An Adaptive Routing Scheme for Wireless Mobile Computing

An Adaptive Routing Scheme for Wireless Mobile Computing Ruixi Yuan C&C Software Technology Center, NEC Systems Laboratory, Inc. 1901 Gateway Drive, Irving, TX 75038 yuan@syl.dl.nec.com Abstract This paper presents a novel routing scheme for wireless mobile networking which is adaptive to the mobile traffic patterns. We propose a concept called friend network to accommodate the need for effective routing information update. Our routing scheme has the advantages of minimizing network bandwidth consumptions, providing optimal routing, and addressing network security concerns at the same time. 1.0 Introduction The advances in computing technology have produced miniaturized computers such as notebook and palmtop computers. Meanwhile, the rapid expansion of wireless communication technologies such as cellular network, wireless LAN are making it possible to support universal network connectivity for mobile computers. Such universal network connectivity offers great promises for general purpose mobile computing. However, network connection between mobile computers presents an entirely different challenge compared to that of the traditional fixed static network. One of the most important problems concerning mobile networking is the addressing and routing scheme that allows mobile hosts to move seamlessly from one location to another. Several proposals have appeared during the past two years [1-5]. For example, the Columbia IPIP protocol [1-2] creates a separate mobile subnet in a campus and uses IP tunnelling for traffic routing. The Sony protocol [3] modifies the TCP/IP stack towards VIP (virtual IP). Recently, Matsushita has proposed using a packet forwarding server (PFS) [6] to handle the mobile traffic. At the same time, location information management has also been an important problem concerning cellular networks and the newly developing PCS systems [7-9]. Another concern caused by the network mobility is security. Due to the constant location change of the mobile station, authentication is needed between the base and mobile stations to prevent unauthorized use of network resources. Furthermore, the continuous location updates of the mobile host also carry significant information of the mobile station, such information can be 1

regarded as private and its propagation should be restricted. This problem has been largely ignored in the present mobile IP proposals. In this paper, we propose a novel routing information update and traffic routing scheme based on the analysis of the mobile traffic pattern as well as concern on the network security. We begin in section 2 with the mobile network configuration and design considerations. In section 3, we present a set of functional requirements on the infrastructure that supports mobile networking. In section 4, we introduce the concept of friend network and describe our traffic pattern based adaptive routing scheme. Performance analysis and other related issues are discussed in section 5. Section 6 concludes the paper. 2.0 Network Configuration and Design Considerations Figure 1 depicts a sample configuration of a network that supports mobility. Here mobile hosts mh1, mh2 served by base station BS1 coexist with a static host sh1 and another mobile host mh3 served by base station BS2 on the same subnet. This subnet connects with another subnet via router G where mobile host mh4, served by base station BS3 is located. mh2 sh1 BS1 BS2 mh1 mh3 migration G BS3 mh4 Fig. 1: A sample network configuration. We have deliberately allowed the arbitrary mixing between mobile host and static host in Fig. 1, because such configuration allows more flexibility in a practical network. Some proposals such as the Columbia Mobile IP, allocate an exclusive subnet for mobile hosts. 2

In a traditional static network, the host address (e.g. IP address) uniquely defines its identity as well as its location. In a mobile network, however, such possibility no longer holds. Therefore, at least two separate parameters are needed to communicate with a mobile host. One serves as the identity of the mobile host, another conveys its location information. How to distribute the location information, to keep it consistent and up-to-date, and to use the information in delivering packets to the mobile hosts are the focusing points of mobile networking. We maintain that the existing static hosts and regular network routers should be able to communicate with the mobile hosts without any changes. Therefore, the burden to support mobile internetworking should be bore entirely by the mobile hosts (MH), base stations (BS) and the mobility support routers (MSR). We also choose to let the MH use its base stations address to represent the location information, because in a wireless environment, packets destined to the MH always transit its base station. This choice eliminates the need of an extra address assignment mechanism for the MH. 3.0 Functional Requirements on the Infrastructure The static network infrastructure is obviously inadequate to support mobile networking. Thus additional functions need to be added to the MHs, BSs, and MSRs to carry out the task. In this section, we list the functional requirements without giving details on how to implement these functions 1. We shall assume these functions exist when describing our routing scheme for mobile networking. A beaconing protocol enabling the MH to communicate with the BS and obtain network connectivity service. MSR routes the packet on a per host basis instead of the traditional network based routing. An encapsulation protocol is used to tunnels the packet for the MH to its current BS or MSR, the BS or MSR then decapsulates the packet and delivers it to the MH. A routing information update protocol to propagate the location information of the MHs between MSRs. This protocol is also used for the MH to inform its home MSR about its new location. A sender redirect mechanism that allows the MSR to inform the packet sender (or the packet encapsulator) about the new location of the MH where subsequent packets should be directed. These requirements are mostly self explanatory. The applicability of the sender redirect mechanism, however, depends on whether the packet sender can support such redirection [6]. 4.0 Traffic Pattern based Routing Scheme Traffic routing decision is made based on the routing information available. Thus the efficiency of a traffic routing scheme depends critically on the propagation of the routing information. In 1. The implementation issues are been actively pursued in the IETF mobile IP workgroup. 3

