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1 Complexity of route optimization and mobility management Catharina Candolin Hannu H. Kari Laboratory for Theoretical Computer Science Helsinki University of Technology P.B. 5400, FIN HUT, Finland Abstract This paper discusses the extra complexity caused by route optimization to mobility management protocols. It will be shown that route optimization leads to extra signaling that is of no or very little use when firewalls are used to protect access networks and home networks. 1 Introduction With the proliferation of wireless networks, the need for mobility management protocols, such as Mobile IP [1][2], has increased dramatically. Wireless networks, in general, support higher rate of mobility than wired networks, and mobile nodes are thus likely to change their point ofattachment totheinternet frequently. Ad hocnetworking adds to this higher flexibility byallowing nodes to establish wireless networks without any predefined infrastructure, but at the same time causing extra complexity to mobility management. This paper focuses on the problem of route optimization in wireless networks that use Mobile IP for mobility management. Our main intention is to demonstrate that route optimization causes excessive complexity to mobility management and extra signaling when two mobile nodes are communicating with each other. 2 Background Different mobility solutions in IP networks can be provided by combining Mobile IP with ad hoc network routing protocols [4]. Mobile IPv4 defines three types of entities: Mobile Nodes (MN), Home Agents (HA), and Foreign Agents (FA). The FAs are omitted in Mobile IPv6. The MN has at least two IP addresses, a home address and a care-of address. The purpose of the HA is to forward packets destined for the home address of the MN to its current location. The MN may beattached to the fixed network via an ad hoc network [5]. This requires a gateway ( ) between the ad hoc network and the fixed network. The gateway provides the MN with a globally routable care-of address. However, within the ad hoc network, an ad hoc network routing protocol is used. To ensure smooth handoffs and lossless data communication over partially unreliable ad hoc networks, MNs can use multipath communication that is an optional feature of the Mobile IP standard. Multipath routing can be performed using alternative paths, as illustrated in Figure 1, with three different combinations: 1

2 HA MN Path 1 Path 2 Path 3 Path 4 = firewall = gateway HA = home agent MN = mobile node Figure 1: Connecting a MN to a fixed network via an ad hoc network HA a HA b a b MN a MN b Figure 2: Two mobile nodes communicating over fixed network ffl two paths between the MN and a of the ad hoc network (path 1 and path 2) ffl two paths between the MN and two s of the same access network via the same (path 1 and path 3) ffl two paths between the MN and its HA via two separate access networks using different s (path 1 and path 4) Route optimization [3] is part of the Mobile IP specification to minimize delays, jitter and load of the core network. Route optimization is done using binding update request/response messages, which are part of the Mobile IPv6 protocol. When a MN changes its point of attachment, it must inform its HA about its new care-of address. To omit the quadrilateral routing of existing sessions, the MN can directly inform its corresponding nodes. This is done by sending an additional binding update request directly to the address of each corresponding node. 3 Mobility management of two mobile nodes We study the case where two mobile nodes are communicating with each other, as illustrated in Figure 2. Both mobile nodes are assumed to be attached to the fixed network via separate ad hoc networks. We also assume that the initial session setup has been done between the two mobile nodes. Firewalls a and b are assumed to allow mobile-to-mobile communication using mobile nodes' permanent IP addresses. Thus, we can concentrate on the optional route optimization phase that is performed in order to get rid of the quadrilateral routing problem. When a connection between mobile nodes is established, the data communication path is as illustrated in Figure 3. 2

3 MN A Figure 3: Data transmission from MN a to MN b MN A (*) Figure 4: First binding update 3.1 First binding update When MN b receives a message from its home agent, HA b, with non-optimal routing, it may send a binding update request to its corresponding node, MN a, as illustrated in Figure 4. The problem arises, at the point illustrated with (*) in Figure 4, in a, since the binding update comes from MN b 's care-of address (of the access network 2). Because of the risk of denial-of-service attacks, a should not accept incoming binding updates from arbitrary IP addresses. There are two alternatives for configuring a. First, HA a can update its firewall instructing it to accept incoming binding updates coming to MN a 's permanent address. This can be done easily since HA a and a belong to the same organization and they can maintain a permanent security association between themselves. The second alternative would be to let MN b make asecurity association with a and let it do the update of its current care-of address to a. The main problem for MN b is to find out the address of a. The second alternative is not very feasible. When the binding update is passed through a, HA a forwards the binding update to MN a and the packet is treated as any mobile terminated packet, thus passing easily through. MN a responds to MN b with a binding acknowledgment sentto MN b 's care-of address. Both a and pass this reply packet with no problems since they have already passed the binding update request in the opposite direction. 3.2 Second binding update The second binding update is performed when MN a detects that the route is still not optimal. The second binding update procedure is illustrated in Figure 5. First, MN a has been informed by HA a about an unoptimal route. The binding update request goes from the MN a 's visited access network directly to of MN b 's visited network. Here, we have the same problem as in the previous binding update, as should know whether the binding update request should be passed through or not. In this case, the additional problem is that MN b and belong to different organizations and thus have no predefined security relationships. Therefore, MN b should first establish a security association with before instructing to allow binding update requests to pass through. When MN b eventually receives the binding update request, it can reply to MN a with a binding acknowledgment that passes through 3

