Problems with Mobile IP and Their Solutions

Mobile IP, the outcome of the convergence of mobile communication technologies and the IP network, is the core technology to implement NGN applications in traditional networks. It supports network mobility, two-way access, and real-time multimedia services [1].

    As a recommended Internet standard, Mobile IP is the earliest solution to mobility management of IP network. The IETF (Internet Engineering Task Force) gives a complete description of Mobile IP in RFC2002. Along with the development of IPv4 to IPv6, Mobile IP is also evolving to Mobile IPv6 [2].

    With the growth of telecommunications, it is inevitable to introduce and develop the Mobile IP technology. The huge market demand drives the growth of Mobile IP[3]. Besides, the technologies of General Packet Radio Service (GPRS), Wireless Local Area Network (WLAN) and CDMA2000 1X are also boosting the development of Mobile IP, providing a platform for the implementation of Mobile IP services [3, 4].

1 Problems with Mobile IP
Although growing rapidly, Mobile IP still has the following problems:
    (1) "Triangle routing" Problem
    The Communication Host (CH) has to send packets to the Mobile Host (MH) via the Home Agent (HA), while the MH sends packets directly to the CH. As the communication in the two directions follows different routes, the problem of "triangle routing" arises, which leads to low efficiency especially when the MH is far away from the HA and the CH is near to the MH.

    (2) Handoff Problem
    Handoff problem means that the HA sends the IP packets of the MH to the original foreign network via the tunnel because it doesn’t know the latest Care of Address (CoA) of the MH during the period starting when the MH leaves the original foreign network and ending when the HA receives the new registration address of the MH. As a result, these dropped IP packets have an influence on the communication between the MH and the CH especially when handoff occurs frequently or the MH is far away from the HA.

    (3) Problem of Intra-Domain Movement
    The frequent intra-domain movement of the MH within a small area will lead to frequent handoff. Consequently, a great amount of registered messages are generated in the network and the network performance is greatly affected [5].

    (4) QoS Problem
    In the mobile environment, it is hard to provide QoS over Mobile IP due to dynamically varying wireless network topologies, limited network resources, unpredictable effective bandwidth and high error rate.

2 Solutions

2.1 Solution to "Triangle Routing" Problem
For Mobile IP, routing optimization is necessary because all packets sent to the MH shall pass through the HA but the route may not be the best. After receiving the packets sent by the CH to the MH, the HA notifies the CH of the binding information about the MH, i.e., the current Foreign Agent (FA) address of the MH, and the CH encapsulates the packets and establishes the tunnel to the FA for transparent transmission. The binding information is transferred via a definite port number. If the MH moves again, the new FA will transfer the updated binding information to the old FA to ensure that the packets are transferred to the new FA. And meanwhile the HA gets the updated binding information so the subsequent packets will be transferred directly from the CH to the new FA. The mobile IP with route optimization sets high requirements on the CH. The CH shall have the ability to obtain the binding information, encapsulate the packets and establish the tunnel. Therefore the CH protocol stack needs lots of modifications.

2.2 Solution to Handoff Problem
The handoff process falls into two stages:
    (1) Mobile Test Stage
    In this stage, a mobile test is conducted for the MH to determine whether it has changed the sub-network for access.

    (2) Re-Registration Stage
    The re-registration stage refers to the period starting when the MH sends a registration request to the HA and ending when the HA receives the request, after the MH confirms the move. The length of the period depends on the distance from the MH to the HA.

    After the above two stages are completed, the MH continues to communicate with the CH. But any dropped packets caused in this period may interact with high-layer protocols, and consequently worsen the communication performance. The interaction with TCP is a typical example. In the mobile IP environment, the dropped packets caused by the handoff will make the interruption duration for TCP connection longer, and thus degrade the TCP performance. The most serious interruption is up to 12 s or so, and meanwhile there are several overtime retransmissions. In a word, the communication performance during the MH handoff depends on three factors: mobile test, re-registration and interaction with the high-layer protocol [6].

    In view of these problems, the concept of achieving local registration through the layered Mobile IP is put forward in the reference document [7], i.e., only when moving out of the area does the MH need re-registration to the HA. This method helps reduce the time delay for re-registration and improve the handoff performance of Mobile IP.

    In the reference document [6], a solution of "original FA notification" is proposed. It can effectively reduce the dropped data packets through the buffer memory. However, how to set the capacity of the buffer memory in the FA is a knotty problem. Also, it is necessary to define a new protocol to support the communication between the old FA and the new one.

2.3 Solution to Problem of Intra-Domain Movement
For the intra-domain micro-movement, improved protocols such as Cellular IP, HAWAII and TeleMIP can be adopted to solve the problem of frequent handoffs, and reduce handoff delay, packet loss ratio and registration information to the HA. For details please refer to reference document [8].

