Analysis of Multicast for Mobility Protocol for IPv6 Networks

Transcription

1 Analysis of Multicast for Mobility Protocol for IPv6 etworks J. M. Corella, A. Mihailovic,. Georganopoulos, A. H. Aghvami Centre for Telecommunication Research, King s College London, Strand, London, WC2R 2LS, UK Tel: + 44 () , jose.corella@kcl.ac.uk Abstract We are going to describe an IP-mobility protocol named Multicast for Mobility Protocol (MMP) for IPv6 and compares it with other protocols: Cellular IP, Hawaii and Hierarchical Mobile IP. Finally, some simulation results will be presented in order to support our affirmations. 1. Introduction Over the past few years, we have seen the explosive growth of the Internet and the increasing popularity of notebook computers that are both portable and extremely powerful. As a result, users are not constrained to use their computers in a single location, and because there are huge collections of information and resources on the Internet, which is becoming indispensable, users require mobility support for their computer as they move around and maintain Internet access at the same time. However, existing Internet protocols do not support seamless host mobility. The network protocols, the TCP/IP protocol suite, have been designed with the assumption that the computers are always attached to the network at a signal physical location. If a Mobile Host moves without changing its address, it will lose the routing, but if the Mobile Host changes its address, it will lose the connection. To solve this problem we can use two addresses: a permanent address (Home Agent s Address) and a temporary address (Care-of Address) as is defined in the IETF s protocol Mobile IP [1]. In Mobile IP the Correspondent Host (CH) sends the packets to the permanent address, which is the Home Agent s Address and the Home Agent forward the packets to the Care-of Address, which is stored in its binding cache. This solution seems to be suitable. evertheless, each time we change our point of attachment in the network we should update the Care-of Address stored in the Home Agent. This procedure in a frequent handover environment would involve huge overhead in the network and the QoS would be unacceptable. Although this problem was mitigated by the introduction of Route Optimisation [2] in Mobile IP, the overhead and delay in the network was still too high. Route Optimisation allow us to avoid the triangular routing through the Home Agent, by establishing a direct routing from the Correspondent Host to the foreign network. In Mobile IPv6 Route Optimisation is implemented in the protocol itself. To solve definitely the problem, the separation between macro mobility and micro mobility environments has been introduced. Whereas Mobile IP will deal with the macro environment, a set of new specifically designed protocols has appeared to deal with the micro mobility environment. The better-known micro mobility protocols are: Cellular IP (CIP) [3][4] Hawaii [5] Hierarchical Mobile IP (HMIP) [6][7] Multicast for Mobility Protocol (MMP) [8] We can see an example of this implementation in a general network in Figure 1. Correspondent Host Mobile IP MMP Home Agent Micro Core Mobile Host Mobility Figure 1: Overall systems architecture. MACRO MICRO 2. Multicast for Mobility Protocol for IPv6 Multicast for Mobility Protocol (MMP) is a micro mobility protocol specifically designed to use a sparse mode multicast protocol called Core Based Trees [8] to handle the movement of the Mobile Hosts. Inside micro mobility domains a Core

2 Based Trees (CBT) routing protocol uses shared-trees always to and from a centre point called the Mobile Core (Core in CBT specifications for IPv4 [9]). In this revision of the protocol for IPv6, Core Based Trees is still the main characteristic of the protocol Routing: MMP relies on Mobile IPv6 for the inter-domain routing, but uses CBT inside the network. In order to explain it clearly the network is divided in the following parts: Macro mobility section: in our proposal Mobile IPv6 will be the protocol that is going to deal with this section. Micro mobility section: MMP deals with this area, as it is specifically designed to manage this environment. Radio access section: MMP manages this section, but uses Mobile IP. The purpose of using Mobile IP packets in the wireless access section is that the Core Based Trees protocol appears transparent to the Mobile Host. As the protocol tries to appear