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IPv6 mobility: Part 1

An overview of the mobility limitations of IPv4.

Most people think that address space requirements will drive IPv6 deployment. However, IPv6's ability to efficiently support many hundreds of thousands of mobile users is another important motivation. By their nature, VoIP cell phones and internet connected PDAs don't remain connected to a fixed network, but connect to the network supported by the nearest cell tower or wireless access point. The designers of IPv6 recognized that mobile devices would be an important technology and designed efficient, scalable support for large numbers of mobile network nodes into the protocol.

Creating and maintaining a network connection is not a problem as long as the connection is initiated from the mobile node, and the node does not move while connected. A wireless laptop operating in a WLAN hotspot acquires an address from the WLAN access point and retains the address for the duration of the connection. The address is local to the hotspot's network. A connection made from the laptop will operate in the same manner as any other IP connection.

Mobile Operation
The complexity comes when making connections to a mobile device or when the mobile device moves from network to network while connected. Restricting VoIP cell phones to outgoing calls is not practical. A calling node needs to have an address to use to initiate the connection, but has no way of finding the address that the mobile device is currently using.

Mobile IPv4 and IPv6
A Mobile IP protocol has been added to IPv4. The problem of how to connect to a mobile node is solved in both Mobile IPv4 and IPv6 by assigning each mobile device a static address called a home address. The home address is an address local to the network maintained by the service provider supporting the device. This might be the network maintained by a cell phone provider. In addition to the home address, a mobile device acquires an address called the care-of-address from the network to which it is currently connected. This network is known as the foreign network. The care-of-address is local to the foreign network from which it was acquired.

A node that communicates with a mobile node is called a correspondent node. To connect to a mobile node, a correspondent node addresses packets to the mobile node's home address. The correspondent node need not be aware that it is connecting to a mobile node, and the correspondent node may also be mobile.

A device called a home agent, usually a router, is located on the home network. When the packets from the correspondent node reach the home network, the home agent intercepts the packets. It then forwards them to a mobile device at its current care-of-address.

Efficiency and Scalability
Up to this point, Mobile IPv4 and IPv6 operate identically. The difference comes when the mobile node sends packets back to the correspondent node. In IPv4 the mobile node must send packets to the correspondent node by sending them to the home agent first. The home agent then forwards the packets to the correspondent node. This means that every packet must pass through the home agent and each packet must traverse the home network on its way to the home agent and on its way from the home agent. A home network would have to have enormous bandwidth to support hundreds of thousands of mobile devices. IPv6 enables more efficient, scalable operation. Part 2 will describe this operation.

David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.

This was last published in June 2005

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