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Mobility Support for the Next Generation Internet
Mobility Support for the Next Generation Internet

As mobile computing becomes more widespread, the need for mobility support for Internet devices increases. The number of mobile computers is expected to soar. Cellular phones that offer IP services based on WAP or GPRS are already available, and their number will increase rapidly. 3G cellular devices will be packet-switched rather than circuit-switched. Therefore, IP services on 3G cellular devices will play an integral role.

Today we face several problems that make roaming with mobile Internet devices difficult. Problems start when a user disconnects his or her mobile device from the Internet in order to connect it elsewhere. Normally the user would not be able to continue communication until configuring the system with a new IP address, the correct netmask, and a new default router.

This problem is based in the routing mechanisms used in the Internet. IP addresses define a kind of topological relation between the linked computers. Today's versions of Internet protocols assume that any node always has the same point of attachment to the Internet. Additionally, the node's IP address identifies the link on which the node resides. If a node moves without changing its IP address, there's no information in its network address about the new point of attachment to the Internet. Existing routing protocols are thus not able to deliver datagrams correctly. Current Internet routing protocols require the network address to change when a host moves to a new location.

Packets sent to the node on link B are always routed across the Internet to link B. If this node moves to another link, it will not receive packets anymore. Packets addressed to this node are still routed to link B.

To support mobile devices, which dynamically change their access points to the Internet, the Internet Engineering Task Force (IETF) currently standardizes a protocol called Mobile IP. There are two variations of this protocol: Mobile IPv4, based on IPv4, and Mobile IPv6, based on IPv6. This article focuses on Mobile IPv6.

Mobile IPv6 allows an IPv6 host to leave its home subnet while transparently maintaining all its present connections and remaining reachable to the rest of the Internet. Mobile IPv6 realizes this by identifying each node, regardless of its current point of attachment to the Internet, via its static home address - the IPv6 address of the node at its home network.

While a mobile node is away from home, it sends information about its current location to an agent system on its home link, the home agent. This is done by Mobile IPv6 first generating a so-called care-of address, an IPv6 address that's temporarily valid at the visited network, and assigning this address, to the mobile node for the duration of its visit. Having assigned this care-of address the mobile node registers this address at the home agent. Now the home agent knows the temporary address of the mobile node at the visited network and starts intercepting packets addressed to the mobile node's static home address. Then it tunnels them to the mobile node's present location.

This mechanism is completely transparent for all layers above IP (i.e., TCP, UDP, and of course all applications). That is, a user doesn't even recognize that he or she is mobile. DNS entries for a mobile node refer to its static home address and don't change if the mobile node changes its Internet access point. In fact Mobile IPv6 influences the routing of packets, but it's independent of the routing protocol itself (RIP, OSPF, ...).

Moving to another visited network, the mobile node has to assign a temporary care-of address at the new network and register this new address again at its home agent.

Principally, the mobility support for Internet devices is possible and standardized for both IP protocol versions, IPv4 and IPv6, but due to the enhanced functionality and later design of IPv6, some features concerning mobility support have been integrated more efficiently in Mobile IPv6 than in Mobile IPv4.

Following are some of the main advantages of Mobile IPv6:

  • Mobile IP has to assign global IP addresses to a mobile node on each point at which it attaches to the Internet. On links that serve for mobile nodes, a set of IP addresses (at least one) assigned as mobile node care-of addresses has to be reserved. Due to an address shortage in IPv4 there may be problems reserving enough global IPv4 addresses on some links; for IPv6 there are enough addresses available.
  • Using stateless address autoconfiguration and neighbor discovery mechanisms Mobile IPv6 needs neither DHCP nor agent systems on the visited networks to configure the care-of addresses of mobile nodes. This substantially reduces costs and requirements for deployment.
  • Mobile IPv6 has the integral support of a route optimization - packets (except the first ones) from a communication partner can be sent directly to the mobile node without using its home agent.
  • Sending packets on a visited network a mobile node that runs Mobile IPv6 uses the care-of address as source address, while a mobile node running Mobile IPv4 has to use its home address. As the home address is not valid on the visited network, the packets could be discarded if filter devices (ingress-filtering) are used in the Internet.
About Wolfgang Fritsche
Wolfgang Fritsche is senior consultant and project manager at IABG and has been responsible for several national and international projects in Internet and cellular communication areas such as IPv6, IP security, mobile IP, multicasting, and 3G cellular networks. Some of his recent activities include leading IABG's involvement in the EU IST-funded 6INIT and 6WINIT projects. Wolfgang has also participated in the standardization work of IETF, 3GPP, ITU-T and ETSI, and represents IABG as a founding member of the IPv6 Forum. He holds a degree in electrical engineering from the
Technical University of Munich.

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