devising a routing information update protocol, it is necessary to keep in mind that excessive routing information update can be wasteful for network resources, while not enough routing information leads to non-optimal routing. This is especially important in a mobile network, as the host mobility demands frequent routing information updates. The network bandwidth used for the routing information update can be greatly reduced while still achieving optimal traffic routing, if we only propagate the routing information to those routers/hosts that need to communicate with the mobile host. For the routers/hosts without the routing information update, a forwarding and redirecting (if the relevant router/host supports such redirecting) scheme can be adopted if the need to communicate with the mobile host arises. Therefore, traffic pattern analysis plays an important role in designing a mobile network routing scheme. For each traffic pattern, there will be an optimal routing scheme that adapts to the corresponding traffic pattern. In a mobile networking environment, we can assume that the majority traffic for each mobile host is from a finite, limited number of networks/hosts. It is obvious that this assumption is very general and covers most of the mobile traffic scenarios. The networks where the majority traffic for the mobile host originated thus should be conceptually separated from other networks. We can create a special set of networks catering to the mobile traffic. Therefore, we introduce the friend network concept for mobile hosts based upon the above analysis. A network is considered a friend network of a mobile host if the following conditions hold: Substantial network traffic exists between the network and the mobile host. The network is a trusted network of the mobile host and equipped with an MSR. With the creation of the friend networks, the routing information can then be distributed much more efficiently. The network security can also be addressed by prohibiting the routing information propagation into unfriendly networks. In the following subsections, we describe the routing information update procedures and the subsequent packet routing in a mobile network. 4.1 Routing information Update When a mobile host migrates into a new guest network, the beaconing protocol enables the MH to obtain network connectivity service from its current BS or MSR. The routing information update is then propagated as follows: 1. The MH informs its home MSR about its new location 1. 2. The home MSR sends routing update to the friend network MSRs. 3. The home MSR may choose to send a sender redirect message to the hosts 2 currently communicating with the MH. 1. This first location update can also originate from the BS or MSR. 2. Sometimes, this sender redirect message is sent to the MSR forwarding packets using non-optimal route. 4

Figure 2 illustrates such routing information propagations. When an MH migrates into friendnet2, the routing information is only distributed among the friend networks, represented by the shaded area. homenet friendnet 2 friendnet 1 migration Fig.2: Routing information propagation in a mobile network. 4.2 Mobile Traffic Routing We adopt a two tier approach for traffic routing in a wireless mobile network. Initially, the packet destined to an MH uses the MH address (which represents its identity) as the destination address. Such packet is routed using normal routing mechanism towards its home network. At some point, the packet reaches a mobility support router (in the worst case scenario, the home MSR), the MSR checks the routing table, if it finds that MH is currently visiting a guest network, the MSR encapsulates the packet and sends it to the BS or MSR currently serving the MH. When receiving the encapsulated packet, the BS or MSR first decapsulates the packet, finds out from its cache if the MH is currently in its service area (cell). If yes, it delivers the packet to the MH via its own mechanism (wireless interface). If the MH has already migrated out of the its cell, the decapsulated packet is routed through normal routing mechanism towards its home network, the packet then repeats the same journey 1. Figure 3 illustrates the packet delivery process. 1. In some mobile routing scheme, a forward pointer is used here to increase efficiency, but in our scheme, such fo ward pointer may not be necessary. Because in most cases, the MSR serving the previous migrated network is a friend network router and shall have the updated location information. 5

sender static infrastructure MSR static infrastructure MSR BS MH MH MSR static infrastructure Fig. 3: Deliver a packet to a mobile host. 5.0 Performance Analysis Many factors need to be considered in the design of a network routing and addressing scheme that supports mobility. One main concern is the backward compatibility with the existing network infrastructure. Protocols require changes on the existing network infrastructure are simply too expensive to be implemented. Another major concern is the performance of the routing scheme, how much network resource it consumes in supporting the mobile hosts, and whether it produces optimal routing for the packets. Other related issues include reliability and security. In this section, we focus the discussion on the performance analysis. We analyze the efficiency of network bandwidth utilization of our scheme and compare it with three other existing proposals: Scheme 1: Friend network based routing information update. This scheme assumes that only those networks considered as the friend network get the updated location information of the MH. Thus traffic from the friend networks can achieve optimal routing, while others need to be forwarded to the MH by the home MSR of the mobile host. Scheme 2: Complete update. All network routers are MSRs, and whenever an MH migrates, the routing information is updated to all the routers. This scheme was first proposed in [4]. Scheme 3: Lazy update. The MH only informs its home MSR about its location, and when a packet destined to the MH arrives at an MSR, it queries the home MSR about the location of the MH. This scheme is similar to the Columbia cold search method [2]. However, it queries only the home MSR instead of flooding all the MSRs. Thus it is more efficient. Scheme 4: No update. This scheme assumes that only the home MSR receives location update from the MH and no extra routing update is performed. Thus the traffic needs to be forwarded to the MH at all times. This scheme was first proposed in [6]. 6

