Future mobility: Mobile IP is the harbinger of untethered computing

Advances in wireless network technology have revolutionized the method and perception of voice communications, making mobility commonplace. Now, developments in wireless data protocols are beginning to do the same thing for data communications.

What cellular technology did for telephony, mobile Internet protocol will do for the TCP/IP-based mode of data transport that has become ubiquitous in wireline environments. Mobile IP is an extension to the "untethered office" concept, allowing unfettered access to the Internet or corporate networks any time, from any location.

This type of mobility is already available today in some forms using satellite access to an Internet service provider. But satellites provide access to just one ISP at a time-and only as long as the user is within the satellite's footprint. Mobile IP provides a way for carriers to offer assured, continuous access to the Internet or a private IP-based network.

Encompassing a set of protocols being developed by the Internet Engineering Task Force (IETF), mobile IP uses the roaming concept of cellular telephony, in which a user can make a single call to an IP-based network and maintain an apparently single connection even as the system is handed off from one IP network to another.

The protocol is being developed for a number of reasons. Most obvious is the explosive growth of the Internet during the last half-decade and the growing number of service providers and vendors whose business is increasingly dependent on the Internet. An equally important factor is the growing use of the TCP/IP suite.

TCP/IP is not just the suite for the Internet, but it has also recently become the most widely used data communications protocol to the desktop. Service providers are closely watching TCP/IP to see if it emerges as the protocol suite for next generation services and networks.

The TCP/IP connection When a client and server-such as a browser and a Web server-communicate over the Internet, they generally employ TCP for reliable end-to-end communication. TCP is connection-oriented, meaning that a logical connection, or virtual circuit, must be created between the client and server before the exchange of data.

When this logical connection is created, a port number is assigned at both the client end and the server end so that these two hosts can keep track of the connection. Every end-to-end TCP virtual circuit, then, is uniquely identified by four values: client IP address, client TCP port, server IP address and server TCP port. Those values must be static for the duration of the connection.

The basic problem with mobile IP is that the client may be moving from network to network. Therefore, the client's IP address may be constantly changing, in which case the four values cannot remain static. If the client's address changes during an open TCP connection, packets cannot be routed back and the connection will eventually be lost.

Solving this problem in mobile IP is analogous to handling cellular telephone calls. Say a wireless phone has the telephone number 802-555-5377. That is the publicly advertised number of the phone, but it may not be the number used for routing purposes because the user may not actually be physically located in the "802" area at any given moment. If the user is in Dallas, the phone must advertise its presence in the 214 area. Although the call setup request may go to 802, calls are completed directly to 214.

Going mobile Several terms and concepts have been introduced to describe mobile IP operation (Figure 1). Mobile IP hosts, or mobile nodes, are assigned two IP addresses. The first is called the home address, which is a static address used to identify the end-to-end connection and used by the mobile node when connected to its home network.

The second address is the care-of address, which is a dynamic address used for packet routing only and used by the mobile node by the foreign network. The care-of address changes every time the mobile node changes to a new network.

Mobility agents are mobile IP-aware servers or routers that know where the mobile node is actually connected. The home agent is the home network's mobile IP agent, which has the responsibility of forwarding the mobile node's packets to the foreign network where the mobile node is actually connected. The foreign agent is responsible for delivering the packets to the transient mobile node.

The mobile node can communicate with any other device on the Internet, mobile or not. The entire mobile IP process, in fact, makes the mobile node's roaming transparent. Thus, other Internet hosts know the mobile node only by its advertised home address and do not know where the mobile node physically resides.

The process by which a mobile node and an Internet host would exchange packets is as follows:

* The Internet host sends a packet to the mobile node using the mobile node's home address.

* The packet is delivered to the home agent.

* If the mobile node is on the home network, the home agent delivers the packet locally. If the mobile node is on a foreign network, the home agent forwards the packet to the foreign agent.

* The foreign agent delivers the packet to the mobile node.

* Packets from the mobile node to the Internet host are sent via the home or foreign agent (whichever is local to the mobile node) using normal Internet routing procedures.

