What is Mobile IP

Mobile IP is a protocol that allows for location independent transmission of IP packets in a wireless communication system. This protocol allows for a mobile device to roam between various LAN sub networks without the need to change its IP address on the global network. This is important because a sudden change in IP of a device can cause many communications errors in applications like VPN, VoIP etc.

In this protocol, a device is associated with two types of addresses. One is home address which is the permanent address of the device on the global network and the other is Care of Address (COA) which is the address allocated to the device visiting a foreign network.

The two types of entities that a network utilizes in this protocol are Home Agent and Foreign Agent. A home agent stores the list of addresses of all the devices that belong to the home network while the foreign agent is associated with the allocation of Care of Addresses to the visiting mobile units.

The steps that then take place in this protocol are:-

Broadcasting of information on the network about the Network Agents after small intervals of time. Recognition of this signal by the Mobile Unit. Feedback to the Network Agent by the Mobile Unit to let it know about its presence. Allocation of a Care of Address to the Mobile Unit by the Foreign Agent in case it doesn’t belong to the Network. Transmission of information from the Foreign Agent to the Home Agent of the roaming device; letting the Home Agent know about the current location of the device.

Now when packets are needed to be transmitted to the above Mobile unit, these are originally associated with the Home address of the device and therefore are routed to the Home Network of the device. The Home Agent which knows that the device is in a foreign network encapsulates these packets with the Care of Address of the device and then tunnels them to the foreign network. The Foreign Agent receives these packets and after removing the header of the care of address, forwards these packets to the Mobile unit. Thus packets are successfully transmitted to the mobile node without any change in its IP on the network.

When a mobile unit needs to transmit some packets, it can do it directly without having to rout the packets to the home network first. This process is known as triangulation. But in some internet gateways, ingress filtering is used which discards the packets in which the sender IP isn’t a part of the subnet of the Network in which the device is present. In this case triangulation is not possible, and the packets are first needed to be routed back to the home network before being transmitted towards their destination.

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