In order to quantify our analysis, we define several parameters pertinent to the mobile traffic pattern and network configuration: 1. Message/mobility ratio: M r. This parameter is similar to the call/mobility ratio introduced in [7]. It is the average number of messages the MH receives per move. 2. Friend network traffic ratio T f. The amount of traffic from the friend networks. which is also defined as majority traffic. It is usually about 80~90%. 3. Friend network ratio: n f. The number of friend networks divided by the number of networks in the entire corporate network. n f and T f are related with each other. 4. Average message*link cost to update the routing information to an MSR: x. This parameter is dependent on the network topology. 5. Average extra message*link cost to forward a packet to the mobile host instead of using a direct optimal route: y. This parameter is also dependent on the network topology. If we consider that MHs and MSRs distribute randomly within the corporate network, x=y is a good approximation. Furthermore, we assume that a sender redirect mechanism exist. After the first packet forwarded, the subsequent packets are routed through optimal route. This assumption allows us to equate the message lengths of a routing update and a data message, because only the first packet of the data message incurs extra link cost. 1 Taking the total number of networks in the corporate network as N. Simple arithmetics yields the extra message*link cost per mobile host migration as in Table 1: Routing Per move cost Effective region (if x=y) Scheme 1 N*n f *x+(1-t f )*M r *(x+y) Nn f /2T f < M r < N(1-n f )/2(1-T f ) Scheme 2 N*x M r > N(1-n f )/2(1-T f ) Scheme 3 2M r *x 0 < M r < Nn f /2T f Scheme 4 M r *(x+y) 0 < M r < Nn f /2T f Table 1: Comparison of extra message*link cost for different routing schemes A simple criterion is also deduced to determine which scheme is the most efficient with respect to the message/mobility ratio. When we consider some sample parameter numbers: n f =0.2, T f =0.8, and N=20. Traffic pattern based routing scheme would be most efficient when 1. In the event where such redirection is not available, forwarding a data message incurs more link costs then that of the routing update packet. This is equivalent to a bigger M r 7

2.5 <M r < 40. This is a rather large range. In addition, by adjusting the friend network ratio n f, our traffic pattern based routing scheme can adapt to the change of message/mobility ratio M r. 6.0 Conclusion In conclusion, we have proposed a novel routing scheme for mobile information network by creating a concept called friend networks. Our routing scheme greatly reduces the network resource consumption by routing information update while providing optimal routing for the mobile traffic. This routing scheme is applicable to a wide range of mobile traffic patterns. In addition, The feature of mobile network security can also be readily incorporated into the scheme by adding policy considerations into the friend network list management. 7.0 Acknowledgments The author is grateful to Dr. Jack Holtzman for bringing Ref. [7] into attention, and would like to thank the anonymous reviewer for valuable comments. 8.0 References [1] J. Ioannidis, Dan Duchamp and G. Q. Maguire Jr., IP based Protocols for Mobile Internetworking, Proceeding of SIGCOMM 91, ACM, September, 1991. [2] J. Ioannidis, D. Duchamp, G. Q. Maguire Jr, and Steve Deering, Protocol for Mobile Internetworking, Draft RFC, June, 1992. [3] F. Teraoka, VIP: IP Extension for Host Migration Transparency, Draft RFC, November, 1992. [4] K. G. Carlberg, A Routing Architecture that SupportMobile End Systems, MILCOM 92, October, 1992. [5] Y. Rekhter and C. Perkins, Short-cut Routing for Mobile Hosts, Draft RFC, July, 1992. [6] H. Wada, T. Ohnishi, and B. Marsh, Mobile Computing Environment based on Internet Packet Forwarding, 1992 Winter USENIX, January, 1993. [7] T. Imielinski and B. R. Badrinath, Mobile Wireless Computing: Solution and Challenges in Data Management, preprint. [8] D. J. Goodman, Trends in Cellular and Cordless Communications, IEEE Communication Magazine, June, 1991. [9] L. J. Ng, R. W. Donaldson and A. D. Malyan, Distributed Architectures and Databases for Intelligent Personal Communication Networks, Proceeding of the ICWC, June, 1992. 8