transparent for both Correspondent Host and Mobile Host, we should consider the following features: The Mobile Core will make the conversion between macro and micro mobility. The Base Station will use Mobile IPv6 The way MMP performs when a new Mobile Host arrives in the network is: Base Stations advertise Mobility Information Options used in IPv6 eighbour Discovery, including the multicast Care-of Address that the Mobile Host will use. When the Mobile Host receives the multicast Care-of Address, it sends a Mobile IPv6 a Registration Request. Once the Base Station has received the request, it sends two messages to the Mobile Core: o It forwards the Registration Request. o It sends a CBT Join Request. When the Mobile Core receives the Registration Request it includes its own Care-of Address in the header of the packet Moreover, when the Mobile Core receives the CBT join Request it acknowledges it with a CBT Join Ack and a routing tree will be established. We should notice that MMP only deals with the downlink and uses normal IP routing for the uplink. This approach is also used in the other micro mobility schemes. We should also notice that the IPv6 multicast Care-of Address has a scope field, which must be filled with a value according to the size of the MMP network Handover: When the Mobile Host receives a stronger beacon from another Base Station it will send a packet with a Mobile IPv6 Registration Request option header. In the same way as for the set up, the new Base Station sends a CBT Join Request. The join request packet will arrive at the cross-over router, which will acknowledge it to the Base Station. As result of the request, a new routing branch will be available from the new Base Station to the cross-over router. evertheless, when a handover occurs, as the old routing is still valid, the packets will be routed through the old branch as well. If we decrease the time-to-live to avoid this network inefficiency, then we will need to update the entries more often, which will increase the traffic. To avoid this problem in MMP we use a new packet, a MMP Instruct message, which will be sent by the Base Station and deletes the entry Soft State When we use CBT the old branch will be alive until the timeto-live Counter of the entry expires. To avoid losing the entry, Mobile Host refreshment packets are sent at specific intervals of time to reset the counter while this entry is still valid. This interval should, of course, be smaller than the entry time-to-live. One of the most important qualities of Multicast for Mobility Protocol is the low overhead produced in the Soft State. In the following lines we are going to model mathematically the overhead generated in MMP for a general hierarchical architecture and compare it with a protocol that does not have this propriety. To be more specific, we have chosen Hawaii for this purpose. ow define: m as the number of Mobile Hosts per Base Station in the network. n as the number of levels of hierarchy. i as the he number of nodes in the level of hierarchy i. For simplicity, we make the following assumptions: All the Base Stations have the same number of Mobile Hosts. All the Base Stations have Mobile Hosts connected. Finally, we can define the routing packet cost per refreshment in a the Hawaii protocol as: m n n (1) where n coincides with the number of Base Stations in the topology, and m n coincides with the total number of Mobile Host. If we take into account the acknowledgements in Equation (1), it is quite straightforward to see that the total routing packet cost per refreshment is: 2 m n n (2) But in a MMP network the total routing packet cost will be: where: and n m n + i i= 2 2 (3)... n 2 (4) 1 m (5)

3 To show the big difference between the two approaches, we apply this formula to the example defined in Figure 2 where the number of Mobile Hosts (m n ) is 12 and the number of hierarchy layers (n) is 4. The way TCP performs is as follows: TCP will increase the throughput rate until the sender advertisements window is reached; also if you limit the size of the window manually, then it will be the limit. When a handover will be produced, TCP will decrease it again. 1 = = 2 8 Tahoe Figure 2: Example of topology For this topology in Hawaii we will have: packets 2 m n 4 = = 96 refresh But MMP generate only: m = ( ) ( ) = 4 4 =12 m= 1 n packets = 276 refresh Take into account that usually we send refreshment messages every 3 ms when a Mobile Hosts is active, we conclude that MMP is much more bandwidth efficient. 