, until it moves away from its network. The detection of the movement is based on the same method explained before.