Route Optimization Problems with Local Mobile Nodes in Nested Mobile Networks

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1 Route Optimization Problems with Local Mobile Nodes in Nested Mobile Networks Young Beom Kim 1, Young-Jae Park 1, Sangbok Kim 1, and Eui-Nam Huh 2 1 Dept. of Electronics Eng. and NITRI, Konkuk Univ., Seoul, Korea 2 Dept. of Computer Eng., Kyung Hee Univ., Seoul, Korea ybkim@konkuk.ac.kr, parkp333@daum.net, sangbok@swu.ac.kr, johnhuh@khu.ac.krg Abstract. As an extension of Mobile IPv6 to the case of network mobility (NEMO), the NEMO basic support protocol has been proposed. However, it suffers from some drawbacks such as pinball routing and large packet sizes in nested mobile networks (MoNET). In this paper we introduce new pinball routing problems associated with local mobile nodes in nested MoNETs and discuss on the effectiveness of several RO schemes in tackling the problems. Among the schemes we demonstrate that the RCS scheme can be an appropriate solution due to its low signaling overhead and short RO setup time compared to other approaches. Most importantly, we show that the RCS scheme is the unique solution for mobility support when a mobile network taking part in a nested MoNET changes its attachment point within the MoNET. 1 Introduction As wireless networking products and services are widely deployed, it is more likely that users carry a number of mobile device and these devices are interconnected with each other to form a small scale mobile network. In some cases, a mobile network (MoNET) can be attached to another mobile network. This pattern of forming a larger mobile network can proceed to any arbitrary depth, constituting a complex hierarchical network generally referred to as nested mobile networks. As an example of nested mobile networks, we can consider a situation where several devices such as mobile phones, laptops, and PDAs brought by a train passenger form a personal area network (PAN) and the PAN connects to the access network installed on the train to establish an Internet connection through one of the access routers available. Fig. 1 shows an example of a nested mobile network. This hierarchical architecture inherently raises several routing problems. Basically, route optimization (RO) aims to resolve the problem of non-optimal routing introduced by the bi-directional tunnel between a mobile router (MR) and its home agent (HA), namely MR-HA tunnel [2]. The non-optimal routing, also known as pinball routing in the literature, inevitably causes increased packet transfer delay, large packet header size, and long packet processing time [3]. Furthermore, in nested mobile networks, the undesirable effects get amplified with each level of nesting. M. Gavrilova et al. (Eds.): ICCSA 2006, LNCS 3981, pp , Springer-Verlag Berlin Heidelberg 2006

2 516 Y.B. Kim et al. CN AR MR 1 MR 2 VMN Internet LFN HA of VMN HA of MR 2 HA of MR 1 MR: Mobile Router HA: Home Agent LFN: Local Fixed Node VMN: Visiting Mobile Node CN: Correspondent Node Fig. 1. A nested mobile network In this paper we first introduce some peculiar pinball routing problems associated with local mobile nodes (LMN) in nested MoNET. To our knowledge, these routingproblems have never been reported in the literature. A LMN can be defined as either a mobile node or a MR, assigned to a home link belonging to a MoNET and which is able to change its point of attachment while maintaining ongoing sessions [4]. We show that the undesirable effects due to non-optimal routing appear in the worst form for LMNs. Next, in order to resolve the non-optimal routing problem associated with LMNs, we propose to use the RCS scheme[10] where binding updates (BU) for MNNs are performed to correlate MNN s home address with the top-level mobile router (TLMR) s care-of-address (CoA). In this way, all packets destined to MNNs are initially directed to the TLMR, bypassing the HAs of all the intermediate MRs in the MoNET. We compare the performance of RCS scheme with those of RRH[5] and RBU+[6] when applied to LMN cases. The advantages we can obtain with the RCS scheme can be summarized as follows: First, the signaling overhead for RO procedure is significantly reduced by confining the internal route information between the TLMR and each intra-monet node within the MoNET. Second, the RO setup time is very short compared to that of RBU+. Third, in the case where a sub-monet (i.e., a subnetwork of the MoNET) changes its attachment point within the MoNET, RRH and RBU can hardly support the mobility (i.e., the binding updates in all CNs and HAs associated with the sub-monet may not be possible) and may cause the Binding Update storm while the RCS scheme can resolve the situation with minimum overhead. The remainder of this paper is organized as follows: The next section presents the new pinball routing problems associated with LMNs in nested MoNETs and introduces related works. In Section 3 we give a detailed description on the RO procedure with the RCS scheme. In Section 4 we discuss the performance of RCS when applied to LMN cases. Finally Section 5 concludes the paper with some comments on further work.