3. Performance evaluation The test network used in ns-2 is shown in Figure 2. Only one Mobile Host has been simulated in order to keep the simulation as simple and clear as possible. The simulation starts when the Mobile Host is in cell 1 and moves from one Base Station to the other until it reaches cell 12. A typical bandwidth value for wireless local networks is the 1 Mbps of the wired network data rate and the point-to-point links have a delay of 1 ms. The handovers are performed at the following times: 6.5, 11.5, 16.5, 21.5, 26.5, 31.5, 36.5, 41.5, 46.5, 51.5 and 55.5 seconds, and the overall simulation time is 6 seconds. A very frequent handover environment has been selected in order to empathize its repercussion in the overall system performance. The transport protocol used in the simulations is TCP. Several TCP schemes have been tested in order to get the one that performs best in a MMP network. The TCP versions tested are: Tahoe,, ew, FACK (Forward ACK), SACK (Selective ACK) and Vegas [1]. Graph 1 shows the throughput of the Correspondent Host, measured in Kbytes per second, versus simulation time, measured in seconds, for the TCP schemes mentioned. Observe that the throughput is strongly reduced when a handover is performed. This decrease in the rate is due the packet losses when a handover needs to occur in the system. Troughput (KBps) Graph 1: Throughput versus Simulation Time using different TCP schemes in a MMP network. It can be observed in the graph that Tahoe and SACK (Selective ACK) TCP are the schemes that more throughput reach in the MMP tested network. Sequence Packets (packets) Simulation Time (s) Simulation Time (s) Vegas ew Tahoe Vegas ew Graph 2: Sent Sequence Packets versus Simulation Time for different TCP schemes in a MMP network. In Graph 2 the sequence packets reaching the Mobile Host are shown versus time. This measure may be the best way to compare the different TCP schemes tested as it takes into account only the valid data that the Mobile Host gets at the end of the simulation. It seems that the TCP schemes that

4 present a better performance are the same as those which reach a higher throughput: Tahoe and SACK TCP. In Graph 3 the maximum sequence number reached by each TCP scheme versus simulation time is presented. Correspondent Host decreases. Besides, the handover distance is one in the first, second, fourth, fifth, seventh, eighth, tenth and eleventh handovers; it is two in the third and ninth; and three in the sixth. Packets Graph 3: Maximum throughput for different TCP schemes in a MMP network. We can also appreciate that in the handovers the number of sequence packets remains the same over a long period. This delay is due to the way that TCP performs: when a handover occurs, the systems loses packets, and when the connection is being re-establish, the sequence number of the incoming packets will not match. As a result of this gap in the sequence the Mobile Host will start to send duplicated acknowledgements, and when those acknowledgments, with the same sequence number as for the last packet received in order, reach the Correspondent Host, it will restart sending packets in order following from the duplicated acknowledgements sequence number. In addition the Correspondent Host will reset its congestion window. Duplicated Acknowledgements (Packets) Tahoe Vegas ew Simulation Time (s) Tahoe Vegas ew Graph 5: Duplicated Acknowledgements versus Simulation Time for different TCP Schemes in a MMP network. Finally, Graph 5 shows the number of duplicated acknowledgements produced during handover for the different TCP schemes versus simulation time. Observe that the number of those packets is lower as the handover distance is bigger. This is due the fact that, as the handover distance increases, the distance between the cross-over router and the 4. MMP compared with other micro mobility protocols As in the core network, Mobile IP has been agreed as the protocol to be implemented in the micro mobility environment, which are, as mentioned before: Cellular IP (CIP) Hawaii Hierarchical Mobile IP (HMIP) Multicast for Mobility Protocol (MMP) In what follows, we provide an overview of the main differences between MMP (Multicast for Mobility Protocol) and others better-known micro mobility protocols: Cellular IP, Hawaii, and Hierarchical Mobile IP MMP vs. Cellular IP: The main characteristic of Cellular IP is the