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Mobile IP

(Zurich, Switzerland - Alvin Wei-Cheng Wong)


- Overview

Mobile Internet Protocol (or Mobile IP) is a communications protocol (created by extending Internet Protocol IP) that allows users to move from one network to another using the same IP address. It ensures that communication will continue without losing the user's session or connection. 


- The Mobile IP Protocol

Mobile IP is an IETF protocol that allows mobile users to move from one network to another while maintaining their IP addresses. The existing two versions of mobile IP protocols are Mobile IPv4 and Mobile IPv6. 

The mobile IP protocol allows transparent routing of IP datagrams on the Internet. Each mobile node (cell phone, handheld, laptop PC, router), is identified by its home address, irrespective of its current location in the Internet. When away from home, a mobile node is associated with a care-of address, which provides information about its current location. 

The mobile IP protocol specifies how a mobile node registers with its Home Agent (HA) and how the HA routes datagrams to the mobile node through a tunnel. Using mobile IP, nodes may change their point-of-attachment to the Internet without changing their IP address, allowing the application and transport-layer protocols to seamlessly maintain connection while moving. 


- The Characteristics of Mobile IP

The general characteristics of mobile IP can be summarized as follows:

  • Transparency of user mobility to the transport and application layer protocols;
  • Interoperability with stationary hosts running conventional IP protocols;
  • Scalability across the Internet;
  • Security by preventing an attacker from impersonating a mobile host;
  • Macro mobility by ensuring long-term connection while away from home agent.

- Types of Entities in Mobile IP

Node mobility is realized without propagating host-specific routes throughout the Internet. Using mobile IP, a mobile device will have two addresses, i.e., a primary or permanent home address and a secondary or temporary care-of address, which is associated with the network that the mobile node is visiting. 

There are two types of entities in mobile IP:

  • A home agent that stores information about mobile nodes whose permanent home address is in the home agent’s network;
  • A foreign agent (FA) which stores information about mobile nodes visiting its network. Foreign agents also advertise care-of addresses.


- Node Mobility

A node that wishes to communicate with the mobile node uses the permanent home address of the mobile node as the destination address for out-bound packets. Since the home address logically corresponds to the network associated with the home agent, conventional IP routing mechanisms forward these packets to the home agent. 

Instead of forwarding these packets to a destination that is physically in the same network as the home agent, the home agent redirects these packets towards the foreign agent. 

The home agent looks for the care-of address in the mobility binding table, and then tunnels the packets to the mobile node’s care-of address by appending a new IP header to the original IP packet, which preserves the original IP header. 

The packets are detected at the end of the tunnel by removing the IP header added by the home agent and are delivered to the mobile node. 

The mobile node directly sends packets to the other communicating node through the foreign agent without involvement of the home agent, using its permanent home address as the source address for the IP packets, i.e., triangular routing. 

The foreign agent can utilize reverse-tunneling by sending the mobile node’s packets to the home agent, which forwards them to the communicating node. 

This mechanism is needed in networks whose gateway routers have ingress filtering enabled, and hence the source IP address of the mobile host needs to belong to the subnet of the foreign network; otherwise, the packets would be discarded by the router. 

The mobile IP protocol defines an authenticated registration procedure through which a mobile node informs its home agent of its care-of address, router discovery (which allows mobile nodes to discover prospective home agent and foreign agents), and the rules for routing packets to and from mobile nodes, including the specification of one mandatory and several optional tunneling mechanisms.



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