Freedom to Travel

Imagine that you are working in your home office with your tablet, which is connected to your home's wireless network. You leave the house for the airport, and while you're traveling in a taxi, the tablet uses LTE to connect with your provider, so you can check your home portal to see whether the heating was turned off. At the airport, the mobile device automatically connects to the public WiFi hotspot, on which you can check the time of departure and check in to your flight. While all this happened, you were connected all the time to your corporate network via a secure tunnel.

This scenario might sound like science fiction, but it is closer than you think. Mobile IPv6 (MIPv6) makes it possible to maintain accessibility on any network with the same address and to switch networks without interrupting connections. The prerequisite for such a setup is an IPv6 infrastructure.

The Roaming Issue

When a connection is established between two nodes, it is usually based on the IP addresses of the communication partners. If one partner changes networks, it inevitably receives a new IP address. As a consequence, the connection is dropped and must be reestablished. In the future, however, the connection between two communication partners will be maintained as a matter of course, despite a changing network connection. This principle is called roaming (Figure 1).

Figure 1: Roaming maintains connections even when the user moves from one network to another.

Depending on the transmission technology, changing the network link without losing the connection is already possible. This capability, however, relies on the network technologies themselves. It can be done, for example, on wireless networks, where the wireless client can move seamlessly between different access points (APs). If the received power of one AP is too low, the client automatically connects to the nearest AP without losing the connection. This is also possible on mobile networks, where appropriate authentication and account measures ensure cross-network billing.

An IP-based solution, however, allows for complete independence of the connection technology and is only fully supported by MIPv6 [1]. Mobile IP is also defined for IPv4 [2], but that specification entails some disadvantages compared with IPv6. For example, MIPv6 uses flexible extension headers to avoid issues with routing, thereby making roaming easier and more flexible. Additionally, MIPv6 basically abstracts itself from the data link layer through the use of Neighbor Discovery – a network layer technology.

Mobile Nodes

A mobile node is referred to in Mobile IPv6 terminology as, you guessed it, a mobile node (MN). The communication partner is the correspondent node (CN) and can be stationary (i.e., a server) or mobile. The MN has a home network, which is referred to as the home link. Attached to this is a router, which binds the fixed address of the MN as the home agent (HA). This fixed address is known as the home address and is a global unicast address. The home agent means that the MN is accessible everywhere via its home address (Figure 2).

Figure 2: Mobile IPv6 terminology.

The MN may reside on its home link or in a different network. Any other network is referred to as a foreign link. On a foreign link, the MN has a different address from its home address; this is assigned by autoconfiguration or DHCPv6. The current address on the foreign link is known as the care-of address. The MN sends its current care-of address to the HA. This message is known as the binding update; it creates a binding on the HA between the home address and the care-of address.

How Mobile IPv6 Works

As long as the MN is using a normal prefix on its home link, it can be reached by standard routing mechanisms, because the home agent with the home address is locally based. On the road, or traveling outside its home link, the MN receives an additional address (care-of address) on any foreign link; it then sends a binding update message to its home agent. The home agent sends a binding acknowledgment (ack) and – thanks to the binding process – knows the care-of address under which the MN is currently accessible. The correspondent node always uses the MN's home address to communicate with the MN; this always leads to the home agent.

The MN can communicate in two ways with the correspondent node. In bidirectional tunneling, packets from the CN are sent to the home agent, which forwards them through a tunnel to the MN. The MN sends the responses through a reverse tunnel to the home agent, which forwards the data to the CN. No support for Mobile IPv6 is necessary on the correspondent node.

When route optimization is enabled, any communication after the initial connection through the home agent is handled directly between the MN and the CN, without going through the HA. A type 2 routing header is used for this process. Route optimization enables more efficient communication because routing can be optimized, instead of detouring via the HA. However, this approach requires Mobile IPv6 support on the CN. Route optimization is one of the main advantages of Mobile IPv6 over Mobile IPv4 because the latter does not allow extension headers (and thus does not have routing headers).