reduction of the control due to the use of the data packets sent by the Mobile Host (MH) as refreshment of the MH location. This property is definitely an advantage when compared with other protocols, such as MMP; with we regard to congestion in network. In the same way, the data packets from the MH are used to maintain the entries in the routing tables of the routers, so the downlink packets can be routed without the need for specific to maintain the entries. This is possible due to the database for the location of the MH, which is maintained with the uplink data packets and which on a hop-by-hop basis. In MMP we do not use this technique and specific is used for this purpose. This is due to two principal causes: 1. In MMP we do not have to worry very much about the overhead in the network. In MMP we use Core Based Trees (CBT) in order to set up the network and in the Soft State, and in CBT we have to maintain branches (paths), not nodes or host. This approach means that we are only going to send one refresh message per multicast group of Mobile Hosts (MHs) and only one refresh message from those that arrives to a node will continue its way up. 2. As very little is used in the Soft State we can maintain the routing tables almost without increasing the overhead in the network and without having to worry about the problem involved in the absence of uplink traffic in Cellular IP. In MMP we maintain a routing table in all the nodes with the following hop-by-hop address in order to reach the MH. In Cellular IP as in MMP we use IP addresses to identify Mobile Hosts, but in MMP those addresses are unicast Care-of Addresses. Cellular IP supports two types of handover scheme:

5 1. Cellular IP hard handover: this approach is simpler but incurs packet losses. 2. Cellular IP semi-soft handover: it is based on the idea that the MH can communicate with two Base Stations, the old and the new BS, during the handover. In this handoff no packets are lost in the TCP applications and loss is minimized in UDP ones. Regarding the MMP approach, only hard handover is consider as it tries to give more priority to more quicker handover than a better QoS during it. In both MMP and Cellular IP we distinguish idle and active mobile hosts, which reduces power consumption at the terminal side, extends battery life and reduces air interface traffic. Also in both protocols, when packets need to be sent to an idle mobile host, the host is paged. A mobile host becomes active upon reception of a paging packet and starts updating its location until it moves to an idle state again. Table 1 shows a simple comparison of CIP, Hawaii and Hierarchical Mobile IP. MH ID odes involved Routing Table Update MH aware of the protocol Mobile IP Means of Update Table 1: MMP compared with Hawaii, CIP and HMIP. MMP Hawaii CIP HMIP Multicast Care-of Home Home Care-of address address address address All the nodes in MMP network All the nodes in Hawaii network Only Cellular IP nodes (some) and uplink data packets odes with FA capabilities (some) o o Yes o Yes Yes o Yes Signalling 4.2. MMP vs. Hawaii: Signalling Uplink data packets Signalling Hawaii, as does MMP, relies on Mobile IP to provide widearea inter-domain mobility. Also in both protocols the Mobile ode retains its Care-of Address unchanged while moving within the foreign domain, thus the Home Agent does not need to be involved unless the mobile node moves to a new domain. MMP and Hawaii also share creating, updating and modifying the location information by explicit messages sent by Mobile Hosts or the Base Stations. But, in contrast with MMP, Hawaii defines four alternative paths set up schemes that control handover between access points. An appropriate path set up scheme is selected depending on the operator's priorities between: 2. Minimizing the handover latency 3. Maintaining the packet order On the other hand in MMP only one set up scheme is provided in order to keep the protocol as simple as possible, which minimizes the handover latency. Hawaii is the protocol that presents most similarity to MMP but, as its principal advantage, MMP presents a Soft State, which incurs much less overheads that Hawaii, as has been explained in section MMP vs. Hierarchical Mobile IP: In the Hierarchical Mobile IP (HMIP) approach we employ a hierarchy of Foreign Agents to handle Mobile IP registration locally. In other words, in HMIP we keep using the Mobile IP protocol in the micro mobility environment