2.4 Solution to QoS Problem
Resource Reservation Protocol (RSVP) and Differentiate Service (DiffServ) have their respective strengths and weaknesses in providing QoS over Mobile IP. But they can be combined to solve the end-to-end QoS problem as shown in Figure 1. The Diffserv is employed in the backbone router, and the RSVP in the access part. When the host originates RSVP requests to the border router of the backbone access point, the border router will divide the requests into certain QoS levels and map them onto the DS field based on the content such as bandwidth and time delay carried by the RSVP requests and the preliminary definition. In the backbone DiffServ domain, the DS field can guarantee the QoS of transmission, and the border router at the backbone output restores the original RSVP requests and sends them to the destination [9].

3 Mobile IPv6
With the background of 3G development, the Mobile IP technology is of great importance in boosting the fast deployment of 3G and IPv6. In contrast to the mobility management mechanism at the link layer, Mobile IPv6 implements mobility management at the network layer.

    Mobile IPv6 supports the host in the same way as Mobile IP, and in the local network the MH works in the same way as any fixed hosts and routers. When the MH enters a foreign network, address auto-configuration defined by IPv6 is adopted to obtain the new CoA. The MH notifies the HA and the CH of its new CoA. The data packets sent by the CH that knows nothing about the MH’s new CoA are routed firstly to the local network of the MH, and the HA transfers these data packets via the tunnel to the MH. When receiving the tunnel packets forwarded by the HA, the MH will send the updated binding information to the CH to inform its new CoA because it knows the original sender of the data packets doesn’t have its address. When receiving the updated binding information, the CH can directly send data packets to the MH. In the reverse direction, data packets sent by the MH are directly routed to the CH [5].

    In comparison with Mobile IP, Mobile IPv6 has the following features:
    (1) Cancellation of FA
    In Mobile IP, multiple MHs may share one FA’s IP address, i.e., FA’s CoA, to alleviate the inadequate supply of IPv4 address resources. In this case, the data packets forwarded by the HA shall pass through the FA before being delivered to the MH. However, the MH in Mobile IPv6 can find a default router in the foreign network to provide routing service. The rich address resources and IPv6 address auto-configuration function make it easy for the MH to obtain the CoA. Therefore the FA is no longer needed.

    (2) Route Optimization
    Mobile IPv6 defines the header of the route that the CH can use directly to send data packets to the MH, making the route optimization a part of Mobile IPv6.

    (3) Security Function
    The enhanced security function of IPv6 is a great improvement to IPv4. Mobile IPv6 makes direct use of the function provided by IPsec, while Mobile IP should solve security problems by itself.
Although IPv6 has better support for mobility than IPv4, it still needs Mobile IP to implement mobility management and provide transparent mobility for the application layer or high-level protocols[4].

    Therefore, changes are made to IPv6 in the following aspects:
    (1) Neighbor Discovery Protocol (NDP)
    NDP enhances the mobility management ability, including CoA auto-configuration of the mobile node, neighbor discovery between the mobile node and the mobile agent and simplified mobile test process by finding the prefix of the network address.

    (2) Provision of Optional End-to-End Messages
    IPv6 provides optional end-to-end messages to reduce bad influence on the performance of the intermediate node router.

    (3) Security
    Security is a basic recommendation of IPv6. Necessary security protocols such as IPsec are required to ensure secure authentication and data transmission.

    (4) CoA
    IPv6 needs no foreign agent and related CoA of the foreign agent, so unlike IPv4, it can avoid problems caused by the network address transfer (NAT). The end-to-end communication is accordingly enhanced.

    (5) Route Optimization
    As a necessary means of communication, route optimization is supported by IPv6.The above mentioned are mobility solutions to single-layer terminals. As for compound multi-layer mobility, its solutions include compound solutions to the network layer and application layer of Mobile IP and Session Initiation Protocol (SIP) and all layer-involved compound solutions.

4 Applications of Mobile IP
Mobile IP is mainly applied in public services, individual information services and business applications.
Public services supported by Mobile IP can provide users with the latest information about weather, news, sports, entertainment, traffic and stock. Individual information services include webpage browse, Short Message Service (SMS), Email, unified message transfer, provision of telephony value-added services, etc.

    Apart from the office application, mobile commerce is the most potential business application. According to the statistics from Yankee Group Europe, a market investigation company, the handsets with genuine 3G functions will not come into the market until the end of 2004, and before 2005, SMS will play a more important role than services like mobile Internet access and webpage browse [10].
"Instant Activation of Remote Backup Host for Nonstop Services", the Mobile IP application in the finance industry, means when the host running in the IP network shuts down due to certain unpredictable factors, in order not to interrupt the service connection with all related clients, the preset remote backup host with the same network setting as the host will be activated to run the Mobile IP client software. With the Mobile IP agent server, the backup host can take over the job of the host in real time. This removal is completely transparent to all clients related to the host.