Note that all packets between the Internet host and mobile node use the mobile node's home address regardless of whether the mobile node is on a home or foreign network. The care-of address is only used for communication with mobility agents and is never seen by an Internet host.

Getting recognized A mobile node has to register its location information with the appropriate mobility agents so that the node can be found. Mobile nodes first have to find a mobility agent. The two ways that this can be accomplished are variations on existing procedures for IP router discovery.

In the first method, the mobile node merely waits for the local mobility agent's periodic broadcast of an agent-advertisement message. In the second, the mobile node broadcasts an agent-solicitation message. In either case, this process allows a mobile node to identify mobility agents, obtain one or more care-of addresses, learn about any special services provided by a foreign agent and determine whether it is connected to its home network or a foreign one.

A mobile node starts on its home network. While attached to the home network, the node is registered with the home agent and operates normally using its home address.

As the mobile node roams, it will be connected to a series of one or more foreign networks. It has to register with a foreign network to advertise its presence and obtain a care-of address. Then it has to register the care-of address with its home agent. This process provides a binding between the home and care-of addresses and defines the lifetime of the registration. Encryption is used to authenticate the registration information.

The mobile node eventually returns home after being registered on a foreign network. At that time, the node has to deregister the foreign network information with the home agent.

Mobile nodes must be able to detect to which network they are connected as they roam from network to network. The simplest mechanism is based upon the registration lifetime. As long as a node stays on a foreign network, the registration will be periodically renewed. If the registration expires, the mobile node can safely assume that communication with the foreign network has been lost. Meanwhile, if the node has moved to another network, it should start to see new agent advertisement messages, at which time it should attempt registration on the new network.

Tunneling around The communication scenario depicted in Figure 2 shows four devices: a host on the Internet, the mobile node, the mobile node's home agent on the home network and the mobile node's foreign agent on the foreign network. When an Internet host sends a packet to the mobile node, it uses the mobile node's home address since that is the address that is advertised.

If the mobile node is currently connected to a foreign network, the home agent forwards the original packet to the registered foreign agent. The original packet arrives at the mobile node unchanged using a scheme called tunneling, whereby the original packet is placed inside of the forwarded packet.

When the Internet host sends a packet to the mobile node, it assembles an ordinary IP packet (Figure 3). The source address field contains the Internet host's address, the destination address field contains the mobile node's home address, and the protocol identifier indicates the next higher layer protocol that comprises the contents of the packet payload. This packet is routed through the Internet to the mobile node's home network.

When the packet arrives at the home network it is intercepted by the home agent, which knows to which foreign network the mobile node is connected and knows the care-of address. The home agent encapsulates the packet so that the original data can be forwarded to the mobile node on the foreign network. This is the tunneling protocol.

The new packet is routed through the Internet to the foreign network using normal IP routing. The foreign agent forwards the tunneled packet to the mobile node using the care-of address and the mobile node retrieves the original packet.

The tunnel is necessary because an end-to-end connection between two host systems must employ the same host addresses in all packets in both directions. In the mobile environment, this is impossible because the mobile node keeps moving and changing its IP address. The tunnel accommodates node mobility by making the dynamic nature of the address transparent to both the Internet host and the mobile node.

Development issues Although the mobile IP specification is under development, it is an emerging technology and several issues must still be resolved.

* The asymmetrical nature of tunneling yields routing inefficiencies. For applications that attempt to throttle themselves based on average round-trip delays, the fact that the one-way delay times are very different may present problems.

* Just as wireless LANs are generally more expensive than wireline LANs on a per-node basis, mobile IP costs will be similarly higher than wireline-based IP.

* Some firewalls may block mobile IP tunnels because they check every packet's source address field, and the home address of a mobile node's packet will differ from the network address of the foreign network.

Mobile IP is still in a growth stage, but its potential importance in years to come should not be underestimated. Although most ISPs do not offer mobile IP support and there are still relatively few products and implementations to date, untethered telecom offers will ultimately become a significant revenue stream to carriers who offer the service.