in spite of using a different specifically designed micro mobility protocol such as Cellular IP, Hawaii or MMP. In both protocols the Mobile Hosts send mobile IP Registration Requests (Registration Updates) to update their respective location information, as both of them use Mobile IP at the wireless interface. But in HMIP the Registration Messages establish tunnels between neighbouring Foreign Agents along the path from the Mobile Host to a Gateway Foreign Agent. In HMIP the packets addressed to Mobile Hosts travel in this network of tunnels, which can be viewed as a separate routing network overlay on top of IP. But in MMP for IPv4 we use tunnelling inside the micro mobility domain, but not in a hop-by-hop basis. In MMP for IPv6 the gateway should put this information into the header because in IPv6 we should not use IP-in-IP encapsulation. In the HMIP network the last Foreign Agents in the hierarchy are called Access Routers and they are the ones that deals with the Mobile Host. As in MMP, in Hierarchical Mobile IP we a can also differentiate between Active and Idle Hosts. In Hierarchical Mobile IP, the paging extensions allow Idle Mobile Hosts to operate in a power saving mode. In HMIP we define the concept of paging area as the domain of a Gateway Foreign Agent, whose Care-of Address is the one registered in the Home Agent. HMIP performs in the following way: after receiving a packet addressed to a Mobile Host located in a Foreign etwork, the Home Agent tunnels that packet to the Foreign Agent, exactly the same as for Mobile IP dealing with MMP. Then, the Foreign Agent sends the packet to the Mobile Host and this recovers the Active state as in MMP. Using this system, as in MMP and Hawaii, we increase the amount of time a Mobile Host can remain in a power saving mode. The main advantages of HMIP over MMP, Hawaii and CIP are: The use of tunnels makes it possible to employ the protocol in an IP network that carries non-mobile traffic as well. The simplicity of the protocol. We do not have a completely different protocol in the micro mobility environment. But HMIP also has the following disadvantage: 1. Eliminating the packet loss

6 It is much less flexible, because the designation of the Foreign Agents is done before the network starts to operate and is not dynamic. In other words, MMP, Hawaii and Cellular IP have much more dynamic routing than Hierarchical Mobile IP. 5. Conclusions We have presented preliminary evaluation of MMP for IPv6. MMP is based on the multicast routing protocol Core Based Trees (CBT), which builds shared-trees to and from a central point in the network called the Mobile Core. MMP relies on Mobile IP for the inter-domain routing and it appears transparent both to Mobile Hosts and Correspondent Hosts due to the use of Mobile IPv6 in the wireless network. MMP appears to be an excellent protocol for bandwidthlimited networks as it introduces very little overhead in the refreshment; and an almost identical performance to Hawaii and other micro mobility protocols in handover efficiency. Finally, some TCP schemes have been tested and compared. Tahoe and SACK (Selective ACK) have performed in the best way considering throughputs achieved. 6. References [1] C Perkins, ed., IP mobility support, RFC 22, [2] D. Johnson and C. Perkins, Route Optimisation in Mobile IP, Internet Draft, 6 July [3] A. Campell, J. Gomez, C-Y. Wan, S. Kim, Z. Turanyi, A. Valko, Cellular IP, Internet Draft, February 2. [4] Z. Shelby, D. Gatzounas, A. Camptell, C-Y. Wan, Cellular IPv6, Internet Draft (work in progress), ovember 2. [5] R. Ramjee, T. La Porta, S. Thuel, K. Varadhan, L. Salgarelli, IP micro-mobility support using Hawaii, Internet Draft (work in progress), 7 January 21. [6] C. Perkins, Mobile IP Local Registration with Hierarchical Foreign Agents, Internet Draft, 22 February [7] H. Soliman, C. Castelluccia, K. El-Malki, L. Bellier, Hierarchical MIPv6 mobility management Internet Draft (work in progress), September 2. [8] A. Mihailovic, M. Shabeer and A. H. Aghvami, "Multicast for Mobility Protocol (MMP) for emerging internet networks, PIMRC (PIMRC 2), September 2, London UK.. [9] A. Mihailovic, M. Shabeer and A. H. Aghvami, "Sparse mode multicast as a mobility solution for internet campus networks, PIMRC 99, Conference Proceedings, September [1] C. Barakat, E. Altman, W. Dabbous, On TCP Performance in Heterogeneous etwork: A Survey, Chapter 1, Systems Research, Project Mistral, July 1999.