    Another application of Mobile IP is to establish voice portal and voice enabled website. The provision of voice applications on the Internet means every one can obtain voice information from Internet via the telephone, which offers the telephone with new applications. It is estimated that there will be 18 million subscribers who will use this kind of voice application in 2005, and the value of its equipment and service market will be up to USD 120 billion in 2006. Some companies, such as Audio Point (USA), have made use of cutting-edge Interactive Voice Response (IVR) technologies to offer a wide range of applications in the fields of media, bank, finance and tourism, allowing subscribers to get the information they want without talking with the operator.

5 Prospects for Mobile IP Technology
Mobile IP is the core technology to implement NGN applications in traditional networks, the new development of IP technologies and the convergence of wireless communication and IP technologies.
Mobile communication technologies are migrating smoothly from 2G to 3G, and now they belong to 2.5G. In this period, the application of the Mobile IP technology is still in its infant stage. The technology was firstly applied in enterprises’ networks, but the limited IP network bandwidth currently restricts the access speed and the development of Mobile IP functions.

    After entering the era of 3G, with the gradual elimination of certain restrictions, Mobile IP will find a wider range of applications and its role will be fully played. The full convergence of Mobile Internet and Mobile IP technology will greatly change people’s lives and work. In the era with ubiquitous information, mobile networks will allow people to really enjoy simple, convenient and fast connection by virtue of the advantages such as unparalleled flexibility, mobility, powerful scalability, fast speed of construction and easy maintenance. Mobile IP makes the network more flexible in mobility. It is attractive with its unique advantages and powerful functions.

    In the 4G era, Mobile IP will be widely applied as a mainstream technology, for the wireless connection speed of the 4G system in the initial stage may reach up to 90 times as high as that of the dial-up modem, being 300 times the speed of the current mobile Internet access, and wireless download rate up to 5-10 Mb/s. The 4G technology will promote the further convergence of mobile communications and IP technologies, making possible the integration of Mobile IP and mobile Internet.
In the future, with the all-IP mobile communication network evolved from broadband CDMA, operators will deliver more diversified services with lower charges. On one hand, in an all-IP network, voice, SMS and other services based on IP packets will undoubtedly reduce the communication cost, allowing subscribers to enjoy services of the same quality at lower expenses. On the other hand, through IP, subscribers can easily combine new communication services with traditional voice services. In this way, a variety of messages can be transmitted simultaneously over an IP network to meet various requirements of the two parties in communication. High-speed access for mobile users and flexible mobility for network users are expected to be the most universal means for service communication in the future, and Mobile IP will become the most pervasive mainstream technology in the communication application.

    We are still a little bit far from the emergence of mobile multimedia. Only through the development of 2.5G systems can the mobile multimedia market become mature, and can we be well aware of the basic service features and most attractive applications of 3G.

    It also should be noted that even at the age of 3G when more powerful and more efficient microcomputers, larger-capacity memories and better displays enable handsets to have those features only today’s desktops have, subscribers will not use mobile phones in the same way as computers. The handset cannot be used so conveniently as the computer due to the difficulty in inputting characters, small screen, short lifetime of the battery and no support for long execution cycle that the computer can provide. Hence the mobile handset can be nothing but a network partner to help people access to the Internet and obtain information through the browser in move.

6 Conclusions
The increasing number of office workers in move, the growing dependence on network computation and the Internet, and the growing convenience and computing ability that portable PCs support are all driving the growth of the Mobile IP technology.

    Since the introduction of Mobile IP in 1996, a number of free-of-charge and commercial Mobile IP applications have emerged. However, there still exist some difficulties in broad commercial applications of the Mobile IP technology and certain problems unable to be solved satisfactorily, such as the problems of security, encryption and authentication, entry filter, node handoff, intra-domain movement and QoS. Certainly, with these problems being solved gradually, Mobile IP technology will play an important role in mobile communications, bringing unprecedented possibility and convenience into people’s free communication and providing new opportunities to develop novel applications in the future.

References
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[4] Guang Xiaoming, Wu Jing. Mobile IP Analysis[J]. China Data Communications, 2003(11).
[5] Yang Xin, Chen Junliang. Mobility Support of the Network and Its Research[J]. China Data Communications, 2004(5).
[6] Ye Minhua, Liu Yu, Zhang Huimin. Handover Technology in Mobile IP[J]. Telecommunication Technology, 2003(12).
[7] IETF Internet Draft. Mobile IPv4 Regional Registration[S].
[8] Shen Min, Liu Zhimin. Mobile IP and Its Improved Technologies[J]. Telecommunications Science, 2003(6).
[9] Feng Yong, Li Fangwei. QoS Guarantee over Mobile IP[J]. Telecommunications Science, 2002(3).
[10] Lei Zhenzhou. Development and Market Trend of Mobile IP Technology[J]. China Radio Management, 2001(6).

Manuscript received: 2004-07-20