3 Route Optimization Problems with Local Mobile Nodes in Nested Mobile Networks Route Optimization Problems with Local Mobile Nodes in Nested MoNET In this section we present the pinball routing problems associated with LMNs in nested MoNET. We consider the case where LMNs change their attachment points within the MoNET and investigate how badly this type of mobility impacts the routing performance. 2.1 Non-optimal Routing Problems Concerning Local Mobile Nodes Let us consider the nested MoNET shown in Fig. 2(a) where MR1 and MR2 together with TLMR constitute a nested MoNET. Suppose a mobile node MN1 under MR1 moves from MR1 to MR2. Then, MN1 becomes a VMN of the subnet under MR2 and its original attachment point, i.e., MR1 can act as MN1 s HA. Note that the mobile node MN1 now becomes a VMN of MR2. Such mobile nodes are termed as local mobile nodes [4]. 7 / V+R$ SUHIL[ V&R$ 7 V+R$ SUHIL[ / V&R$ V+R$ SUHIL[ VV&R$ V+R$ SUHIL[ V&R$ MN1 s HA V+R$ V&R$ (a) (b) Fig. 2. (a) The movement of a local mobile node (MN1) within a nested MoNET. (b) The binding update procedure invoked by the movement of MN1. Now, the movement of MN1 invokes the binding update procedure and a BU message should be sent to MN1 s HA, i.e., MR1. If the NEMO basic support scheme is applied, the path taken by the BU message would be "MN1 MR2 MR1 TLMR HA_TLMR HA_MR1 HA_MR2 HA_MR1 HA_TLMR TLMR MR1" as shown in Fig. 2 (b). 2.2 The Packet Delivery Route Between a LMN and a CN Upon completion of the BU procedure, a packet delivery from a CN residing in the same MoNET to MN1 is performed following a very complicated route. This

4 518 Y.B. Kim et al. 7 / MN1 s HA Fig. 3. The non-optimal route for packets sent from CN to MN1 procedure can be described in two steps. In the first step, a packet sent from CN is initially forwarded to MR1, which is the HA of MN1, and follows the route "CN MR3 TLMR HA_TLMR HA_ MR3 HA_MR1 HA_TLMR TLMR MR1" as denoted by the dotted line in Fig. 5. In the second step, using the binding information in its binding cache, MR1 knows that the packet should be forwarded to MR2 and the packet now takes the route MR1 TLMR HA_TLMR HA_MR1 HA_MR2 HA_MR1 HA_TLMR TLMR MR1 MR2 MN1" as denoted by the solid line in Fig. 3. It should be noted that the routes for binding updates and packet delivery can be extraordinarily complicated when a LMN moves within a MoNET when the bi-directional tunnels are used as suggested in [2]. 3 Route Optimization Using RCS Scheme In this section we describe the RO procedure when the RCS scheme is applied to the LMN cases. Whenever MN1 notices its changes of attachment point, it will issue a BU message toward HA-MR1 to bind its new care-of address with its home address. At each intermediate MR from MN1 to TLMR, the source address field is recursively substituted by the MR s CoA and finally at the TLMR, by the TLMR s CoA as shown in Fig. 4. During the CoA substitution process, each MR records the routing information in its own forwarding table that the address MN1-HoA corresponds to the address specified in the source address field of the BU packet. This information is useful later when packets heading for MN1 should be forwarded from TLMR to MN1. This process is repeated from MR2 to TLMR. At this point, the forwarding table entries at each MR may look like those in Fig. 5. Finally, MR1 stores the information which correlates MN1-HoA to TLMR s CoA, in its binding cache when receiving the BU message from MN1.

5 Route Optimization Problems with Local Mobile Nodes in Nested Mobile Networks / 65& &R$ '67 +$ +R$ V+R$ V&R$ 65& &R$ '67 +$ +R$ 65& &R$ '67 +$ +R$ 65& &R$ '67 +$ +R$ 'HVW2SWLRQ +R$ ILHOG %8PHVVDJH Fig. 4. The binding update procedure from MN1 to MR1 7 / 9r 6qqr Ir C ƒ DAI HI C 6HS 8 6 7vqvt8hpur V+R$ V&R$ 9r 6qqr Ir C ƒ DAI HI C 6HS!8 6 7vqvt8hpur V+R$ V&R$ 9r 6qqr Ir C ƒ DAI HI C 6HI 8 6 Fig. 5. The status of binding caches and forwarding tables at intermediate routers and CN after the completion of binding updates in MR1 and CN If a packet sent from CN arrives at MN1 after the BU process, MN1 will send another BU message toward CN for route optimization. After the BU process, any packet sent from CN to MN1 will be first forwarded to the TLMR using the type 2 routing option header and then forwarded to MN1 using the information in the forwarding table prepared in the BU process. Fig. 6 (a) depicts the packet route from CN to MN1 after the completion of binding update in HA-MR1 and CN when the ingress filtering is taken into consideration.