4 MN A (*) Figure 5: Second binding update MN A Path a Path b Binding update request 1 Binding update request 2 Figure 6: Multipath data transmission between two nodes without any problems. For security reasons, the mobile nodes do not know the address of their visited network's firewall and, especially, they are not allowed to reconfigure the firewall as they wish. 3.3 Multipath support When a MN uses an ad hoc network to connect to the fixed network, the reliability of the wireless path from the MN to a is significantly lower than when the MN is directly connected to the. This is not only because the MN is moving but also all the other nodes between the MN and the can be moving, as illustrated in Figure 1. Therefore, mobile nodes are very likely to use multipath communication to ensure uninterrupted data delivery over ad hoc networks. This leads to an avalanche of signaling packets when the mobile nodes are both using ad hoc networks, as illustrated in Figure 6. In this example, MN a uses two paths with route optimization and MN b starts a second path. Two binding updates are required, since the MN b wants to be sure that the binding update reaches MN a. Then, there will be four alternative paths between MN a and MN b. To support route optimization, mobile nodes need to make also the above mentioned security associations with the access networks' firewalls for every path. Hence, the total number of extra signaling messages is high, while the probability that route optimization is ever needed is low. If route optimization is not used, only two paths are needed in both ad hoc networks and no extra signaling is needed beside the update of the HAs. 3.4 Optimizing the number of signaling messages Binding update requests are optional messages in Mobile IP. This means that if the firewalls are not updated, the core Mobile IP protocol still works, but packets are not sent via the shortest path in the fixed network, since route optimization will fail. Thus, the mobile nodes need to decide whether they should send or omit these optional signaling messages. This leads to an optimization problem where the first aspect is the additional load in the wireless networks due to extra signaling and the second aspect is the unoptimal routing in the fixed network. Which one is more optimal, depends on several parameters, such as the mobility frequency of the mobile nodes, the number of simultaneous sessions, the amount of transmitted data, and the number of used paths. 4

5 4 Conclusion When two mobile nodes are communicating, route optimization can be performed to minimize the data routing in the fixed network. This unfortunately significantly increases signaling by the mobile nodes over the wireless links. When firewalls are used, the route optimization requires also signaling between firewalls and home agents/mobile nodes. The signaling load gets even worse when the mobile nodes are moving frequently or when they are using multipath routing due to the wireless ad hoc access networks. Therefore, the complexity and overhead of route optimization are much higher than the benefits they can provide, especially when the capacity of the fixed backbone network is typically several order of magnitude higher than the one of the wireless access networks. Acknowledgments This research has been funded by the Finnish Defence Forces. We also thank Maarit Hietalahti for valuable comments on the paper. References [1] David B. Johnson and Charles Perkins. Mobility Support in IPv6. Internet Draft, IETF, draft-ietf-mobileip-ipv6-15.txt (work in progress), [2] Charles Perkins. IP Mobility Support for IPv4. Request for Comments 3220, [3] Charles Perkins and David B. Johnson. Route Optimization in Mobile IP. Internet Draft, IETF, draft-ietf-mobileip-optim-11.txt (work in progress), [4] J. Macker S. Corson. Mobile Ad hoc Networking (MANET): Routing Protocol Performance Issues and Evaluation Considerations. Request for Comments 2501, [5] Perkins C. Nilsson A. Tuominen A. Wakikawa R., Malinen. Global connectivity for IPv6 Mobile Ad hoc networks. Internet Draft, IETF, (work in progress),