Mobile IP

Nowadays a device might belong to a home network which means that it maintains a permanent address known as its home address and a temporary internet address assigned to it when joining another network.
This permanent unique address is used to communicate with such hosts.
When a host with a permanent address is attached to another network, the data received at the home network should be forwarded to the visited network.

There are several methods to keep track of migrating hosts such as:

• A server that keep the binding between the permanent address and temporary address. However this method has several limitations, mainly it is impossible to provide an on-line migration, transport protocols need to know about the new address in order to reestablish communication. It increase network traffic to update the mapping and caches if existed.
  

• Broadcast solution: when a host wants to send data to a migrating host, it broadcasts a query packet in a network and the migrating host reply with its temporary address. This mechanism can only be applied to small networks.

There are three well know problems involved when dealing with host migration:
[Internet draft NETLMM problem]

• Update latency: the update of the new temporary address is always necessary when a host migrate, though this update might take time if the migrating host did not notify about his mobility, network traffic, crashed routers,etc. During this time, the home network will forward received packets to an old temporary IP address until the new mapping occurs.
• Signaling overhead: when moving to another network, the migrating host notifies its home network and acquire a new address. The configuration and notification can be expensive depending on the method and the node resources (bandwidth, battery dependent etc).
• Location privacy: the change in temporary address exposes the migrating host topologically.
  

"The on-line Migration":
With the huge development of Wireless networks, a host can migrate from one network to another in a short time while still connected to other hosts. This is called "on-line" migration.
When migrating several applications need to stop and reestablish the communication with distant hosts using the new assigned address. This causes service or application interruption.

This interruption of service is encountered because in the OSI layers, the IP address is used for both host identification and routing (data delivery).
In the transport layer (TCP, UDP) the IP address is used as an identifier along with other parameters in order to uniquely address a host or a communication session.
As for the network layer, the IP address is used as a way to locate a host and deliver data.

The following paragraphs describes some solutions for the host migration.