6 520 Y.B. Kim et al. 7 / 9r 6qqr Ir C ƒ DAI HI C 6HS 8 6 7vqvt8hpur V+R$ V&R$ 9r 6qqr Ir C ƒ DAI HI C 6HS!8 6 7vqvt8hpur V+R$ V&R$ 9r 6qqr Ir C ƒ DAI HI C 6HI 8 6 (a) 7 / 9r 6qqr Ir C ƒ DAI HI C 6HS 8 6 7vqvt8hpur V+R$ V&R$ 9r 6qqr Ir C ƒ DAI HI C 6HS!8 6 7vqvt8hpur V+R$ V&R$ 9r 6qqr Ir C ƒ DAI HI C 6HI 8 6 (b) Fig. 6. The packet delivery routes after the binding update in CN: (a) When the bi-directional tunnel is used between CN and TLMR to avoid the ingress filtering (b) When the ingress filtering is not applied 4 Performance Evaluations For end-to-end packet delivery, in case of RRH, the routing within the MoNET is performed using the source routing information packets carry in their own headers, while in case of RBU+ and RCS, it is done by the MoNET itself, thereby reducing the packet header size. However, in case of RBU+, a number of BU messages should be sent from the mobile node and each intermediate MR to the CN until the CN completes the RO process. The total number of BU messages issued increases with degree of nesting. The three schemes have the similar end-to-end delay after the completion

7 Route Optimization Problems with Local Mobile Nodes in Nested Mobile Networks 521 Table 1. Performance comparison results (N denotes the degree of nesting) RRH RBU+ RCS Amount of Routing Info. O(N) O(N) O(1) Number of BU Messages O(1) O(N) O(1) BU Packet Header Size O(N) O(1) O(1) Data Packet Header Size O(N) O(1) O(1) RO setup time O(1) O(N) O(1) of RO procedures. However, they differ in the RO setup time which represents the time elapsed until a CN finds the optimized route. The RO setup time should be much longer for RBU+ because at least N BU messages are required for RO at the CN for nested MoNETs with nesting degree N. Table 1 summarizes the performance comparison results in terms of the amount of routing information exchanged, number of BU messages issued per RO process, BU packet header size, the header size of data packets, and finally RO setup time. We now compare the performance of RRH, RBU+, and RCS in terms of memory size required for address bindings. Consider the nested mobile network shown in Fig. 7 (a). In the network MR1 takes the role of TLMR. We assume the basic address mapping in the form address A address B takes S bytes of memory. In order to support the mobility of mobile nodes MN1~MNr in the case of RRH scheme, the following binding information should be provided in the HA of MR N and CNs of MN1~MNr: MR1 s subnet prefix MR1 s CoA, MR2 s subnet prefix MR2 s CoA,, MR N s subnet prefix MR N s CoA. Thus, the total memory space required M RRH would be M RRH = S½ ½N + S½N = S(r+1)N (1) On the other hand, for RCS scheme the binding information should be stored as follows: MN1 s HoA MR1 s CoA in the HA of MN N and the CNs of mobile nodes MN1~MNr, and MR i s subnet prefix MR i s CoA in intermediate router MRi (i=1,2,,n). Therefore, the memory space required M RCS is given by M RCS = S(r+1+N) (2) Fig. 7 (b) depicts the required memory sizes for RRH and RCS schemes when r=10 and S=50 bytes. Let us consider a somewhat troublesome case where a mobile network taking part in a nested MoNET moves within the MoNET. In Fig. 8 (a) the mobile network formed by MR2, MN1, and MR4 (namely sub-monet ) moves from the subnet of MR1 to that of MR3. Under this circumstance, binding updates should be performed in HAs and CNs of all the mobile nodes (including MR2 itself) belonging to the sub-monet. However, this is not possible for RRH (and/or RBU+) scheme because the MNNs under MR2 can not notice the movement of MR2. Even if we assume the MNNs do notice the movement of MR2, a considerable number of BU messages should be sent out of the nested MoNET and this may cause a Binding Update storm. In RCS scheme this situation can be resolved as follows: After MR2 obtains a new CoA from MR3, MR2 sends a BU message. However, no BU messages need to be

8 522 Y.B. Kim et al. 1 U (a) Fig. 7. (a) A nested mobile network with degree of nesting N (b) Required memory sizes for RRH and RCS schemes (b) 7 / 7 / V+R$ V&R$ %8 PHVVDJH V +R $ V&R$ (a) Fig. 8. (a) The movement of MR2 and its own mobile network within a nested MoNET. (b) A BU message is issued by MR2 toward the TLMR. sent out of the MoNET because no binding information change has occurred with respect to the outside of the MoNET. This greatly helps in reducing the signaling overhead. Nonetheless, since internal paths for all MNNs under the sub-monet have changed, the forwarding tables at the intermediate MRs on the new routes from TLMR to the MNNs should be updated. To this end, it is required that a list of the HoAs (and subnet prefixes) of all the MNNs (and MRs) under MR2 be included in the BU message issued by MR2. This list is available from MR2 s forwarding table. Each intermediate MR updates its forwarding table when receiving the BU message. If the addresses in the list are new, the BU message is further forwarded to the upper MR (b)

9 Route Optimization Problems with Local Mobile Nodes in Nested Mobile Networks 523 Dest. Address Next Hop Interface No. Dest. Address Next Hop Interface No. MN1-HoA MN1-CoA 3 MN1-HoA MR2-CoA 2 MR4 s subnet prefix MR4-CoA 2 MR4 s subnet prefix MR2-CoA 2 (a) MR2 MR2 s subnet prefix MR2-CoA 2 Dest. Address Next Hop Interface No. (b) MR3 MN1-HoA MR3-CoA 7 MR4 s subnet prefix MR3-CoA 7 MR2 s subnet prefix MR3-CoA 7 (c) TLMR Fig. 9. The status of the forwarding tables of the MRs on the route from TLMR to MR2 after the BU process initiated by MR2 following the procedure described in Section 3. Otherwise, the BU message forwarding stops on this MR. For the case of Fig. 8 (a), the BU message is forwarded from MR2, to MR3, and then finally to TLMR where the BU message is not forwarded any further. After the BU process initiated by MR2 is complete, the forwarding tables of the MRs on the route reaching from TLMR to MR2 will look like those in Fig. 9. The routing information in the forwarding tables need to be maintained as a soft state such that the information gets invalid if it is not referenced for some amount of time. 5 Conclusions The NEMO basic support scheme in nested MoNETs produces a long transfer delay and excessively large packet sizes due to non-optimal routes and multiple encapsulations. In this paper, we introduced some pinball routing problems associated with local mobile nodes and discussed on the effectiveness of several RO schemes in tackling the problem. Among the schemes we demonstrated that the RCS scheme can be an appropriate solution due to its low signaling overhead and short RO setup time compared to other approaches. Most importantly, we showed that RCS scheme is the unique solution for mobility support when a mobile network taking part in a nested MoNET changes its attachment point within the MoNET. The RCS scheme has some drawbacks when applied to LMN cases in that some changes are required in existing mobile IPv6 standards in order to make MRs maintain the forwarding tables. Also we need a more efficient way for binding updates in the case of a sub-monet s movement inside a nested MoNET rather than including the HoAs of all the MNNs under the sub-monet in BU messages. These remain as the future work.

10 524 Y.B. Kim et al. Acknowledgement This research was supported by the MIC(Ministry of Information and Communication), Korea, under the ITRC(Information Technology Research Center) support program supervised by the IITA(Institute of Information Technology Assessment)" (IITA-20-C ). References 1. D. Johnson, C. Perkins, J. Arkko, "Mobility support in IPv6", RFC 3775, June Devarapalli, V., Wakikawa, R., Petrescu, A. and P. Thubert, "Network Mobility (NEMO)Basic Support Protocol", RFC 3963, January P. Thubert et al., "Taxonomy of Route Optimization models in the Nemo Context", Internet Draft (draft-thubert-nemo-ro-taxonomy-02), Internet Engineering Task Force, February T. Ernst et al., Network Mobility Support Terminology, Internet Draft (draft-ietf-nemoterminology-04), Internet Engineering Task Force, October 24, P. Thubert et al. "IPv6 Reverse Routing Header and its application to Mobile Networks", Internet Draft (draft-thubert-nemo-reverserouting- header-), Internet Engineering Task Force, June Hosik Cho, Eun Kyoung Paik, and Yanghee Choi, "RBU+: Recursive Binding Update for End-to-End Route Optimization in Nested Mobile Networks", Lecture Notes in Computer Science (LNCS), Springer-Verlag, pp , June E. Baccelli, T. Clausen, R. Wakikawa, "Route Optimization in Nested Mobile Networks (NEMO) using OLSR," NCS T. Clausen, P. Jacquet, "Optimized Link State Routing Protocol," RFC 3626, ietf.org/ rfc/rfc3626.txt, Hanlim Kim, Geunhyung Kim, Cheeha Kim, "S-RO : Simple Route Optimization Scheme with NEMO Transparency", ICOIN 20, LNCS 3391, 20, pp Y. B. Kim, K.-Y. Lee, H. Ku, E. Huh, "Route Optimization via Recursive CoA Substitution for Nested Mobile Networks ", Preprint for publication, 2006